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Old 11-23-11, 10:28 AM   #1
wernst
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Cracking along the seat tube under the QR seat clamp. 4130 ChroMo. Thoughts?

All,

So I have a 2006 Dahon folding bike with a 4130 steel frame, and two days ago I noticed a small crack on the seat tube, just under the clamp. I'm not the original owner, so there's no warranty to work with. I've taken two photos (one with flash, one without).





I've marked the end of the crack with a Sharpie to determine the rate of crack growth, and while there's been no change since I noticed it, and I don't know how long it's been there, I assume it is going to get worse with time, and that I should take steps to get it repaired early.

So I'm here posting on Framebuilders to confirm that such a crack is indeed repairable, and perhaps to get a reference to someone in the Los Angeles/Orange County area who might do a quality repair, and maybe to get a sense of the costs I'm looking at (and I don't really care how it looks - this is my daily commuter bike.)

Thanks all,
Warr
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Old 11-23-11, 11:17 AM   #2
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To me, it looks like the installed seatpost is too small. The crack at the stress relief hole is obvious, but it looks as though there may also be a crack right under the clamp.

IMHO, a reasonable fix would be to cut a piece of 1mm wall 4130 tubing with an inside diameter that's close to the O.D. of the seat tube, and cut to fit around the rear half of the seat tube and extending to about a half inch below the stress relief hole, braze it to the seat tube as a "sister" reinforcement, and redrill the stress relief hole and cut the slit in the reinforcement.

If there is a crack just below the clamp, you might have to take the sister to the top of the seat tube (all around the top above the top tube weld) and get a new clamp with a larger diameter to fit the sister.

Make sure you're using the correct diameter seatpost.

It wont be pretty, but it'll be functional.
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Old 11-23-11, 11:28 AM   #3
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agree with Scooper about the seatpost size and fix.

don't think there will be any other cracks though.

Someone always says to stop drill the crack, and I'd just like to point out that the crack emanates from a stop drill
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Old 11-23-11, 01:02 PM   #4
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Thx for the opinions, folks. Please keep 'em coming.

Just to clarify things: the seatpost is the correct diameter, and is the factory seatpost. There is also a plastic shim between the seatpost and the seat tube (you can see it in the gap - it's black).

There's no cracking under the clamp itself - that's just the paint.

As a folding bike, the seat post clamp gets clamped and unclamped about 8 times a day for the commute.

-Warr
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Old 11-23-11, 04:01 PM   #5
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is it possible the shim is wearing out/deforming?
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Old 11-24-11, 02:07 AM   #6
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This is a crappy solution for a stock-only-one-seatpost production line. There is no way a plastic shim can withstand the pressures created by the seatpost-binder. You have two real solutions:

1. replace plastic shim with aluminium shim. Measure OD of seatpost, real ID of seat-tube (measure 2-3" down). Subtract the two and divide by 2 for shim thickness.

2. replace seatpost with larger-diameter that fits precisely into seat-tube.

Before doing either of these, you must expand the top of the pinched seat-tube so that it's the same diameter as the rest of the tubing.
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Old 11-24-11, 07:58 AM   #7
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In the pictures, it looks like the two sides of the slot are a lot closer at the clamp than down at the bottom, at the hole. If it is not an illusion caused by the camera, that's the cause of the crack without a doubt. My guess is that you need a bigger seatpost without any kind of shim in there. Your LBS should br able to help find the correct size( .2 mm less than the ID of the seattube). The sizing will be complicated if the tube has been deformed under the clamp but a good mechanic should be able to figure it out and reshape the tube back to original. I will say that it's curious that it came from Dahon that way.
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Old 11-24-11, 10:53 AM   #8
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my guess is that the plastic shim is the manufacturer's attempt at a bearing so that the seat post is easy to remove. I can imagine a few ways that bulge at the bottom of the slot might happen, all of which would be fixed by sistering on some more material.
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