Quote from Shotglass :

because any torque in excess will just serve to increase the engines rpm and pass through the clutch more or less without doing anything to it
same way by which the stress on the clutch should drop if you spin the wheels during acceleration

pardon?

Maybe I didn't clarify what I mean with the car's inertia: inertia of mass, drag and rolling resistance/tyre friction... Only the first one will act differently on accelerating or braking, the other two will always slow you down, no matter what... So the torque difference at the clutch will be bigger when those three forces act in the complete opposite direction to the engine's torque output compared to engine braking, where only the inertia of mass will counteract to the engine braking (drag and tyre friction even helping the braking)...

Quote from gezmoor :

[...]
That's my reasoning anyway.

I don't really understand what you're trying to describe in your example... When you're accelerating or braking, there can't be total balance of forces (high gear, low rpms and wide open throttle in your OP reads like you mean acceleration)... Again, I probably wasn't clear what forces I meant in my previous posts... Let me take your example, but for acceleration this time: you get an engine torque output of 200 Nm on one side of the clutch... When we assume a total resistance of 400 N at the wheels, you get the 100 Nm you quoted on the other clutch plate with your gear ratio of 1:2... Thus, there is a difference of 300 Nm between the two plates... If you take a lower gear with a ratio of 1:4, the resisting torque drops to 50 Nm, which means less difference/stress between the two plates...

I concede that you should never get a clutch to slip that way (unless the plates are unloaded or worn/glazed/whatever), and given I'm very much guessing here I might be totally wrong too...

Ok you've lost me. Gearing works in opposite multiplication factors depending on the side of the gears the force is input.

A gearbox with a ratio of:

Input 1:2 Output

will take a 100Nm Engine torque input and output 200Nm to the transmission/wheels.

The same gearbox will work in the opposite way on forces coming from the transmission/wheels:

Output 1:2 Input (or Input 2:1 Output)

will take that reactive 200Nm coming back from the road to the wheels, (as newtons third law states it must - even under conditions of acceleration) and it will be reduced to 100Nm on the engine side.

The gearbox doesn't mulitply the torque/forces with a ratio of 1:2 in both directions. It multiplies in one direction and divides in the other, that's how gearing works.

ie

Engine to wheels:

Engine (100Nm) - Clutch (100Nm) - Gears - (200Nm) - Wheels (200Nm) = 400N applied to road.

Wheels to Engine:

400N reactive force applied to tyre = Wheels (200Nm) - Gears - (100Nm) Clutch - Engine (100Nm).

So the clutch see's balanced forces. It must do otherwise by definition it would have to be slipping.

But, having said all that I can't corrolate any of the above with the fact that at 40mph, (for example), the gearbox side clutch plates must be revolving at a different speed if 6th gear is selected to if 2nd gear is used. So I know I'm missing something important here.

Edited to add- The forces are balanced at the contact patch and through the transmission, even under acceleration. Not to be confused with the forces acting on the car as a whole, which obviously are not balanced (ie drive force from the wheels are greater than the rolling resistance, air resistance etc presented to the car as a whole).

Ok, I wont pretend to know anything about cars, because I dont know much about them. But I am studying engineering (aerospace) and know a bit (a small bit) about how gears work. Also feel free to correct me if its wrong, I only just finished first year and have only done 1 unit which included gears.

Lets work in metric so my head doesn't explode and I'll try and keep it simple. The easiest way I can think of doing it is in terms of power, speeds and torques, you can do forces and gear ratios if you want and I hope you'd come to the exact same answers.

Lets say car is moving 20m/s, which is a bit more than 60kmh and the resistive forces at the wheels are 4000N (doesn't matter whether the car is accelerating or not). This means the power which needs to be supplied to the wheels is 20x4000=80000=80kW.

This is what the engine needs to supply to the wheels, and we'll ignore any power losses (of which there are many).

If you're in a gear where the engine is revving, say, 500rad/s (a bit under 5000rpm, so probably 1st or 2nd gear), and the engine then has to be supplying 80kW, so it must be delivering 80000/500 = 160Nm of torque.

Now lets say the engine is going 100rad/s, probably 5th or 6th gear, the engine now still needs to supply 80kW, but the torque it needs to supply is 80000/100 = 800Nm.

This means the clutch, which is connected to the engine, has to be transferring 5 times as much force. Or if the clutch is engaging/disengaging it has a change in force which is much greater (though this change in force depends on the speed and momentum of the car and engine, clutch, gearbox, etc when the clutch is engaged/disengaged and isn't hugely simple to calculate).

Now I didn't actually use any gear ratios, simply the difference in angular velocity of the engine, which is obtained by having different gear ratios, if you knew the diameter of the wheel you could calculate the gear ratios from it and you'd get the same result (I hope, unless I've made some fatally stupid mistake, which I often do especially given its absurdly late at night, so feel free to correct me).

Conclusion: Driving in 6th gear at low speed places larger torques on the engine, clutch and gearbox.

EDIT: Actually I'm an idiot... it is easier to explain in terms of gear ratios, I'm just not thinking straight.

Imagine the torque at the rear wheels is, say, 1600Nm (due to friction, wind resistance, acceleration, etc). Lets say you're in first gear with a gear ratio of 10:1... that means for every 10 revolutions of the engine, the rear wheels revolve once. This means the torque applied back through the gearbox changes from 1600Nm down to 160Nm. Now imagine the gear ratio is 2:1 (more than likely an overdrive 6th gear). The torque at the rear wheels is still 1600Nm (the car has the same resistances and same acceleration), but after gearing down the torque at the engine is now 800Nm.

Gezmoor, you're right the forces at the clutch have to be balanced otherwise the clutch is slipping. What you're neglecting is the fact the gearbox changes the amount of torque delivered from the wheels before it ever reaches the clutch. You can think of it in a few different ways. One that power = force (or torque) x speed (or rotational speed), if the speed decreases force must increase in a system where power (and thus energy) is not lost. Another way is to consider the forces at the contact patches where the gears mesh, these also have to be balanced so the gears dont slip, for 2 gears of different sizes (number of teeth) for the force at the contact patch to be balanced, the torques on the gears themselves must be different (torque = force x distance from rotational center).

Wolfracer, you're right, to drive at a constant velocity or constant acceleration requires less torque from the engine the lower the gear is. But the thing is that this is negligible in terms of clutch wear. I think tristan was the first to point out that clutch wear is caused by clutch slipping (some racing clutches can only deal with a handful of starts but can be driven hard all day long). So you may be putting a high load on the clutch if you are in such a high gear that you need to put on throttle to stop the engine from stalling.

Low rpm torque as a characteristic of turbo engines is irrelevant here as this low end torque is only the max torque at a given rpm. The real torque is defined by the position of the driver's right foot.

Apart from that I have to say there is quite some confusion in this thread. There can't be different amounts of torque on either side of the clutch unless it's slipping. Check out Free Body Diagrams, equilibrium of forces and Newton's third law.

there are a few mathematical formulas that one can use to explain this, but the easyest way to explain this (imo) is, lets say you put a piece of paper on a table and press on the center of it with the eraser side of a pencil (vertically), and spin the paper with the pencil (the contact of the eraser with the paper is the clutch in this case). then replace the piece of paper with a book (-_-) and press down on it with the pencil (same force as on paper) and try to spin the book (applying the same torque as you did on the paper). obviously you "clutch" is gonna slip. and btw slipping is the only way that i am aware of that might damage a clutch (because of the engine torque).

Quote from J.B. :

There can't be different amounts of torque on either side of the clutch unless it's slipping.

Even when a clutch is slipping, there's no difference in the amount of torque at the clutch plates. Angular acceleration versus angular inertia of the clutch plates themselves could cause a short period of difference in input and output torques, but this couldn't be sustained.

Using higher gears at slow speeds isn't usually an issue when the car is driven on a flat surface, but when it's driven uphill (especially when velocity decreases and/or the angle of the ascent increases).

Here's a graph attached that isn't actually made for this application but could give you an idea what's it about. Y-axis value is the actual force at the tread that's moving the car for each gear, plotted percentage curves are the total driving resistance (climb, drag, rolling) for each degree of climb (10% equals the climb of 10m in 100m). X-axis is the velocity..

When a car is driving uphill, the driving resistance is working to slow the car down, and if the driver needs to maintain a constant velocity the car is close to or at full throttle. If the gear is too high, there simply isn't enough force to overcome the resistance and the car begins to slow down. If the clutch is strong enough to withstand the needed torque from the engine, the engine will just slow down. Or, like it is when this actually happens, the safety margin designed into a clutch has disappeared during use and whenever the needed force to maintain a velocity is higher than the engine output, the clutch will slip.

EDIT: I had an idea when I started typing this, but can't really tell if it ended up helping with anything at all.

Quote from Huru-aito :

Using higher gears at slow speeds isn't usually an issue when the car is driven on a flat surface, but when it's driven uphill (especially when velocity decreases and/or the angle of the ascent increases).

Here's a graph attached that isn't actually made for this application but could give you an idea what's it about. Y-axis value is the actual force at the tread that's moving the car for each gear, plotted percentage curves are the total driving resistance (climb, drag, rolling) for each degree of climb (10% equals the climb of 10m in 100m). X-axis is the velocity..

When a car is driving uphill, the driving resistance is working to slow the car down, and if the driver needs to maintain a constant velocity the car is close to or at full throttle. If the gear is too high, there simply isn't enough force to overcome the resistance and the car begins to slow down. If the clutch is strong enough to withstand the needed torque from the engine, the engine will just slow down. Or, like it is when this actually happens, the safety margin designed into a clutch has disappeared during use and whenever the needed force to maintain a velocity is higher than the engine output, the clutch will slip.

EDIT: I had an idea when I started typing this, but can't really tell if it ended up helping with anything at all.

I can't think of a European or Japanese car made in the last 40 years that would have a clutch with so little margin for additional load. I can't even think of a US car either, but feel they are more likely to have had the problem

In short - this may technically be true, but you won't find a car so badly designed. The clutch is rarely the actual weak-link. More than likely you'd get wheelspin or stall before the clutch gave up.

hmm ok. Well it seems we haven't reached a conclusion.

Just to clarify however, I'm refering to the situation where the rpms aren't so low as to cause the engine to stall. eg. my car at 60mph in 6th gear is revving at 2k rpm. It's this kind of scenario I'm refering to. People are saying that the car shouldn't be driven at, (and certainly not accelerated from), those kinds of rpms in high gears as it's bad for the clutch. They argue that you should be in 3rd or 4th gear. I'm thinking it doesn't add up. Remember we're talking about clutch damage/wear not ideal acceleration.

I can imagine the engine being less efficient at such low revs but from the discussion so far I see no reason to believe there would be any effect on the clutch. Have "these people" tried to back up their arguments with any reasons/facts?

Quote from tristancliffe :

In short - this may technically be true, but you won't find a car so badly designed. The clutch is rarely the actual weak-link. More than likely you'd get wheelspin or stall before the clutch gave up.

That's why this whole high gear longevity thing is far fetched, at least in the way I understood the original post (low speeds, flat surface). In this case, most of the time the engine would just stall, or the tyres would give up.

Like I said, the clutch needs to be already worn out for this high gear slip to happen. And it wont happen on a flat surface. But in real life this does happen, as I've experienced it myself a few times. Twice with a VW T4 while towing a car on a trailer (still within max towing capacity), and once with a Fiat Uno 1.1 Both at top gear, highway speed (or a bit more than that in the Fiat!). Clutch stopped slipping after I changed down. I don't know what kind of abuse the T4 had been enjoying of, but it never slipped on the same roads without the extra weight being towed.

EDIT: Load up your car to the max laden weight, and find some steep hills to ride up and down and really try out the engineered torque capabilities of a clutch - most of the time our cars are driven way below the engineered limits of different parts.

so will gearing a rallyX car with closer ratios help it accelerate faster?

Wow, that's a sudden burst of off-topic.

Yes, if you space the ratios closer (for a given top gear) your through the gear accelerations will be quicker, albeit at the expense of very low speeds/standing starts, where the enforced high first gear will become a disadvantage.

You also won't have to use quite as many rpms before the change as well.

Quote from gezmoor :

My car at 60mph in 6th gear is revving at 2k rpm. It's this kind of scenario I'm refering to. People are saying that the car shouldn't be driven at, (and certainly not accelerated from), those kinds of rpms in high gears as it's bad for the clutch.

Those people have very likely not seen what a clutch looks like and are imagining some sort of magical device that glows with ethereal pulses of energy in their vague mental image of the arcane miracle which is called Ye Drivetraynne.

Quote from atlantian :

so will gearing a rallyX car with closer ratios help it accelerate faster?

Closer ratios only work if shift times are very fast (like the 30ms to 50ms shift times of Formula 1 and GT cars with computerized shifters). If the shift times are relatively long, then wider gearing works better. For example, the old record at Blackwood with a LX6 was set using a 3 gear setup, 4th, 5th, and 6th, since shift times were losing more than what would be gained by using a closer ratio 4 gear setup.

Quote :

clutch - the weak link only on some cars

In the how realistic is clutch overheating thread, there were two videos made of stock cars that had weak clutches. On the other hand, most USA based peformance cars have relatively strong clutches, and some drivetrain component will most likely break before the clutch slips if the clutch is dumped in a higher gear.

Quote from gezmoor :

hmm ok. Well it seems we haven't reached a conclusion.

Just to clarify however, I'm refering to the situation where the rpms aren't so low as to cause the engine to stall. eg. my car at 60mph in 6th gear is revving at 2k rpm. It's this kind of scenario I'm refering to. People are saying that the car shouldn't be driven at, (and certainly not accelerated from), those kinds of rpms in high gears as it's bad for the clutch. They argue that you should be in 3rd or 4th gear. I'm thinking it doesn't add up. Remember we're talking about clutch damage/wear not ideal acceleration.

Yes, if you spend most of your normal driving in high gears you will indeed be putting more torque on your clutch. This has nothing to do with "low end torque" or something like that, the reason is described by wolfracer.

The question is just how much this matters. I maintain that unless the clutch is already worn out by excessive slipping it doesn't matter at all.

Quote from Bob Smith :

I can imagine the engine being less efficient at such low revs...

That is what you might think just based on the awful noise coming from a low revving engine. But in fact this is exactly how you drive for max efficiency. The reason is that the lower the revs the more throttle you will need to get the same power. And the more throttle you use the more efficient the whole combustion process is. That's actually the reason small engines are more efficient than big engines: during normal driving the smaller engine spends it's time much closer to full throttle than the big engine.

Quote from J.B. :

the more throttle you use the more efficient the whole combustion process is.

Full throttle at the torque peak results in the highest ratio of power generated versus fuel consumed. However, only engines designed for near constant rpm usage (such as aircraft or generators) are operated like this. In the case of cars, the engines can generate much more power than is required to maintain speed even at 80mph. Friction losses are more than 1/2 the total losses until speeds reach between 35mph and 40mph, depending on the car. The result is many cars get their best gas milage around 45mph. Higher powered, lower drag cars would get their peak gas milage at higher still speeds.

But that doesn't change the fact that petrol engines are more efficient at full throttle than part throttle for the exact reasons explained above.