Cycling UK Forum

Hi,

I had a 31.8 mm seat post clamp on my bike but never felt as though it was tight enough (I could still turn the seat). I then measure the diameter of the post and it was 28.6 mm so I replaced the clamp with one of this size. To measure the diameter I wrapped a piece of paper around the tube, marked the overlap, converted the measurement to mm, and divided by PI. To get the new clamp onto the top of the tube I had to pry it open with a pair of pliers. I managed to get the seat post in with a very little force, tighten the clamp, and the saddle is now secure. I am concerned however that I may cause damage to my bike if the clamp is too tight. Should the clamp just slide onto the tube when installing it or is there always a little force required?

P.S. The bike has a steel frame.

Thanks,

Richard

dick220369 wrote:Hi,

I had a 31.8 mm seat post clamp on my bike but never felt as though it was tight enough (I could still turn the seat). I then measure the diameter of the post and it was 28.6 mm so I replaced the clamp with one of this size. To measure the diameter I wrapped a piece of paper around the tube, marked the overlap, converted the measurement to mm, and divided by PI. To get the new clamp onto the top of the tube I had to pry it open with a pair of pliers. I managed to get the seat post in with a very little force, tighten the clamp, and the saddle is now secure. I am concerned however that I may cause damage to my bike if the clamp is too tight. Should the clamp just slide onto the tube when installing it or is there always a little force required?

P.S. The bike has a steel frame.

Thanks,

Richard

are these QR clamps or hex bolts? all the clamps I've used just dropped on the the seat tube. did you measure the external diameter of the seat tube (on the bike frame) or the seat post (the removable post the saddle clamps onto)? the seat post diameter will be smaller than the seat tube diameter.

Hi,

The clamp uses hex bolts and I measured the external diameter of the seat tube (on the bike frame). I don't think there will be any damage to the frame because, even though I had to pry open the clamp to get it on, I still managed to slide in the seat post. And even then the clamp required some additional tightening to secure the post. I just though it was strange that I hade to pry open the clamp in the first place. I didn't know if this was normal.

Thanks,

Richard

When you buy a seat clamp the size/diameter (often marked on the seat clamp) refers to the outside diameter of the seat tube, and not the seat post that goes inside the seat tube.

I checked the old seat clamp and it says 27 on it so I assume this mean 27 mm. If this is correct then the new 28.6 mm clamp should slide on easily without having to pry it open.

Outside diameter of seat tubes:-

1 1/8" = 28.6mm
1 1/4" = 31.8mm
1 1/2" = 34.9mm

Hi
Aldi do a digital caliper for under a tenner (I think). They were on sale earlier this month and my local store still have stock
Regards
tim-b

the seat post clamp will normally have a internal lip that stops it from sliding down the seat tube. If this is too large (too large for the wall thickness of a steel frame) then when the clamp is tightened, the lip will bear directly against the seat pin instead of the clamp bearing down onto the seat pin via the seat tube.

To cure this you have three main options

1) get a new seat clamp -and you should measure carefully- most clamps are built for aluminium frames and have big lips...

2) file the lip down so it is smaller

3) use a shim between the clamp and the seat tube.. beer can is good...

cheers

Brucey's right, as usual
It shouldn't take very much force at all to fit a seatpost clamp onto a frame. Sometimes I've had to spread a clamp with a screwdriver
Your frame almost certainly takes a 30.0mm seatpost clamp if it's TIG welded steel and takes a 27.2mm seatpost. I wouldn't worry too much about overtightening if it's a standard alu post in a steel frame. If it's a very light alu or carbon seatpost you need to be careful. 5-10NM is normal for seatpost tightening torque, you'd need to be going hard to put more than that in with a 4mm or 5mm allen key