Bicycle Mechanics - Determining seatpost size

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TimJ
12-07-05, 11:32 AM
I've got a specialized hardrock frame from what year I don't know. It doesn't have a seatpot. I've searched all around and can't find anything online to determine the age and what seatpost size it might take. Is it possible to meaure it accurately? Seatpost sizes are so weird, I'm guessing if I just measured the inside of the seatube the figure wouldn't be the actual correct seatpost size, like it needs to be slightly smaller or something. Is there a proper way to do it or am I overthinking it?

How would an lbs determine it?

In case someone with excyclopedic knowledge is reading, the frame is red with "Specialized" on the downtube in gold, Hardrock on the top tube in black, and round tubes. It's labled as "ax" aluminum. It didn't have suspension, had a threaded fork, and nexave derailluers. Cantilever brakes. Gripshift. Non-replacable derailluer hanger. I suspect it was one of the first aluminum hardrocks.

phantomcow2
12-07-05, 12:46 PM
you use a caliper. Get a caliper, use the side to measure ID, and measure the inside diameter of your seat tube
phantomcow2
12-07-05, 12:50 PM
and be careful when measuring. Measuring the Inner diameter of stuff with a caliper can be tricky, so take several measurements to compare. ANd make sure you are free of debreis. Hold the jaws so they are actually measuring the diameters, thats the tricky part

ofofhy
12-07-05, 02:09 PM
http://www.sheldonbrown.com/seatpost_sizes.html

Check here.
ethanfry
12-08-05, 10:53 AM
Don't listen to these guys (listen to _me_).

Take it to your LBS. They have a tool that looks like a shatf with a stepped taper. Oh wait, it _is_ a shaft with a stepped taper! Drop it in, and it'll settle at the largest step that fits into your seatpost. That is the size seatpost you want to use.

DO NOT just find a seatpost that 'kind of' fits and stick it in there. If you have any sort of noticeable play, your post is too small. We're talking about tenths of millimeters here. A too-small seatpost will result in your misshaping the seat tube (at least where the binding bolt is), and that sort of thing is virtually impossible to correct correctly. What's more, I'm going to say it could contribute to seatpost (and perhaps seat tube) failure. Use the proper tool, get the proper result, buy the proper post.

The caliper method may give you the right measurement, but I'd say it's a crap shoot at best.

And while we're shooting craps, I'm gonna go with 27.2.