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Fork length, and frame design

Mr RowlieMr Rowlie Posts: 41
edited June 2010 in MTB workshop & tech
If a frame is designed to take a 120mm fork, is it designed for the complete length of the fork, or is the length of the fork and sag taken into account?

The reason i ask,

A 2010 RS Reba, set to 120mm, is said to have an A2C of 493mm
A 2010 RS Revelation, set to 140mm is said to have an A2C of 519mm

Now assuming frame designers use the complete length of the fork to design head angles etc, the frame is designed around an approximate A2C of 493 (Without complicating matters of different fork manufacturers)

Now if a RS Rev at 140mm was to be put onto this frame, and set to ride at 20% sag, the A2C (with 20% sag) would be approximately 519mm - 28mm sag, resulting in 491mm A2C.

So therefore, is it safe to assume that the longer fork would cause no more stress to the frame than the smaller 120mm fork?


And also another consideration, on a full suspension frame would it be correct in saying that as the rear suspension compresses, the head angle slackens? (assuming the fork does not compress)
Therefore the frame must be designed for a range of head angles through the rear shocks travel?


  • nicklousenicklouse Posts: 50,675 Lives Here
    for the answer you will really have to talk to the frame makers.

    but no it would put more strain on the frame as it can still extend further and so come into contact with the ground and exert a force at a slacker angle than a shorter fork.

    basically see what the AC length is for the supplied fork and keep to about 10-20mm of that.

    re fullys that does not happen. as the front changes first and then the rear.

    the idea of suspension is to isolate the rider from the ground ( no change in attitude) does not always work like that though.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • thelonegrooverthelonegroover Posts: 1,073
    I think your theory appears sound. However, frames actually designed for 140mm folks tend to have slacker head angles and are therefore built to be stronger. It depends how you plan to ride the bike and if the frame fails are you going to pursue a claim.
    Planet X Kaffenback 2
    Giant Trance X2
    Genesis High Latitude 2x10
    Planet X n2a
    Genesis Core 20
  • Mr RowlieMr Rowlie Posts: 41
    The bike would be used for trail centres aswell as light xc rides, if the frame did ever fail, I would not be looking into claiming at all, as ofcourse it is my decision to extend the travel.

    Now ofcourse, the simple answer would be that a reba would be fine for my riding, but the difference in weight between the reba and rev seem very small. Basically getting an extra 20mm travel for very little wieght difference.

    Nick, although the fork would be at full length hitting the ground, surely the majority of the force would be when the fork is at a more compressed state? Or have I got this wrong.

    Can't say i'm the hardest rider on components, so trying to determine the degree of risk involved.
  • thelonegrooverthelonegroover Posts: 1,073
    The only potential problem I can see is, on steep climbs you may find the front lifting, if its set too long.
    Planet X Kaffenback 2
    Giant Trance X2
    Genesis High Latitude 2x10
    Planet X n2a
    Genesis Core 20
  • supersonicsupersonic Posts: 82,708 Lives Here
    you can reduce the rev internally anyway if you wish.
  • Mr RowlieMr Rowlie Posts: 41
    I think that is the best way to go, im assuming theres a minimum and maximum amount of travel for the revs though?

    shall have a google.
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