a bike/math question...attn. math gurus!!- Mtbr.com
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  1. #1

    Join Date
    Dec 2007

    a bike/math question...attn. math gurus!!

    if a frame has a 70deg head angle using a 100mm susp fork, how much would the head angle change by putting a rigid 80/90 mm fork on it. the seat angle is 73 deg.

    i have a steel frame on the way and i want to use a carbon rigid fork and build this up as a winter rig. i am sure i'll try it out this summer as well.

    another ? how is a rigid fork measured? i have seen one w/ 445mm on the legs but when i measured it from top of crown where the race sits to mid point where the axle sits i came up w/ a longer measurement. since this would be mostly for winter i don't want to do suspension. any recomendations on a carbon fork that would meet this problem.

    is there a ratio, example, so many mm of fork length-/+ = so many deg of change -/+

    please let me know,thx,geo.

  2. #2
    is buachail foighneach me
    Reputation: sean salach's Avatar
    Join Date
    Sep 2007
    what is the axle to crown length of the rigid fork? and the suspension fork?

  3. #3
    Wood chips are stupid
    Reputation: akdeluxe's Avatar
    Join Date
    May 2004
    The method you used to measure the fork was correct. A shorter length fork will,of course,steepen the head angle. How much...I cant tell you that.Try the frame builders forum. Do yourself a favor and get a fork with a blade length AND RAKE that the frame was built for.

    "Trust me,you don't want a big baby."


  4. #4
    Beware of Doggerel
    Reputation: Adam's Avatar
    Join Date
    Jan 2004

    Rake etc...

    In the end who cares?

    The best idea is to just ride the bike and see if you like it with the new fork. I don't mean to be a smart a$$ (it just comes naturally), but seriously my experience is that whenever you plan this stuff by the numbers it never comes out quite like you expect. Sometimes its better than you expect sometimes its worse. The best thing to do is just forget about the numbers and just try the fork, then you can always drop and angle finder on there and see how it measures up.

    For me the thing that always weirds me out about shorter forks is not the head angle or steering changes, but the changes in bar height and the lengthening of the effective top tube. That factor may not matter to you, but maybe it will. The point is that changing the fork will change a variety of factors on the bike and there is no real substitute to just taking the plunge.

    Maybe you could find someone who has a fork with similar dimensions that you could borrow? Or pick up a cheap fork with the same dimensions before you drop the coin for carbon fiber.

    Finally for fork recommendations, my wife has one of those bontrager carbon fiber forks on her winter bike and she loves it. I think they come in a couple different lengths too.

    I wanna say I'm sorry for stuff I haven't done yet, things will shortly get completely out of hand --T.M.G.

  5. #5
    let's ride...
    Join Date
    Dec 2006
    The math is readily doable if you have all of the numbers--axle race to crown length and offset (rake) of the existing forks and the new forks. But, ultimately, unless you have uber amounts of experience and detailed knowledge of a variety of setups, the additional math isn't going to accurately translate into how you'll perceive the riding characteristics.

    I agree with Adam. Buy a cheap fork that closely mirrors the setup that you have--if you like the way it rides--and see how it feels. Losing one centimeter over an effective length 100 cm(? what is a typical lenght from the top of the head tube, following its centerline, to the ground?) is a rather subltle change.

    Whatever the degree change in headtube, it will be the same in the head tube.

    Of course, if you are a professional frame builder or perhaps a mechanical engineer, you might be quick to point out that a small change in one are of a machine can have a magnified effect on another portion. Again, I agree with Adam. Experiment on the cheap and upgrade when you found what you like.

    happy hunting.

  6. #6
    mtbr member
    Join Date
    May 2006
    I agree with Adam. I did a 29er conversion on my pugs and according to the math I was supposed to use a regular fork. However I had gotten a good deal on a Bonartager switchblade 29er fork. To make a long story short, it worked out great, the bike rides fine and I saved myself almost $100 bucks.

  7. #7
    Alaska Turner Mafia
    Reputation: EndoRando's Avatar
    Join Date
    Sep 2004
    General rule of thumb used: one inch of axle to crown difference makes about one degree difference. The differences just don't change at the head and seat tube angles though; it affects ETT, BB height, and wheelbase too.

    The fork length to try and match is your suspension fork's length minus sag. Your situation may get you there, break out the tape and give it an estimate. Then, like Adam said, you try it out and see how it handles.

    Deceleration Trauma is my middle name

  8. #8

    Join Date
    Dec 2007
    i do not have either fork for this frame yet. the frame builder... pipedream.com says the sirius frame will work best w/ a fox talas fork. reviews of the frame by other bike mags say the frame handles best w/ a 100mm travel fork. this is where i got my info about the fork.
    frame w/ no fork is 69deg ht/ 72.5 st. tt is 605mm/ 23.75 in. b/b hiegth is317.5 or 12.5 in. since this is a budget project i would like to try to get the right fork or close the 1st time around. my upgrades to my nrs have given me the parts to build a rigid/winter rig. out of pocket cost must be low. let me know guys, thx,geo.

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