Can I use a 34.9 clamp on a 34.4 seat-tube?

keef66

Yesterday faffing with the seatpost on the CR1 and the seatpost clamp stripped it's threads. Quick switch to the winter bike for the afternoon ride, which was fortuitous since I was dodging heavy showers and glad of the mudguards.

Anyhoo, I always thought the clamp looked a bit flimsy, so I thought I'd take the opportunity to fit something a bit better. Measuring the ID of the clamp and the OD of the seat-tube I get 34.4mm, but the closest aftermarket ones I kept finding were all 34.9, and I've ordered a Thomson one.

This morning I found that Westbrooks do actually have a 34.4 clamp (for Scott models newer than mine) :roll:

Why didn't I look there first?? And it's cheaper than the one I've bought

Actually I'm not sure I'd have bought one had I spotted it sooner; it looks to be the same cheese-based construction as the one which failed.

So is the 34.9 clamp likely to work, or will it nip up before it's exerting sufficient clamping force on the tube / post? Could I shim it in some way? Strip of coke can or a wrap of insulating tape?

Should I send it back and buy the Scott part from Westbrooks, and treat it with kid gloves??

slowbike

I'd probably just do the insulating tape - although it will probably just move about.
I used quite a bit of it for the child seat clamp - and some more for the followme mount point - neither of these are position critical though - although you could say that there's more twisting force on the child seat clamp.

The tension in the cable to shift the mech out is quite high - especially when you hit the stop - that's what is going to drag the mech down (or up depending on the lead in) - I'd probably just try it and see ...

keef66

Cable tension isn't an issue; it's the seatpost clamp I'm talking about, nothing to do with the front mech.

Ben6899

Slowbike wrote:

I'd probably just do the insulating tape - although it will probably just move about.
I used quite a bit of it for the child seat clamp - and some more for the followme mount point - neither of these are position critical though - although you could say that there's more twisting force on the child seat clamp.

The tension in the cable to shift the mech out is quite high - especially when you hit the stop - that's what is going to drag the mech down (or up depending on the lead in) - I'd probably just try it and see ...

I think Keef is talking about the seat post clamp?

I'd try the 34.9mm, Keef. Half a mil is sweet FA.

slowbike

keef66 wrote:

Cable tension isn't an issue; it's the seatpost clamp I'm talking about, nothing to do with the front mech.

Ah flip ...

yer - not much cable tension in a seat post clamp ...

If it was the seat post I'd say nope - get the right one - for the clamp - you'll be fine because it ... clamps ?

keef66

Well the Thomson clamp has arrived on my desk. Comes in a little drawstring bag with a proper instruction leaflet. It tells you to grease the bolt threads before use. And it's quite particular about preparing the top of the seat tube, and which way round to fit the clamp on metal vs carbon frames.

It is a thing of beauty so I'm determined to make it work...

PhilipPirrip

The Thomson clamp is a lovely thing and because of their design they don't need the masses of torque to achieve enough clamping force so, hopefully if it works, no more stripped threads for you.

keef66

Yes, I did a double-take when I read the 2.8Nm recommended torque.

proto

Get the old seat clamp helicoiled for not much money at a local engineering company and put it back on is my advice.

keef66

So to answer my own questions in the interests of anyone reading this in future:

is the 34.9 clamp likely to work, or will it nip up before it's exerting sufficient clamping force on the tube / post?

The second one. That's exactly what happened. Seems 0.5mm too big is a lot in seatpost clamp terms.

Could I shim it in some way? Strip of coke can or a wrap of insulating tape?

John Smiths can actually; strip folded in half to give a double thickness then trimmed precisely. Invisible when fitted, and the clamp's now holding at its 2.8Nm torque.

@ Proto: I did briefly consider drilling / tapping the old one to take an M5 bolt, but think there's too little alloy there to play with. Ditto the Helicoil, and I suspect it would cost as much as a Scott replacement (£10)

Ben6899

Finally! Good use for a John Smith's can!