fork lengths (a-c) and proper frame fitting
- Thread starter RockiMtn
- Start date
was either hoping someone had a useful link on the web, or perhaps start compiling something as a guide.
be nice to know approx what fork length you should be looking at for a certain period of bike. and perhaps what fork were available during that period.
be nice to know approx what fork length you should be looking at for a certain period of bike. and perhaps what fork were available during that period.
I'm kind of looking at this as well - wondering about it anyway.
As far as i've thought about it there's a couple of options.
Find out what forks the frame came with and get the A-C size from that.
Get the frame angles from catalouges and set up the frame with wheels and possibly a random pair of forks. Use something like an inclinometer to set the angles as per the spec and measure the A-C size required.
Hey presto, a probably complicated way to get your size. That said, i'm sure someone will post an incredibly simple way of doing the same.
Can't help with any web links - sorry.
As far as i've thought about it there's a couple of options.
Find out what forks the frame came with and get the A-C size from that.
Get the frame angles from catalouges and set up the frame with wheels and possibly a random pair of forks. Use something like an inclinometer to set the angles as per the spec and measure the A-C size required.
Hey presto, a probably complicated way to get your size. That said, i'm sure someone will post an incredibly simple way of doing the same.
Can't help with any web links - sorry.
I think 39cm was fairly common for non-suspension adjusted frames, although some were built for 38cm forks.
The initial suspension-adjusted frames were made generally for 41cm, more rarely 42. A 60mm fork will tend to have an unsagged a-c of 43cm, sagged c41.5, so building the frame for 41 was a reasonable compromise.
It's complicated by the myth that a 71 degree head angle is 'right', whereas in fact many earlier mtbs with 39cm forks had 70 or 70.5 angles. The head angle reduces by c0.6 degree for every cm longer a-c, so e.g. a frame built to give a 70 degree head angle with a 39cm fork would need a 37.5cm fork to give a 71 degree angle.
Also many makers cottoned on to the market belief that 71 degrees was 'right' and just said all their frames had 71 degree geometry, even if they didn't. It's called marketing. And back in those innocent times, many buyers would buy a 71 degree frame and then fit a long fork to it, thinking the head angle would still be 71 degrees. Which is called boll*xx.
One decent approximation of what type of fork a frame was designed for is to fit any old fork and a pair of wheels and measure the bb drop with that fork (i.e., the height of the bb centre relative to the hub centre). XC frames were invariably designed for a 30-35mm bb drop, so if you have a bb drop of say 15cm that is a clear indication that the fork is longer than the frame was designed for. The bb is about 40% of the way from the rear hub to the fork, so 15-20mm too high a bb indicates that that particular fork is 40-50mm too long, which gives you a general idea of what length fork to fit to give the intended head angle.
It also suggests that the head angle if you stuck with that fork would be quite slack - not the end of the world of course, just fit a short stem and swept bars and the steering will be as lively as if you still had the 71 degree angle.
The initial suspension-adjusted frames were made generally for 41cm, more rarely 42. A 60mm fork will tend to have an unsagged a-c of 43cm, sagged c41.5, so building the frame for 41 was a reasonable compromise.
It's complicated by the myth that a 71 degree head angle is 'right', whereas in fact many earlier mtbs with 39cm forks had 70 or 70.5 angles. The head angle reduces by c0.6 degree for every cm longer a-c, so e.g. a frame built to give a 70 degree head angle with a 39cm fork would need a 37.5cm fork to give a 71 degree angle.
Also many makers cottoned on to the market belief that 71 degrees was 'right' and just said all their frames had 71 degree geometry, even if they didn't. It's called marketing. And back in those innocent times, many buyers would buy a 71 degree frame and then fit a long fork to it, thinking the head angle would still be 71 degrees. Which is called boll*xx.
One decent approximation of what type of fork a frame was designed for is to fit any old fork and a pair of wheels and measure the bb drop with that fork (i.e., the height of the bb centre relative to the hub centre). XC frames were invariably designed for a 30-35mm bb drop, so if you have a bb drop of say 15cm that is a clear indication that the fork is longer than the frame was designed for. The bb is about 40% of the way from the rear hub to the fork, so 15-20mm too high a bb indicates that that particular fork is 40-50mm too long, which gives you a general idea of what length fork to fit to give the intended head angle.
It also suggests that the head angle if you stuck with that fork would be quite slack - not the end of the world of course, just fit a short stem and swept bars and the steering will be as lively as if you still had the 71 degree angle.
My 1994 Marin Pine Mtn came with a 390mm fork (or thereabouts). It always felt nervous and was dramatically improved with a 63mm travel suspension fork. Last summer I ran a 410mm P2 in the frame and again it felt right.
My suspicion is that Marin designed the frame for suspension but then were forced for whatever reason to omit it.
So I think there is a lot of trial and error as well, and if it feels right it probably is!
My suspicion is that Marin designed the frame for suspension but then were forced for whatever reason to omit it.
So I think there is a lot of trial and error as well, and if it feels right it probably is!
Anthony":3535o6pa said:
You sir are a genius!!!
I've been trying to work out the right forks height for a 2003 XC/trail frame for ages. 99 pace forks seemed low (racy feel but twitchy and not great d/h) and 2006 + 2007 forks of different manufacturers, yet all with same travel and intended purpose were too tall (great d/h handling but poor climbing and straight line pedalling efficiency) . According to the manufacturer the frame geometry didn't change right up to 2009 when it was discontinued.
As you say the manufacturer only gives the usual "designed for 100mm forks giving head angle of 71". Although they did send the frame angles/measurements to me, i didn't have an accurate way of measuring angles. Plus they don't say if the 71 degrees is the standing angle or the riding angle (i.e. with fork sag)
I'm going try your theory and see what I get. If only they included forks on the frame diagram this would have been so much easier!
Thanks for this
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