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Old 09-08-10, 10:04 PM   #1
wunderkind
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Forks: Axle to crown measurement

Hello,
I'm thinking to swap out my suspension fork for a rigid one. My current suspension fork has a 'uncompressed' axle to crown length of 550mm. The rigid fork is 440mm. Would it work? Both are 1 1/8" threadless.
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Old 09-09-10, 12:39 AM   #2
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That's too much of a difference. The way to figure the suspension fork's equivalent length is to deduct 20 to 24% of the suspension travel to compensate for the static sag when it's adjusted correctly for your weight. Even if the suspension fork is a 130mm travel that's only 30'ish mm from 550 equals 520. The 440 mm fork is 110mm shorter. Unless the frame with the 550mm fork was made to have a really slack steering angle from such a fork length there's just no way you would want to put that short a fork onto that frame. But study the geometry of the steering head and the rise of the head tube to see if by some chance it's the same as the others in the line that use the shorter forks. Sometimes they set up the serious big drop and downhill bikes with these longer forks and accept the really slack head angle that comes with them.
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Old 09-09-10, 01:55 AM   #3
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If you want a somewhat more definite number, put a zip tie on the upper fork leg, just where it disappears into the lower. Then get on the bike, maybe even ride a little on flat ground. Measure how far up the leg compression has pushed the zip tie. Subtract this distance from your uncompressed axle-to-crown measurement.
Apart from that I agree with BCrider, your odds arent' good.
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Old 09-09-10, 06:31 AM   #4
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Hmmm, in the other thread, you noted your susp. fork was this one, which is specced with 50mm of travel.

The difference between Axle to crown measurements between that and the rigid sure seems big for such a short travel susp. fork.

I'd imagine you'd want that difference to be 40mm or less for most bikes. Preferably within 25mm for quite a few bikes.

Is your suspension fork for 700c wheels?

Can you take pictures of actual bike and replacement fork?
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Old 09-09-10, 08:59 AM   #5
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Not the best pic... But this would give you an idea. It is 26". This fork is spec to accept both 26" and 700c.
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Old 09-09-10, 09:03 AM   #6
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Quote:
Originally Posted by LesterOfPuppets View Post
Hmmm, in the other thread, you noted your susp....
+1 on the "Hmmmm..." Even 29er rigid forks are 468~470mm typically, 440mm is pretty standard replacement for a modern 26" ~100mm suspension, the Surly 1x1 comes in two legnths, 453mm and 413mm:
http://surlybikes.com/parts/1x1_fork/
the Surly Insigator is 447mm:
http://surlybikes.com/parts/instigator_fork/
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Old 09-09-10, 09:05 AM   #7
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The rigid fork... It has a bit of a bend at the bottom towards the axle. So it's not a straight blade.

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Old 09-09-10, 10:02 AM   #8
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You may want to re-measure that suspension fork. It does not look like the sort to be 550 mm from axle to crown. My full suspension all mountain style bike has a 120mm travel fork on it that is very obviously longer than the one in your picture and it's only 508mm axle to crown fully extended. So your 550mm figure would appear to be wrong.
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Old 09-09-10, 10:11 AM   #9
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Just Math : Head angle gets steeper, with shorter fork blades, then the trail changes, trail is part of steering geometry,
the re-rake of the fork compensates, but only with a custom fork do you have any choice in the rake.

You Can add a spacer under the crown race and lengthen the distance between , as if the blades are longer ..

Last edited by fietsbob; 09-09-10 at 11:46 AM.
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Old 09-09-10, 10:43 AM   #10
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Put the fork in and ride it, I bet it's fine....
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Old 09-09-10, 11:29 AM   #11
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Quote:
Originally Posted by BCRider View Post
You may want to re-measure that suspension fork. It does not look like the sort to be 550 mm from axle to crown. My full suspension all mountain style bike has a 120mm travel fork on it that is very obviously longer than the one in your picture and it's only 508mm axle to crown fully extended. So your 550mm figure would appear to be wrong.
Well I measure from the middle of the QR all the way up to the crown. See pix
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Old 09-09-10, 11:46 AM   #12
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550mm = about 21.5"
I figure about 13" from axle to the top of your tire. That leaves 8.5" from top of tire to approx. bottom of headtube to get 550mm. That top of tire to bottom of headtube dimension doesn't look anywhere near 8.5" in the photo. And this is still without sag we're talking about.

I think your 440 replacement fork is probably close enough for government work.
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Old 09-09-10, 09:10 PM   #13
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Quote:
Originally Posted by LesterOfPuppets View Post
550mm = about 21.5"
I figure about 13" from axle to the top of your tire. That leaves 8.5" from top of tire to approx. bottom of headtube to get 550mm. That top of tire to bottom of headtube dimension doesn't look anywhere near 8.5" in the photo. And this is still without sag we're talking about.

I think your 440 replacement fork is probably close enough for government work.
I am an idiot and a blind one too! I re-measured the axle to crown again. It is actually 470mm uncompressed! So I think it should match!
The steerer tube is 230mm.
However the rigid fork is 270mm.

What do you think?

Last edited by wunderkind; 09-09-10 at 10:02 PM.
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Old 09-10-10, 04:58 AM   #14
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.. I re-measured the axle to crown again. It is actually 470mm uncompressed!
470 uncompressed to 440 rigid sounds like a good match.

Quote:
Originally Posted by wunderkind View Post
.. The steerer tube is 230mm. However the rigid fork is 270mm.
So? Means you have to cut the rigid fork's steerer down by 40 mm to keep the same stem/spacer assembly as you're currently using(assuming they're put in right). Or add 40 mm worth of spacers somewhere. Now is the time to consider if you're happy with your current riding position.
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Old 09-10-10, 07:49 AM   #15
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Put the fork in and ride it, I bet it's fine....
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