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Technical Heavy clutch pedal (slave question)

Discussion in 'Traditional Hot Rods' started by marx, Jun 22, 2014.

  1. marx
    Joined: Oct 8, 2005
    Posts: 475

    marx
    Member

    I'm currently running a 3-speed Saginaw in my roadster, behind a 324 Olds, and my clutch pedal is much heavier than I'd like it to be. I'm running a 1940 Ford brake master cylinder, and running that to a CNC push slave cylinder via braided steel line. The stock master is, I believe, a 1.0625" bore, and the slave is a 7/8" bore. Is my heavy pedal problem just a matter of disproportionate bore sizes? Or is the CNC slave just junk? Both? master.jpg slave.jpg
     
  2. dave lewis
    Joined: Dec 12, 2006
    Posts: 1,378

    dave lewis
    Member
    from Nampa ID

    Most clutch master cylinders are 3/4" bore...
     
  3. dana barlow
    Joined: May 30, 2006
    Posts: 5,115

    dana barlow
    Member
    from Miami Fla.
    1. Y-blocks

    Smaller master cly size will make it EZer to push,but you will have to push the pedal more = if your pedal moves about 3in. now,with a smaller master cly it may move around 4 or 5in.
     
  4. The 7/8" slave cylinder is too small for that master. Get a '60 Chevy pick-up slave cylinder, works good and the mounting is similar.
     

  5. Those cylinder sizes give you about a 1.5 to 1 disadvantage.
     
  6. marx
    Joined: Oct 8, 2005
    Posts: 475

    marx
    Member

    Like this one here, Rich? These apparently have a 1 1/16", bore, so this will put my ratio pretty close. If I'm understanding correctly then, what I want is that ratio to be as close as possible then, yes?
     
    JohnEvans likes this.
  7. marx
    Joined: Oct 8, 2005
    Posts: 475

    marx
    Member

    Ooops. Here's the photo I meant to attach... slave2.JPG
     
  8. ago
    Joined: Oct 12, 2005
    Posts: 2,199

    ago
    Member
    from pgh. pa.

    The 60 Chevy pickup is for the passenger side of the bell housing. I have a CNC 7/8" slave and a Willwood 3/4" clutch master, and it works fine. 1 1/16" M/C is too large.


    Ago
     


  9. Sure the small bore slave work would fine with the small bore aftermarket master; but I think it looks like Marx is wanting to have a traditional looking master cylinder on his firewall.

    And while Chev originally mounted the truck slave on the right side, it is easy to mount on the left side. We mounted several of them so far this year using either the Chev dual or early Ford masters.
     
  10. Stole this picture from tb33anda3rd's Back to the Fifties post, if you look close you can see the Chev slave cylinder we used on Chad's 324. upload_2014-6-23_22-9-18.png
     
  11. fab32
    Joined: May 14, 2002
    Posts: 13,985

    fab32
    Member Emeritus

    Rich B, How do you get away with running hard line from the master to the slave? With engine torque and movement it will be a short time before there is a fracture in the line. You can only bend a wire so many times before it will break. I would use a flex line in at least a portion of the run.

    Frank
     
    kidcampbell71 likes this.
  12. Ulu
    Joined: Feb 26, 2014
    Posts: 1,775

    Ulu
    Member
    from CenCal

    One of my early engineering jobs was programming the tubing bender at Manlift. We bent a lot of hydraulic line. We made a lot of flex lines as well.

    You can run hard lines if you put in a special loop or two or otherwise route an extended hard line correctly, which will allow for the proper flex in the correct directions. A straight run run will leak for sure, when the flare cracks loose one end or the other, or it cracks at the bend.

    Unfortunately the longer line will often vibrate objectionably more at some point in the RPM range, so a flex line a few inches long is always used by the factory on cars AFAIK. That also makes the cylinders easier to mount on the assembly line.

    For us it was an unnecessary expense, even with the heavy vibration of industrial engines, and flex lines were avoided except where absolutely essential. Also we normally had lots of room to route stuff & it didn't have to win shows either, ;)
     
  13. We always use a flex line. Usually run a very short piece of 1/4" steel line from the port on the cylinder to a tab on the slave cylinder bracket, then a short rubber flex line to a tab on the body, and another 1/4" steel line up to the master. In tb33anda3rd's picture I posted, follow the line down from the outer port on the master and you can see the tab, clip, and the end of the rubber hose. Rich
     
  14. ago
    Joined: Oct 12, 2005
    Posts: 2,199

    ago
    Member
    from pgh. pa.

    Rich B.
    I like that the Oldsmobile engine is a real engine. I don't understand why some people fake an engine that it really isn't. I also believe that if a single M/C brake system is (carefully) maintained


    Ago
     
  15. marx
    Joined: Oct 8, 2005
    Posts: 475

    marx
    Member

    Rich has it right. I'm running side-by-side early Ford masters because I love the look. Plus, it was a lot of work snuggling them onto that T firewall! Any solution I'm looking for will lie solely on the slave end of the equation. Otherwise I'll just leave things as they are, and let my left leg get much stronger than my right.
    So the next question I have for you guys who clearly understand the math more than I do: does the i.d. of the hard line and braided hose contribute anything in the equation here? Or is it fairly insignificant in the overall scheme?
     
  16. Halfdozen
    Joined: Mar 8, 2008
    Posts: 632

    Halfdozen
    ALLIANCE MEMBER

    The diameter of the hard line and hose have no bearing on clutch operation, unless the flex line is rubber, very long, and swells under pressure. Like brake calipers, you'll want the bleeder facing up when you bleed a hydraulic clutch setup.
    It's all about mechanical advantage. The pressure plate doesn't have to move far to release the clutch disc, maybe .070- .080", depending on the clutch. About 5- 6" of pedal travel works well. At each point in the system you have mechanical advantage, or leverage if you want to call it that. The release fork will have some leverage ratio, the distance from the pivot to the release bearing will probably be shorter that the distance from the pivot to the outer end where the linkage or slave cylinder attaches. Using hydraulic cylinders, a small bore/ long stroke master cylinder will have mechanical advantage over a larger bore/ shorter stroke slave. The pedal will have leverage built in, once again measure the distance from the pivot to the point where the pushrod attaches, and from the pivot to the point where your foot pushes the pedal. It'll probably be in the 5:1 to 6:1 ballpark. It's best to run the math before choosing components, multiply all the ratios together. It's just like trying to loosen a tight bolt, you can apply a lot of force to a short ratchet, or less force to a longer breaker bar.

    If your pedal's heavy it's probably a very short stroke too. More mechanical advantage will ease the pressure required to operate the clutch and increase the pedal travel. I'd suggest you work backwards from the clutch. See how much travel it takes at the outer end of the release fork to disengage the clutch. If you must keep the existing master cylinder you can gain leverage by increasing the bore of the slave cylinder and/ or increasing the leverage at the pedal.
     
  17. Ulu
    Joined: Feb 26, 2014
    Posts: 1,775

    Ulu
    Member
    from CenCal

    Hydraulic cylinders allow you to trade travel for force. For instance (I'm using convenient numbers for the example 'cause nobody has a 4" master cylinder ;) )

    Say you have a 4" dia master and a 2" dia slave.

    Square the radius of each and compare to find the ratios.

    Radius of 4"cyl =2" & radius of 2" cyl =1"


    2 squared = 4 and 1 squared = 1 so the ratio is 4 to 1 for travel but only 1 to 4 for force.

    A 4" master will allow a 2" slave 4x the travel of the master with only 1/4 the force.

    You'd work hard to push that clutch but a mere 1/4" travel of the master provides 1" travel of the slave.

    Reverse the sizes and it all reverses.

    a 2" master pushing a 4" slave give you 4x the force applied, but only 1/4" of travel for 1" of master cyl travel.

    I'm ignoring the levers, as you all know how to figure leverage. As previously stated, you multiply the leverage together.

    If you have 5 to 1 at the pedal, say 1 to 1.5 from the hydraulics, and an awful 1" to 6" ratio at the fork, you'd have 5 x 1.5 over 6, which = 1.25,

    ...so 100lbs foot pressure = 125 lbs pressure on the throwout bearing for this example.
     
  18. ago
    Joined: Oct 12, 2005
    Posts: 2,199

    ago
    Member
    from pgh. pa.

    Good info ULU
     
  19. Ulu
    Joined: Feb 26, 2014
    Posts: 1,775

    Ulu
    Member
    from CenCal

    View attachment 2652451
    That's a lovely engine bay. I'm really impressed.
     



  20. I guess, I would disagree with the statement that line size has no bearing on clutch operation. While it's true that the actual hydraulic pressure will be equal through out the system, If you use 3/16" hardline with the larger bore master and slave cylinders, the clutch release will be slower to due to restricted return flow. 1/4" hardline "flows" more and the clutch will release quicker. Not much, but it can be noticeable under some circumstances.
     
    Eric Satterfield likes this.
  21. Atwater Mike
    Joined: May 31, 2002
    Posts: 11,625

    Atwater Mike
    Member

    Pull that clutch master off, take it to a GOOD machine shop, and have them bore and sleeve it down to 11/16" or 3/4", whichever is the correct ratio of proportion for the slave. NAPA can get you a m/c kit for the smaller bore. (If you want to do it right, and keep that master...)
     
  22. I'm sure that would work too; but the Chevy slave cylinder usually costs around $40, and I'm guessing the machine shop charge along with a smaller kit would be quite a bit more, and then you would have a "special" master cylinder, not a "regular old" Ford cylinder that you can get a kit or a rebuilt pretty quick at most any auto parts store. From Marx's picture, it looks like it would be pretty easy to fit the Chevy slave to his bracket. Also, he could probably sell the CNC slave to recoup part of his $. Just saying
     
    marx likes this.
  23. Ulu
    Joined: Feb 26, 2014
    Posts: 1,775

    Ulu
    Member
    from CenCal

    Another alternative, if the slave has enough travel, might be to lengthen the tail of the clutch fork.
    That will reduce pedal pressure quickly on a fork with an unfavorable leverage ratio.

    But at some point the slave cyl will top out, so you can't make it too long.
     
  24. marx
    Joined: Oct 8, 2005
    Posts: 475

    marx
    Member

    On the Chevy truck slaves: between Bendix and Dorman, each makes a 1" and a 1.0625" bore. The 1.0625" would put me at a 1:1 match with the M/C, so can I assume, from all the math so far, that this is the ideal ratio I want to start with?
     
  25. dirtracer06
    Joined: Sep 29, 2009
    Posts: 198

    dirtracer06
    Member

    Install the larger one....it will be a night and day difference....did the same on my 34 with the 60 chevy master and a 7/8 slave...worked but my left leg was so wore out after driving it i could not hardly walk...stepped up to the 1 1/16 th slave and it is easly to work now...

    Posted using the Full Custom H.A.M.B. App!
     
  26. Ulu
    Joined: Feb 26, 2014
    Posts: 1,775

    Ulu
    Member
    from CenCal

    Going from the 7/8" to the 1 1/6" slave doesn't seem much but it's a 47% increase in leverage for you, (ignoring both pedal and fork leverage.)
     
  27. marx
    Joined: Oct 8, 2005
    Posts: 475

    marx
    Member

    Thanks for the astounding instructional in clutch physics, guys! This is good stuff. I wish I'd asked when I built the car, but....if you don't ask, you can't learn. Now I know. I'm going with the 1.0625 Chevy slave, and will re-run 1/4" line and 1/4" hose to it. I'll update here on how things work out!
     
  28. Ulu
    Joined: Feb 26, 2014
    Posts: 1,775

    Ulu
    Member
    from CenCal

    Well it's not what you don't know, but what you don't know you don't know, as they say.

    In other words, if we knew all the right questions to ask, right up front, life would be a lot easier all around.

    ;)
     
  29. marx
    Joined: Oct 8, 2005
    Posts: 475

    marx
    Member

    Just wanted to update the thread. As advised, I pulled everything and started from the top. Replaced lines with 1/4" and the slave with the 1.0625 Chevy slave. Replaced the braided stainless hose with an early Ford rubber brake hose. Rather than start over on the bracket I'd first made, I was able to trim and modify it to work nicely with the new set-up.
    Long story short: it's like driving a totally different car. Problem solved! dirtracer06 put it right: it used to wear my leg out like crazy. Driving in slow heavy traffic sucked. It's more fun to drive my car than it ever has been! Going through the gears is a blast!
    Huge thanks to Ulu, Rich B and Titus for all your help and advice!
    Now I just gotta get used to the smooth, easy pedal, before I push my foot all the way through the firewall!
     

    Attached Files:

  30. Ulu
    Joined: Feb 26, 2014
    Posts: 1,775

    Ulu
    Member
    from CenCal

    Good news!
     

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