Hi all,

Can anyone help me to understand the effect of getting a longer / shorter fork?

I'm replacing the rigid, 1 1/8th rigid threaded fork on my '97 Trek (26 inch wheel) with a 1 1/8th rigid threadless fork. I'm running lovely old canti brakes, but want a disc-ready fork to be ready for the future when disc brakes rule the world.

There are several options out there, but they all have either a shorter or longer axle-crown length than the old fork: Most are longer, made to replace suspension forks.

The old fork: 405mm A-C

Possible replacements:
- Identiti Tuning Fork: 395mm A-C
- Surly 1x1 - Shorter version: 413mm A-C OR Longer version: 453mm A-C
- Surly Instigator: 447mm A-C
- Surly Troll: 453mm A-C
- Surly Big Dummy: 425mm A-C
- Gusset Jury: 405mm A-C

I had my eye most on the shorter Surly 1x1, but they're out of stock everywhere and I suspect they're possibly phasing them out as the trend goes to longer forks. The Identiti fork is the cheapest option.

I don't want to make the bike noticeably lower at the front end as 90s MTBs are already quite aggressive in their saddle-handlebar heights (bars inches below , but the way it's been described to me, a 10mm shorter fork will partly bring the front end closer to you as the bike pivots around the rear axle, so won't have the same effect as say, using 10mm less in spacers below the stem.

Having said that, I put the front wheel on a 50mm thick book to measure the increase in handlebar height of a 50mm longer fork, and it raised it 4.5cm as marked on a wall, so it seems the height of the front end is still affected quite a bit...

I don't mind making the front of the bike a bit higher, for comfort, though have seen lots of people saying that increasing the fork length puts extra stress on the head tube welding, which I'd be worried about on an older bike...

Any input appreciated,

Sb88

for minimum change of bike behaviour, you want to match both ATC and rake/trail of the replacement fork.
Sometimes different ATC and different rake/trail pretty much cancel each other out, sometimes they reinforce each other.
Looking at ATC only, both the 395 and the 413 looks like viable options that shouldn't change behaviour much.
I think there's a Mosso fork at 413 mm too on EBay.
But I think you got it wrong with shorter fork bringing the front end closer.

The rule of thumb that I have heard for suspension forks is 20mm. I would tend to error on the longer a-c side than shorter, but as mentioned above, there are other factors to consider, especially if you are thinking of going 50mm longer. I can't imagine that not screwing up your steering.

Here is source for forks that I have used. I picked up a Kona P2 from them with a 1" steerer.

Bikeman: Rigid Mountain Fork

John

from your other redundant thread ..

axle to crown race seat changes? frame head angle changes, up or down ..
head angle is part of the steering trail dimension, effecting how the steering feels.




you can keep the trail the same if you change the rake/offset as the HTA changes ..
(no mention of that dimension in all your picks, get that data)


draw it all out on a big piece of paper to see what difference it makes .


NB: HTA and Axle Plumb line cross before the 2 lines reach the ground plane.

Thanks for your input!

I'm no expert, but I think what you're saying is that the slacker head angle of a longer fork can be counteracted by a 'straighter' fork with less offset/rake...?

But will have a more careful look into this tomorrow.

I don't mean necessarily with straight fork legs, as I know the offset could be created by the dropouts / crown construction, but you get my drift.

Not hard to find on Google - see if the tool below works for you.


Bicycle Calculator - compare frame sizes and calculate head angle

generally (lets say the HTA is a constant and the axle to crown distance is too ) with more rake the trail is decreased .

heavy front load Porteur rack bike riders like that .

Bike Friday and Brompton are both low trail and small wheel ... front loads really make them handle really well

The Trek Tech manuals have trail and offset. I couldn't find the a-c, but you can measure that pretty easily.

Trek, Fisher, Klein, Lemond bike catalogs, bicycle brochures

John

Cheers John - I found that doc too from when I bought the bike second hand - to know what I was getting size-wise.

After much tinkering with online calculators, and asking in the LBS where I volunteer, (who suggested I could an ''inch or three'' if it's being used for commuting/touring) went for the Surly Big Dummy which is 20mm longer A-C and seems fairly bullet-proof...

Cheers for advice people.