Clutch bummer

[Tony Radford]

I'm sure this has been covered before, but I'm too depressed to search. I just changed my 3/4" master to a 1" in hopes of getting more travel out of my 7/8" slave. I haven't finished bleeding yet, but the pedal pressure is already at NASCAR plus. How much harder should it have gotten?

[Burgs]

I can't help you with the different size hydraulic components, but may be able to help with other components. I'm not sure if yours is already be set up like this, but if not this might help.

The geometry is clearly better, as it relates to travel not pedal pressure, with a drivers side fork pivot and a short fork. The short fork can cause clearance issues with a slave mounted forward of the fork, so a pull type slave mounted to the rear might be necessary. Using a threaded rod to increase the overall length of the slave assembly, and a heim joint at the mounting end allows you to mount directly to the frame without side loading the slave from engine movement from torque rotation. The difference in the amount of travel compared to a passenger side pivot, with the longer fork, is night and day.

[Cobrabill]

Master bore size will have zero effect on pedal travel.Bore size increase will increase pedal effort.

[Maurice Johnson]

Cobrabill,
How can that be? Tony was looking for more slave travel. For the same pedal travel, he increased his fluid transfer into the slave by 75%. The pedal load might increase due to loss of mechanical advantage, but woudn't he get more travel from the slave piston? This presumes his component assembly allows more travel. JMHO.

[Cobrabill]

Slave travel is dictated by the...slave.Your last sentence is the great un-known.

[Tony Radford]

Well, the good news is that I now have all the travel I need. Shifting is smoother than it's ever been with this long-throw clutch. But damn! The pedal pressure is stout! My left leg will soon be much larger than the right.

Maurice, I'd be happy to come by the shop while you're there if I'm in town. I spend half of my life in State College, PA. Let me know when you plan to be there and I'll do my best. You can't go wrong with a Unique. I've had mine for around 7 years and I'm the third owner. Southern Automotive makes a mean stroker as well. You'd do well with the package.

[Maurice Johnson]

Tony,
Happy to hear the problem is solved--- except for the left leg thing. LOL!!. Will contact you closer to May 2nd. Kinetic has all the bells and whistles, corner scales, laser alignment, full welding and machine shops,etc. They have fine tuned some Cobras in the area. I will ask Russell which ones they were.

[mr0077]

The ratio between the master and the slave cylinder bores has a direct relation to pedal movement and effort. With no change to the slave or linkage geometry, a smaller master will increase the pedal travel for a given clutch movement and reduce the effort for that travel, and a larger master will reduce the pedal travel (for a given clutch movement) and increase the effort at the pedal.
You can change the master or slave to make the changes, but the ratio of the piston areas determines the relative movement ratios. Slave travel is dictated by the total system. If he isn't getting enough travel at the slave for the available pedal travel, he could either increase the master or reduce the slave.

[Tony Radford]

Ken, that doesn't seem right. I increased the size of the master and got a lot more slave travel with less pedal movement. It's WAY harder to push, but for the first time, I get more than sufficient clutch travel to allow smooth shifts. It makes sense to me that I'm pumping more fluid into the slave just due to the greater volume in the master. With the smaller master, I had to overdrive the piston travel to get less than full travel in the slave. This lead to premature distruction of the master. Now, I don't even need the full travel of the 1" master to move slave enough to fully engage the clutch.

[Humpty Dumpty]

Psi is based on area, or Pi R Squared. .25xPi for the 1", and .14xPi for the 3/4...a little less than twice as hard, ouch.

[mr0077]

I "think" we're saying the same thing...I'm saying that with your larger MC you would get the same clutch travel with less pedal movement but the pedal effort would be greater, so with the same movement of the clutch pedal as before the change, you would get more slave and clutch movement. Going from 0.75" to 1" would be a 77.8% increase in effort at the pedal and movement at the clutch (1" squared divided by 3/4" squared = 1.7777), so that's why your pedal is like an exercise machine for the Incredible Hulk.

[Rick Parker]

Maybe a 7/8" or 15/16" Master???

[Tony Radford]

I think I'm going to use a coil spring as my throttle return so I can work my legs out symetrically. I can tell that this is going to get old. At least I'm not grinding any gears. If I had to look at another master cylinder, I think I'd puke.

[mr0077]

A 7/8" would give you 36% more effort and travel than the 3/4", and 15/16" would give you 56% more...how about a 13/16" for about 17% more travel and effort?