[V8Vairs] V8 Clutch effort

Jim Acker jim.acker at comcast.net
Fri Sep 28 13:45:11 EDT 2012


Late to the discussion, but I've had some experience here that may be useful.  With a stock pedal assembly, it is very difficult to get a reasonable pedal effort with a master cylinder larger than 5/8".  I have tried 3/4" and 1", both of which were way to hard to use, both in terms of pedal effort and torque on the supporting infrastructure under the dash.  This also needs to be qualified with an understanding of how my pedal is set-up.  In my case, I am using the popular approach of using the stock pedal assembly with the lever connecting to the cable reconnected to become a push lever on a master cylinder mounted roughly in the center of the front firewall. So, we have already introduced two concepts; one is hydraulic leverage and the other is mechanical leverage represented by the lever activating the master cylinder.  Physics is physics and you can't escape the trade-off between these forces.

So, assuming you have the same set-up I have, you want to design the lever to ensure that when you push the pedal down, you use the entire stroke of a 5/8" master cylinder.  In my experience, there are a limited number of bore and stroke combinations for after market, universal slave cylinders.  Most, like CNC and Wilwood are 7/8".  That is what I used on my Porsche set-up.  If you are running a Saginaw or similar GM style set-up, I would strongly recommend using a hydraulic clutch release bearing vs. something pushing on the stock clutch lever attached to a stock release bearing.  One reason, is that it is clean and the other is that you eliminate the mechanical leverage equation of the clutch lever.  That point will be clear in a minute.

Regardless of the clutch release mechanism you use you know how much fluid you are pushing from the other end.  It is simply the volume of a cylinder equation (look it up on the internet) based on the bore and stroke of the 5/8" master cylinder.  You can also calculate the same for the slave.  Assuming a similar stroke as the master, the volume of the master is obviously less than the slave.  You can then do some math to actually determine exactly how much of the slave volume is filled by the entire volume of the slave (because you are using the entire stroke).  That will lead you to the stroke you will get from your slave.  If that is enough to disengage your clutch, either by direct push from a hydraulic release bearing, or by pushing on a clutch lever, you are done.  The clutch lever introduces the additional variable of mechanical leverage.  The longer the lever is on the slave side of the lever relative to the release bearing side, the less your slave will move the bearing side of the lever.

In the end, this is a pain in the butt, but you can get there.  Again, using a 5/8" master with the stock pedal set-up as I described, and a hydraulic release bearing, you have the easiest set-up.  If you use an external slave, you have some more math to consider.

Jim


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