MEASURING THE SEAT POST DIAMETER????

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MEASURING THE SEAT POST DIAMETER????

Postby kenster » Wed Dec 01, 2010 6:44 pm

HI guys, bit of a basic question BUT how do you measure the seat post r.e from outside to outside or inside measurement? got an older bike that needs a new post got a 27mm but still a little too small. i have seen i can source 25.4mm,26mm, 26.2mm,26.4mm,26.8mm so i want to get it exactly right :D thanks folks
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Re: MEASURING THE SEAT POST DIAMETER????

Postby baltipine » Wed Dec 01, 2010 6:53 pm

I believe it is just outside to outside but decimals of mm are crucial. You need a vernier - with a ruler you may just about be able to choose one of the more popular options but you may still get it wrong.

I would say the seat post should be a tight (snug) fit before you do the bolt up.

Yours is probably a 27.2 but could be 27.4.......

Borrow a vernier!

BP.
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Re: MEASURING THE SEAT POST DIAMETER????

Postby thirdcrank » Wed Dec 01, 2010 6:56 pm

SB illustrates the correct tool here, but I've never seen one, and I've never seen one advertised.

http://www.sheldonbrown.com/seatpost-sizes.html
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Re: MEASURING THE SEAT POST DIAMETER????

Postby Greybeard » Wed Dec 01, 2010 6:59 pm

kenster wrote:HI guys, bit of a basic question BUT how do you measure the seat post r.e from outside to outside or inside measurement? got an older bike that needs a new post got a 27mm but still a little too small. i have seen i can source 25.4mm,26mm, 26.2mm,26.4mm,26.8mm so i want to get it exactly right :D thanks folks


The array of seatposts is a little bewildering - and does nothing to endear the various frame manufacturers to me either :roll:
A digital caliper could be handy - not too expensive from places like Lidl/Aldi who sometimes have them for around a tenner or so. This is the sort of thing - a standard, non-digital one is even cheaper :P
http://cgi.ebay.co.uk/6-150mm-Digital-L ... 230c24194a
A cheaper alternative - which could even be free if you already do a bit of car maintenance. If you know the diameter of the seatpost you have and it is smaller than you need - try slipping a range of metric feeler gauges down the gap until you're happy with the fit. Add the measurement to the diameter of the post you have, and Bob's yer uncle. A cheap set of gauges should be available from your local car parts shop for a quid or two
http://cgi.ebay.co.uk/DRAPER-73988-10-B ... 230bcf1530

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Re: MEASURING THE SEAT POST DIAMETER????

Postby kenster » Wed Dec 01, 2010 7:14 pm

Thanks gents, love this forum soo helpful. Just looked up the model on the table etc from link. the model says it's 26.6mm so guess the one to go with is 26.4mm or would you say go 26.2mm???? thanks jon
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Re: MEASURING THE SEAT POST DIAMETER????

Postby robgul » Wed Dec 01, 2010 7:24 pm

thirdcrank wrote:SB illustrates the correct tool here, but I've never seen one, and I've never seen one advertised.

http://www.sheldonbrown.com/seatpost-sizes.html


I would suggest that this method is probably unreliable as variations on the collar at the top of the tube, corrosion in the tube etc would have an impact [I didn't see the gadget at Sheldon's workplace when I went there in about 2002!]

A Vernier caliper, as mentioned, is the tool to do it. OR it's possible to get a pretty good measure by laying the seat pin on a flat surface and sliding two square edged items to either side and measure the distance apart that they are ..... if you're still with me!

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Re: MEASURING THE SEAT POST DIAMETER????

Postby MikewsMITH2 » Wed Dec 01, 2010 7:42 pm


thirdcrank wrote:SB illustrates the correct tool here, but I've never seen one, and I've never seen one advertised.

http://www.sheldonbrown.com/seatpost-sizes.html




That would be the ideal tool as you can try which is too tight/too loose. I measure them with a micrometer, which gives the greatest accuracy. The problem is that they are not always made to any great precision and the size that your micrometer reads doesn't exactly correspond to one of the standard sizes, so you still have to guess which standard size is the closest. If the measurement falls between 2 standard sizes, I always assume that the manufacturer would have erred on the small size when making them.
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Re: MEASURING THE SEAT POST DIAMETER????

Postby pete75 » Thu Dec 02, 2010 11:24 am

In the absence of a vernier caliper or a mike an accurate way might be to measure the circumference with a tape measure and then divide that figure by pi which will give the diameter.
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Re: MEASURING THE SEAT POST DIAMETER????

Postby Wesh-Laurence » Thu Dec 02, 2010 11:49 am

A cheap vernier is worth it's weight in gold.

I use it for measuring, headsets, BBs, bars, bolts, axles etc as well as seatposts.
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WHICH POST TO B UY THEN PLEASE????

Postby kenster » Thu Dec 02, 2010 12:23 pm

which one to get then please? I have established the size from the manufacturer 26.6mm what post should i get therefore? 26.4mm,26.2mm or 26 mm?? much appreciated thanks
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Re: MEASURING THE SEAT POST DIAMETER????

Postby matt2matt2002 » Thu Dec 02, 2010 12:45 pm

pete75 wrote:In the absence of a vernier caliper or a mike an accurate way might be to measure the circumference with a tape measure and then divide that figure by pi which will give the diameter.

Is that my meat pie or my apple pie?
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Re: WHICH POST TO B UY THEN PLEASE????

Postby CREPELLO » Thu Dec 02, 2010 1:19 pm

kenster wrote:which one to get then please? I have established the size from the manufacturer 26.6mm what post should i get therefore? 26.4mm,26.2mm or 26 mm?? much appreciated thanks

The answer is 26.6mm, as the manufacturer says. Why do you want to go smaller? Best not go for a smaller size unless you know that the 26.6 is absolutely too tight a fit. You mustn't go too small, otherwise you will risk distorting the frame around the clamp, risk the post slipping, experience creaking and not get a good seal against water ingress.
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Re: MEASURING THE SEAT POST DIAMETER????

Postby kenster » Thu Dec 02, 2010 5:50 pm

Thanks your ALL LEGENDS :D :D
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Re: MEASURING THE SEAT POST DIAMETER????

Postby Ash28 » Thu Dec 02, 2010 5:56 pm

Do you know the make and model of the frame as someone may know the dia, I think 531 tubing has a seat post dia of 27.2.
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Re: MEASURING THE SEAT POST DIAMETER????

Postby PW » Thu Dec 02, 2010 7:08 pm

*MOST* 531 d/b is 27.2mm. Sometimes it's 27.0 Depends on the reamer used by that builder. Plain gauge 531 (off the top of my head) is usually 26.4mm.
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