Bicycle Mechanics - Am i measuring for my seatpost correctly?
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05-16-09, 02:33 AM
Inner diameter of the downtube right? I measured it, well a couple of days ago now and it was something like 24.4mm??? its an old peugeot (1982) im just curious because the smallest ive run across are 27mm. I really hope im just doing it wrong.
Thanks for any help:o
05-16-09, 03:30 AM
It is NOT the downtube (from the bottom-bracket that goes up from the pedals to the stem on the front of the frame) that you measure to discern the diameter of the seatpost. It is the SEAT-TUBE. The vertical tubing that runs from where your Bottom-Bracket (where the arms and pedals go through), and the seat (saddle) is attached.
Measure this with a metric device. Such as a metric-caliper. Doing so should solve your question.
+1 Are you using a micrometer? If not, be sure to use one, or just take it to your favorite bike shop and have them do it. They will probably do it for free, or close to it. Seat posts usually come in 0.2 mm increments, which you need a good micrometer to measure.
05-17-09, 04:17 AM
There are bikes that use seatposts smaller than 24mm. This (http://www.sheldonbrown.com/seatpost-sizes-m-z.html#p) might help as a check for your measurement.
05-17-09, 09:34 AM
It is possible that 24.4 is the correct size. This '74 uses a 24.4:
05-17-09, 01:26 PM
You'll want to be careful with a micrometer on old frames. It's not uncommon for someone to have slammed down the wrong size seatpost. Examine the rear seattube cuttout area for deformation - experience will tell you if some idiot installed the last seatpost, or not. If the seattube is slightly mangled (e.g corrosion/garbage in there) this will also interfere with sizing the correct seatpost. Just some things to be aware of.
05-18-09, 09:41 AM
Instead of just measuring, I'd probably take it to the LBS and get them to use their seatpost sizing tool and try different posts.
05-18-09, 01:09 PM
^^good idea. Thanks guys
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