I was testing my seat and as I was sitting on the bike on the highest level of the seat post.

My seat post came off and bent my seat post, so I ordered another, but now my seat tube doesn't allow my new seat post to fit in even though I bought the same size.

I got a wrench and made my seat tube bigger and finally it fit but now my seat clamp doesn't fit.

Now I ordered a 34-point-something seat clamp which finally worked but now my problem is my seat clamp isn't tight enough.

My seat goes left and right and when riding for a long time my seat goes down. Is there anything I can do?

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My seat tube used to be 28.6 after this incident my old clamp didn't go back in, so I got a 30mm seat clamp but that still didn't work.

So I bought a 34.9mm seat clamp as I thought maybe if I get a bigger size and then really tighten it down it might work as I don't know my bike seat tube size, Then the seat tube in the frame was out of shape.

Could you give me advice on what clamp size to get?

I just bought 2 clamps 28.6 and 31.8 hope one fits.

  • 3
    Sounds like you deformed your frame’s seat tube. I’m not exactly sure it’s safe to continue riding this frame.
    – Michael
    Commented Oct 18, 2020 at 12:02
  • 1
    The clamp appears to be too large. The ends are bumping together. Commented Oct 18, 2020 at 18:27
  • You should be using the correct size seat clamp (the original one). You might need to gently spread it open a bit to fit. Watch for cracks in the seat tube, which would indicate the metal is damaged.
    – MaplePanda
    Commented Oct 18, 2020 at 19:00
  • 4
    Looking at the photos I would be very concerned about work hardening and subsequent fatigue cracks. If you wat to keep riding the bike, get a seat post long enough it has a lot of post inserted into the frame (200+mm, more is better) and regularly inspect for cracks.
    – mattnz
    Commented Oct 18, 2020 at 20:18
  • 5
    I’m also curious as to what you were trying to do originally - were you trying to see what it would feel like riding with the seatpost all the way out or something? There is a “minimum insertion” line for a reason.
    – MaplePanda
    Commented Oct 18, 2020 at 22:35

1 Answer 1


What’s happened here is that you have expanded the diameter of the top of the seat tube, in the process it looks like you have made some small cracks on the top edge, and created a number of burrs. That’s why the original seat post clamp will not fit anymore.

The larger clamp obviously does not work as its diameter is too large to clamp down on the tube with enough force.

Given that there is a reasonable length of seat tube above the top tube, I’d say that this is rescuable by carefully cutting the deformed area away. Removing 5mm from the top of the tube and smoothing the area below to remove burrs may be enough. The slot in the back of the tube may also need to be extended to allow the seat clamp to compress the tube.

No offense meant, but given that you’ve made a big mess of the seat tube with your own efforts, I’d enlist the help of experienced bike mechanic, who will also be able to assess whether the seat tube can be cut back safely.

  • 4
    I'd also be concerned about the damage to the frame - this bike is on the way to being a write-off/parts donor.
    – Criggie
    Commented Oct 18, 2020 at 23:51
  • I'd take a metal file and gently face the top of the tube until you reach a clean surface. Probably safer than a hacksaw as it takes material away in smaller steps, then again, filing is also an art :)
    – user53407
    Commented Oct 20, 2020 at 8:54
  • As a last resort a skilled welder could probably also take the top part off, weld on a new tube part, but I guess the frame is less worth than what that would cost.
    – user53407
    Commented Oct 20, 2020 at 9:08

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