Calculating proper belt size

Discussion in 'Motorbike Technical Discussion' started by rricciardi, Jul 21, 2005.

  1. rricciardi

    rricciardi Guest

    Hey All,
    Can someone give a quick explanation on how to calculate the correct
    belt size. I will be using a 113" motor with a 32 tooth tranny pulley.
    The rear pulley has 65 teeth. Any help would be appreciated. Rich.
    rricciardi, Jul 21, 2005
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  2. Depends on how many pies you've eaten lately, surely?

    Or, in other words, your question reads, "How long is a piece
    of string?"

    To give even an approximate answer, it's necessary to know:
    a) What is the tooth pitch on the belt/pulleys
    b) What is the distance between pulley centres?

    Once these are known, elementary geometry gives the answer, but
    probably based on the circumference at the root of the belt teeth; a more
    definitive answer also needs to know what is the thickness of the belt from
    there, and how the hell do the manufacturers specify length anyhow?

    Oh, and motor capacity (113" -- I presume that's cubed -> 1.85 l)
    enters into the equation not at all.

    Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
    Brunel University. [email protected][|] Room 40-1-B12, CERN
    GSX600F, RG250WD, DT175MX "You Porsche. Me pass!" DoD #484 JKLO# 003, 005
    WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
    KotPT -- "for stupidity above and beyond the call of duty".
    Dr Ivan D. Reid, Jul 21, 2005
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  3. I suppose this is a hardtail chopper you're a-buildin' there.

    Have you got a belt catalog yet? Engineers build stuff out of catalogs
    and computer assisted drafting programs. Good Ol' Boys have been
    building stuff from chalk lines on their garage floors for a long time

    It doesn't matter how many *teeth* the pulleys have, what matters is
    the inner *diameter* of the two pulleys and *how far apart* they have
    to be.

    Draw a straight line on your garage floor with chalk or a crayon.

    Lay the pulleys down on the line the right distance apart from center
    to center.

    Draw a chalk line all the way around the pulleys. They probably have a
    raised rim all the way around them to keep the belt on the pulley
    instead of sliding off to the side. That's the inner diameter you need
    to figure on working with.

    Draw a line dividing each circle in half at a right angle to your first
    straight line.

    Draw two chalk lines connecting those lines a certain distance *inside*
    the outer edges of the two pulley circles you've drawn. How far inside
    those outer lines you go depends on the height of those raised rims I
    mentioned before.

    Measure the two long lines you just drew with a tape measure.
    Measure accurately to within 1/16th of an inch. Both lines will be the
    same length.

    Write the numbers down twice in a column like you were going to add
    them up. That's what you're going to do.

    Measure halfway around each pulley. The belt you use is going to wrap a
    little more than halfway around the big pulley and a little less than
    halfway around the small pulley. You'll get two numbers, one for the
    big pulley, one for the small pulley.

    Write down both numbers and add up all four numbers and that's the
    inside length dimension of the belt you want. At least it's in the ball

    Good Ol' Boys who build choppers from scratch in their garage are happy
    when they get a part to come out within a quarter of an inch of what
    they intended.

    Then look into your belt catalog and see which belts are long enough to
    do what you need to do.

    Read up on how much those belts will stretch because you're going to
    want to start out with the rear wheel in the middle of the adjustment
    and still have enough adjustment to take up the slack when the belt

    And that middle of the adjustment range is going to affect how long
    that first line you draw on the garage floor is going to be.

    Study on what you need before you buy your belt and maybe make a deal
    with the supplier to return it if it turns out to be a leetle bit too
    short or a leetle bit too long...
    krusty kritter, Jul 21, 2005
  4. rricciardi

    rricciardi Guest

    Thanks for the info guys. I'm glad I asked. I just placed the tranny
    in the frame and ran a piece of string around both pulleys and was
    going to use that measurement to order a belt.
    rricciardi, Jul 22, 2005
  5. I didn't know you had your motor in the frame or I would have told you
    to just wrap a steel tape around the pulleys and take that number and
    call it the inside length of the belt. The belt company should be able
    to deal with that dimension, since it would be to root of the teeth
    that Dr. Reid was speaking of.
    krusty kritter, Jul 22, 2005
  6. rricciardi

    rricciardi Guest

    Sorry, I should have mentioned that. I guess I was on the right track
    then. Thanks.
    rricciardi, Jul 22, 2005
  7. rricciardi

    Matt Guest

    Measure it with something (eg wire or a tape measure) that doesn't stretch.
    Matt, Jul 23, 2005
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