Effect of Axle-Crown increase/decrease

sb88

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 / Longer version: 453mm A-C
- Surly Instigator: 447mm A-C
- Surly Troll: 453mm 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 seat-handlebar position, 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 fewer spacers below the stem.

I don't mind making the front of the bike a bit higher either, 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

nicklouse

every 25mm changes the head tube angle by about 1 degree.

steeper sharper slaker lazier.

sb88

Nicklouse that's great, thanks. So one of those longer forks would change it by around 2 degrees... But I'm right in thinking that it's not the same as raising/dropping the handlebars using spacers above the headtube?

The Rookie

Correct, it's not the same, the steering responce is affected by the head tube angle, raising or lowering the bars has no effect on the HT angle.

Chunkers1980

No, because it changes relative angles of all else - from the seat tube, bb height, chain stays ect

sb88

But I'd be right in thinking that, given the bike 'pivots' around the rear axle in an 'arc' when increasing/decreasing length of forks, a significant enough change would result in some increase / decrease in the height of the bars, just not as much as the increase in fork length. E.g. a 5cm increase in fork length might result in a 2cm raise of the front end of the bike. I suppose I could check this by putting something 50mm deep, e.g. a book, under the front wheel and marking on a wall how much it makes the front end rise by...?

cooldad

The bars will be higher. By the same amount, but that's a different question.

sb88

You're right: I put the front wheel on a 5cm book and the bars raised about the same amount, as marked on a wall. The next question is, will it do the frame any harm to put add roughly 50mm to the axle-crown length (especially the down-tube <--> head tube weld?

Chunkers1980

Who knows? What's its rated max?

cooldad

I wouldn't. Non corrected 90's frame is not designed for a few inches longer.

sb88

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19698584#p19698584]cooldad[/url] wrote:

I wouldn't. Non corrected 90's frame is not designed for a few inches longer.

Ha - I was going to say there's a version of my bike with suspension forks, which i'd assume would be 80mm, so I could probably risk an 80mm susp-corrected rigid fork - but looking at the 97 Trek tech docs from RetroBike.com, the forks were 56mm travel!

A compromise might be the Surly Big Dummy - 425mm A-C, which is within the 20mm leeway recommended for suspension fork changes. Designed for the Big Dummy cargo bike but can't see why wouldn't work: beefy, disc and canti tabs, rack eyelet and cable guides... Hmmmm

Monty Dog

How about this (they also do a carbon version)?

http://www.ebay.co.uk/itm/eXotic-Rigid-Lightweight-Alloy-XC-Mountain-Bike-Fork-42-5cm-Disc-Specific-/200895190693?hash=item2ec64956a5

sb88

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19700276#p19700276]Monty Dog[/url] wrote:

How about this (they also do a carbon version)?

http://www.ebay.co.uk/itm/eXotic-Rigid-Lightweight-Alloy-XC-Mountain-Bike-Fork-42-5cm-Disc-Specific-/200895190693?hash=item2ec64956a5

Hey - thanks for the suggestion - looks decent and I see they do a disc + canti-mount version, but I'm after a steel fork for comfort / strength.

Have ordered a Surly Big Dummy fork at 425mm A-C. FWIW, lots of people, inc. the local bike shop where I help out, have suggested a couple of inches extra would be fine, so the roughly 20mm extra on the Big Dummy should be good...

The Rookie

Carbon is stronger than steel.....

sb88

The Rookie wrote:

Carbon is stronger than steel.....

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19701286#p19701286]The Rookie[/url] wrote:

Carbon is stronger than steel.....

Not gonna bite on this one - that's a whole new thread!