Seat Post Diameter

[mat205125]

I'm looking to buy a new saddle in the near future as the one that came as standard with my bike is not the lightest, and has it's fair share of rips and scrapes.

Not sure that the bank account will stretch far enough, but was thinking about a seat post at the same time.

I've measured the diameter of the current post (specialized MTB), and it is 30.6mm. Looking on ebay and such like, this doesn't appear to be a common size.

Is this a size that is common off the shelf, or is there an option for fitting a slightly smaller diameter with a shim?

[Trooper2]

30.6 MM is not all that common, I know that Kalloy make a seat post that size, but that maybe what you already have. You can definitely shim to get a wider selection. U.S.E. and Post Moderne both make shims to fit a 27.2 MM (very common) seatpost to a 30.6 MM seat tube.

http://www.use1.com/products/seat_post_shims/shim_...

http://www.zyro.co.uk/product_detail.asp?pid=635

[snotrag]

Many manufacureres will make a 30.6m ( think one of my bikes is a 30.6?) but probably wont be on the shelves in the shops. Online places will help here.

However, it might be easier to shim it. Some people are not a fan, its never been a problem for me. I have shimmed one of my bikes to a 27.2 to match the other, thus, can swap seatposts if needs be.

DO NOT be temppted to think 'ah only .6 of a mm' wont do any harm and use the wrong size - you'lll bugger your frame!

[mat205125]

Cheers for the info guys. Shimming seems a sensible option, and like your idea about shimming to a common size for when you have multiple bikes.

No need to worry about dismissing "just" 0.6mm. An engineering training means that I appreciate that 0.6mm is loads,

When using a shim do you just apply a tiny bit of oil between shim and frame to stop it jamming? What about inside for an Alloy / Ti / or Carbon post?

[Trooper2]

Grease between the shim and seat tube and seatpost and shim no matter what the seat post is made out of, or you run the risk of welding it all together (galvanic corrosion).

[pdV6]

Trooper2 said:

Grease between the shim and seat tube and seatpost and shim no matter what the seat post is made out of, or you run the risk of welding it all together (galvanic corrosion).

Unless you're running a carbon frame, in which case no grease at all.

[Trooper2]

pdV6 said:

Trooper2 said:

Grease between the shim and seat tube and seatpost and shim no matter what the seat post is made out of, or you run the risk of welding it all together (galvanic corrosion).

Unless you're running a carbon frame, in which case no grease at all.

Most manufacturers of carbon posts and frames say do not use grease, they are afraid of some greases that are slightly acidic causing the breakdown of the clear coat and eventually causing the carbon resin to swell, seizing the post in the frame. Some manufacturers (Trek for sure) say that acidic greases won't effect their carbon fiber products and they are only concerned with seatpost slippage. If you don't use grease and ride in the rain or live in an area with high humidity (especially salt air) you will get galvanic corrosion if you have a mix of steel, aluminum, titanium and/or carbon fiber.

I was a bike technician for 15 years and always greased carbon fiber frames and seat posts if they were going to touch a metal. I have never seen a swelling issue with either carbon frames or seatposts. I have however seen many stuck seatpost do to the lack of grease. You could search out a non-acidic grease if you want to grease but are concerned about it. There are also carbon fiber assembly pastes that can be used. If you have a carbon frame (with no metal or fiberglass inserts in the seat tube) and a carbon seatpost there is no reason to grease.

http://www.velonews.com/tech/report/articles/11799...

http://www.velonews.com/tech/report/articles/9023....

Edited by Trooper2 on Thursday 5th July 14:47

[snotrag]

IIRC it is Lithium based (usually white) grease that promotes corrosion/seizing when two metals are in contact.

[Rico]

Sure its not 30.9mm? Lots of specialized frames have that size?

[mat205125]

Rico said:

Sure its not 30.9mm? Lots of specialized frames have that size?

30.6 with my digi vernier. Having seen how uncommon the size is for posts for sale, I've checked and double checked.

[Rico]

Is the seatpost original? Might be a post thats a couple of .mm too small? Ie the frame is designed for a 30.9 but someone fitted a 30.6?

I may be telling porkies and the frame is a 30.6, just playing devils advocate a bit

[catso]

Rico said:

Sure its not 30.9mm? Lots of specialized frames have that size?

Indeed, my Hardrock has a 30.9mm seat post. It seems odd that there would be another size just 0.3mm smaller (of course I could be wrong) maybe yours is slightly undersize and a 30.9 would fit?

Edited by catso on Thursday 5th July 16:17

[BOR]

Does a seat post sold as a 30.9mm actually measure 30.9mm, or is it actually a 30.6mm, which is suitable for a 30,9mm frame ? There has to be a difference in diameter between the two, otherwise they wont fit.

[mat205125]

That is a very good point BOR.

I can see me and the vernier taking a trip to the bike shop to have a measure of a 'x'mm post.

Does 30.9 = the size of the post, or the size of the frame?

[catso]

BOR said:

Does a seat post sold as a 30.9mm actually measure 30.9mm, or is it actually a 30.6mm, which is suitable for a 30,9mm frame ? There has to be a difference in diameter between the two, otherwise they wont fit.

My 30.9mm seat post measures 30.9mm, would imagine that it's 30.9 to fit into a 31mm tube? If so 30.6mm is quite alot smaller, don't know if it would clamp up sufficently?

[pdV6]

Frames and posts are quoted in terms of the size of the post. I doubt there would be even as much as 0.1mm difference between the external diameter of the post and the internal diameter of the seat tube.

[mat205125]

Best not get a 30.9 then, unless I plan to take a big hammer to it, or ream the frame (joking)

I'd guess that the cam on the quick release has at least 1mm of "lift" (waiting for the "does my bike have cams then?" questions) so can't imagine their being much clearance between a post and frame diameter.

Question to all the carbon posted peeps. Do they scratch badly when you move them up and down? I'm not a constant fiddler with seat height for climbing and decending, but still wouldn't want to see marks on something that costs a fair old wedge of cash.

[pdV6]

mat205125 said:

Question to all the carbon posted peeps. Do they scratch badly when you move them up and down? I'm not a constant fiddler with seat height for climbing and decending, but still wouldn't want to see marks on something that costs a fair old wedge of cash.

They shouldn't do too badly, assuming that tube and post are clean.
Having said that, mine's got some decent scratches in the clearcoat due to seat tube abuse by the frame's previous owner (it's a very tight fit now).