Sorry if this has been asked already. I have searched but can't find an obvious answer.

Ok first this isn't specifically a LFS related question it relates to real cars. Reason I'm asking it here is that there are obviously some people on here with the required technical knowlege to answer it.

Finally to the question:

I often see on various car forums that it is extremely bad for the clutch to use high gears at low speeds (ie low revs). Especially to give it some throttle under these circumstances, (particularly in turbocharged cars which make 90% of their peak torque down at (say) around 2k rpm). The oft given reason for this, is because of the high levels of torque at low rpms is bad for the clutch.

Now, my physics is rusty but my knowlege of car engines and gearing makes me think this doesn't quite add up. Surely a car engine makes the same amount of torque for any given rpm/throttle opening combination irrespective of the gear it's in. So surely then the "load" on the clutch will also be the same for any given rpm/throttle opening irrespective of the gear it's in??

That all makes seems reasonable to me, but then I also know that higher gears are harder to move, (as anyone that has ever ridden a bicycle would know). Now, I think what people assume is that this means the clutch must be under a lot more load from the engine when in a high gear. But, surely the clutch should be rated to deal with the maximum torque produced by the engine, (plus obviously a significant amount of over engineering for reliability reasons). This being the case, combined with the fact that the engines output is the same irrespective of gearing, can someone explain to me in technical terms how the clutch can be under more stress if you open the throttle full at low speed/revs in a high gear than if you do the same thing in a low gear.

Looking at a diagram of a clutch assembly would solve this for you I think.

The forces from the engine may remain constant irrespective of gearing, but the forces coming back from the wheels are not. In higher gears there is less gearing reduction, so the forces coming the other way are larger, so there is the potential for more load on the clutch.

I don't know wether that is really done with some cars, but if the clutch is wet running on motoroil and it's connected to a dry-sump lubricated engine very low oil pressure could mean the clutch isn't well lubricated and the next gear change would harm the friction pads very much.

However the dry-sump + wet running clutch is propably rather rare.

Vain

My Aprilia has a dry sump/wet clutch system

Quote from Bob Smith :

The forces from the engine may remain constant irrespective of gearing, but the forces coming back from the wheels are not. In higher gears there is less gearing reduction, so the forces coming the other way are larger, so there is the potential for more load on the clutch.

I wouldn't say the forces are stronger in a higher gear - how would that be achieved? The lower gearing reduction in the higher gears is just not enough to sufficiently "overpower" the inertia of the car, and the clutch is and will always be the weakest link in the chain, so it'll slip...

Quote from bbman :

I wouldn't say the forces are stronger in a higher gear - how would that be achieved?

From the tires?

Consider that the clutch only wears when it's slipping/partially engaged (although it will wear/heat when engaged, but pretty damn negligably), then at low speeds, low rpm the clitch will have to be slipped. Trying to accelerate without making the engine either stall or leap out of the mounts means more slipping (and more revving) = more wear. In a lower gear you are more likely to be able to engage the clutch more quickly with less revving, and hence get less wear.

But if the clutch is engaged I think it's pretty unlikely to do any 'damage'. Unless you're clutch is weak/worn already and slips even with no release bearing pressure.

Quote from wheel4hummer :

From the tires?

I asked "how", not "from where"... The tires certainly aren't thinking "oh, he's in fifth, we have to be stickier now and create much more rolling resistance"...

Quote from bbman :

I wouldn't say the forces are stronger in a higher gear - how would that be achieved? The lower gearing reduction in the higher gears is just not enough to sufficiently "overpower" the inertia of the car, and the clutch is and will always be the weakest link in the chain, so it'll slip...

Exactly. With more force multiplication (or rather, less force division), and this multiplication applies to inertia as well give the different angular velocities, the clutch is more likely to slip, hence more wear.

Quote from Bob Smith :

Exactly. With more force multiplication (or rather, less force division), and this multiplication applies to inertia as well give the different angular velocities, the clutch is more likely to slip, hence more wear.

but hes talking about accelerating in different gears to the transfered torque is coming from the engine and identical in any gear
since you will be far below the torque band of your engine in a gear thats too long the stress on the clutch should be lower

afaik its because youre need to "grind" the clutch too get the car too roll, otherwise you stall the engine

The engine torque is the same regardless of the gear, assuming that the tires don't spin. The main issue is the length of time the clutch is slipped if launching in a higher gear, not the amount of torque involved. In a higher gear, the clutch will be slipping for a longer period of time.

The OP didn't mention clutch slipping at all - merely the application of full throttle in a high (numerically low) gear, at low revs (and hence relatively low road speed).

Don't add slipping to it, which will cloud the issue based on time, vehicle mass, engine output, clutch material etc.

Quote from Bob Smith :

The forces from the engine may remain constant irrespective of gearing, but the forces coming back from the wheels are not. In higher gears there is less gearing reduction, so the forces coming the other way are larger, so there is the potential for more load on the clutch.

btw is this strictly true at all?
i agree that the forces comming from the wheels are larger but to put any load on the clutch there also has to be a countermoment from the engine ie engine braking
all additional force from the wheels that will speed up the engine shouldnt have the potential to cause clutch slip since theres nothing twisting against it

Thanks for all the replies.

Bob where is the difference in angular velocities coming from? the clutch is engaged in the situation I'm refering to. The wheels have the same angular velocity at 40mph in 6th gear as they do in 2nd and the engine has the same angular velocity at 2k rpm in any gear, and exactly the same torque production as well.

Why is the clutch the weak point? it's been designed to handle maximum torque output of the engine + a margin for safety and reliability. Surely the higher gearing only means less force production at the wheels and therefore less acceleration.

I think I half get it. What we're saying then is that higher gears mean more reactive force at the clutch plates from the wheels because the wheel side clutch plates want to spin at a faster speed at say 40mph in 6th than they do in 2nd gear so the angular velocity differential between the engine speed and clutch plate speed is larger therefore the sum force acting on the plates is larger because of this??

I'd still like to see how the angular velocity part translates in to forces though. Any links to some equations?

Quote from tristancliffe :

.....
But if the clutch is engaged I think it's pretty unlikely to do any 'damage'. Unless you're clutch is weak/worn already and slips even with no release bearing pressure.

This is my thinking too, (though I kind of understand what bob is refering too).

Remember the argument is that in turbo, (especially turbo diesel), cars there is a lot of torque available at low revs. This specific fact is being given as the reason not to use a lot of throttle at low revs in high gear.

Makes no sense to me, surely the higher torque available at low revs in turbo cars means they are far more suitable to use more throttle at low revs in high gear than a normaly aspirated car not vice versa. Exactly because there is more torque available.

Quote from gezmoor :

Why is the clutch the weak point? it's been designed to handle maximum torque output of the engine + a margin for safety and reliability.

If you take a look at the whole drivetrain, it soon becomes apparant that the clutch has to be the weakest link: it's the only thing relying solely on friction to transfer torque - everything else uses a rigid shaft or some form of leverage (except a some diffs maybe)...

I think you view the problem from the wrong end of the stick: you needn't look at the torque output of the engine on the clutch, but the inertia from the car acting via the gearbox on the clutch... The higher the gear, the more pronounced this inertia will get, eventually leading to a torque difference too big for the clutch plates to handle...

Quote from bbman :

The higher the gear, the more pronounced this inertia will get, eventually leading to a torque difference too big for the clutch plates to handle...

that torque difference cant go beyond the maximum braking torque of the engine

The reason why is is not recommended to run at slow speeds in higher gears is due to oil pressure. An engine builds oil pressure by either running off a gear connected to the distributer, or via a crankshaft mounted pump. Sufficient oil pressure is needed to protect rod and main bearings from wear. It is not smart to run at excessively low RPMs.

The clutch is not going to care about low rpm operation. Go too high and the fingers will flatten out causing lost pressure on the friction material.

Quote from Shotglass :

that torque difference cant go beyond the maximum braking torque of the engine

Why not? Given that both forces would be directed backwards, that difference even has to be less, don't you think?

Quote from gezmoor :

... the fact that the engines output is the same irrespective of gearing, can someone explain to me in technical terms how the clutch can be under more stress if you open the throttle full at low speed/revs in a high gear than if you do the same thing in a low gear.

It isn't. The stress on the clutch would be the same in this case. In some GM cars, (Camaro, Firebird, Corvette), there was a lockout on manual transmissions to force a 1st to 4th gear shift if below a certain throttle pressure, and within a relatively low speed range (like 15mph to 19mph) to squeeze an extra 1 mile per gallon for the EPA rating, but here the ideal was to use minimal throttle at low speed in 4th gear.

Quote from bbman :

Why not?

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

Quote :

Given that both forces would be directed backwards, that difference even has to be less, don't you think?

pardon?

Quote from PAracer :

The reason why is is not recommended to run at slow speeds in higher gears is due to oil pressure. An engine builds oil pressure by either running off a gear connected to the distributer, or via a crankshaft mounted pump. Sufficient oil pressure is needed to protect rod and main bearings from wear. It is not smart to run at excessively low RPMs.

You only need oil pressure to overcome the rotation of the crackshaft, to get oil to the main bearings. Obviously the oil is assisted getting up to the big end, piston and little end. A well designed crank barely needs ANY oil pressure. At lower rpms you don't need anywhere NEAR as much oil pressure, which is why they are geared to give lower oil pressure at low revs, and sufficient at high rpm, with a PRV to dump excessive pressure (which is also bad).

In short, I think you are wrong - low rpms are not bad from a oil pressure and bearing wear point of view.

Quote from PAracer :

The clutch is not going to care about low rpm operation. Go too high and the fingers will flatten out causing lost pressure on the friction material.

To high on what? RPM? The inertia of the clutch won't cause the fingers to flatten, and the clamping force (which would be the only thing to flatten fingers) doesn't vary with RPM. Sorry, but you're wrong again (or I misunderstood you).

Quote from bbman :

If you take a look at the whole drivetrain, it soon becomes apparant that the clutch has to be the weakest link: it's the only thing relying solely on friction to transfer torque - everything else uses a rigid shaft or some form of leverage (except a some diffs maybe)...

Ok fair point, from that perspective the clutch must be the weak point. I was thinking more from the perspective of the clutch being able to handle the engines output, and that somehow this was capable of over coming the frictional resistance of the clutch (presuming a non-worn clutch) in some way which didn't make sense to me for the reasons I already stated.

Quote :

I think you view the problem from the wrong end of the stick: you needn't look at the torque output of the engine on the clutch, but the inertia from the car acting via the gearbox on the clutch... The higher the gear, the more pronounced this inertia will get, eventually leading to a torque difference too big for the clutch plates to handle...

I can see how this might make sense. But, if we take inertia as being the force fed back by the wheels through the gearing then newtons third law comes in to play. The force from the engine passes through the clutch then the gearbox where the force (more correctly torque) is altered, (lets assume multiplied) and passes to the wheels to drive the car forward. Now at the wheels the resulting force will be balanced at the contact patch, (assuming friction hasn't been overcome), by the reactive force applied to the contact patch by the road surface. This reactive force then passes back through the drive train, via the gearbox where it is altered, (in our example divided as the gearing is multiplying the force form the engine so the gearing must work in the opposite way on forces from the wheels), and ends up at the clutch plates as an exactly equal reaction to the force coming from the engine. All of the above is assuming no losses throught the entire transmission system for the sake of simplification.

The important point being that the gearbox has a fixed factor of multiplication or division in any given gear. So the affect on the forces coming from either the engine or wheels is negated.

eg

Take a gear box with a ratio of 1:2. The torque output of the engine at say 2k is say 100Nm. Then the torque seen by the wheels is 200Nm. This translates to a Force at the contact patch(s) of the wheels of say 400N (assume a 0.5m wheel radius). Now newtons laws say the reactive force in return must be 400N, which translates back to a torque 200Nm at the wheel and therefor the gearbox which then reduces it by a factor of 1:2 resulting in 100Nm torque at the clutch plates. An exact balance of the engines torque.

As you can see from the above example the ratio of the gearing is irrelevant. All that matters is what the engine output gets translated to at the road and the resulting accelaration given the mass of the car. Now obviously if the car is at a standstill or has the brakes on then the torque at the wheels, (and therefore the gearing ratio) becomes very important as to whether the car even moves. But in the situation I'm refering to the car is already moving so all resitance in the transmission or rolling resistance in the wheels etc has been overcome and we're left with basic F= ma.

That's my reasoning anyway.