What causes over-long travel forks to damage frames?

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  • What causes over-long travel forks to damage frames?
  • Premier Icon Harry_the_Spider
    Subscriber

    Longer fork causes greater bending moment at the headtube.

    avdave2
    Member

    Well you might just chuck it down something you’d never attempt with shorter forks for one thing.

    Think I might stick some Revs on my Rockhopper to see what happens.

    Check your dental insurance. 🙂

    edit – I see you have checked

    rp16v
    Member

    depends really u can get away with 10mm here or there i run my 100mm geo frame at about 110ish and it actualy handles better but depending on companys like switching from marazochi to r/s the a-c will be less i had 100mm mx comps that are only slightley shorter than my 115 recons.

    elgee
    Member

    would handle like a pig. stress on the headtube. kiss goodbye to lifetime frame warranty. 🙂

    not worth it dude!

    stevious
    Member

    Actually, in reality I have some u-turn Rebas on their way for my ‘hopper and they’re 120mm max instead of the 100mm recons that just died. Fairly confident the frame can take it (spesh used to do an AM version with 130mm forks with this frame IIRC) but will probably end up running the forks short most of the time anyway.

    Anyway, anyone else want to weigh in on the actual question that I asked? (not that I’m dismissing you answer, of course, HTS. I suspect you’re probably right).

    TandemJeremy
    Member

    Has anyone ever seen this happen?

    40 mm longer fork is only less than 10% longer. Most of the serious stress will happen when the fork is bottomed out anyway. I cannot believe the safety margins for the head tube are only 10%.

    I run a bike with a much longer fork than it was designed for

    jedi
    Member

    i always used my hummer with 130mm forks. rides like a dream

    TurnerGuy
    Member

    might also be a correlation between people who put overlong forks on a frame and then riding bigger stuff which results in more stress on the frame.

    juiced
    Member

    run 80mm zocchis on my sunn. Original had a whopping 30mm.
    Ran 100 originally before reducing travel as it didn’t like clmbing and the wheel used to tuck under a little as it was pretty slack.

    cynic-al
    Member

    TJ is right, all the stress occurs when bottomed out.

    It’s because bigger fork = bigger riding. Brant has stated this previously IIRC (and therefore it is gospel)

    Premier Icon oldnpastit
    Subscriber

    40 mm longer fork is only less than 10% longer

    Is the relationship between deflection and length linear for a cantilever? I thought it was a cube law?

    http://en.wikipedia.org/wiki/Euler%E2%80%93Bernoulli_beam_equation

    So, make your cantilever (forks) 10% longer and the deflection will be (roughly) 30% greater.

    cynic-al
    Member

    TJ is right, all the stress occurs when bottomed out.

    It’s because bigger fork = bigger riding. Brant has stated this previously IIRC (and therefore it is gospel)

    TandemJeremy
    Member

    oldanpastit thats the amount it bend the fork is it not – the amount of stress on the headtube will be simply a bending moment – force x distance

    Premier Icon oldnpastit
    Subscriber

    oldanpastit thats the amount it bend the fork is it not – the amount of stress on the headtube will be simply a bending moment – force x distance

    Yes, I think you’re right. The forks will be bent much more but the frame will be just fine.

    andrewh
    Member

    I am running an experiment on this for you.
    I have a 1999 Marin Attack Trail which came with 100mm Manitou X-Verts.
    These were swapped in 2001 for 100mm Pace RC36-ProClass2.
    These were swapped in 2006 for 125mm RST Hi5’s. Also gained bigger brakes and tyres. Note: despite claiming 125mm these have rarely given any travel at all, but the leverage principle is the same.
    Today I have fitted 150mm Junior Ts.

    I’m building up gradually (increase of 50mm over 12 years) and so far no problems. I’ll let you know if I die.

    stevious
    Member

    Sweet. Bendy forks = even more suspension. Right?

    stevious
    Member

    Just a quick ponderous question. Is the damage caused by having more leverage from a bigger crown -> axel distance or is there a more complex picture from the additional travel.

    Think I might stick some Revs on my Rockhopper to see what happens.

    EDIT: Not really.

    martinxyz
    Member

    I ran a 125 vanilla on my pace rc200 for months.Bit odd at first but it was only myself that was riding it and never thought that i could be at risk. Besides.. a good functioning fork thats around 40mm too long probably stresses the headtube less than a thrashed short travel fork that barely moves!

    Premier Icon jam bo
    Subscriber

    Has anyone ever seen this happen?

    yes, but not for a very long time and then it was when triple clamps first started coming out and people were fitting them to inappropriate bikes and ripping headtubes off..

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