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How to find seatpost size?(9 posts)

How to find seatpost size?LC
Jul 3, 2003 8:56 AM
I have a old Windsor frame that I need to get a longer seatpost. There is no markings on the current seatpost, so how do I determine what size I need? From my seatpost collection all I know is 26.4 is slightly too big and 25.0 is way too small.

I did try looking at Sheldon Brown's database, but he thought it might be 26.4 and I know that is not correct.
re: How to find seatpost size?TJeanloz
Jul 3, 2003 9:01 AM
You can measure your current seatpost with calipers, or use the tried-and-true method of bringing it to the bike shop and trying every size they have.
Your LBS should have a tool to check the post size....nmMR_GRUMPY
Jul 3, 2003 11:43 AM
re: How to find seatpost size?Atombomber
Jul 3, 2003 5:12 PM
Fill the seat tube with sand until it is just to the top of the seat tube. Dump the sand onto a scale a weigh it. Find the density of the sand on the Internet by using one of the search engines. Once you know the volume, divide this figure with the length of the seat tube. This will give you the area of the cross section of the seat tube. Divide this figure by Pi (3.1415926etc), and then take the square root of that result. Multiply by two, and you will have the diameter. Convert to metric if measurements were in imperial.

Or, take a set of Vernier Calipers and measure the inside of the seat tube or the outside of the post you have.
Atombomber in Mensa?bburgoyne26
Jul 3, 2003 6:20 PM
I think Atombomber took his physics class way too serious!

Sometimes the seatpost has the size etched on it...I guess you already checked that?
Mensa is for smart folkAtombomber
Jul 3, 2003 7:42 PM
I'm just a wiseacre.
me stupidbburgoyne26
Jul 4, 2003 10:29 AM
"Sometimes the seatpost has the size etched on it...I guess you already checked that?" DOH! you already said that, my bad. I'm an idiot...no Mensa for me
Seriously now.Atombomber
Jul 4, 2003 7:04 AM
Use some calipers to measure the old post, or the seattube inner diameter. I don't know the bike, but if the reading you get does not correspond with a current size, you might need to increase the size of the seat tube to accomodate the the next larger post, or forgo the boring and shim up a smaller post.
Seriously now.xxl
Jul 4, 2003 8:06 AM
Measure the old post w/calipers, (assuming it fits). I wouldn't measure the seat tube, unless you have some deep inside calipers/mikes, to take your measure several inches past the seat tube clamp; over time, the once-round seat tube opening becomes ovalized. Not much, say, oh, 0.2mm or so, but enough to screw up your measurements. Plus, the manufacturing tolerances of the companies that make the low-end aftermarket seat posts older bikes usually have to be retrofitted with aren't exactly tight.

Nashbar sells cheap seat posts in odd sizes and extended lengths.