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Correct seatpost clamp sizing

Posted: 26 May 2019, 3:20pm
by belgiangoth
I need to replace a seatpost clamp. Am I correct in believing that they are sized by diameter? What is the tolerance?

Re: Correct seatpost clamp sizing

Posted: 26 May 2019, 7:17pm
by PT1029
If you are talking about the clamping collar where the seat post enters the frame, diameter is usually 28.6mm, 31.8 or 34 (or is it 34.something?) - the same as clamp on front gears (unless you have some tapered exotic something or other).
Seat posts come in 0.2mm diameter increments, the decimal point always being an even number*, these you need to get exact. (*Some Giants use 30.9mm diameter seat pins though).

Re: Correct seatpost clamp sizing

Posted: 27 May 2019, 12:22am
by Brucey
standard seat tube sizes are indeed

28.6mm (1-1/8")
31.8mm (1-1/4")
34.9mm (1-3/8")

but tolerances vary because
- not all clamps are made the same way
- not all clamps accept the same size seat pin inside the clamp
- not all frames use standard sized seat tube sizes at the top

on the last point tubes are often externally butted and this means that the ID of the seat binder collar may vary from the nominal diameter of the seat tube where the FD fits.

In short the clamp could need to be almost any size, so assume nothing, measure everything.