What diameter is my seatpost?

Jamey

On my seat post clamp it has the little symbol for diameter (circle with a diagonal line bisecting it) and next to that it says 31.8mm but that could be any one of three things - the seat post itself, the seat tube or the seat post clamp.

Which one is it?

And, if it's not the seat post then can you please tell me if there's a way to work out the diameter given that I know the clamp is 31.8? Like, is it always the case that the post itself is a certain number smaller than the clamp, or something?

Slow Downcp

I'd say the clamp - the seatpost size should be stamped on it somewhere. have you taken it out and checked?

Clever Pun

in the same vein, I've got an old frame how do I tell the width (It's smaller than my existing oversized ones)

Jamey

I've removed the seatpost before but I can't remember if it said anywhere. Will check again when I get a chance.

In the meantime if anyone else has a Specialized Tricross Sport (2008 if it makes a difference) and happens to know, please let me know.

Slow Downcp

It's a 27.2 according to their website.

http://www.specialized.com/bc/SBCBkMode ... spid=32209

emdeef

..and the clamp is 31.8

whyamihere

It's a 27.2.

whyamihere

Clever Pun wrote:

in the same vein, I've got an old frame how do I tell the width (It's smaller than my existing oversized ones)

Verniers.

Jamey

Thanks guys.

Now to get a cheap seatpost I can use for clamping the bike in a workstand.

Clever Pun

whyamihere wrote:

Clever Pun wrote:

in the same vein, I've got an old frame how do I tell the width (It's smaller than my existing oversized ones)

Verniers.

?

whyamihere

http://www.tickintimeworldofwatchtools. ... -457-p.asp

Slow Downcp

Jamey wrote:

Thanks guys.

Now to get a cheap seatpost I can use for clamping the bike in a workstand.

Why, what's wrong with using the post you've got? As it's alloy wrapped in a carbon layer, you're not gonna crush it.

Jamey

Slow Downcp wrote:

Jamey wrote:

Thanks guys.

Now to get a cheap seatpost I can use for clamping the bike in a workstand.

Why, what's wrong with using the post you've got? As it's alloy wrapped in a carbon layer, you're not gonna crush it.

Oh really? I thought that was a possibility. I'm sure someone on here said it was fairly common to use a cheap/old/knackered seatpost for clamping to avoid damaging the main one.

Slow Downcp

Jamey wrote:

Slow Downcp wrote:

Jamey wrote:

Thanks guys.

Now to get a cheap seatpost I can use for clamping the bike in a workstand.

Why, what's wrong with using the post you've got? As it's alloy wrapped in a carbon layer, you're not gonna crush it.

Oh really? I thought that was a possibility. I'm sure someone on here said it was fairly common to use a cheap/old/knackered seatpost for clamping to avoid damaging the main one.

If it's a carbon post, then yes - but if it's alloy with a carbon wrap you're not gonna do any damage unless you REALLY tighten it up.

Clever Pun

whyamihere wrote:

http://www.tickintimeworldofwatchtools.co.uk/vernier-calipers-150-mm-x-002-mm-457-p.asp

wouldn't a ruler work?

Jamey

Slow Downcp wrote:

If it's a carbon post, then yes - but if it's alloy with a carbon wrap you're not gonna do any damage unless you REALLY tighten it up.

It's the stock post that came with the Tricross, which says it's carbon wrapped.

Why do they do that, then? Is it lighter than alloy but not as expensive as pure carbon, or something?

alfablue

Just cosmetic I would guess

dodgy

Clever Pun wrote:

whyamihere wrote:

http://www.tickintimeworldofwatchtools.co.uk/vernier-calipers-150-mm-x-002-mm-457-p.asp

wouldn't a ruler work?

It can be quite difficult to measure cross sections with a ruler, it's hard to know when you're measuring the widest part although trial and error will get the right answer eventually. Vernier Calipers are very cheap now, I just bought a digital one off eBay for £6 (+£3 delivery) and it's really handy for measuring stuff on the bike, particularly cartridge bearings because you can measure the 3 important dimensions really quickly, easily and accurately. It would be possible with a ruler, but possibly not accurate enough.

Slow Downcp

Jamey wrote:

Slow Downcp wrote:

If it's a carbon post, then yes - but if it's alloy with a carbon wrap you're not gonna do any damage unless you REALLY tighten it up.

It's the stock post that came with the Tricross, which says it's carbon wrapped.

Why do they do that, then? Is it lighter than alloy but not as expensive as pure carbon, or something?

It will be slightly heavier than an alloy post probably, as it's an alloy post with extra carbon stuck on - it's purely for looks.

Jamey

Crafty beggars.

Cheers.

rb1956

Clever Pun wrote:

whyamihere wrote:

http://www.tickintimeworldofwatchtools.co.uk/vernier-calipers-150-mm-x-002-mm-457-p.asp

wouldn't a ruler work?

It's difficult to measure the diameter of a round tube accurately with a ruler. Better to wrap a tape round, measure the circumference and use d = c/π to work out the diameter, but it's not easy to find a tape that is accurate and flexible, especially when measuring small tubes. And it won't work to measure inside diameters anyway.

A vernier calliper is a very useful tool, and they're cheap. Don't bother with the expensive dial or digital callipers. Once you've learned how to read the vernier scale, they're accurate to about 1/10 millimetre which is quite good enough.

Konan

Depends on the carbon wrap. It's quite common to use a lighter grade alloy and skin carbon over. This works out at a similar weight to alloy (less, in some cases) and retains some of the damping properties of carbon but not all the price properties.

Sometimes.... it's show.