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Seat post completely slipping due to cracked bike tube - Options?

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Seat post completely slipping due to cracked bike tube - Options?

Old 05-07-18, 08:41 PM
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Seat post completely slipping due to cracked bike tube - Options?

Hi guys, I recently acquired a bike from someone who was less than honest about the shape of the frame. Basically, i quickly learned that the seat tube is cracked and now the seat post doesnt hold up at all.
On this old frame, the clamp is a part of the frame so I imagine I don't have an option to install a separate clamp for the seat post? What are my options here? the current clamp that's a part of the frame doesn't hold the seat post in place regardless of torque. Is this frame rendered useless?



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Old 05-07-18, 09:17 PM
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Get your local shop to carefully measure and supply the correct sized seat post, and a new clamp bolt. The crack that's visible should not preclude tightening the correct post in there.
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Old 05-07-18, 09:24 PM
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Originally Posted by Prehyp
Hi guys, I recently acquired a bike from someone who was less than honest about the shape of the frame. Basically, i quickly learned that the seat tube is cracked and now the seat post doesnt hold up at all.
On this old frame, the clamp is a part of the frame so I imagine I don't have an option to install a separate clamp for the seat post? What are my options here? the current clamp that's a part of the frame doesn't hold the seat post in place regardless of torque. Is this frame rendered useless?



Failing that, you could weld a (steel) seatpost onto the frame...
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Old 05-07-18, 09:50 PM
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Nothing much wrong with the frame, you have the wrong sized seat post...
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Old 05-07-18, 10:16 PM
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Not sure the post is undersized from only the photo data. Also doubt the crack is influencing the post's hold.

Let me explain- First the crack- The fix is simple. Lengthen the slot down about 1/8 to 1/4" below the crack's visible bottom/root. File a slightly larger then the slot is wide key hole to help further remove the stress concentrating at the slot's new bottom. I strongly suspect that the brazing temp was a bit high and the effective wall thickness change (the lug and tube VS just the tube) made the thinner tube only get well over heated. Done this boo boo myself. That the slot is so short, not even extending below the lug's edge, makes the distortion from clamping focus right there. Poor execution of a bad design.

Now the post's holding- far more experienced builders then I have commented on how a too short slot can contribute to a slipping post. What I don't see is the slot's width showing the classic closing up when a too small post is used. The slot is parallel from bottom to top. When a seat lug is tightness around a too small post there is typically a closing up of the slot, more at the slot's top then the bottom. I suppose the post could be only a couple of tenths (of a mm) to small and this isn't much so maybe if this is the case the slot could look parallel when relaxed... Often a closing up slot also has a distorted bulge at it's base, I don't see any evidence of this too.

Now there are other factors that could be at paly too. Like the high heat also caused a distorting of the lug/tube shape. So a round post is being fitted into an ovalish hole. The binder bolt could be bottoming out on it's self if a classic Campy like one. But I would thing this would be the first thing to check out.

Certainly I agree that an experienced guy (gal) should be involved for any repairs. If a hack wrench suggests reaming the tube to make it round they better understand about tube wall losses and future worse problems this can lead to. So much of this type of fix involves judgement and not numbers. Andy
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