Seat Clamp Size Advice

[EasyPeez]

Hi,

I think I need a 30mm seat clamp but would like some reassurance that I'm right before ordering.

Bike is a 2015 Genesis Equilibrium Disc - Mjolnir steel tubing. Spec list claims a seatpost diameter of 27.2mm.

I have measured the removed clamp and the seat tube (see below) and to my eye both seem to be a hair's breadth under 30mm across at the widest point, hence I was about to order a 30mm Hope clamp (options listed are 28.6, 30, 31.8, 34.9).

However I then noticed that the clamp I am replacing has '27' stamped on the bottom (see pic). Wondering about this prompted be to do a bit of internet searching, whereupon I found that CRC tells me I should be buying a 28.6mm clamp -

The seat clamp diameter needs to match the diameter of your seat tube rather than your seatpost, which is a little smaller. For example, a 27.2mm seatpost (a size commonly found on road bikes) fits inside a 28.6mm diameter seat tube, so it’s a 28.6mm clamp you need

I'm aware of vernier calipers but would prefer not to splash out as I can't think of anything else I'd use them for.

Is CRC right? Or me? Or are we both wrong?

Thanks in advance for any help.

 

[Threevok]

I think you need a 30mm

28.6 would be too small

 

[Milkfloat]

Looking at the top of the seat tube has it opened up? In other words, does the post go in too easily for the first inch?

 

[biggs682]

Use a set of calipers for more accuracy would be my advice

 

[vickster]

Ask Genesis

 

[si_c]

I'd go for the 30mm, for the simple reason that seat tubes tend to be slightly more than 0.7mm thick at the top. If you're buying online then you can simply return it if needed.

 

[EasyPeez]

I think you need a 30mm

28.6 would be too small

That's what I thought.

Looking at the top of the seat tube has it opened up? In other words, does the post go in too easily for the first inch?

I don't think so. I'll check tonight and have a see. Is there something about the pic that makes you think this might be the case? Is it a common thing/problem?

Use a set of calipers for more accuracy would be my advice

I take your point, but I can't think of one other thing I'll ever use them for. £20-30 on a one-use item, and a few days more waiting to fit my seat clamp....from the pic provided I can't see how it can be anything other than a 30mm that I need. You have a lot more experience of bike repair and maintenance than me so am I being daft? Do you look at that pic and think it could just as easily be a 28.6 or a 31.8 that I need?

 

[Andy_R]

Buy one of each, and send back the one that you don't need.

 

[Milkfloat]

That's what I thought.
I don't think so. I'll check tonight and have a see. Is there something about the pic that makes you think this might be the case? Is it a common thing/problem?

Nothing obvious, but if you suspect it could be smaller it may be worth investigating. Although having said that CRC saying that 27 or 27.2 = 28.6 clamp cannot always be right as it depends on the thickness of the seat tube. Personally, I would borrow or buy a vernier caliper, they are very useful for all sorts of thins and incredibly cheap.

 

[Threevok]

You will have a little leeway if the clamp you use is slightly bigger

My Inbred requires a 29.8mm but I have a 30mm HOPE on it and it fits just fine

 

[Pale Rider]

If the 30mm is a bit sloppy you could shim it.

A neatly cut strip of Coke can would be barely visible, although I can understand you'd prefer your shiny new Hope clamp to fit without modification.

 

[EasyPeez]

CRC saying that 27 or 27.2 = 28.6 clamp cannot always be right as it depends on the thickness of the seat tube

Yeah, that was my thinking as I was measuring my seat tube - sure, 27.2mm might be a standard size seat post for a road bike, but road bikes come in steel, aluminium, carbon, titanium...surely the thickness of the seat tube is not going to remain exactly the same across all the variables of tubing material and brand?!

My Inbred requires a 29.8mm

Wow! Do clamps come in loads of minutely different size options? I thought the sizes I listed in the OP were the extent of the standardises size options.

If the 30mm is a bit sloppy you could shim it.

That's what I was thinking - get one that's too small and it's useless, but slighty too big and a shim should sort it. I have rubber strips in various thicknesses that came with a front light (to allow the bracket to be fitted to different size handlebars) - one of them should do the trick.

 

[Threevok]

Wow! Do clamps come in loads of minutely different size options? I thought the sizes I listed in the OP were the extent of the standardises size options.

Yes they do and it's more down to the thickness of the frame tubing, rather than seatpost diameter

The last four MTBs I have owned all had a 27.2 seatpost size, but all had a different clamp sizes, because of the difference in the thickness of the metal of the frame's seat tube.

However, because these differences are so small - you can sometimes get away with the closest larger size standard

 

[Deleted member 26715]

You should really be measuring the post not the hole

 

[Profpointy]

You need a micrometer or a vernier. Trying to estimate to 0.1mm accuracy with a ruler is optimistic as is wrapping a tape measure round and dividing by pi - which I've seen suggested

Either borrow a mic or vernier or take the post into a bike shop.

I'd be inclide to pony up for a couple of mics or a vernier on fleabay as they're always handy for measuring bolts and so on even for an occasional dabbler