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On the premise that one look is worth a thousand words, and because this seems to be of great interest - let us talk about clutches. I will refer to the following picture (please excuse my lame art):



This is NOT the clutch in an M5, but it is similar and OK for purposes of discussion.

<h2> The Parts </h2>

1. Not shown in the photo, dark blue in the diagram is the flywheel - that is the heavy, round plate at the rear of your engine which is connected to the crankshaft - when the engine is running, it is always turning.

2. The pressure plate assembly bolts to the flywheel - so it is always spinning too. The pressure plate itself is not actually visible in this picture - it is inside the housing you see, and is a smooth metal plate with a hole in the center, not unlike a brake disk. (It is represented as turquoise in the diagram.) The pressure plate can move forward and back, but is connected to this assembly and is also always spinning with the engine. Very, very strong springs attached to the housing you're looking at push it forward unless you have your foot on the clutch pedal, see below.

The clutch disk (dark red in the diagram) is sandwiched in between the pressure plate and the flywheel. It is only held in place by being squeezed; it is not physically connected to the engine in any way. Note that it is lined with material similar to brake lining. Its center connects to a splined shaft coming from the transmission, called the transmission input shaft.

The throwout bearing (green in the diagram) rides between the transmission and the pressure plate, and it "floats" over the transmission shaft and slides along it.

The clutch pedal is connected to the throwout bearing, and moves it along the transmission input shaft.

<h2>How it works</h2>

When your foot is OFF the clutch pedal, the engine is effectively connected to the transmission. The clutch disk is squeezed so hard it cannot spin. Unless you're in neutral, the rear wheels are going to turn. The engine speed is exactly the same as the speed of the transmission input shaft. (The reason you have a clutch in the 1st place is so that you can put the car into gear while the car is stopped, and then get it moving. Once the car is moving, the clutch isn't really needed. With perfect engine synchronization you can even shift gears without the clutch - but I don't recommend trying it in the M5!)

When you put your foot down on the clutch pedal, the throwout bearing slides toward the pressure plate and presses on those fingers you see radiating into the center. Those fingers are levers that pull the pressure plate back against its springs, releasing the "squeeze" on the clutch disk. Now the engine, flywheel and pressure plate assembly are spinning freely around the clutch disk, which is free to spin at its own speed. Its speed is a function of the speed of the wheels and the gear you're in. (If you're stopped and in gear, its speed must be exactly 0 RPM.)

When the pedal is part way down, the engine tries to turn the clutch disk at one speed, while the transmission tries to spin it at another. The difference is made up in slip. The associated friction generates heat. The more power transmitted while this slip is occurring, the more heat will be generated.

But once the clutch is fully engaged, there is no spinning - it is like one solid unit. So there is no heat generation. This is why, once the clutch is fully engaged, it can handle your 400HP with no problem. But when it is slipping as you get the engine and transmission speeds synchronized, you want to help it by modulating the throttle, trying to get the engine to be going the same speed as the tranny. The closer you are, the less slip is neccessary.

Note that you could have more clutch lining, stronger springs, etc, - but it would still be up to your foot to determine how much the clutch slips while you are in fact using the clutch. Once the clutch disk gets badly glazed, the springs in the pressure plate assembly aren't strong enough to create enough friction to prevent slippage. You could have stronger springs, but then you'd need more pedal effort... etc.

Hope this helps.
 

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To throw a few extra monkeywrenches into the works...

1) A slipping clutch not only causes wear of the clutch disc, but can also cause warpage of the flywheel and pressure plate if you get it hot enough.

2) Given sufficient torque, a clutch can slip even when fully engaged. At this point you start looking for more spring, more aggressive facing material, or both.

3) You get into serious tradeoffs at that point, though, because most high-torque friction materials engage with a neck-whipping suddenness (like the Kevlar-puck clutch from my SHO that just went into the garbage) and stiffer springs make left-pedaling the car even worse in traffic.

4) At which point you end up with a multi-plate clutch, which can get much higher torque-handling capacity into a smaller package at the cost of more parts, more complexity, etc.

5) F1 cars use, last time I heard anything about it, a dry multiplate clutch 4" in diameter, this allows much lower rotational inertia and a lower powertrain-package mounting.
 

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Greg,

As always your posts are both informative and entertaining! Keep em comin!

Andy M.
 

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And another word on clutches... in case you are all up to it?!


I was a long distance trucker many years ago before I saw the light and returned to school to obtain my Engineering degree (ok yes, I am a tad bit over 40 in case you are wondering??) ...and we ALWAYS tried to match the engine RPMs while downshifting....

this serves two fundamental and very important functions...

1) it protects the engine from the sudden and sometimes severe disruption caused by dropping the clutch and suddenly decreasing RPMs by 2-4K or so... your engine will not be your friend for life if you practice this daily?!!!

2) the clutch will also last longer if you match RPMs! not to mention you will not get that nasty odor all the time!


Two good reasons to be kind to your clutch!


Andy M.
 

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Maybe this is childish.....but matching your revs while downshifting also produces a sweet sounding blip form the engine...and if you're lucky, some bubbling of the exhaust!
 

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Greg,
you rule with explanations like this

thanks a lot, it is useful to know how any mechanism works to be able to use it at the maximum and optimal capacity.


------------------
SL
 

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thx for the super info... for someone like me that isn't a mechanic it is a very valuable post...

Dany



'01 M5 - lemans blue (Delivery in a few days - Status 190)
'97 M3 - cosmos black
 
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