Viewing 30 posts - 1 through 30 (of 30 total)
  • running longer forks than the frame is designed for…
  • ir_bandito
    Free Member

    obviously its going to slacken the head angle, which makes climbing less good, but descending more fun.
    But what about the frame? Is 6% increase in fork length going to cause that much damage to a frame? If its running that close to yield strength of the material, surely there are other issues to worry about…

    (Thinking of putting 130mm Marzocchi's on a frame designed for 100mm forks)

    Kuco
    Full Member

    According to Commencal yes it does make a difference and that was with a fork only 10mm more.

    davidtaylforth
    Free Member

    130mm marzocchis are usually pretty long, the same length as 140mm Rock Shox pikes

    I'd say it will make a pretty noticeable difference to the handling, head angle will slacken, bb height will raise, seat angle will slacken so climbing wont be as good and your weight will be further back.

    As for strength, the extra length will put more stress on the frame for sure, dont know how much though.

    You could alwyas run them soft with extra sag.

    What frame is it?

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    ir_bandito
    Free Member

    Thorn Raven. Not too fussed about the slacker head angle – I like it that way. Seat angle is fairly steep anyway – 73deg, so losing a bit won't eb the end of the world.
    Forks are about 500mm axle to crown, so I deduce 6% (30mm) increase from 100mm forks.
    I assume head tube and welds are sized purely in bending, ie if the wheel hits something at speed. Obviously forks compressing will take some, but the rest of the force is taken as bending moment across the length of the head tube.
    So, a 6% increase in Bending moment therefore equals a 6% increase in stress. (A touch more if there's any shear, but I suspect its very little). And if that 6% knocks it above yield stress of 853 tubing, then it was under-designed in the first place. I'm only 80kg, so unless there's a 85kg weight limit, I should be right.

    I'm going to fit a Crank Bros headset to knock a few mm out of the height, and as you say, if I run the forks with, say 40mm of sag, the overall shouldn't be much difference to 100mm.

    nuke
    Full Member

    Given the difference in axle to crown height of different forks with the same travel, I don't know why frame manufacturers don't give a recommended axle to crown height rather than travel length.

    davidtaylforth
    Free Member

    500mm? What forks are they?

    My marzocchis are the same as pikes which is 520mm i think

    TroutWrestler
    Free Member

    I run Pikes (519mm) on a Whyte 19 designed for Maverick SC (494mm), so an extra inch in length. The handling is great, and I did a whole bunch of calculations about the head angle, and concluded that the variation was within the range possible by using varying tyres and adjusting the dropouts.

    taka
    Free Member

    this is what happens http://www.youtube.com/watch?v=GS-yQSrETc4

    😆

    ir_bandito
    Free Member

    '05 AM2s.
    498.5mm according to 'zocchi website.

    Ben_Haworth
    Full Member

    Putting longer forks on will also raise the BB height and can actually damage downhill handling, especially when cornering IME.

    What is the BB height of the bike when running 100mm forks?

    ir_bandito
    Free Member

    Benji – don't know for certain. According to the Thorn website, BB "drop" is 25mm.
    and looking at the 'zocchi website, 100mm forks have 477mm axle-crown, so its only 22mm longer…

    davidtaylforth
    Free Member

    Nah, the 498.5mm will be with the ETA on, wind it off to get your 130mm and you end up with a 518.5mm A2C.

    nuke
    Full Member

    Try using this…

    http://bikegeo.muha.cc/

    larry
    Full Member

    I had a 2001 rockhopper which I loved, I put some 120mm bombers on it and a couple of weeks later the headtube cracked!! Rode great for those two weeks though! 😉

    racefaceec90
    Full Member

    well i run fox float 120 forks on my stumpy hardtail.(i phoned spesh uk first,to confirm that it was ok to do so).the bike handles fine,in my opinion!!!

    ir_bandito
    Free Member

    davidtaylforth – definitly not got the ETA on and it measures roughly 500mm. Confirmed here:
    http://www.marzocchi.com/Template/listSPAForkStatistics.asp?IDModelYear=8357&Tipology=AM&LN=UK&Sito=mtb&IDFolder=229&Submit=Select
    Its the AMIIs

    davidtaylforth
    Free Member

    I think the Marzocchi site is lying

    ir_bandito
    Free Member

    Clearly. How come I measured them at around 500mm then? I'll grant you AMIs and AMIIIs are 518.5mm

    davidtaylforth
    Free Member

    Doesnt really make any sense. They use the same lowers and uppers as the other models and have the same travel so i cant see how they are 20mm shorter.

    buzz-lightyear
    Free Member

    Cripes that vid is bad. The whole front snapped off.

    cynic-al
    Free Member

    IMO the fork length warranty thing is about how you will use the bike. Stresses are high only when the fork is compressed.

    This is pretty much what Jeff Steber (Intense) says in this month's mba.

    AlasdairMc
    Full Member

    cynic-al – Member

    IMO the fork length warranty thing is about how you will use the bike. Stresses are high only when the fork is compressed.

    This is pretty much what Jeff Steber (Intense) says in this month's mba.

    So a2c doesn't make a difference whatsoever, it's a2c when the fork is bottomed out?

    taka
    Free Member

    must of stung a little that would put my confidence down a bit 😆

    mamadirt
    Free Member

    cynic-al – Member

    IMO the fork length warranty thing is about how you will use the bike. Stresses are high only when the fork is compressed.

    This is pretty much what Jeff Steber (Intense) says in this month's mba.

    Which issue of MBA is that cynic-al?

    Wozza
    Free Member

    I've upped my forks from 100 to 120 on my Blizzard. I guess i could argue that the head tube is under less impact stress since i'm not bottoming out nearly as much, or am I just kidding myself?

    I guess what i'm asking is, how good are the welds on steel frames and am I seriously looking at face-planting soon? 😯

    tinsy
    Free Member

    I went from 100 manipoos to 120mm zochis on an 04 1oomm stumpjumper FSR, was all good but found a lockdown feature was your freind on climbs.

    In fact I would rate a lockdown feature on a fork way above lockout or travel adjust.

    All that shear % stuff is right in a way, but unless you jumping a lot and landing flat, its the braking bumps and such like that is putting most stress on the headtube and so the angle your hitting stuff is more inline with the fork travel. There is also the thought that fork stiction is greater on slack head angle bikes so it might be a bit less supple on small stuff and be harder on fork bushings etc, but obviously will pay off when it gets rough and downhill.

    in answer, do it, see if you like it.

    hora
    Free Member

    I'd Run 130 Revs on a 100mm frame as they are lighter(?) and as said the Mazz are longer forks and heavier.

    ir_bandito
    Free Member

    davidtaylforth – I may have to conceed to you, on closer inspection my forks are a little over 500mm (perhaps 518.5?) so the marzocchi site could be wrong…

    Anyway, as for the geometry, quick check and confirmation with Thorn, the longer forks will slacken the angles off and raise the BB. To approximatly the same as an Orange P7. And that gets rave reviews. And as the 853 tubeset is stronger (and lighter) than the standard Cro-moly the Orange is made out of, I suspect the headtube ought to be ok without the big gusset. I'm not planning on doing any DH races, or jump sessions on it, if it breaks, it breaks. We shall see….

    ir_bandito
    Free Member

    Oh, and its all very well suggesting running RS forks, but thats not what I own (or just paid £90 to have serviced).

    cynic-al
    Free Member

    AlasdairMc – Member

    So a2c doesn't make a difference whatsoever, it's a2c when the fork is bottomed out?

    I'm not saying no difference whatsoever, but it seems to me if you do exactly the same riding on 2 different forks, the high stresses on the frame are not going to be massively different

    Mama – most recent, carbon GT on cover.

Viewing 30 posts - 1 through 30 (of 30 total)

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