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Thread: Measuring trail

  1. #1
    Upstanding member. Mike T.'s Avatar
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    Measuring trail

    Does anyone have a good method for measuring trail on an existing bike?

    Getting the point on the ground perpendicular to the hub axle is easy (plumb line) but extending the steering axis is a bit tougher. I usually eyeball a straightedge down the centre of the headtube for this one. There has got to be a more accurate way. Hmmmm, maybe if I made a little jig that would hold the straightedge at 1/2 the diameter of the headtube...............

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    Trail = (R * cos(H * pi / 180) - K) / sin(H * pi / 180)

    R = tire radius
    H = head tube angle
    K = fork rake
    i ride bikes.

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    Senior Member Bing's Avatar
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    click on the "Download XCL file" link inside this hyperlink to Anvil Bikeworks calculators to get an excel spreadsheet that does the trig for you.

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    Senior Member Bing's Avatar
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    what are you doing up so early?

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    Upstanding member. Mike T.'s Avatar
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    Quote Originally Posted by fore
    Trail = (R * cos(H * pi / 180) - K) / sin(H * pi / 180)
    You are assuming a couple of things here Fore.
    1) I have the ability to do something with that formula stuff.
    2) I know the head angle.

    I have an angle finder and a 3' level (to find a level piece of floor) but the graduations on the angle finder are so fine as to make its reading inaccurate at best. I would think that even a mildly incorrect head angle reading injected into your formula would produce a very inaccurate answer.

    I do think and actual "on the floor" measurement would be the way to go. Thanks for your formula (or whatever that is!) anyway.

  6. #6
    Prefers Cicero cooker's Avatar
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    I did a rough estimate this way: I took a digital photo of my bike ...I leaned it against a wall with the front wheel straight, and I stood some distance from the bike and lined the camera up on what I thought was the front wheel axle line. Then opened the photo in MS-Paint, and drew lines on it down the head tube, to the ground, and vertical from the axle to the ground. I measured the trail distance between the two ground points using Paint's pixels as units, and the diameter of the rim in the photo and calculated their ratio. Then I multiplied that by 27" since it's 27" wheel. It ended up being about 1.5 " or about 4 cm, but it may have been out by about 1 cm

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    Upstanding member. Mike T.'s Avatar
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    Quote Originally Posted by Bing
    click on the "Download XCL file" link inside this hyperlink to Anvil Bikeworks calculators to get an excel spreadsheet that does the trig for you.
    Hey that works great !! Thanks. Even my mathematically challenged brain can use it. At school they tried to tell me pie are square and I knew that was bull**** as they are all round.

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    Industry Maven Thylacine's Avatar
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    Except for those square pies, they're definitely square.
    Have you earned your stripes? <<click here / Questions about custom frames? Chat me! - warwickg71 (AIM/iChat) ThylacineCycles (Skype)

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    Upstanding member. Mike T.'s Avatar
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    Quote Originally Posted by Thylacine
    Except for those square pies, they're definitely square.
    Momma always made round pies. I ain't never seen a pie square.

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    Matthew Grimm / Flunky Kogswell's Avatar
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    Quote Originally Posted by Mike T.
    Does anyone have a good method for measuring trail on an existing bike?

    Getting the point on the ground perpendicular to the hub axle is easy (plumb line) but extending the steering axis is a bit tougher. I usually eyeball a straightedge down the centre of the headtube for this one. There has got to be a more accurate way. Hmmmm, maybe if I made a little jig that would hold the straightedge at 1/2 the diameter of the headtube...............
    tire diameter:

    hold a ruler up to your front wheel and measure from the ground to the center of the front axle - then multiply by two for the tire diameter

    head angle:


    get a $0.49 plastic protractor - they have a hole at the origin - put a string in the hole and tie a weight to it - makes a dandy angle meter

    fork offset:


    tricky - use a ruler and eyeball it

    calculation:


    convert everything to millimeters and use our trail calculator - you'll need to have Flash installed:

    click here to load the trail calculator



  11. #11
    Upstanding member. Mike T.'s Avatar
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    Thanks Kogs that's a dandy trail calculator! It's within 3mm of what I crudely measured on the ground.

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