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Some Facts and Numbers on Inadequate Clutch Pedal Travel Range
A common complaint I get is that I cant get the tranny into reverse without grinding the gears, even when I adjust the clutch linkage to the end of its travel!
Here are a few interesting facts and numbers:
The C2/C3 Corvettes (and most GM passenger cars) have a total movement range at the throwout bearing, when the clutch pedal is depressed its full stroke, of about .580 (just over 9/16). That means that you have to use a pressure plate that fully disengages the clutch with about ½ of pressure plate diaphragm movement. Full clutch disengagement is considered to be a .050 gap between the pressure plate and the clutch disc.
To test this, I disassembled the clutch system out of a 67 Vette that was experiencing clutch disengagement problems: The clutch would simply not disengage fully, even with the linkage set up at the tightest possible position.
With the clutch, pressure plate, and the flywheel removed from the vehicle, I re-assembled the clutch and pressure plate to the loose flywheel. This assembly was then placed in a hydraulic press with the throwout bearing in place, and a dial indicator gauge was placed on the throwout bearing to measure its total stroke against the pressure plate.
The bearing was compressed .100 at a time: At .100, the clutch was locked in there solid. No change at .200, .300, .400, or .500. At the .500 distance, the disc should have been loose, with the .050 spec clearance. But it was locked solid. At .650 the disc just barely came loose, and it hit the required disengagement spec of .050 at a total bearing travel of .780 !
Obviously, with only .580 of travel available in the Vette linkage system, this clutch and pressure plate system is completely unusable. It doesnt matter if you put in a taller ball stud, install a longer clutch rod, or relocate your springs: All youre going to get is .580 travel, and you cant use a pressure plate with a .780 disengagement spec. This pressure plate was a popular, name-brand unit.
So I procured a new clutch and pressure plate system. This one an 11 McLeod Street Pro (modest entry-level system good for about 450 horsepower) and tested it to the spec:
Here is the test setup just like I tested the system that came out of the car: Hydraulic press is pushing against the TO bearing, and I have a lever arm sandwiched in there for the dial indicator. Here Im right at zero and ready to test:
The tech at McLeod had promised me that their pressure plate would have full .050 disengagement at .480 TO bearing movement, so here Ive cranked it down to .470:
The .050 feeler gauge slides tightly between the pressure plate and the disc exactly as advertised:
Being fully disengaged at .470 bearing travel means that this system will operate perfectly with the Corvettes clutch linkage travel range. The linkage system will still need to be checked and adjusted for overall geometry, and Ill be installing an adjustable fork ball stud to correct any geometry errors. But this is going to work perfectly.
On an interesting note, it should be remembered that the pressure plate disengagement system operates like a cam lobe, so the disengagement is a bell-shaped curve: If you dont have enough travel (less than .480), the clutch wont fully disengage. However, once you go over .520 (on a properly-speced pressure plate), the pressure plate starts closing the gap again, decreasing the amount of disengagement. On a good pressure plate like this, if you tighten your .580 stroke linkage up to the limit, you can actually cause the clutch to not disengage by pushing the pedal to the floor: It will be over the hump on the bell-curve, and will re-engage instead of dis-engage.
So check your pressure plates before installing them: Make sure they produce .050 gap on the disc with a .480 - .520 compression. And then make sure you adjust your linkage correctly so your pedal does not produce more than the .520 max travel.
Does anyone want a cheap, brand new clutch system that needs .780 bearing travel ?
I hate to hear that.......as I have always felt that Hays clutches were one of the best. But maybe that ended years ago when the business was sold off and the founder retired.
Thanks for another excellent article and technical evaluation.
Excellent article. Now what's needed is a list of clutch/pressure plates that meet the criteria of C-1/C2/C3.
A few years back when I did a frame off. I changed the clutch. It was a Hayes set-up that I replaced for the installed (same) Hayes that was in the car. Immediately had a dis-engagement problem. Finally got it to work for a short time, but as it wore there was no further available adjustment.
After a lot of hassle and in and out solutions I went with the often suggested Luc set-up (My car is a '62 with a modified older style LT-1).
Although it works, it really is a flimsy set up for what I have. In time I will try Lars' suggestion.
One thing I found out with all the problems others have (myself included) is that the choice of throwout bearings is very limited. Years ago there was a wide range of choices for specific cars and engines. Now there are only a few to choose from. The Corvettes seem to be the most sensitive for the lack of sizes.
The clutch geometry for the early Corvettes is very sensitive towards any change from the original. I found that the angles of the components also affect the operation. One solution that I found that works in some instances is to put a half inch or so slot in the bracket that bolts to the bellhousing. Sometimes it takes only a slightly difference angle to make it disengage properly.
I also found that you can go to a minimum gap clearance of .040 to have it disengage
One thing I found out with all the problems others have (myself included) is that the choice of throwout bearings is very limited. Years ago there was a wide range of choices for specific cars and engines. Now there are only a few to choose from. The Corvettes seem to be the most sensitive for the lack of sizes.
Chevrolet has used only two sizes of throwout bearings for decades - 1-7/8" for passenger cars and trucks (flat-finger clutches), and 1-1/4" for Corvettes (bent-finger clutches).
Chevrolet has used only two sizes of throwout bearings for decades - 1-7/8" for passenger cars and trucks (flat-finger clutches), and 1-1/4" for Corvettes (bent-finger clutches).
John - I realize those are the two sizes, but, depending on the supplier the actual sizes can vary, i.,e. the 1 1/4" can be 1.35, or less or more. That slight difference can be just enough to make things not work.
I long ago lost the desire to plow new ground and take care of new problems with each new JC Whitney purchase so I gave it up and buy OEM.
JC Whitney.....................I haven't heard that name in a few years. They were located only a few miles from where I grew up. Sort of like a neighborhood auto parts store.
JC Whitney.....................I haven't heard that name in a few years. They were located only a few miles from where I grew up. Sort of like a neighborhood auto parts store.
I know "of" Michael Hanson. I have not met him in person, but have followed his threads and advice for years on various forums. I do not know either Buxbaum or Gomer.
Why not just buy stock replacement parts and make sure your 50 years old clutch linkage isn't worn out instead of going through that drill?
I'm actually an advocate of using stock and OEM parts on stock cars. But when you're working on a customer racecar with all non-stock components, you have to select components that will hold up to the new requirements, and that may entail use of aftermarket parts. But this is the reason I call this "The 50% Hobby:" Only about 50% of the aftermarket parts you buy will fit, operate, or perform as advertised and as needed...
I'm actually an advocate of using stock and OEM parts on stock cars. But when you're working on a customer racecar with all non-stock components, you have to select components that will hold up to the new requirements, and that may entail use of aftermarket parts.
07-06-2015, 09:50 PM
Some Facts and Numbers on Inadequate Clutch Pedal Travel Range
