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Thread: Clutch pedal force

  1. #21
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    You can put me down all you want as to how smart you are and how dumb you think I am. All I did was follow the stupid instructions and understand how it works. The master and TO bearing both operate well within the maximum travel ranges. The master never bottoms out on the internal stop, and the bearing which doesn't use any spring return is setup with a max 1/16in clearance, so it doesn't bottom in the return position. In operation the bearing just floats on the clutch fingers with zero clearance and no pressure. I went with the recommended 3/4 master, and it would have been fine, but I wanted a little less travel so I could keep my heel planted on the floor which makes it much easier for modulating initial engagement, so I tried a bigger recommended 7/8 cylinder which I determined by observation instead of just math which sometimes is important but doesn't always work out that way in the real world, until you try it. Plus, the travel specs are in the instructions.
    And how can you calculate the friction added by your pivot points? Also, unless your slave cylinder has a spring return it should be setup so the slave doesn't bottom out in the return position and then your bearing will float on the fingers with no pre-load which is the way most hydraulic clutch setups operate and self-adjust for clutch wear.
    I have a C5 Corvette and have learned a lot from dealing with that hydraulic setup and there was nothing you could calculate to deal with their issues, which you are finding out. A hydraulic TO bearing is still the best way to go. I didn't have t worry about any of it by installing an auto in my 55 which was the best choice for that car. A 4-speed computer-controlled auto transmission would have been a huge hit back in 1955.
    IMG_3299.jpg
    Last edited by 55 Rescue Dog; 03-25-2023 at 05:32 PM.

  2. #22
    Registered Member chevynut's Avatar
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    Okay RD, I think I now understand where I went wrong on the calculations for your clutch setup. I remember reading somewhere that you needed a 3/4" MC with most hydraulic throwout bearings, and that gave a 1:1 correlation between master cylinder stroke and slave (hydraulic TO bearing slave) stroke. In doing some more research I think that 1:1 was bad information or a bad assumption on my part.

    I looked at the Howe HTOB installation instructions and they say that for a 3/4" master cylinder, you get .43" of TOB stroke with 1" stroke of the master cylinder. That makes the equivalent bore of the HTOB slave cylinder 1.144" in diameter.

    I also found this at Modern Driveline:

    Here are the volume displacement requirements for the Tilton bearing assembly:
    TravelVolume (cu/in)
    0.40.483
    0.450.543
    0.50.604
    These numbers give a bore area of 1.208" and an equivalent bore diameter of 1.24" for the throwout bearing slave cylinder. So these numbers vary quite a bit from one HTOB to the other.

    Now it all makes sense. With your original 3/4" master cylinder your pedal force with an assumed 400 pounds at the clutch fingers would only be 27.5 pounds. You would only have .413" of TOB travel with 6" of pedal travel, which is what I think you were having problems with. When you changed to the 7/8" bore the pedal force went to 37.4 pounds and you get the same .413" of TOB travel with only 4.4" of pedal travel. Sorry for the mistake.

    By the way, Mcleod says you can't use their HTOB with a clutch that has weights on the fingers like the Centerforce diaphragm clutches have. I didn't know that. I'm not sure if that's a limiter with other HTOBs.

    I think my TOB could be traveling too far, but it's just about exactly what Novak said to use and what I designed for. Mcleod and Centerforce both say you need far less. I'll probably try to go to a smaller bore slave which will drop my pedal force and still operate my clutch correctly. Part of it depends on how much pedal travel I actually want when the clutch is released.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


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    56 Chevy 2-door BelAir sedan
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    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  3. #23
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    How far off the floor is your pedal when the clutch disengages? You can check while someone turns a rear tire, push the clutch pedal in/up and check the engage/disengage points. I like mine around 1 inch or so from the floor, adjusted with the threaded pushrod.

  4. #24
    Registered Member chevynut's Avatar
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    Today I checked the slave pushrod travel at my clutch fork. I tried setting up a dial indicator to do this but there's no room so I used a 6" steel rule. I have about .1" of movement of the clutch fork before the throwout bearing touches the clutch fingers. Then pushing the pedal to the floor I get 1" of additional travel at the fork. I'm not sure if I'm actually hitting the floor or if I'm bottoming out the master cylinder. I need to check that. My master cylinder has 1.4" of travel.

    Those fork travel measurements correspond mathematically to about 0.43" of free pedal travel and .050" clearance between the TOB and the fingers. The additional 1" corresponds to 4.30" of pedal travel. So I mathematically have a total of about 4.73" of pedal travel from the up stop. Also, the 1" of fork travel should give me 0.500" of TOB travel.

    However, I think I have closer to 5.25-5.5" of actual pedal travel so I'm either getting some deflection or slack somewhere, or I still have some air in the system. I hope it's the latter.

    Funny that my wife, who pushed the pedal when I measured, doesn't think the clutch is too stiff. LOL

    According to my numbers, I have a 8.64 overall ratio between the pedal and TOB with the .750" master cylinder. So for a 400# clutch my pedal force is theoretically 46 pounds, and for a 450 pound clutch it's 52 pounds. That seems to be in the higher range of what my research suggests it should be.

    The .500" of TOB travel I'm getting with 4.3" of pedal travel after touching the fingers is 1/8" more than what Centerforce says I need. But I don't think I want to have to push the clutch all the way to the floor to disengage it, so it might be just about perfect where it is. Theoretically it should engage 1 1/8" off the floor. I could possibly move to a .700" booster that would drop my pedal force about 6 pounds if I can still get enough TOB travel. I think I'm going to wait to see where it engages before I try to do anything with it but I may try bleeding the system again and see if fork travel changes.

    I could also decrease the pedal free play by tightening up the TOB gap. Some TOBs are meant to run all the time but I'm not sure mine is. Lots of guys say the TOB will outlast your clutch running against the fingers all the time. That's another .050" of travel I could gain.

    Maybe I need some kind of clutch booster. There actually is such a thing.
    Last edited by chevynut; 04-04-2023 at 09:26 PM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  5. #25
    Registered Member chevynut's Avatar
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    Quote Originally Posted by 55 Rescue Dog View Post
    How far off the floor is your pedal when the clutch disengages? You can check while someone turns a rear tire, push the clutch pedal in/up and check the engage/disengage points. I like mine around 1 inch or so from the floor, adjusted with the threaded pushrod.
    I think that's a good idea and planned to do it since I read that's how some guys suggest doing it. I can adjust free play two places on mine, at the master cylinder pushrod and at the slave pushrod. I think I'd rather adjust the slave pushrod. I also need to install a clutch pedal return spring to bring the pedal up against the stop because the master cylinder doesn't seem to do it like I thought it would. That also might indicate that there's still some air in there. I have a small spring pulling the clutch fork toward the slave.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  6. #26
    Registered Member chevynut's Avatar
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    I did a little research on clutch boosters and springs and found out why my Porsche has such a light pedal...

    https://www.youtube.com/watch?v=YfjD4a_m900

    Also occurred to me that tri5's have a "clutch helper spring" as well. I always thought it was a return spring, because that's what everyone calls it, but it works in both directions just like the Porsche spring. Looks like this is a really common setup and my car doesn't have one anymore. I'm not even sure I have room to put it back in if I wanted to. I imagine those springs reduce the clutch pedal force quite a bit.

    Last edited by chevynut; 04-05-2023 at 11:18 AM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  7. #27
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    Pictures of your pedal and linkage setup would help. Shouldn't need any power assist if setup right.

  8. #28
    Registered Member chevynut's Avatar
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    Quote Originally Posted by 55 Rescue Dog View Post
    Pictures of your pedal and linkage setup would help. Shouldn't need any power assist if setup right.
    I could show you a picture of my linkage, and it wouldn't make any difference. You probably would ask why I did this and that and whatever, and tell me how good yours works and how simple it is. Mine is working as-designed. I had specific objectives, one of which was a hidden master cylinder. And I've posted some pics of it before.

    The only question left is whether I can get by with less travel at the TOB so I can use a 0.700" master cylinder instead of the 0.750" one. I won't know that until I try it out driving the car. I can't put a stock "helper" spring on it because I have too much stuff under the dash, like a bellcrank to move the brake master cylinder over, and my clutch switch, and Raingear. I may want to try to design something different if I think the pedal is too stiff.

    Like I said you can't get something for nothing. You trade off pedal force and travel for throwout bearing travel....it's that simple. If I need to move the TOB 0.375" against 400 pounds, it takes 150 inch-pounds of work. If you want to move the pedal 3" to disengage the clutch by moving the TOB .375", it takes 50 pounds of force to do it. There's nothing you can do to change that but add a booster that uses an outside source of energy like vaccum, pressure, or a spring (which stores energy). You can change levers, master cylinders, slave cylinders, etc. all you want and it's still going to take 160 inch-pounds of work. A smaller master cylinder will decrease the pedal force, but it will increase the pedal travel needed.

    With the same clutch and the same pedal travel, your car is going to take exactly the same force as mine to work the clutch unless you have an assist spring. It doesn't matter if it's mechanical linkage, a cable, a master and slave, or a hydraulic TOB. It's simply physics.

    The only tradeoff is how far you have to move the pedal. I want the clutch to have a slight amount of free play, but I might eliminate that if I feel comfortable with the TOB spinning all the time. So the question is where I want the clutch to engage and disengage. There's a total of 6" of possible travel between the pedal and the floor as far as I can tell. I think I want it to engage about 2" off the floor. That leaves 4" of travel to disengage it, and if I add any free play it's reduced further. The only way I'm going to know for sure is to drive the car.
    Last edited by chevynut; 04-06-2023 at 08:34 AM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  9. #29
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    Sounds like you have it all figured out all by yourself, and that you shouldn't have chosen a heavy-duty street/strip clutch.
    Last edited by 55 Rescue Dog; 04-07-2023 at 07:30 AM.

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