Seatpost clamp queery

[breakwellmz]

I ordered one of these the other day as part of a mini rack/saddlebag support i am making.-

http://www.ebay.co.uk/itm/351181073920? ... EBIDX%3AIT

I had assumed that the smaller of the two diameters(You can probably see that it has a step in the bore) would clamp on to my 27,2mm seatpost and i would have to add a sleeve to the rest of it to to take it down to the same size.
What turned uo has diameters of 31,8mm and 30,5mm, clearly a long way off the size i need.
Reading their description, have i been sent an incorrect item, or have i got this all wrong?

Cheers

[Audax67]

The clamp has to pinch the frame in onto the seatpost, so it has to fit over the top of the seat tube. That's why there's a shoulder visible inside this one, to stop it slipping down.

31.8 - 27.2 = 4.6 which means it's expecting a 2.3 mm thickness of metal at the top of the tube. 1.65 mm if you use the 30.5 mm option.

Both of those look feasible, especially if the seat tube is reinforced at the top. I'd say try it.

[Brucey]

such clamps never clamp to the seat pin itself, only ever to the frame. The shoulder is just a location and must always be clear of the seat pin, but can be so by a variable amount. If it did clamp there then it would do so only by about 1mm or so; not enough to be useful anyway.

The reason they are sized in the way indicated is that there are two common sizes and they are associated mainly with those seat post sizes. If they were not indicated thus then 'normal people' i.e. those without vernier calipers would get confused.

I agree the description could be improved, by adding the actual clamp diameters.

If you want it to work for you then I would suggest that you make a shim, perhaps form a scrap frame or something?

cheers

[gerrymcm]

I think these things are normally determined by the frame e.g. ext diameter of the top of the seat tube. I'm looking at a salsa vaya at the moment and they state.
Seatpost 27.2mm
Seat Collar 30.0mm. Salsa Lip-Lock Included

I "think" that this means that if you measured the ext diameter of the top of the seat tube it would be 30mm and therefore the inside of the clamp must be 30mm in order to securely clamp on it. Also I'm sure that these things work by clamping the seat tube not the seat post. I think the lip is designed to simply stop the thing sliding downwards when loose not to clamp the seat post.

I can only think that they've tried show what seat posts it could be used with and that it's been badly described. Surely an internal size would be much more useful to prospective purchasers?

I'd ask for a refund.

Gerry

[stewartpratt]

They're quoting the seatpost size (as you rightly deduced) but it's a clamp to go on the frame's seat tube. Their two sizes will be designed for 28.6mm and 29.8mm (or 30mm, especially once painted) seat tubes, which are the common steel tubing sizes for 25.4 and 27.2mm seatposts respectively. So you've bought a seat clamp for a 29.8mm seat tube.

Their listing is, I would agree, a little misleading. For a start, an aluminium frame which takes a 27.2mm post won't use a 29.8mm seat clamp. I would return the clamp: if you're within the time defined by remote selling legislation you're entitled to (at your own cost) even without any dispute over their bizarre listing (a return on the basis of which would be at their cost).

[breakwellmz]

Thanks chaps.
A combination of my naivety and not the best of information from the seller then.
It`s sitting atop of the seatpost lug(Lugged frame)
What dimensions should their smaller version be would you think?

Cheers

[stewartpratt]

What dimensions should their smaller version be would you think?

I'd be 99% confident it'd be 28.6.

[stewartpratt]

Wait, sorry, I misread your first post, I think. You've got a 31.8 clamp...? Is that the larger of the inner diameters or is that the outer diameter? 31.8 is another common clamp size, often for aluminium frames taking a 27.2. (Not sure there are many aluminium frames that take a 25.4, hence my assumption of steel sizes.) If it's 31.8 inner then I'm not sure I'd want to guess what the "25.4" version is.

*sigh*

In any case, their listing is silly. They're basically listing the parts by a completely daft dimension: like listing a pair of trousers by chest size.

[breakwellmz]

They're quoting the seatpost size (as you rightly deduced) but it's a clamp to go on the frame's seat tube. Their two sizes will be designed for 28.6mm and 29.8mm (or 30mm, especially once painted) seat tubes, which are the common steel tubing sizes for 25.4 and 27.2mm seatposts respectively. So you've bought a seat clamp for a 29.8mm seat tube.

Their listing is, I would agree, a little misleading. For a start, an aluminium frame which takes a 27.2mm post won't use a 29.8mm seat clamp. I would return the clamp: if you're within the time defined by remote selling legislation you're entitled to (at your own cost) even without any dispute over their bizarre listing (a return on the basis of which would be at their cost).

I was`pipped at the post`

[breakwellmz]

Their smaller(25.4mm) replacement turned up today, it was just the size i wanted for my 27,2mm seatpost!-