Effect of changing fork A-C and offset

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  • Effect of changing fork A-C and offset
  • Premier Icon roverpig
    Subscriber

    There are various threads discussing the advantages and disadvantages of forks from Fox, Rockshox etc, but rarely any mention of A-C length or offset differences. For example, I’ve been looking at 120mm 29er forks and as far as I can tell for a Fox 29 the A-C is 521mm and the offset is 44mm. For a Revelation 29 at 120mm the A-C is 528mm and the offset id 46mm. So what difference (if any) could you expect by going from one to the other?

    Premier Icon roverpig
    Subscriber

    Is there a lead balloon award. I think I may have a candidate 🙂

    Premier Icon mikewsmith
    Subscriber

    Very little at those numbers. However it is worth checking I have forks of the same travel over 1inch longer which does change things up a little

    nick1962
    Member

    7mm drop in A-C height and 2mm difference in offset.
    If you get a protractor, draw some lines,take some measurements,then do some geometry and maths you can convert that to degrees and see if it maks any difference.
    Or you could just buy the fork with the best colour and ride your bike.

    cynic-al
    Member

    17mm (sagged) = approx 1 degree, dunno about trail.

    Premier Icon roverpig
    Subscriber

    Thanks. I guess it was more the effect on trail that interested me. The difference between “standard” and G2 offset only seems to be around 5mm, so I wasn’t sure if a 2mm difference would actually be noticeable.

    Premier Icon jameso
    Subscriber

    Both forks produce ~the same trail. The offset reduces what the a-c adds, very small differences tho. a-c will vary anyway, when riding and between forks of the same model.

    Premier Icon roverpig
    Subscriber

    Thanks. I found a useful calculator here

    http://yojimg.net/bike/web_tools/trailcalc.php

    That doesn’t let you enter A-C so you have to estimate the effect of the longer A-C on HA, but it does look as though the extra 7mm on the A-C and the extra 2mm offset in going from Fox to RS pretty much cancel out in terms of trail.

    Cheers,

    Andy

    macb
    Member

    the fork AtoC will rotate the frame around the rear axle and I tend to equate 20mm to 1 degree on frame angles. It will also lift/lower the BB and has an impact on saddle position and ETT.

    Trail numbers are altered as follows:-

    increasing the HT angle decreases the trail and vice versa
    increasing the fork offset decreases the trail and vice versa
    decreasing the tyre size decreases trail and vice versa

    You can work out all the specifics using something like BikeCAD online.

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