Effects of headangles and bb height?

Discussion in 'Observed Trials Discussion' started by airmj, Jul 16, 2004.

  1. airmj

    airmj Guest

    hey guys, are there any really big disadvantages to slapping on a 390mm rigid fork on a frame made for 100mm travel suspension forks?

    First of all the headangle would steepen a lot, bring it to maybe about 75-76? not too sure though. And second, it's gonna drop the bb below the centerline of the 2 hubs.

    From what i've researched,

    1. A slack headangle of maybe 69-70? makes the bike more stable at higher speeds, but less responsive at lower speeds. It's also harder to do static moves like pivoting, rocking, trackstanding and the likes. It makes it easier to pull the front end up also, right?

    2. A steep headangle makes the bike handle much more responsive at lowspeeds, but rather twitchy at high speeds, makes pulling up the front end more difficult? But it's easier to do static moves.

    3. A lower bb height will lower the rider's cg, improving balance, right? Which in turn makes trackstanding? It also decreases the ground clearence?

    4. A higher bb will bring the cg of the rider up, but the bike will feel more response, more flickable? But some riders feel it's over twitchy or uncontrollable.

    So back to the question, is it wise to put a rigid fork on a frame made for long travel forks? My current situation and buget allows me to get the rigid fork at a perfect price, but if i were to get the suspension fork it would be over my budget. Any advice? Any of u riders out there have tried both types of geometries/ headangle/ bb height differences?

    The bike will be used for street and a little bit of trials.
  2. Elan

    Elan steve french

    a 390mm rigid fork is way too short, even too short for a trials bike really. your cranks would drag constantly. it would be a way bad dea, i recommend at least a 420mm, or suspension corrected 440mm rigid fork. webcyclery.com has the tange rigid fork for like $40, and a few other rigid forks made for converting a suspension bike to rigid, that would suit you way better.

  3. airmj

    airmj Guest

    mmm, i can't order from webcyclery since it would cost a bomb to ship it to where i live :(

    But yea thanks for the input... anyone know how long the Kona P2 fork is? can't find the length of it anywhere.

    their website did say something about "Offset=1.74'" or something like that on the fork info. What does that mean?
  4. bwagner

    bwagner New Member

    the offset they list is probably the forks rake. The rake is measured from a virtual line straight down the steerer tube to the bottom of the fork legs, and then measure from the center of the axle to the virtual line. that would be equal to the 1.75" thing you've seen on the site. Also, remember that most bikes designed around a suspension fork these days are pretty slack, meaning that if you put a shorter rigid frok on there, it'll probably only bring it to where a modern Trials bike is, around 71-72 degrees. I think every inch you drop it steepens the angle by like a degree or so, not very much.
  5. echoteam13

    echoteam13 Guest

    it would be practically impossible to lift the front end up in any stressful situation wit a fork that short.
  6. Shipley

    Shipley ..........

    If it's a Kona rigid fork chances are it was made for frames with suspension so it should work for you lengthwise. If you check it out and it's short or not in your price range.. head to your local shop and have a look through their distributors catalogues, there are plenty of rigid forks out there that are suspension corrected for cheap prices. BTI (distributor that your shop can get) even carries the long Planet-X forks now which would be perfect.

    Good luck..
  7. xxxfr


    kona fork is 404mm.
    as far as the head angle and bb height, going from a 100mm suspension fork, to a 400mm rigid fork, you head angle will drop ~4 degrese and you bb will drop ~1". You wheelbase will also shorten by ~1.1", and obviously your bars will drop by 4".

    So, if you got a frame like that, build it up with a 400-420mm fork and itll work well, but if youve been riding it w/ the 4" fork, its going to feel unrideable for a few days. went from my stp, 3" travel fork, 8 deg rise stem, 2" rise bars, to a zebdi w/ a 400mm rigid, 0 deg stem, and .5" rise bars, and its harder to pull up, but theres so much more control
  8. airmj

    airmj Guest

    yeap, i found some kona p2s and px knifens and my local shop. But the knifens were way out my of budget though. In the end i got some cheapo second hand for for $5 just to ride around before i can get enough money for a 420mm knifen.

    Thanks a lot for the replies guys :D i'd be sure to try it and post the results here again :D