Bicycle Mechanics - Seatpost shim?
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10-20-09, 06:34 AM
I've a 27.2mm seatpost that gradually slips down into the seat tube despite the clamp being tightened all the way to the torque limit of the clamp bolt. Obviously, the bike manufacturer had poor manufacturing tolerances on my particular frame.
The steel seat-tube shims that the bike shop sells are MUCH too thick to work. The mechanic suggested making a semi-circular shim from a coke can with a pair of tin snips. This seems doable.
My question is, how do I form a "lip" at the top of the shim to prevent it from sliding down into the seat tube? My thought was to position in in the frame & use a small flat-tip screwdriver as a chisel point to bend a part of the shim over the top of the seat tube. Is there an easier option?
10-20-09, 06:54 AM
I doubt that your frame has enough space to add a Coke-can shim if you are using a 27.2mm post in a frame designed for a 27.2mm post. If it does, why not try a 27.4mm post? Assuming you don't have a grossly oversized seat tube, I'd try a few things. First, get a new seatpost clamp. It's possible that yours was overtightened at some point deforming the clamp. Second, try some fiber Grip by Finish Line on the post. Not only will it help with corrosion, but the friction material in the grease will give your clamp some extra bite on the post. Third, if you are a really heavy guy, try taking your weight off the saddle over bigger bumps. Your seatpost and rear wheel will thank you.
FWIW, my dad (~220 lbs.) had a similar issue with a quick release seatpost clamp on his Trek 7300FX. The problem was solved a non-quick release clamp tightened to spec.
10-20-09, 06:58 AM
Had not thought about the Fiber Grip, great suggestion, thanks!
10-20-09, 10:11 AM
I have a stretched Trek seat lug combined with some small 27.2 posts. I use .003 brass from a hardware store as a shim. The idea is to fill in the hole to the point where the bolt ears are parallel when the bolt is torqued adequately, then never overtorque the bolt again. So far, so good.
I made a lip as follows: my shim wraps about 300 degrees around the post, and sticks out around 1/4 inch above the seat lug. I bent the corners of the shim down to prevent it falling in. The other thing that helped was to grease teh interface between the shim and the frame, and the one between the shim and the post. The frame-side grease seems to be "gluing" the shim to the frame. I tend to hold the shim in place when I R&R a seat post, to make sure it doesn't get distorted very much.
I tried a 27.4, and it was too big to be inserted. I chose a brass shim because it had the potential of being compressed to properly fill the gap, compared to steel or stainless steel. Plus it is chemically stable, unlike aluminum.
10-26-09, 01:54 PM
...try some fiber Grip by Finish Line on the post...
An inexpensive solution! I just ordered a tube.
The clamp looks OK, so I don't think that it's damaged.
I do try to stand when I can anticipate bumps, but sometimes, they are hidden. Also, since I sometimes ride on the shoulder of a heavily-travelled road, I sometimes encounter debris (limbs, etc.) hidden by grass. Even when I'm on smooth pavement, though, about 3-4 hours of riding compresses the seat post into the tube by as much as 1-1.5 inches.
I'm hoping the Finish Line dope will do the trick. If not, I'll try a brass or aluminum shim.
Since my frame is aluminum, I'm just suspecting that loose manufacturing tolerance is to blame.
THANKS for the replies - they are appreciated! FarHorizon
10-26-09, 02:12 PM
Have you considered that it might be the post that maybe undersized? If it's a cheap post this might not be surprising.
10-26-09, 02:16 PM
I had a bike that was supposed to be 27.2 also and had the exact same issue.
I bought a 27.4 post from harris cyclery and it fits like a glove! Kinda stupid that the original didn't work.
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