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Old 05-13-11, 01:03 PM   #1
GrayJay
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Fork Crown / steertube failure

I recently converted a vintage Bottecchia road bike into a cyclocross bike, added cantilevers to the fork.

http://www.bikeforums.net/showthread...=#post12603264



After several miles of use, I noticed that something felt wierd with the steering and there was a mild shudder when braking. Then, rather unexpectedly the front brakes locked up and I flipped over the bars. Inspection revealed that the fork steer tube had separated from the cast fork crown, looked to be really insufficient factory brazing. I suspect that the shuddering cantilevers stressed the joint more than the original fork crown mounted centerpulls ever had. Nothing was actually bent or damaged in the crash (besides my hands), the crown was so loose I was able to remove it from the steer tube by hand. Once I cleaned away the scant remaining brass, the OD of the steering tube is 25.4mm and the ID of the cast hole in the crown is 26.0mm, makes for a really loose fit between the two. Is a 0.6mm loose fit at all typical at the fork crown? If I were to braze it back together and completely fill the void between the tube and crown with brass, should it hold together OK with such a large brazed gap or are there other measures I should take to fix?
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Last edited by GrayJay; 05-13-11 at 01:13 PM.
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Old 05-13-11, 02:01 PM   #2
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That is not a typical gap, IMO. Should be a close sleeve fit, but not a press fit. Sounds like an ISO steerer and a JIS fork crown? Or was JIS 27.0?
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Old 05-13-11, 02:27 PM   #3
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Sounds like the caliper brake bolt had been holding things together, so when it was gone...
If the steerer/crown was so badly brazed, I wouldn't trust the blade to crown joints either.
New fork time.

Last edited by Live Wire; 05-13-11 at 06:04 PM.
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Old 05-13-11, 04:13 PM   #4
unterhausen
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ouch. I'm not sure it follows that the fork blades are a problem, but I would certainly have a hard time trusting it. It takes a lot of heat to braze a fork crown/steerer with brass. And it's not a job for a beginner.
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Old 05-13-11, 04:31 PM   #5
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Quote:
Originally Posted by mudboy View Post
That is not a typical gap, IMO. Should be a close sleeve fit, but not a press fit. Sounds like an ISO steerer and a JIS fork crown? Or was JIS 27.0?
AFAIK, 1" steerers are 25.4mm, regardless of headset dimensions. The 26.4/27.0 dimension is the diameter of the collar at the top of the crown. But it DOES sound like a mighty sloppy fit. I wouldn't trust it.

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Bend, OR
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Old 05-13-11, 07:52 PM   #6
GrayJay
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The crown casting does not have a collar around the steertube above the flat section of the casting. The race was held in place with a thin 26.4mm shim around the steertube. If I try to fix it, I will probably also add an external fillet around the steer tube to provide a bit stronger connection and cut it to correct 26.6mm size to retain the crown race without a shim.
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Old 05-14-11, 10:49 AM   #7
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That's way too much gap for my money. In your shoes, I'd either start over, or buy one of Soma's traditional cyclocross forks.
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