Road Bike, Cycling Forums banner
1 - 8 of 8 Posts

·
Registered
Joined
·
70 Posts
Discussion Starter · #1 ·
I need a new seatpost and want to get a clamp as well. The size for the seatpost is 27.2mm, which seems pretty standard. The specs for my bike say the stock clamp is 30.7mm (Im assuming that is outside diameter).

The seat post I am looking to get is a Thomson and they come in 27.2. The Thomson clamps sizes are 28.6, 31.8, and 35mm. None of those match either the inside or outside diameter for my bike or Thomson's own seat tube size.

This is probably just my own ignorance on how these things are measured, but any help in understanding how to match seatpost clamps with seat tubes would be greatly appreciated.
 

·
Registered
Joined
·
20,820 Posts
I need a new seatpost and want to get a clamp as well. The size for the seatpost is 27.2mm, which seems pretty standard. The specs for my bike say the stock clamp is 30.7mm (Im assuming that is outside diameter).
27.2mm is the O.D. of the Seatpost.
30.7 is the O.D. of the seat tube on the frame and the I.D. of the clamp. The clamp goes on the frame, not the seatpost.
 

·
Registered
Joined
·
7,492 Posts
Seat tube outside diameter will vary a little depending on material, but 31.8 mm is the matching clamp for a 27.2 mm post.
 

·
Registered
Joined
·
70 Posts
Discussion Starter · #4 ·
27.2mm is the O.D. of the Seatpost.
30.7 is the O.D. of the seat tube on the frame and the I.D. of the clamp. The clamp goes on the frame, not the seatpost.
I should have been more clear. I know the OD of the seatpost is 27.2mm. The clamp size is spec'd from the manufacturer as 30.7mm, which should be the OD of the seat tube and ID of clamp at the point the clamp sits on the seat tube.

In looking at clamps, specifically the Thomson ones, there isn't a 30.7mm option. Will sizing up to a 31.8 cause problems?
 

·
Registered
Joined
·
756 Posts
May be time to use some calipers to double check your current clamp and seat tube diameter to make sure.

Using way too big a clamp means it will probably bottom out on itself and not clamp tight enough. Or if it does hold, it could be over stressed as well as concentrating at a pinch point on your seat tube possibly damaging both your frame and the clamp.
 

·
Forever a Student
Joined
·
4,963 Posts
Seatpost clamps should come from the FRAME manufacturer, end of story. I would not use any aftermarket clamp on any bike I ever owned. How are they going to know the proper torque spec for the frame and the proper orientation? They aren't.

If for some reason you don't have a seatpost clamp for your frame, get one from the company the made the frame and follow their instructions to the letter.
 
1 - 8 of 8 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top