Rear end swap question

Discussion in 'Got gears?' started by gary sell, Apr 21, 2017.

  1. gary sell

    gary sell Active Member

    So if the new Buick cylinders I have (not the up37170) are the same size as what was on the LeMans originally, I will have the same volume and pressure. But the Buick brakes are 2 1/2'" as opposed to the LeMans 2". Would that cause a problem ?
     
  2. gary sell

    gary sell Active Member

    The UP37170 cylinder has a 1" bore. The new Buick cylinder I have #45999 that is the same as the Lemans cylinder has a 15/16" bore. Would a 1/16" make a difference if I used either of the 2 bore size cylinders ? Of course no matter which bore I use, I would use a matched pair on the rear wheels. Your Thoughts ?
     
  3. a local Buick restoration shop who has been around these cars for years has been telling me that I may regret using sportwagon rear brakes because on a lighter car such as a skylark they will lock up fairly easy. with that in mind maybe the 15/16 wheel cylinders might help because there would be less surface area in the cylinder thus less pressure to the shoes.
     
  4. wkillgs

    wkillgs Gold Level Contributor

    Wider tires on the rear would have more surface area on the ground = less likely to lock up. (?)
    SW rears may complement a front disc upgrade for more braking power all around.
    Guess you never know until you try. I wouldn't over think it unless you are using original size tires.
     
    GranSportSedan likes this.
  5. i'm going to try it first and then if there is an issue I will install a adjustable proportioning valve for rear brakes
     
  6. BrianTrick

    BrianTrick Brian Trick

    I haven't done a bunch of them,maybe 10 or so,but from the ones that I have done,I have not had any complaints from the customers,or reports of issues.
    Another possibility is the wheel cylinder from the mid-70's A-bodies,with the optional 11" diameter rear brakes. That might have been another one that I used. I do know that I found a cylinder with the correct bore size,but the spacing for the two small bolts was different. The one I used had the same bolt spacing,I just need to make the hole round.
     
  7. gary sell

    gary sell Active Member

    Would anyone else like to respond to my question in post #22 ? I would appreciate it !
     
  8. wkillgs

    wkillgs Gold Level Contributor

    I replied in post #24. I can elaborate a little.
    Going from a 1" cylinder to a 1-1/16" cylinder is an increase of about 12.8%. Force is proportional to area, so about 12.8% more force from that cylinder. But you are also applying that force to a larger brake shoe!
    I wouldn't use a smaller cylinder on the larger brake shoes.....you would be applying a smaller force to a larger area!
     
  9. gary sell

    gary sell Active Member

    I guess my math skills are weak ! 1 1/16 is 12.8% larger than 1. ??
     
  10. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    No one and a 1/16" is larger than one inch, by 1/16". Looking at it another way, 17/16 is larger than 16/16. Wouldn't that be 6.25% larger?
     
    Last edited: May 15, 2017
  11. gary sell

    gary sell Active Member

    Larry70gs: My math skills are not that bad ! What you said is very easy to understand. What I don't understand is 'wkillgs' post #28 ? ?
     
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Above is post #28. There isn't an upper case number, so it looks like 11/16, but there is a dash there, 1-1/16. Is that what you are referring to?
     
  13. gary sell

    gary sell Active Member

    I don't understand how a 1-1/16 inch bore can be 12.8% larger than a 1 inch bore ?
     
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    It is larger by 1/16", which by my math is 6.25% larger.
     
  15. wkillgs

    wkillgs Gold Level Contributor

    No, you have to compare the AREA of the different piston sizes.
    Area of 1" piston = 0.785 square inches
    Area of 1-1/16" piston = 0.886 square inches, about 12.8% larger.

    To put that 12.8% in perspective, The weight over the rear wheels changes about 9% depending on whether your gas tank is empty or full!

    You have the opportunity to improve your brakes, why not do it?
     
  16. wkillgs

    wkillgs Gold Level Contributor

    Edit... You were comparing 15/16" wheel cylinders to 1" wheel cylinders...... that's a 13.8% difference.
     
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Ahh yes, the area, I was considering piston diameter only:D
     
  18. gary sell

    gary sell Active Member

    Well, it's been about 5 weeks since I started this thread. I finally finished the conversion. I installed a '65 Buick Sportwagon rear end housing with a 3:08 posi in my 3:55 posi Pontiac Lemans. There were a few hiccups along the way, 1 of 2 wheel bearings that I purchased on the net was wrong. Had to wait for a replacement. Had to slightly alter the backing plates due to new wheel cylinders for Buick housing not being available. Used slightly smaller Pontiac cylinders with small change to backing plates. Everything else lined up perfectly and installation was fairly easy. Now have 2-1/2 inch wide rear brakes as opposed to previous 2 inch. I will see how much rpm difference it makes at 70 mph. At this time the only issue I have is that in order to have an accurate speedo reading I would need to replace the drive gear, or buy a ratio adapter to make speedo reading correct. According to calculator at present I will be going 80 at speedo reading of 70. That is a 15% ratio change needed. Thanks to everyone who posted replies on this thread. Thanks also to my brother-in-law Dave for all his assistance with the install. Might post again after driving for awhile.
     
  19. wkillgs

    wkillgs Gold Level Contributor

    For the speedo... 1 tooth on the driven gear is about 3 mph.
    I can look up the BUICK drive gear vs driven gear combo. Is this a ST300 trans?
     
  20. gary sell

    gary sell Active Member

    wkillgs: It's a muncie 4 speed. It apparently has the smaller Drive gear, because it presently has the larger Driven gear. This limits the # of Driven gears available to correct the speedo issue. They must be used in smaller/larger combination or larger/smaller combo. I would rather not have to pull trans to change to the Larger drive gear/Smaller driven gear. That combination would give me more gear/teeth options for the Driven gear. Sounds crazy, but I think it's correct !! The ratio adapter route would be easier/cheaper than changing the gears. I just might keep in mind that the speedo is 10 mph off for the time being. It's always something !
     

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