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Seatpost diameter enigma - mid 90s Giant Boulder Alu Lite 6061 -

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Seatpost diameter enigma - mid 90s Giant Boulder Alu Lite 6061 -

Old 06-01-13, 07:22 AM
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Seatpost diameter enigma - mid 90s Giant Boulder Alu Lite 6061 -

Having a major WTF moment today

My current bike came with a diy seatpost shim with no measurements stamped on it. It looks like it's 30mm at its widest; word on the street is that Giant's standard seatpost sizes are 30.9. The widest diameter on my bike frame appears to be 30mm, I don't see how a 30.9 seatpost would fit without stretching the current seatpost tube. I tried a 1 1/8" shim (28.6mm) inside my bike's seatpost tubing, and that appears to be off by ~1mm, which makes my potential size 29.6. Given that I found 29.8mm seatposts for sale, that seems like a safe bet. But then I see 30mm seatposts. Will that .2 mm make a difference? Is this an exact science?

If not, how much room for error do I have? Any tricks using household objects or coins I can do to determine the proper fit? Thanks.
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Old 06-01-13, 08:05 AM
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"If not, how much room for error do I have? Any tricks using household objects or coins I can do to determine the proper fit?"

You need to determine your seatpost diameter to within 0.1mm or so. 0.2 mm will make a big difference. Digital calipers are available for as little as $10 which can be used. Many bike shops have a graduated rod for determining the correct post diameter. Trying to clamp a too-small post risks damaging the seat tube.
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Old 06-01-13, 08:34 AM
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The difficulty in accurately measuring for seatposts arises from distortion at the top of the seat tube.

If the OP has a caliper (vernier, dial or digital are equally accurate) he should attach the seat binder bolt, and tighten it until the slot is the same width at the top and bottom, which sets the correct diameter, then measure his seat tube's ID in 2 directions at right angles. If both are the same that's the size. If not, the tube is ovalized (common, and not an issue) and the right size is the average.
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Old 06-01-13, 08:51 AM
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Yes, it is an exact science. .2mm will make a huge difference between a loose seatpost that slides down and a properly fitting seatpost that doesn't damage the frame or slide down.

I remember some guy came into our co-op with a cutout of the seatpost that he made with a hammer and a piece of cardboard. I told him that would not suffice at all and he didn't believe me. Came back the next week with the bike and we got the right post fitted.
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Old 06-01-13, 10:04 AM
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thanks guys. I took the bike in to the store that pulled my bottom bracket and headset.

Which measurement is most critical? Seatpost tubing diameter or the DIY shim's diameter? (Remember: Ther was no original seatpost fitted to this bike when I got it other than the DIY shim.) Do bike component specs, such as seatpost diameter, vary depending on which region the bike is sold in (Europe, US, Asia-Pacific)?

ok, took it to the bike shop (who have already done some work on my bike). They were kind enough to measure it with a pair of (non-digital) calipers. They measured the DIY shim's diameter, since that's what the previous owner used as a seatpost shim (nearly 1' long). Here's where it gets weird: calipers measured 30mm. But both guys shook their head (this is Europe) in disbelief, and concurred on the correct diameter being 29.8mm; not 30mm, as the DIY shim's diameter measured. What do you guys make of this?

I'm under the impression that this bike (Giant Boulder Alu Lite 6061) was mostly distributed in Europe and the UK. Most of my searches indicate that the only people that sell them used hail from either the Benelux countries or the UK. I haven't found anyone in the US selling them used (is this relevant?). Could this fact somehow correlate to the diameter? I came across several threads that indicated there is such a thing as 30mm seatpost diameter. One of the mechanics seems pretty old school, so I'm guessing he's clever enough to recognize the likelihood of a 30mm DIY seatpost actually being a shim for a 29.8 hole. I don't know.... It looks like the bike dates from the mid-90s, there's a possibility the frame's seatpost tubing expanded over a 20-year lifetime - isn't there?

Again - thanks for all the input.
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Old 06-01-13, 10:17 AM
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It could be measurement error if, for example the shim flexed away from the post slightly. Also, since the tube is slotted, it is possible to jam something slightly oversize in a bit, but when you get to the bottom of the slot, the error becomes obvious as it won't go in farther.

It sounds like the 29.8 post is right, and the acid test is that it's a snug, but slip fit in the true size tubing below the end of the slot.

Another solid indicator is the width of the slot when clamped. If the original ID of the tube matches the post, the tube won't close much when clamped tight, so the width of the slot at the top will be very close to that at the bottom.
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