Fed up on my constantly slipping seat QR clamp, so doing the sensible thing and going for a bolted clamp.

I remove the old clamp, see the number 27 etched on it, so I peruse Wiggle for a 27mm sized clamp. There's nothing there of that size – just a couple at 28.6 and various sizes in excess of 30mm.

I find the tape measure and check out this '27 mm' clamp. And guess what – it measures much more than 27mm. It's about 30mm inside diameter when clamped an about 31mm when open.

So what size am I after?

28.6mm? 31.8mm?

I know I'm probably being an idiot, but hey, it's a Sunday and you lot can have common sense on my behalf.

In case you're interested, the seat tube in question on an 853 Genesis Altitude frame. In horrible horrible green.

Steel frames are sometimes 28.6 but most likeley 30.0mm.

Buy a caliper, and measure it (and the seat post)!

Sounds like the seatpost is to small, hence yer clamp slippage…..

real_ben – Sound fair. Thanks.

b r – Good idea for accuracy, but no caliper to hand and can't be bothered to buy one right now. Cheers anyway.

LsD – Unlikely, as the post is original spec on the bike.

Thanks all. Looks like I'll take a punt on 30.

Clive, I wouldn't completely rule out what LsD says. I had a frame ages ago that was supposed to take a 27.0 seatpost but that kept slipping. A 27.2 wouldn't fit so I had to get it reamed out to 27.2, after that it was fine with a 27.2 post.

Even if the post is the otignal spec on the bike it might still be this.