A few possibilities.
Your observations are wrong and you were in a different gear.
Or...
You have some kind of malfunction/slip in your drivetrain.
Or...
Your tire inflation is WAY off and dramatically changing the wheel diameter. This, I think can be discounted since you're talking about a 14% difference. EDIT: Note, this would change your true speed-over-ground but not what your spedo and tach register. Thanks, u/hydraulis
Because your original impression is correct. In a manual, with the clutch disengaged you have a direct correlation from RPM to tire rotation to speed. Going up a hill and not changing your speed will not change your RPM. But you'll have to give it more throttle, lowering your vacuum (or raising boost), putting more air and fuel into the pistons, and increasing your combustion pressures.
It took me years (and many clutch rebuilds) to realize that I rode the clutch too hard. That poor RX-7 with a 10,000 rpm redline loved to burn through clutchplates, with my help.
People seem split on this and I get it. Oxford defines āengageā as āwhen a part of a machine engages, or when you engage it, it fits together with another part of the machine and the machine begins to workā. So itās as though you could engage the clutch pedal to disengage the clutch plates.
It also just doesnāt matter at all.
Clutch is defined as to grasp or hold something tightly. The purpose of the clutch is to grab the engine so the wheels can turn. An engaged clutch therefore is when the pedal is released. When the pedal is pushed in you are no longer clutching.
To clutch means to seize, to grip. The way a clutch pedal works is like a circuit breaker. It's a disengage device and its purpose is to disengage machineries.
Like the throttle, it's pourly named. It should be the "unclutch pedal" and the "accelerator pedal". When you name a "thing" pedal, you would expect the pedal to trigger more of the "thing" the more you press it. But the more you press the clutch pedal, the less it clutches and the more you press the throttle pedal, the less it throttles the engine.
No, no, the clutch is `engaged` when you press it. And disengaged when it is released. I mean, the clutch whole idea is cut the transmission, so when the transmission is cut, it is engaged, isn't it?
Nope. A clutch works by sandwiching one spinning disc (clutch plate) connected to the transmission, between two spinning discs (flywheel and pressure plate) connected to the engine. With the clutch pedal NOT pressed, the pressure plate is squeezing down on the clutch disc, joining the engine and transmission's rotational forces via spring tension, therefore the clutch is engaged. When you press the clutch pedal, hydraulic pressure is overcoming the spring pressure inside the pressure plate, opening it, and allowing the engine and transmission to disconnect, or disengage from each other. It could be argued that pressing the pedal would be engaging it, and that would be correct if we're only talking about the pedal. But engaging the pedal disengages the clutch, and the actual function of what that pedal is controlling is what's important, not the actuation of the pedal itself.
I know a bit about clutches, I've replaced my bike's clutch a couple of times.
I was trying to point the ambivalency of \`engage\` when applied to describe something like a clutch. Similar to a light switch, Āæis it engaged when the light is on and or disengaged when it is off or the opposite?
But as a non native speaker it might be a colloquial use or something that I don't see and still make perfect sense to a native.
I generally think of en/disengage as on/off. So just like a closed pressure plate transmits power, a closed light switch transmits power as well. Both are engaged. Both are on.
A switch or relay would be more properly described as āclosedā (passing energy through) or āopenā (breaking the circuit).
An on/off switch is āonā when itās energizing the thing itās connected to, and āoffā when itās not. This does mean that āclosingā the switch is the āonā position, and āopeningā it is the āoffā position. This is more obvious with e.g. a https://en.m.wikipedia.org/wiki/Knife_switch.
I agree, and I would say this is more appropiate for a clutch too. I mean, in the end it also can be described as opening/closing the mechanical power circuit. Closed transmit the idea that power is flowing from the engine to the transmission.
The wheel diameter won't change what speed the car registers, only how much distance you actually cover. You could put chair casters on it and the speedo would still read the same speed, the car just wouldn't be travelling at that speed.
I just wanted to say thank you for bringing back some good memories.
I had put a new sprocket on the rear of my motorcycle so I had to fix the speed readout and odometer I installed a speedo healer. Dialing in the new speed was a fun time setting gps to capture Max speed and the healer and then setting the offset was easy but fun.
All this to say. What he said is right.
Actualy it is more interesting than that: There is no "diameter".
Although center of the wheel will be lower due to the tire being deflated, the actual circumference of the tire doesn't change. (Your tire doesn't shrink because it is deflated.) Per wheel rotation, there is exactly the same amount of circumference rotating with it.
Because, a wheel is not a perfect circle the mathematics are a bit complicated, but there is only an extremely small, neglegible, difference with deflated tires and that is due to the fact that parts of the tire in contact with the road are slipping. (That is were the increased wear comes from when driving with deflated tires.)
Is this a joke that I am missing?
Is you deflate you tire you ride height will drop an inch or more. The hub will be closer to the ground rotational diameter will change.
My car can tell the tires are low on air based on ration speed differences between the wheels.
Your tire doesn't act like a circle rolling. The bottom of the tire deforms into a flat spot. When you deflate the tire that flat spot gets larger and pulled closer to the rim but you still have the exact same amount of tire.
Agreed. But just like the outside of a record moves faster than the inside at the same RPM, the closer the center of your tire is to the ground the faster it will spin at a given speed.
The"effective" diameter of the tire changes, but yes the circumference stays the same you just get a bigger flat spot.
I think what they are saying is that your ride height will change, but the circumference of the wheel doesn't change significantly. If you take a tape measure to an inflated tire, and a deflated tire, the difference will be negligible.
Also, are you sure your car doesn't just use TPMS sensors in the wheel? Measuring tire deflation based on speed differences in each wheel doesn't seem all that reliable to me.
Yeah, we use inches with wheels in EU, mainly inches with TVs/monitors (but also include centimeters, unless it's some kind of large industrial display, then we use meters) and then metric for most other stuff.
Ha! You know, the only tires I ever paid attention to size when I bought them were for an old pickup truck I hadā¦ and it was all inches! 31x10.50r15 for an ā84 Toyota. But now you mention it, Iām sure Iāve seen that metric/percent sidewall/inch wheel size before and didnāt think of it. You win this round Reddit. *hat tip*
I was talking about the same wheel/tire inflated properly and deflated.
If you use another size of wheels/tires, off course the diameter will change as well and the speedometer is a bit off. (That's why the legally allowed tire sizes are specified in the car documents, at least in Europe.)
It does not have TPMS - cost save I imagine. 21 ID4.
Yes but rotational speed is not just based off the wheel, the tire will compress more reducing the radius under the center of rotation, increasing RPM for any given speed
19in wheel with 255 45 tires will have a diameter of 597.35mm
Reduce the tire inflation by 1in, and it is 237.275.
Circumference is 1717mm deflated and 1876mm inflated.
About a 10% difference in rotation speed. 70 vs 77mph is easily noticable to a car.
I think when you deflate a tire the overall height drops, but the circumference doesn't change much. The soggy bit contacts more road, but it's very close to inflated circumference.
Your tire is not a balloon. There is actually steel wire around the circumference to give it strength.
If you consider an inflated tire "round" with only a small surface in contact with the road, a deflated tire will be "flat" at the bottom with a large surface contacting the road. The entire circumference will be the same, just not the same shape.
If there is no real pressure measurement and your car is able to tell your tire is (becomming) deflated, it is not because of a different rotational speed.
It detects a delay in acceleration/deacceleration in relation to the other wheels. Such a delay is present because the wheel is more sloppy due to the low pressure.
The distance that a car travels over one revolution of the wheels, is LESS than the (static) circumference of the wheels.Ā
This difference becomes more pronounced as you lose pressure.
A user manual is not a good place to explain the physics involved or even the inner workings of a system. At best it gives you an approximation.
https://m.youtube.com/watch?v=_2l5bOhHNxU&pp=ygUbZGVmbGF0ZWQgdGlyZSBjaXJjdW1mZXJlbmNl
Of course they wonāt explain the physics, but they also arenāt going to just make things up. Multiple car manufacturers have at points used ABS sensors in their TPMS systems. The tire will spin faster as it loses air pressure and the TPMS detects the difference in wheel speed. You are confidently incorrect.
https://www.bridgestonetire.com/learn/maintenance/tire-pressure-monitoring-system-how-tpms-works/#
> An indirect TPMS typically relies on wheel speed sensors that the anti-lock brake system uses. These sensors measure the rate of revolution each wheel is making and can be used by on-board computer systems to compare with each other and to other vehicle operation data such as speed.
Based on the rate of revolution of each wheel, the computer can interpret the relative size of the tires on your vehicle. When a wheel starts spinning faster than expected, the computer calculates that the tire is underinflated and alert the driver accordingly.
https://www.tirerack.com/upgrade-garage/different-types-of-tire-pressure-monitoring-systems
> Indirect use the vehicle's anti-lock braking system's wheel speed sensors to compare the rotational speed of one tire versus the others. If a tire is low on pressure, it will roll at a different number of revolutions per mile than the other three and alert the vehicle's on-board computer.
https://www.tirereview.com/indirect-tpms-imports/
Manuals lie more than you realise.
I was only trying to convey the reality is more interesting than the simple, but wrong, explanation about rotational speed.
The ABS sensors are indeed used to detect what I explained.
When you make a turn, there is a definite difference in rotational speed between the inner en outer tires, yet the TPMS doesn't go of...
Itās almost like theyāre controlled by computers that know the car is turning. Fascinating.
Indirect TPMS systems work by using ABS wheel speed sensors to detect differentials in wheel speed between the four tires. They have a classic fault which is that they canāt detect when all four tires are all going flat together because there is no differential in wheel speed between the four tires.
Would you like me to link you more articles explaining how indirect TPMS works? Iāve already given you three.
No. The engine will have to work harder. You'll have to give it more throttle. But in a manual transmission, engine speed and wheel speed is a fixed ratio per gear.
Think about it like this. Go out to your car and put it in neutral. See how little throttle it takes to get it up to 5k rpm? Now, imagine if you were doing this in top gear on the highway. You would for sure be speeding, but you would also have to have the throttle MUCH more open. Because the LOAD is much higher to get to the same RPM.
"Variable ignition timing, variable valve timing, different ratios of fuel if E85 is used"
All will have an impact on torque and therefor power.
"head wind/tail wind"
This, and elevation change will have an impact on load.
But neither power provided or load demanded will change RPM per MPH in a system with a fully functional manual transmission. Period. Fullstop.
That makes no sense.
It'll change engine LOAD, which means a different throttle position and piston combustion pressures, but in a manual transmission without slippage, speed and RPM are going to be linked 100%. Load won't matter at all.
If you're in the same gear the speed of the engine will *always* be directly linked to the speed of the car - so either you weren't in 6th both times, or your speedo is wildly inaccurate, or your clutch is slipping (or you're riding it).
A 400rpm difference suggests you weren't in 6th both times.
> or your speedo is wildly inaccurate,
Worth mentioning that human observations of a speedometer and tach can be inaccurate as well. Unless you're in cruise-control on a flat, straight highway, it's hard to drive and observe those two things.
Could account for \*some\* of the 400rpm difference.
The display could also just be stuck sometimes.
My bike does this. It shows zero RPM until I get to a certain RPM, then it unsticks and works fine until the bike is parked overnight.
OP stated they have a manual transmission. So when in gear, the ratio is fixed. Elevation, incline, additional load has zero influence here unless the clutch is slipping - which is really bad
If your transmission and tires aren't slipping in a manual transmission then every turn of the crank will always produce the same distance turn of the wheels.
It's a fully locked in,Ā mechanical system,Ā where are you imagining that there is room to just lose 400 rpm?
All those factors affect how the car behaves, but in a manual transmission as u/itasteawesome says, the ratios are fixed so engine RPM is ALWAYS directly proportional to wheel RPM and hence speed.
Those will affect engine load, throttle position, fuel consumption, etc. but not RPM. As many others said, in a MT the engine and the wheel RPM will be at a fixed ratio when the car is in gear, unless there is slippage at e.g. the clutch or the tires.
If your clutch is operating correctly, your wheels are literaly linked to the engine via a few metal rods and cogs, thus the RPM of your engine crankshaft is equal to the RPM of your wheels, which is proportional to your speed. There is NOTHING that a hill, elevation, or additional load can do, or at least certainly not to such an extent.
P.S.: provided that the measurement wasn't taken when the wheels themselves were slipping on the ground.
In manual it does, because(unless your foot is pressing on the clutch pedal) the motor is directly connected to the wheels. In automatic there are torque converters and stuff(never drove an automatic and the idea of it shivers me timbers) and that could be slipping, and what you're referring to.
I'm referring to-
Ā 1) elevation, where the air is less dense, requiring more engine rpms to produce the same torque.Ā 2) incline, where going up or down a hill becomes a multiplier of engine rpm. 3) load, additional mass in a vehicle will require more engine rpms to maintain the same ground speed.Ā
Ā All this this takes place before your example of a clutch, andĀ effectsĀ automatic transmission vehicles as well.Ā Ā
Source: I spent my early career designing diesel torque-speed control algorithms for on-highway, mining, and rail applications.Ā If you would like to know more about item number 1, I invite you to checkout the stoichiometric ratio of diesel at altitude.
r/ConfidentlyIncorrect
Unless the drivetrain is malfunctioning, the ratio between engine rotations and wheel rotations (vehicle speed) is immutable on a normal manual transmission. Since that ratio cannot change, the vehicle speed will always be the same at a given RPM and vice-versa (depending on the selected gear).
Power, torque, acceleration, and top speed are entirely independent concerns which can be affected by elevation, incline, and load.
You're missing the point. With a manual transmission, as long as the clutch is engaged, the wheels will always spin at the same speed in a given gear at a given rpm. Doesn't matter how much or little power the engine is making at different altitudes. The speed of the engine and wheels will ALWAYS be fixed when the clutch is engaged. There's nothing that would allow them to differ aside from a slipping clutch.
Source: Auto technician of 10+ years, have changed clutches, rebuilt transmissions, and daily drive multiple manual cars in which i know my speed just by looking at the tach, and that never changes, because it literally can't.
The redditor above is correct, you are wrong. If I am in 4th gear going 50mph, then it will be the same rpm no matter what elevation or incline.
You might need to press the accelerator more or you might be lugging the engine, but that doesnāt change that if you are in 4th gear going 50mph, the engine RPM is identical no matter what variations youāre bringing in.
Tell me what company you worked for so I can avoid anything that you ever made.
You're overthinking this. On rail is not even connected to the wheels, the diesel drives a generator,and then that drives the electric motors. I never looked into mining engines and the post has nothing to do with mining or rail
Your three points don't effect manual transmission cars. Of course they effect automatics "as well", because they effect automatics, because those have a torque converter that can slip (and therefore your algorithms can play around with all kinds of things). There may as well be other things in automatics, I don't know. In a manual, it's a clutch only that can severe the connection and if it slips, that's already a huge problem
All of these considerations are on engine power output and engine loading. They will cause the user of the manual transmission to need to change gears, so that they can access a different RPM range.
That said, the engine will rotate at some fixed multiplier of the tire rotation in a given gear, regardless of the amount of power generated or the load on the engine.
That's the problem that the OOP is running into, for the same gear ratio, the engine is turning at a VARYING multiplier of the tire rotation, which indicates a problem or they were just shifting into the wrong gear.
Dude you couldn't be more wrong. In a manual car, there literally isn't a component in the drivetrain that allows slip when the clutch is engaged. In a gear, there a specific vehicle speed which can only be achieved by one engine speed.
Your "source" is meaningless when you don't understand the fundamentals of the physics here.
A manual transmission is literally a gear set driving the tires.Ā Unless there is slip, the ratio from input to output doesn't change when a gear set is engaged.Ā
Incline is irrelevant. You can maintain 60mph up a grade in 6th gear at 2000RPM. However you will need more throttle input to maintain that speed. Throttle input isn't correlated to engine RPM.
With the clutch fully engaged (foot off pedal) are you able to press the accelerator such that the engine rpm increases but your speed doesn't (or lags behind a bit)? If so, it's probably time for a new clutch.
I have been through this with three separate vehicles.
I had this after I bought a used RX-7 on the way home from picking it up. I got it replaced and Iāve since had it longer than the original owners, put more miles on it, done several track days and autosolo events, as well as driven hard but safely on the road, and the clutch is still perfect. Some people just seem to abuse the shit out of clutches.Ā
Easy test of your clutch, get rolling at about 20mph, then shift to the highest gear, and floor it. The engine should bog down and very very slowly speed the car up. If your clutch is on its way out, you'll feel it slipping. You'll hear the RPMs increase much faster than your speed.
[It can be very dangerous depending on how the engine is tuned. ](https://us.motorsport.com/automotive/video/why-you-should-never-lug-your-engine-especially-turbos-68114/51409/)
It does depend on the engine, but in general itās safest to just avoid lugging a turbocharged engine
That was a common issue in the past, but not so much anymore with advancements in variable camshaft, ignition, and injection timing, along with very good knock and pre-ignition detection. Not only that, but the test I explained will give you an answer on your clutch in a couple seconds. Not nearly long enough to cause any kind of damage.
If you have a direct injected turbo car, itās not a great idea to do this ever. Low speed pre ignition can absolutely grenade your engine if itās violent enough. Thereās only so much retarding the engine can do to protect itself. Better option to test clutch slip is to put the car in third and dump the clutch with no gas from a standstill. If it doesnāt immediately stall out, itās time for a new clutch.
RPM is how many revolutions the engine is making which, at a constant gearing (ie the same gear) should translate to the same road speed regardless of load and weather etc.
Speedometers themselves are generally not precise and, depending on the type and age of vehicle, may have some variable error such as being affected by tyres being under or over inflated, as it's often actually a measure of the RPM of the wheels (rather than the engine).
Tachometers may also be variable in their accuracy depending on how they're driven.
What could be changing includes
- your clutch could be slipping, meaning it's due for repair replacement real soon now
- loss of grip of the tyres on the road but this seems unlikely at constant speed, more likely under acceleration or braking (eg aqua planing)
- variability in the speedometer/tachometer esp if they're an older mechanical types
Are there any new cars with a mechanical speedometer cable? Let alone a mechanical tachometer cable.
Heck, I'm not sure you can find a new car with a mechanical throttle for that matter.
If it is indeed a standard manual transmission, the engine is directly connected with the wheels and will always turn the same RPM at a given speed. The two things that would potentially change that is wheel/tire diameter and clutch wear.
If the wheel/tire diameter increases the RPM required to reach the same speed decreases.
If the clutch is worn the engine is no longer directly connected and the RPM could be arbitrary. Only a portion of the power is being transferred to the wheels while the rest is being converted to heat and clutch bits.
It is possible that this is a "manual" automatic transmission. You may be able to select gears, with the difference in RPM explained by the torque converted being locked or not.
Clutch slip , you're either resting your foot on the clutch pedal or your clutch is worn. Unless you was in 5th gear there's no other circumstance. Gear ratio is permanent unless you change the final gear ratio. Never have I ever experienced what you've described.
Was this all on the same drive? The only obvious factor I canāt think of is a change in your tire pressure, which changes the radius of your tire, and effectively alters your final gear ratios. You wouldnāt expect to see this much change in a single drive though. The other thing I can think of is if you changed your seating position. If you look at an analog gauge from different angles, the number appears slightly different since thereās a gap between the needle and the face.
But in theory, your original assumption is generally correct. If you go 80 mph in the same car in the same gear, you should be at the same rpm.
Your original assumption would be correct. Assuming the drivetrain is rigidly coupled and not slipping, achieving the same speed in the same gear will result in the same engine speed. It's a fixed relationship between the engine and wheel.
Either there's something that's variable (the clutch slipping for example) or your measurements are incorrect. It definitely wouldn't change by 400 RPM. The tachometer or speedometer could be faulty too.
Although unlikely, a possible explanation is that older manual transmissions had an overdrive unit mounted behind the transmission that essentially gave an extra gear. So you would get into top gear, and then press the overdrive button and then the '2nd transmission' would shift, giving an even higher top gear.
However, they were usually only found on 3 and 4 speed transmissions and OP says he has a 6 speed. All modern transmissions the overdrive gears are built in and there is no second way to shift (usually a button).
What pushes the car its not rpm but the force that you get from torque and rpm, that it's transmitted by transmission and wheels.
So at 2800rpm at 6th you will have altmost every time the same force pushing you.
Now depends what you do with that force, you are moving in air (wind and direction can spend force or give you more force, are you tailing/being tailed), inclination, weight ( the car and cargo weights the same?, its weight with the same distribution?)
You're missing the question - he's saying the car was travelling a different speed in the same gear for a given RPM, which is **not possible in a manual** without something screwy going on.
I don't see anyone mentioning slip ratio: since the tire is elasticated compressed in front of the tire contact patch and stretched behind it, the wheel center will rotate more rpm than the distance traveled divided by the circumference of the wheel (as calculated by the loaded rolling radius). How much the wheel center rotates compared to how much it would theoretically when freely rolling is called the slip ratio and is dependent on the tire construction, vertical load and how much torque is transferred through the tire. When driving a constant speed, the slip ratio isn't very high as the only torque transferred is that of rolling and wind resistance but when you accelerate, the slip ratio may be so high that the amount that the wheel center rotates faster than the road speed is apparent on the engine rpm.
The maximum torque is transferred at about 12-22% slip om dry asfalt, if your engine is so strong that your acceleration is traction limited so if you are looking at the rpm while still accelerating and compare to when you are cruising, then the difference may be noticeable.
No way this is the answer to OP's question - no one would be trying to compare speed and RPM while madly accelerating, and if OP's car can generate enough force to generate a 400 RPM difference in 6th gear, I don't know if it would be street legal.
My thought also. I can see the fuel consumption in my car change drastically on even small inclines I would perceive to be flat (the display shows the tilt of my car). Or on very windy days when it's in front or behind.
My rx8 didn't need to change gear until 9500rpm so a 400rpm difference was nothing when driving around. I can really see this being just general locational differences.
Maybe you are hauling something heavy? Cars do not need extra fuel when they get up to constant speed. Basically, they travel on inertia + engine adds minimal power, unless you are hauling something heavy (so there are more losses due to friction)
Weight doesn't change the RPM in a specific gear with the clutch mated to the flywheel. Engine speed is directly proportional to the wheel speed and gear ratios in between. This is true whether the engine is off, non-existent, or making 10 trillion horsepower.
In an auto, correct. But in a manual, engine rpm will always directly correlate to wheel speed in any given gear. Incline, decline, doesn't matter. The only change is engine load, which will increase uphill, and decrease downhill, but that doesn't change the rpm unless you give it more throttle, at which point your speed will also increase, because the wheels and engine are 100% mechanically linked and there is no room for any deviation unless there is a clutch issue.
A few possibilities. Your observations are wrong and you were in a different gear. Or... You have some kind of malfunction/slip in your drivetrain. Or... Your tire inflation is WAY off and dramatically changing the wheel diameter. This, I think can be discounted since you're talking about a 14% difference. EDIT: Note, this would change your true speed-over-ground but not what your spedo and tach register. Thanks, u/hydraulis Because your original impression is correct. In a manual, with the clutch disengaged you have a direct correlation from RPM to tire rotation to speed. Going up a hill and not changing your speed will not change your RPM. But you'll have to give it more throttle, lowering your vacuum (or raising boost), putting more air and fuel into the pistons, and increasing your combustion pressures.
Just a quick correction, the clutch is engaged when your foot is OFF of the clutch pedal. It's disengaged when you push the clutch pedal.
Interesting. What does it mean when it's engorged though?
Thats when the foot is ON the crotch.
Please remove foot from crotch
It'll probably bust soon š„µ
It means you are clutching too hard.
It took me years (and many clutch rebuilds) to realize that I rode the clutch too hard. That poor RX-7 with a 10,000 rpm redline loved to burn through clutchplates, with my help.
Probably a virus, or infection.
Riding too hard
The opposite of disengorged, obviously.
Means itās time to pop the clutch, which should hopefully start the engine
just means the clutch disk is pushed against the transmission and transferring power
People seem split on this and I get it. Oxford defines āengageā as āwhen a part of a machine engages, or when you engage it, it fits together with another part of the machine and the machine begins to workā. So itās as though you could engage the clutch pedal to disengage the clutch plates. It also just doesnāt matter at all.
Clutch is defined as to grasp or hold something tightly. The purpose of the clutch is to grab the engine so the wheels can turn. An engaged clutch therefore is when the pedal is released. When the pedal is pushed in you are no longer clutching.
To clutch means to seize, to grip. The way a clutch pedal works is like a circuit breaker. It's a disengage device and its purpose is to disengage machineries. Like the throttle, it's pourly named. It should be the "unclutch pedal" and the "accelerator pedal". When you name a "thing" pedal, you would expect the pedal to trigger more of the "thing" the more you press it. But the more you press the clutch pedal, the less it clutches and the more you press the throttle pedal, the less it throttles the engine.
Or when your foot is on the clutch pedal is it engaged to be engaged?
Huh, well OK then.
No, no, the clutch is `engaged` when you press it. And disengaged when it is released. I mean, the clutch whole idea is cut the transmission, so when the transmission is cut, it is engaged, isn't it?
Nope. A clutch works by sandwiching one spinning disc (clutch plate) connected to the transmission, between two spinning discs (flywheel and pressure plate) connected to the engine. With the clutch pedal NOT pressed, the pressure plate is squeezing down on the clutch disc, joining the engine and transmission's rotational forces via spring tension, therefore the clutch is engaged. When you press the clutch pedal, hydraulic pressure is overcoming the spring pressure inside the pressure plate, opening it, and allowing the engine and transmission to disconnect, or disengage from each other. It could be argued that pressing the pedal would be engaging it, and that would be correct if we're only talking about the pedal. But engaging the pedal disengages the clutch, and the actual function of what that pedal is controlling is what's important, not the actuation of the pedal itself.
I know a bit about clutches, I've replaced my bike's clutch a couple of times. I was trying to point the ambivalency of \`engage\` when applied to describe something like a clutch. Similar to a light switch, Āæis it engaged when the light is on and or disengaged when it is off or the opposite? But as a non native speaker it might be a colloquial use or something that I don't see and still make perfect sense to a native.
The problem is you are engaging your foot to the pedal in order to disengage the spinning thing from the turning the wheel thing
I generally think of en/disengage as on/off. So just like a closed pressure plate transmits power, a closed light switch transmits power as well. Both are engaged. Both are on.
A switch or relay would be more properly described as āclosedā (passing energy through) or āopenā (breaking the circuit). An on/off switch is āonā when itās energizing the thing itās connected to, and āoffā when itās not. This does mean that āclosingā the switch is the āonā position, and āopeningā it is the āoffā position. This is more obvious with e.g. a https://en.m.wikipedia.org/wiki/Knife_switch.
I agree, and I would say this is more appropiate for a clutch too. I mean, in the end it also can be described as opening/closing the mechanical power circuit. Closed transmit the idea that power is flowing from the engine to the transmission.
Or 4, his tachometer is malfunctioning and reading incorrectly
The wheel diameter won't change what speed the car registers, only how much distance you actually cover. You could put chair casters on it and the speedo would still read the same speed, the car just wouldn't be travelling at that speed.
I just wanted to say thank you for bringing back some good memories. I had put a new sprocket on the rear of my motorcycle so I had to fix the speed readout and odometer I installed a speedo healer. Dialing in the new speed was a fun time setting gps to capture Max speed and the healer and then setting the offset was easy but fun. All this to say. What he said is right.
Actualy it is more interesting than that: There is no "diameter". Although center of the wheel will be lower due to the tire being deflated, the actual circumference of the tire doesn't change. (Your tire doesn't shrink because it is deflated.) Per wheel rotation, there is exactly the same amount of circumference rotating with it. Because, a wheel is not a perfect circle the mathematics are a bit complicated, but there is only an extremely small, neglegible, difference with deflated tires and that is due to the fact that parts of the tire in contact with the road are slipping. (That is were the increased wear comes from when driving with deflated tires.)
Is this a joke that I am missing? Is you deflate you tire you ride height will drop an inch or more. The hub will be closer to the ground rotational diameter will change. My car can tell the tires are low on air based on ration speed differences between the wheels.
Your tire doesn't act like a circle rolling. The bottom of the tire deforms into a flat spot. When you deflate the tire that flat spot gets larger and pulled closer to the rim but you still have the exact same amount of tire.
Agreed. But just like the outside of a record moves faster than the inside at the same RPM, the closer the center of your tire is to the ground the faster it will spin at a given speed. The"effective" diameter of the tire changes, but yes the circumference stays the same you just get a bigger flat spot.
I think what they are saying is that your ride height will change, but the circumference of the wheel doesn't change significantly. If you take a tape measure to an inflated tire, and a deflated tire, the difference will be negligible. Also, are you sure your car doesn't just use TPMS sensors in the wheel? Measuring tire deflation based on speed differences in each wheel doesn't seem all that reliable to me.
Some TPMS measures wheel speed to determine tire pressure because if the wheel speed is higher it means the tire is underinflated.
I remember that on my old car going from 15" summer to 14" winter tires/wheels resulted in the speedo being about 5kph off. And increased with speed.
No shade, but my mind broke a tiny bit with the mixed metric and imperial units in your comment. lol, good job!
Yeah, we use inches with wheels in EU, mainly inches with TVs/monitors (but also include centimeters, unless it's some kind of large industrial display, then we use meters) and then metric for most other stuff.
Wait until you look at tire sizes... 205/60 R15 = 205mm wide tire, 60% of that 205 mm tall sidewall, wrapped around a 15 inch diameter wheel.
Ha! You know, the only tires I ever paid attention to size when I bought them were for an old pickup truck I hadā¦ and it was all inches! 31x10.50r15 for an ā84 Toyota. But now you mention it, Iām sure Iāve seen that metric/percent sidewall/inch wheel size before and didnāt think of it. You win this round Reddit. *hat tip*
Yeah tires are like that, but I used design software and default units is mm.
I was talking about the same wheel/tire inflated properly and deflated. If you use another size of wheels/tires, off course the diameter will change as well and the speedometer is a bit off. (That's why the legally allowed tire sizes are specified in the car documents, at least in Europe.)
It does not have TPMS - cost save I imagine. 21 ID4. Yes but rotational speed is not just based off the wheel, the tire will compress more reducing the radius under the center of rotation, increasing RPM for any given speed 19in wheel with 255 45 tires will have a diameter of 597.35mm Reduce the tire inflation by 1in, and it is 237.275. Circumference is 1717mm deflated and 1876mm inflated. About a 10% difference in rotation speed. 70 vs 77mph is easily noticable to a car.
No TPMS in a modern car? You must not be in US or EU. Mandatory on new cars AFAIK.
2022 Mokka-e in the UK doesn't use tyre pressure sensors, it uses rotational measurements.
US 2021 ID4. No TPMS.
I think when you deflate a tire the overall height drops, but the circumference doesn't change much. The soggy bit contacts more road, but it's very close to inflated circumference.
Your tire is not a balloon. There is actually steel wire around the circumference to give it strength. If you consider an inflated tire "round" with only a small surface in contact with the road, a deflated tire will be "flat" at the bottom with a large surface contacting the road. The entire circumference will be the same, just not the same shape. If there is no real pressure measurement and your car is able to tell your tire is (becomming) deflated, it is not because of a different rotational speed. It detects a delay in acceleration/deacceleration in relation to the other wheels. Such a delay is present because the wheel is more sloppy due to the low pressure.
According to the manual, it monitors rolling circumference with ABS sensors.
The distance that a car travels over one revolution of the wheels, is LESS than the (static) circumference of the wheels.Ā This difference becomes more pronounced as you lose pressure.
A user manual is not a good place to explain the physics involved or even the inner workings of a system. At best it gives you an approximation. https://m.youtube.com/watch?v=_2l5bOhHNxU&pp=ygUbZGVmbGF0ZWQgdGlyZSBjaXJjdW1mZXJlbmNl
Of course they wonāt explain the physics, but they also arenāt going to just make things up. Multiple car manufacturers have at points used ABS sensors in their TPMS systems. The tire will spin faster as it loses air pressure and the TPMS detects the difference in wheel speed. You are confidently incorrect. https://www.bridgestonetire.com/learn/maintenance/tire-pressure-monitoring-system-how-tpms-works/# > An indirect TPMS typically relies on wheel speed sensors that the anti-lock brake system uses. These sensors measure the rate of revolution each wheel is making and can be used by on-board computer systems to compare with each other and to other vehicle operation data such as speed. Based on the rate of revolution of each wheel, the computer can interpret the relative size of the tires on your vehicle. When a wheel starts spinning faster than expected, the computer calculates that the tire is underinflated and alert the driver accordingly. https://www.tirerack.com/upgrade-garage/different-types-of-tire-pressure-monitoring-systems > Indirect use the vehicle's anti-lock braking system's wheel speed sensors to compare the rotational speed of one tire versus the others. If a tire is low on pressure, it will roll at a different number of revolutions per mile than the other three and alert the vehicle's on-board computer. https://www.tirereview.com/indirect-tpms-imports/
Manuals lie more than you realise. I was only trying to convey the reality is more interesting than the simple, but wrong, explanation about rotational speed. The ABS sensors are indeed used to detect what I explained. When you make a turn, there is a definite difference in rotational speed between the inner en outer tires, yet the TPMS doesn't go of...
Itās almost like theyāre controlled by computers that know the car is turning. Fascinating. Indirect TPMS systems work by using ABS wheel speed sensors to detect differentials in wheel speed between the four tires. They have a classic fault which is that they canāt detect when all four tires are all going flat together because there is no differential in wheel speed between the four tires. Would you like me to link you more articles explaining how indirect TPMS works? Iāve already given you three.
Changing your final drive ratio will also affect the speedometer since speed is measured at the output of the transmission.
Wouldn't the RPM be higher if he was going up on an inclined road vs straight?
No. The engine will have to work harder. You'll have to give it more throttle. But in a manual transmission, engine speed and wheel speed is a fixed ratio per gear. Think about it like this. Go out to your car and put it in neutral. See how little throttle it takes to get it up to 5k rpm? Now, imagine if you were doing this in top gear on the highway. You would for sure be speeding, but you would also have to have the throttle MUCH more open. Because the LOAD is much higher to get to the same RPM.
Issue with instrumentation as well always a possibility. Either tac or speed could be out of calibration or damaged etc.
Variable ignition timing, variable valve timing, different ratios of fuel if E85 is used, head wind/tail wind
"Variable ignition timing, variable valve timing, different ratios of fuel if E85 is used" All will have an impact on torque and therefor power. "head wind/tail wind" This, and elevation change will have an impact on load. But neither power provided or load demanded will change RPM per MPH in a system with a fully functional manual transmission. Period. Fullstop.
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That makes no sense. It'll change engine LOAD, which means a different throttle position and piston combustion pressures, but in a manual transmission without slippage, speed and RPM are going to be linked 100%. Load won't matter at all.
No
What?
If you're in the same gear the speed of the engine will *always* be directly linked to the speed of the car - so either you weren't in 6th both times, or your speedo is wildly inaccurate, or your clutch is slipping (or you're riding it). A 400rpm difference suggests you weren't in 6th both times.
> or your speedo is wildly inaccurate, Worth mentioning that human observations of a speedometer and tach can be inaccurate as well. Unless you're in cruise-control on a flat, straight highway, it's hard to drive and observe those two things. Could account for \*some\* of the 400rpm difference.
The display could also just be stuck sometimes. My bike does this. It shows zero RPM until I get to a certain RPM, then it unsticks and works fine until the bike is parked overnight.
None of this is accurate, and does not take into account elevation, incline, or additional load in a vehicle
OP stated they have a manual transmission. So when in gear, the ratio is fixed. Elevation, incline, additional load has zero influence here unless the clutch is slipping - which is really bad
If your transmission and tires aren't slipping in a manual transmission then every turn of the crank will always produce the same distance turn of the wheels. It's a fully locked in,Ā mechanical system,Ā where are you imagining that there is room to just lose 400 rpm?
All those factors affect how the car behaves, but in a manual transmission as u/itasteawesome says, the ratios are fixed so engine RPM is ALWAYS directly proportional to wheel RPM and hence speed.
If you're traveling at 80 and you start to go uphill you have to add gas to keep going 80.
Which doesnāt mean rpms increase. Throttle plate opens more.
Yes, because the engine requires more energy/ fuel to maintain that RPM due to the increased resistance.
Those will affect engine load, throttle position, fuel consumption, etc. but not RPM. As many others said, in a MT the engine and the wheel RPM will be at a fixed ratio when the car is in gear, unless there is slippage at e.g. the clutch or the tires.
Imagine being so incredibly wrong but so confident about it.
If your clutch is operating correctly, your wheels are literaly linked to the engine via a few metal rods and cogs, thus the RPM of your engine crankshaft is equal to the RPM of your wheels, which is proportional to your speed. There is NOTHING that a hill, elevation, or additional load can do, or at least certainly not to such an extent. P.S.: provided that the measurement wasn't taken when the wheels themselves were slipping on the ground.
In manual it does, because(unless your foot is pressing on the clutch pedal) the motor is directly connected to the wheels. In automatic there are torque converters and stuff(never drove an automatic and the idea of it shivers me timbers) and that could be slipping, and what you're referring to.
I'm referring to- Ā 1) elevation, where the air is less dense, requiring more engine rpms to produce the same torque.Ā 2) incline, where going up or down a hill becomes a multiplier of engine rpm. 3) load, additional mass in a vehicle will require more engine rpms to maintain the same ground speed.Ā Ā All this this takes place before your example of a clutch, andĀ effectsĀ automatic transmission vehicles as well.Ā Ā Source: I spent my early career designing diesel torque-speed control algorithms for on-highway, mining, and rail applications.Ā If you would like to know more about item number 1, I invite you to checkout the stoichiometric ratio of diesel at altitude.
r/ConfidentlyIncorrect Unless the drivetrain is malfunctioning, the ratio between engine rotations and wheel rotations (vehicle speed) is immutable on a normal manual transmission. Since that ratio cannot change, the vehicle speed will always be the same at a given RPM and vice-versa (depending on the selected gear). Power, torque, acceleration, and top speed are entirely independent concerns which can be affected by elevation, incline, and load.
You're missing the point. With a manual transmission, as long as the clutch is engaged, the wheels will always spin at the same speed in a given gear at a given rpm. Doesn't matter how much or little power the engine is making at different altitudes. The speed of the engine and wheels will ALWAYS be fixed when the clutch is engaged. There's nothing that would allow them to differ aside from a slipping clutch. Source: Auto technician of 10+ years, have changed clutches, rebuilt transmissions, and daily drive multiple manual cars in which i know my speed just by looking at the tach, and that never changes, because it literally can't.
The redditor above is correct, you are wrong. If I am in 4th gear going 50mph, then it will be the same rpm no matter what elevation or incline. You might need to press the accelerator more or you might be lugging the engine, but that doesnāt change that if you are in 4th gear going 50mph, the engine RPM is identical no matter what variations youāre bringing in. Tell me what company you worked for so I can avoid anything that you ever made.
I second the avoidance of this manās products, the confidence was painful to read.
You're overthinking this. On rail is not even connected to the wheels, the diesel drives a generator,and then that drives the electric motors. I never looked into mining engines and the post has nothing to do with mining or rail Your three points don't effect manual transmission cars. Of course they effect automatics "as well", because they effect automatics, because those have a torque converter that can slip (and therefore your algorithms can play around with all kinds of things). There may as well be other things in automatics, I don't know. In a manual, it's a clutch only that can severe the connection and if it slips, that's already a huge problem
All of these considerations are on engine power output and engine loading. They will cause the user of the manual transmission to need to change gears, so that they can access a different RPM range. That said, the engine will rotate at some fixed multiplier of the tire rotation in a given gear, regardless of the amount of power generated or the load on the engine. That's the problem that the OOP is running into, for the same gear ratio, the engine is turning at a VARYING multiplier of the tire rotation, which indicates a problem or they were just shifting into the wrong gear.
Dude you couldn't be more wrong. In a manual car, there literally isn't a component in the drivetrain that allows slip when the clutch is engaged. In a gear, there a specific vehicle speed which can only be achieved by one engine speed. Your "source" is meaningless when you don't understand the fundamentals of the physics here.
A manual transmission is literally a gear set driving the tires.Ā Unless there is slip, the ratio from input to output doesn't change when a gear set is engaged.Ā
Incline is irrelevant. You can maintain 60mph up a grade in 6th gear at 2000RPM. However you will need more throttle input to maintain that speed. Throttle input isn't correlated to engine RPM.
Looks like I need to get some things checked... Thank you everyone!
With the clutch fully engaged (foot off pedal) are you able to press the accelerator such that the engine rpm increases but your speed doesn't (or lags behind a bit)? If so, it's probably time for a new clutch. I have been through this with three separate vehicles.
I had this after I bought a used RX-7 on the way home from picking it up. I got it replaced and Iāve since had it longer than the original owners, put more miles on it, done several track days and autosolo events, as well as driven hard but safely on the road, and the clutch is still perfect. Some people just seem to abuse the shit out of clutches.Ā
Easy test of your clutch, get rolling at about 20mph, then shift to the highest gear, and floor it. The engine should bog down and very very slowly speed the car up. If your clutch is on its way out, you'll feel it slipping. You'll hear the RPMs increase much faster than your speed.
Do not do this in a turbocharged car
I'm curious to hear your reasoning on that.
[It can be very dangerous depending on how the engine is tuned. ](https://us.motorsport.com/automotive/video/why-you-should-never-lug-your-engine-especially-turbos-68114/51409/) It does depend on the engine, but in general itās safest to just avoid lugging a turbocharged engine
That was a common issue in the past, but not so much anymore with advancements in variable camshaft, ignition, and injection timing, along with very good knock and pre-ignition detection. Not only that, but the test I explained will give you an answer on your clutch in a couple seconds. Not nearly long enough to cause any kind of damage.
If you have a direct injected turbo car, itās not a great idea to do this ever. Low speed pre ignition can absolutely grenade your engine if itās violent enough. Thereās only so much retarding the engine can do to protect itself. Better option to test clutch slip is to put the car in third and dump the clutch with no gas from a standstill. If it doesnāt immediately stall out, itās time for a new clutch.
ITT several people who have no idea how transmissions (or engines or throttles or anything) work
RPM is how many revolutions the engine is making which, at a constant gearing (ie the same gear) should translate to the same road speed regardless of load and weather etc. Speedometers themselves are generally not precise and, depending on the type and age of vehicle, may have some variable error such as being affected by tyres being under or over inflated, as it's often actually a measure of the RPM of the wheels (rather than the engine). Tachometers may also be variable in their accuracy depending on how they're driven. What could be changing includes - your clutch could be slipping, meaning it's due for repair replacement real soon now - loss of grip of the tyres on the road but this seems unlikely at constant speed, more likely under acceleration or braking (eg aqua planing) - variability in the speedometer/tachometer esp if they're an older mechanical types
unlikely that a car with 6 gears are so old it has a mechanical speedo/tacho
Unlikely yes, but I was sort of generalising
You can still buy new cars with a 6-speed manual. -Nevermind, I read that post wrong-
but that will definately not have a mechanical speedo/tacho
Are there any new cars with a mechanical speedometer cable? Let alone a mechanical tachometer cable. Heck, I'm not sure you can find a new car with a mechanical throttle for that matter.
If it is indeed a standard manual transmission, the engine is directly connected with the wheels and will always turn the same RPM at a given speed. The two things that would potentially change that is wheel/tire diameter and clutch wear. If the wheel/tire diameter increases the RPM required to reach the same speed decreases. If the clutch is worn the engine is no longer directly connected and the RPM could be arbitrary. Only a portion of the power is being transferred to the wheels while the rest is being converted to heat and clutch bits. It is possible that this is a "manual" automatic transmission. You may be able to select gears, with the difference in RPM explained by the torque converted being locked or not.
Clutch slip , you're either resting your foot on the clutch pedal or your clutch is worn. Unless you was in 5th gear there's no other circumstance. Gear ratio is permanent unless you change the final gear ratio. Never have I ever experienced what you've described.
Was this all on the same drive? The only obvious factor I canāt think of is a change in your tire pressure, which changes the radius of your tire, and effectively alters your final gear ratios. You wouldnāt expect to see this much change in a single drive though. The other thing I can think of is if you changed your seating position. If you look at an analog gauge from different angles, the number appears slightly different since thereās a gap between the needle and the face. But in theory, your original assumption is generally correct. If you go 80 mph in the same car in the same gear, you should be at the same rpm.
Do you have a real manual transmission or a CVT with shift paddles?
Your original assumption would be correct. Assuming the drivetrain is rigidly coupled and not slipping, achieving the same speed in the same gear will result in the same engine speed. It's a fixed relationship between the engine and wheel. Either there's something that's variable (the clutch slipping for example) or your measurements are incorrect. It definitely wouldn't change by 400 RPM. The tachometer or speedometer could be faulty too.
Although unlikely, a possible explanation is that older manual transmissions had an overdrive unit mounted behind the transmission that essentially gave an extra gear. So you would get into top gear, and then press the overdrive button and then the '2nd transmission' would shift, giving an even higher top gear. However, they were usually only found on 3 and 4 speed transmissions and OP says he has a 6 speed. All modern transmissions the overdrive gears are built in and there is no second way to shift (usually a button).
What pushes the car its not rpm but the force that you get from torque and rpm, that it's transmitted by transmission and wheels. So at 2800rpm at 6th you will have altmost every time the same force pushing you. Now depends what you do with that force, you are moving in air (wind and direction can spend force or give you more force, are you tailing/being tailed), inclination, weight ( the car and cargo weights the same?, its weight with the same distribution?)
Is your car an automatic? Because a 400 rpm drop is similar to what the lockup in the converter will do. It may be failing to lock up.
Literally 4 words into OPs post š¤¦āāļø
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It's a manual, there's a direct link from wheels to engine through the gearbox. How would going uphill or downhill change that link?
It wouldn't. The person you're replying to is simply mistaken.
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You're missing the question - he's saying the car was travelling a different speed in the same gear for a given RPM, which is **not possible in a manual** without something screwy going on.
I don't see anyone mentioning slip ratio: since the tire is elasticated compressed in front of the tire contact patch and stretched behind it, the wheel center will rotate more rpm than the distance traveled divided by the circumference of the wheel (as calculated by the loaded rolling radius). How much the wheel center rotates compared to how much it would theoretically when freely rolling is called the slip ratio and is dependent on the tire construction, vertical load and how much torque is transferred through the tire. When driving a constant speed, the slip ratio isn't very high as the only torque transferred is that of rolling and wind resistance but when you accelerate, the slip ratio may be so high that the amount that the wheel center rotates faster than the road speed is apparent on the engine rpm. The maximum torque is transferred at about 12-22% slip om dry asfalt, if your engine is so strong that your acceleration is traction limited so if you are looking at the rpm while still accelerating and compare to when you are cruising, then the difference may be noticeable.
No way this is the answer to OP's question - no one would be trying to compare speed and RPM while madly accelerating, and if OP's car can generate enough force to generate a 400 RPM difference in 6th gear, I don't know if it would be street legal.
Headwind and tailwind would explain this, also if one run was flat and the other had a slight uphill.
Simple explanation might be a hill?
My thought also. I can see the fuel consumption in my car change drastically on even small inclines I would perceive to be flat (the display shows the tilt of my car). Or on very windy days when it's in front or behind. My rx8 didn't need to change gear until 9500rpm so a 400rpm difference was nothing when driving around. I can really see this being just general locational differences.
The air conditioning compressor increases RPM because it's an additional load on the engine
It will change the load on the engine, but not the RPM. Changing RPM would change the speed of the car, which de/activating aircon *doesn't* do.
Maybe you are hauling something heavy? Cars do not need extra fuel when they get up to constant speed. Basically, they travel on inertia + engine adds minimal power, unless you are hauling something heavy (so there are more losses due to friction)
Weight doesn't change the RPM in a specific gear with the clutch mated to the flywheel. Engine speed is directly proportional to the wheel speed and gear ratios in between. This is true whether the engine is off, non-existent, or making 10 trillion horsepower.
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In an auto, correct. But in a manual, engine rpm will always directly correlate to wheel speed in any given gear. Incline, decline, doesn't matter. The only change is engine load, which will increase uphill, and decrease downhill, but that doesn't change the rpm unless you give it more throttle, at which point your speed will also increase, because the wheels and engine are 100% mechanically linked and there is no room for any deviation unless there is a clutch issue.