Fork Travel and Bike Geometry

Jonesy.

Hoping someone can answer this...

I have a bike with an 80mm travel fork that has an axle to crown length of 425mm.

If I replaced it with another 80mm fork, but with an axle to crown length of 450mm, is that going to upset the geometry and handling of the bike (even slightly)?

Cheers,
J

nicklouse

it will slacken the head angle by 1 degree.

each inch changes the average bikes head angle by 1 degree.

so ity will make the steering slacker etc...

how it will feel does depend on the bike....

Jonesy.

Thanks for the info.

Hmm...I'm really not keen on doing anything to effect the steering or handling, other than enhancing it (it's a late 90s Klein, and rides like a dream, other than the forks not matching up to the quality of the frame). I may just have to search around for a fork that matches the existing one in terms of length.

nicklouse

what is the fork with a 425 AC height?

Jonesy.

It's an ancient (10 year old) Rock Shox Jett XC.

Perhaps they don't even make them with such short legs anymore. I've seen rigid forks with that axle to crown length but so far no suspension forks...

supersonic

I thought the Jett was 425 for the 63mm version.

nicklouse

I remember the old long travel version of the Jett that cam on my old Scott.

swapped it out for an 80mm Marz Z3 Bam 80 and fitted a slightly shorter stem. Way Way better.

supersonic

Jetts were rubbish - based on the Indy, but heavier and just as flexy!

Jonesy.

Yep, they're rubbish, that's why I'm keen to replace it!

I've measured the fork legs and they're definitely 425mm, and the bike was supposed to have 80mm travel forks. All modern 80mm forks appear to have 450mm legs.

Perhaps fitting the 450mm length fork and then reducing the stem height will solve the problem? At the moment there are some spacers between the stem and headset so I reckon there's room to play with...

supersonic

It will alter the bar height but not the head angle.

I'd have thought with such a low C2A height the crown would hit the tyre at full compression lol.

nicklouse

Mmm SID 1998 63mm : 435mm

supersonic

Jonesy, how much stanchion is exposed at full extension?

Jonesy.

Guys, I must apologise.

Like the half-wit that I am, I've been measuring the fork legs from axle to top of leg, rather than to top of crown along the centre line. These will obviously give me different lengths. I've just measured from the top of crown, along the centre line (e.g. where the wheel would be), down to the axle line, and the length is 450mm.

So it appears that I've been stressing over nothing! Sorry about that, I'll fetch the dunce hat.

The stanchion is exposed about 4.5 inches / 12cm. What would that mean?

Anyway, if I've now measured it correctly, the good news is that I can go check out some Magura Durin Race forks which I hear are pretty tasty!

supersonic

Means it is the 100mm travel fork - but little consequence now, we know the real height lol.

nicklouse

go have fun.

Jonesy.

supersonic wrote:

Means it is the 100mm travel fork - but little consequence now, we know the real height lol.

Just to get this straight in my head - is the important factor when replacing the fork, the axle to crown length? So if I get an 80mm travel fork with the same a-to-c length as my existing one (even if the existing one has 100mm travel), that'll still be fine, right?

nicklouse

it will be fine

yes A to C is the most important number.

Jonesy.

Thanks for clarifying.

Magura, here I come...