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Re: Clutch Construction 101 (archive)

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Posted by kwillmorth on April 24, 2000 at 09:52:50:

In Reply to: Clutch Construction 101 posted by David on April 24, 2000 at 07:20:06:

First, I understand how a clutch works. I've had my hands in them for more than 30 years in various forms from racing to bone stock junk. Their is no mystery to the basics, but in the detail of each different specific auto makers design, in an attempt to make them act nicely for novice drivers, a great deal of difference exists.

Second, the point I was making is, that in street cars, to make the clutch act in such a way as to be comfortable, their is a progression in the mechanism that is not linear. The point was, that the further you push down the peddle, the less it moves the clutch, so, what feels like extra space, is actually the progressive action of the mechanism. From the point it "feels" like it has released (slipping), to fully released (maximum air space from clutch face to pressureplate and flywheel), is further than you might think, or are able to feel through the mechanism.

Regardless of Clutch Construction 101, the clutch can be partially engaged. At the point it slips, it is partly disengaged, and is caused simply be engiune power overcomming the tractive force of the clutch materials and flywheel/pressure plate surfaces). At this point friction of the clutch is not enough to feel like it is acting on the car, but it is enough to create heat and wear (I thought that was your point in the first place)..

The reason the clutch on a street car has this progressive mechanism or feel to it, is to make engagements, shifting, getting under way from an uphill start, etc... smoother. This is dopne by making the mechanism move the clutch more slowly at the designed engagement point, and faster leading in and out of it. This can be done in the lever arm design, the position of the various pivot points, the design of the hydralic mechanisms involved, and the design of the pressure plate, to name just a few. I am assuming, because the BMW clutch feels very good and is very smooth, something on this level must be part of the design. Because BMW clutches have almost always been this way, I also assume it is for a reason, that a design flaw that is carried on through many generations of car from the 2000 up.

The point I made about racing clutches is, they tend to be IN or OUT, with short stiff actions. This is great on the track, creates as little wear as possible, is able to handle large amounts of power (I know this comes from heavier forces applied at the pressure plate, etc....), repetitive actions when very hot and on and on.... but does not make for a very nice street setup.

I was attempting to support your original point that limiting clutch throw may not be a good idea, unless you are certain that the clutch is fully disengaged, and not just slipping. Friction surfaces take very little interferance to create a lot of heat. They are not simple on and off devices, they can be partly engaged (if not, then leaving at a light would be a more violent experience). Heat is the killer of brakes and clutches, more actually than wear from friction (slipping is minimal actually). If you limit travel and are creating heat that was not part of the design of the device, you will limit life... but won't know it for some time.

Which is what I assumed you were saying originally... Sorry for attempting to support your comments, I will show greater restraint in the future.


: It seems that not everybody understands how a clutch works. The friction disc is the meat in a sandwich, being between the flywheel in front and the pressure plate behind. The disc (with lining on both sides) is free to slide fore and aft along the splines of the transmission input shaft. The pressure plate is a metal ring with a flat face that's pressed against the back face of the disc by a powerful spring. The release (or throwout) bearing sits around the center of the shaft and is on the end of the clutch fork. When you step on the clutch pedal, you press the bearing forward against the center of the pressure plate, where a radial lever system of some kind causes the pressure plate's bearing surface to move away from the rear of the disc (there are several different kinds of pressure plates, the commonest one today being a circle of radial spring "fingers" - old English cars have three discrete levers with riveted pivots and coil springs).

: The entire clutch assembly is bolted to the back surface of the flywheel and spins with it. Power is transferred to the transmission through the splines on the input shaft. Depressing the pedal does not move the disc directly. It releases the pressure hlding the disc against the flywheel, decoupling the two. Although the disc does move away from the flywheel some thousandths of an inch, the gap itself is not important as long as there's no pressure between disc and flywheel. The last inch of pedal travel may be releasing the last bit of pressure.

: Over-travel of the throwout bearing against the pressure plate can actually cause pressure against the hub of the friction disc. In some cars, the back of the rotating friction disc will rub the release mechanism (throwout bearing etc) and cause damage and an AMAZING noise.

: I hope this helps you understand my point of view.

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