What kills your clutch, and how to avoid it
Riding the clutch is such a bad idea. You probably don’t even know the full extent of the problems this might cause. Let me explain why…
This report is inspired by the dwindling number of manual transmission drivers out there in the world, dreading the demise of the third pedal.
You’ve probably seen it, in traffic, where a stereotypical tradie (perhaps not unlike yourself) will, with a tray full of equipment and a trailer laden with tools and supplies, will rock the entire overloaded entourage by balancing on the friction plate of his clutch.
Or the boy racer who think’s he’s a talented driver.
This is, of course, mechanical torture of the most henous kind, punishable by early breakdown en route to the job site at 6am on a cold, wet winter’s morning.
But it’s not just tradies. Boy racers do this too, by trying to pretend they’re about to launch away from the traffic lights in some show of unbridled masculinity, only to arrive at the next set of traffic, three seconds before the rest.
Here’s how the question of clutch killing by way of egregious mechanical abuse was put to me:
Excellent question there, Emy, but what I actually said was that it saves excessive wear on the thrust brace. In fact, I think the clutch springs will be fine, because they’re quite durable, like most springs are.
There is, however, a very good reason not to de-clutch endlessly when you’re stopped in a manual, which we’ll get to in just a sec. But it’s not about clutch spring longevity. Let’s talk about this spring thing, first, based on Emy’s second point:
Think of steel like a cake - there’s a thousand different kinds. Spring steel in terms of its broad behaviour, operates in a ‘stress’ and ‘strain’ system, which is essentially just ‘load’ and ‘response’, ‘load: response’.
Obviously the load varies and that has a certain relationship to strain. The first landmark on this journey for a piece of steel is the ‘Yield Point’. Bend a ruler within its YP, it returns to its original shape and it’ll do that endlessly, which is the ‘spring’ you desire when making springs of any kind.
Apply a significant load which goes past the YP, you simply bend it. However, when you do this, you still get a certain amount of that desirable spring.
But steel can only take a certain amount of bending into what’s called the ‘plastic’ region, where it’s permanently deformed, before it snaps entirely.
This is why springs in your car are very carefully designed to operate only inside the safe zone before the yield point. Springs are ubiquitous, they’re everywhere - in your drop-saw, in your tyre pressure gauge and inflator, there’s one in your toaster, your stapler, anything with a valve or a trigger.
Actual springs are really quite durable, if you don’t over-stress them and bend them permanently. Just stay away from the yield point. Of course, in most decent mechanical designs, there are protections in place to stop you over-stressing critical springs.
In the case of coil springs, or leaf springs, in suspension systems, there are bump stops. The range of motion is managed by the geometric limitations in the mechanism, which keep the spring conservatively in the elastic zone.
Even in less critical designs, like a spring closing a pool gate, you usually can’t move the gate more than 180 degrees - because that’s when it runs into the fence.
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How your clutch works
In the case of a clutch mechanism, the clutch fork has a defined maximum range of motion, just like the one in a clothes peg - and that’s the maximum possible deflection of the clutch spring. You can’t really over-stress it.
Obviously the material and the design and the heat treatment has to be right - but subject to that, the clutch spring will out-last the friction plate.
A clutch actually works like this: The three major components are the machined surface on the rear end of the flywheel, which is bolted to the rear end of the engine’s crankshaft. Then there’s the pressure plate, which is bolted to the back end of the flywheel.
And then there’s the friction plate, which people also call the ‘clutch plate’, which is sandwiched between the pressure plate and the arse end of the flywheel.
Those finger-shaped things comprise a thing called a diaphragm spring, which is essentially a plate-shaped spring that squeezes the pressure plate onto the flywheel, with the friction plate sandwiched in between, which is how torque is transmitted, when your foot is off the clutch.
It’s that spring pressure that locks the clutch mechanism up and transmits drive from the crankshaft to the gearbox.
In the centre of the friction plate is this very neat little splined hole (see above), and, during powertrain assembly, because the gearbox loves the friction plate very, very much, he places his input shaft deep into the friction plate’s splined hole, in the hope that, together, they can one day crank out a bunch of totally fat burnouts - on private property, because doing that on a public road would be illegal. Obviously.
The gearbox and the clutch plate remain in this sandwich for as long as possible.
So, when your foot is off the clutch pedal, the whole mechanism is locked up, under pressure, transmitting drive. The pressure plate and the face of the flywheel clamp the friction plate down, and thanks to the hi-tech miracle of spliney clutch copulation, drive goes straight into the gearbox.
Even if you’ve got an engine making 800Nm, it’s really not that hard to restrain the torque, because it’s not that much. A fat man standing on the end of a diving board, is a couple of thousand Newton-metres. Engines don’t actually make that much torque, in absolute terms. Their big party trick is that they can make it while they’re spinning at 4000rpm.
When you reach about 6000rpm in second gear, or something, and you change up, you decouple the engine from the gearbox, briefly, to select third, without a big, expensive crunch. The whole mechanism, including the splined coupling and those radial finger diaphragm springs, is spinning around 100 times per second. This is so fast, it’s hard to conceptualise.
When you press the clutch pedal, that effort gets transmitted (by hydraulics or a cable) to a a clutch fork, which reaches in and presses down on the diaphragm spring, which loads up and releases the friction plate, decoupling the flywheel from the spline shaft, allowing you to change gear without breaking a lot of expensive components.
And the problem that needs to be overcome is that the diaphragm springs are spinning so fast and the clutch fork is completely stationary. How?
There’s this fairly simple, but clever, part in there, called a thrust race, or a thrust bearing, which allows the diaphragm springs to stop spinning around and lets the clutch fork transmit the thrust that bends the springs and decouples the gearbox from the crank, without tearing the whole assembly into small, formerly expensive pieces.
This is a thrust brace. Like most equipment used for thrusting, it should be treated with mechanical respect.
Most bearings deal with radial loads, which are loads at 90 degrees to the axis the shaft spins on. Thrust bearings deal with axial loads, which are loads in line with the shaft, basically. Thrust races are reasonably durable, because they get used a lot. Every time you change gear, basically.
But if you stop 60 times a day, for 60 seconds, and if you leave your foot on the clutch, with first gear selected, getting ready to take off, that’s an hour a day of extra load on the thrust race, times five days a week, times 52 weeks a year, it adds up to hours of unnecessary heavy wear for the thrust race.
You can avoid this increased wear entirely, simply by selecting neutral and taking your foot off the clutch, and waiting patiently for the green light.
The second reason for selecting neutral, which most people don’t understand, is that it’s potentially very bad for your engine, when you just sit there, stopped, with your foot on the clutch, fully de-clutched.
It’s bad because the load you’re putting on the clutch mechanism gets resolved by the crankshaft. That’s Newton’s third law. If you push on the rear end of the crankshaft, using the clutch, the crank has to push back, which means it suffers additional wear.
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Cleverly, one of the main bearings in the engine is designed to absorb thrust, and provide that reaction force, keeping the crank in place to a fairly high tolerance, in the backwards-forwards domain.
All the main bearings (on the crank) absorb radial loads from the sucking, squeezing, banging and blowing upstairs in the combustion chambers. But only one of them is typically designed to keep the crank from floating backwards and forwards. That thrust bearing is the one, typically, with the faces that wrap around.
So, every time you de-clutch, that bearing in the engine does a little bit of extra work, pushing back. And if the clutch fork mechanism is adjusted too tight, not only does the thrust race in the clutch mechanism work unnecessarily hard, all the time, so does the thrust bearing inside the engine. That’s definitely not ideal.
The inconvenient truth about thrust bearings on crankshafts is that they’re not especially good. They’re not pressure fed as well as the radial load faces.
The radial loads are huge in an engine. All that fire, pushing the pistons down hard. But thrust bearings are generally adequate, at best.
Ideally, you don’t want the crankshaft to float longitudinally because if it were to do that, it would over-stress a lot of expensive engine components that would destroy an engine. Conrods, and things of that nature.
Conclusion
Every time you sit there, stopped, with your foot on the clutch, that thrust-absorbing main bearing on the crankshaft is working really hard to push back, taking all that extra, unnecessary load.
Sure, you can replace the thrust race inside the clutch fairly easily. Fairly cheaply. It’s not fun, but it’s a pretty simple repair. Mechanics do it all the time and you can be in and out in a day, with spliney love re-established. You just need to tap and pay.
But replacing the thrust race in the engine, that’s an ‘engine out; total rebuild’ proposition, at best. That’s if you diagnose it before it lets go, and that hard to do.
If you break a con rod because the crank floated a bit too much one day, while the gearbox was making sweet sandwich love to the friction plate, it’s easy to diagnose, but you’ll have a heart attack when you ask ‘How much?’
So if you’re gunna stop for more than a few seconds, select neutral and take your foot off the clutch. This is one small way to make your life better.
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