Home Forum Bike Forum Effect of changing fork A-C and offset

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• Effect of changing fork A-C and offset
• 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?

roverpig
Subscriber

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

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.

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.

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.

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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