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Re: Maybe you shouldn't limit clutch pedal travel. (archive)

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Posted by kwillmorth on April 23, 2000 at 23:28:41:

In Reply to: Maybe you shouldn't limit clutch pedal travel. posted by David on April 23, 2000 at 19:37:51:

Another issue is the pivot point geometry in both the master and slave cylinders and the relationship with the pedal and clutch throwout. I've seen setups where the last inches of travel have a diminishing effect due to the geometry between the parts. I have always assumed this was why the BMW clutches (virtually every one) has such a longish throw from engagement to the full-in position... Like you said, trimming this means the clutch would not fully disengage. On our race cars, we always used aftermarket assemblies, that were specifically designed for short throws, first by geometry, second by cylinder sizing, third by clutch face and materials design. In the end you have a great race clutch, that would be tiresome for every day use to be sure...

Something else to look into I guess.

: All the notes about shortening the clutch pedal travel kinda bothered me a bit,and (as Bessie Smith might have said) I can't stay silent no more. You probably shouldn't remove much, if any travel. Although the clutch may not feel like it's doing anything once you sense disengagement, the space between the lining and the flywheel continues to widen as the pedal moves beyond this point. With inadequate clearance between disc and flywheel, your clutch, throwout bearing and synchro lives may all be shortened. There isn't usually any useless throw in a new clutch because too much pressure or movement at the pressure plate can damage things, too - and that's what would result from excess travel. As the clutch wears, you can take up the space vacated by any worn off lining, just as brakes adjust for lining wear and retain normal pedal travel before engagement. But the actual space between clutch and flywheel isn't very big (fractions of an inch) and wear amounts to thousandths of an inch. There's not much room to play around.

: The length of pedal travel is part of the equation between pedal force and clutch disc travel. The factory chooses a pedal travel that allows what they think is optimum required force on the pedal. Shorter pedal travel means higher required force if you change it the right way - it's a system based on leverage, both mechanical and hydraulic.

: We set up race cars with short clutch pedal travel by changing the throwout fork geometry. This increases the required pedal pressure as well as the stresses on the fork, so we machine them from better material than stock. But the actual travel of the clutch disc (and, therefore, the pedal) is dictated by the clutch itself. Carbon button clutches and competition clutches like 10,000 RPM, Tilton and CenterForce specify their desired range as well.

: I'm not familiar enough with the MZ3 clutches to know exactly how little travel is too little and whether it's safe to remove any - but the above facts suggest that it's probably not a real good idea unless you know FOR SURE that BMWs will tolerate less than factory clutch travel. You get a very rough guide by engaging reverse directly from neutral, because reverse is not synchronized. If there's ANY noise, the clutch isn't fully disengaging. Properly functioning synchromesh will quiet engagement of all forward gears for many miles even if your clutch isn't adequately disengaged. You risk serious damage this way and you won't know it until it's too late.

: If somebody out there truly knows that there's excess travel in the BMW clutch disc, NEVER MIND (as Roseanne Roseannadanna would have said).

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