Lighten aftermarket clutch effort?

I ended up using a Clutchmasters single mass flywheel with their organic, sprung hub disc and non-self adjusting pressure plate in the 525iT. The Valeo single-mass conversion was backordered until May - ignore that BavAuto sent one 2 days after the car was finished...

I didn't think the pedal would be more than slightly heavier than stock - I have this exact clutch on my e36 M3 track day car, albeit on a 10# JB flywheel, and it's not heavy.

The one in the 525iT though, is crazy heavy. As in hurts my knee, makes my foot sore heavy.

The take up and feel are exceptional. Car definitely spools up quicker and feels "sportier". I like it, but the effort is just too heavy for traffic.

Car has 19mm master cylinder bore, 22mm slave. Later e34 8 cylinder cars had a 26.9mm slave bore. Would I still get full disengagement if I switch?

What do old people in Europe who drive manual do when they have a knee replaced?

You can calculate this with maths if you know the stroke of the master and the required stroke of the slave.

If my maths are working, that's a pretty significant difference. Like 50% bigger. So it's unlikely you'd have enough travel.

Is there anything we can add to our guesstimate from the current pedal position when friction starts in earnest, and/or how much (if any) free travel there is at the top before you start moving hydraulic fluid?

motomoron wrote: Car has 19mm master cylinder bore, 22mm slave. Later e34 8 cylinder cars had a 26.9mm slave bore. Would I still get full disengagement if I switch?

No. Larger master would lessen the effort as long as the stroke is the same or a little shorter.

Lengthen the pedal.

There's zero play - BMWs are spring loaded against the master w/ a helper spring. The clutch is fully disengaged in the first 1/4 of pedal travel.

motomoron wrote: The clutch is fully disengaged in the first 1/4 of pedal travel.

Then either a larger master or smaller slave would decrease the pedal effort.

Currently you have a 19mm master and a 22mm slave so the slave is ~115% of the master. Any combination that gets closer to 100% should help some.

Hal wrote:

motomoron wrote: Car has 19mm master cylinder bore, 22mm slave. Later e34 8 cylinder cars had a 26.9mm slave bore. Would I still get full disengagement if I switch?

No. Larger master would lessen the effort as long as the stroke is the same or a little shorter.

I'm pretty sure you are wrong on that- smaller master or larger slave lessens effort, providing the pedal pivot remains constant.

Area of 22mm slave: 11^2 ~= 121 * Pi ~= 380
Area of 26.9mm slave: 13.25^2 ~= 180 * Pi ~= 568

380 / 568 ~= 0.67

So, the other slave option has about half again as much area (I think this is what Keith noted) as the current slave. So for each unit of motion on the master, the alternative slave would move about 2/3 as far...

Can we take that to mean that since 1/4 travel disengages now, it would take 150% of the current travel, or 3/8 travel to disengage the clutch with the 26.9mm slave?

I suspect that if you were sure it was that sample, you wouldn't be asking here because you could work that much out for yourself.

So, I had fun working the problem, but I'm sorry I don't know anything more specific about the application...

Hal wrote:

motomoron wrote: The clutch is fully disengaged in the first 1/4 of pedal travel.

Then either a larger master or smaller slave would decrease the pedal effort. Currently you have a 19mm master and a 22mm slave so the slave is ~115% of the master. Any combination that gets closer to 100% should help some.

Other way around. The master has to be smaller, or the slave lager to decrease effort at the pedal.

BTW - it's area, not diameter that shows the difference.

19mm master=283mm/2 22mm slave=380mm/2 26.9mm slave=568mm/2

The ratio now is about .75 - going to the bigger slave gets down to .50.

A 24mm slave would probably be about right.

Somehow I suspect the solution will involve me boring and honing a slave, making a piston, and cross referencing as 24mm rubber cup from something else.

Streetwiseguy wrote:

Hal wrote:

motomoron wrote: Car has 19mm master cylinder bore, 22mm slave. Later e34 8 cylinder cars had a 26.9mm slave bore. Would I still get full disengagement if I switch?

No. Larger master would lessen the effort as long as the stroke is the same or a little shorter.

I'm pretty sure you are wrong on that- smaller master or larger slave lessens effort, providing the pedal pivot remains constant.

Yep, you are right. That's what I get for trying to post in a hurry before going somewhere.