Axle to crown distances for rigid fork replacement

[ChrisEyles]

I've been looking at replacing the old elastomer fork on my late 90s Marin for a rigid steel fork.... and quite frankly I am getting very confused about axle to crown distances!

The current fork is around 450mm at full extension, or 443mm with sag (when I'm sat on the bike).

It's only a 50mm travel fork, and the online recommendations for <80mm travel forks all seem to suggest a rigid fork of around 410mm axle to crown. This would obviously lower the front end of my bike considerably, and to be honest the front end already feels low enough to me, with the steering nice and sharp.

I was looking at a set of surly 1x1 forks with 453mm axle to crown, but this has a suggested suspension correction of >100mm - much more than I've got.

So I'm confused! Should a rigid fork be matched to the height of the suspension fork (minus sag), or should you assume that the fork is going to be partially compressed a lot of the time you're riding and go a bit shorter?

I've seen a nice pair of period forks with an axle to crown of 425mm at a good price - is this likely to be a happy medium or am I likely to find the steering overly twitchy (and live in fear of flying over the bars on a steep downhill!).

Thanks for the help!

 

[Yellow Saddle]

Do you know the offset of your current and proposed fork? Although the length of the fork obviously affects the angle of the front end and therefore the self-steering sensitivity, the offset is more important and needs to be factored in as well.

 

[ChrisEyles]

Ah, I hadn't even thought of that! Will have a measure tonight.

 

[ChrisEyles]

Well, both the current and replacement fork have 45mm of offset/rake.

I'm still not sure what that's going to do to the handling! But I've bought the fork and will just have to give it a go and see what happens. Will be a fun bit of tinkering to do while the weather's too awful to ride in at least

 

[si_c]

Well, both the current and replacement fork have 45mm of offset/rake.

I'm still not sure what that's going to do to the handling! But I've bought the fork and will just have to give it a go and see what happens. Will be a fun bit of tinkering to do while the weather's too awful to ride in at least

Been thinking about a rigid fork for my MTB, just for funzies, am curious as to what the difference would be on a trail.

 

[ChrisEyles]

In an abject fit of nerdyness, I've just written a bit of code to calculate the change in head tube angles and trail distance when changing the axle-to-crown and offset/rake of a fork.

Assuming my current head-tube angle is around 70 degrees (this is a bit tricky to measure, but I did my best with a protractor and a plumb-bob), then my trail is currently around 88mm (higher than I expected - but then I am used to thinking about road bikes) with the 455mm A2C 45mm offset current forks. Switching out to the 425mm A2C 45mm offset forks steepens the head-tube angle to 71.6 degrees and reduces the trail to around 76mm.

If anyone's interested I can stick a copy of the equations/code up (in Fortran) and a copy of the executable.

Of course the real question is what that 12mm reduction in trail and 1.6 degree steepening in head-tube angle is going to feel like!

I'll report back when I've made the swap. Haha, and I shall be quite disappointed if after all these calculations I can't feel the difference

 

[graham bowers]

It depends on the trail. I used to ride a rigid fork MTB and on fast rocky descents the muscles at the back of my upper arms used to shake so much it hurt. I replaced the rigid forks with some elastomer sprung rock shock suspension forks and fast rocky descents became painless. Also, on rocky ascents, the rigid forks would bounce the bike off line if I hit a rock, but the compliance of the suspension forks soaked up the bumps to some extent and kept the bike on line. I was biking on the North York Moors at the time so nothing as severe as modern downhill courses.

I've been out on a rigid fork mtb this afternoon in NW Leicestershire, around the trails at Hicks Lodge and other interconnecting trails. Suspension forks would just have been unnecessary weight.

If your suspension fork is worn, you may expect more accurate steering.

 

[ChrisEyles]

A quick update for anyone who may find this information useful.

I switched from the following:

455mm Axle-to-Crown 45mm offset suspension forks (effective sagged A2C ~ 445mm)
70 degree HT angle, 88mm trail

to

425mm Axle-to-Crown 45mm offset rigid forks
72 degree HT angle, 76mm trail

The switch was a big success. The shorter forks are slightly noticeable in a lower front end and slightly lower BB (I had to watch for pedal strikes a little more carefully). The handling was perhaps a touch sharper, but this could be just a result of the suspension to rigid switch rather than a change in geometry. High speed stability just as good as before too, although I did get the impression I was having to make more of an effort to get back over the rear wheel on the steep downs.

So I'd say if you're looking at switching forks, then 20mm or so difference in axle-to-crown measurement isn't going to totally change the way your bike handles - go for it!