Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

Seatpost slippage!

Old 05-05-08, 11:09 AM
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powpow
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Seatpost slippage!

OK, aluminum Thomson 31.8 seatpost, Ti frame, normal clamp. I greased the seatpost initially, but is started slipping even with a good amount of torque. Then I cleaned the post and wiped the seat tube to get rid of the grease, and it still slips a little bit. I don't have a torque wrench, but I used a LOT of torque this time, and it still slipped about 3mm on an 85 mile ride yesterday

Any ideas?
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Old 05-05-08, 11:17 AM
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Campag4life
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Don't use a lot of torque. Slippage is due to coef. of friction between mating surfaces. If you know the saddle height you like, mark the post with tape, lightly sand the Thomson post with 600# wet paper below the tape mark, wipe both inside of seat tube and post with alcohol and install. It shouldn't slip. Last resort? Hairspray...yes hairspray. Mark the post with tape again and spray the post with hairspray below the tape line, let dry for about 5 minutes and assemble. It won't slip. Thomson posts shouldn't slip because they are lightly serrated by how the post is machined by design intent.
HTH.
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Old 05-05-08, 11:20 AM
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You've done all you can. Take the post back to the store and have them measure/gauge it to see if it's in spec. If it's not, it should be covered under warranty. If it is in spec, try a single layer of electric tape around the post right above the seat tube.
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Old 05-05-08, 11:23 AM
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My thomson slipped like mad in my Ti frame. I used some tacx carbon assembly paste I had laying around and it hasn't budged since.
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Old 05-05-08, 11:25 AM
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Originally Posted by Campag4life View Post
Don't use a lot of torque. Slippage is due to coef. of friction between mating surfaces. If you know the saddle height you like, mark the post with tape, lightly sand the Thomson post with 600# wet paper below the tape mark, wipe both inside of seat tube and post with alcohol and install. It shouldn't slip. Last resort? Hairspray...yes hairspray. Mark the post with tape again and spray the post with hairspray below the tape line, let dry for about 5 minutes and assemble. It won't slip. Thomson posts shouldn't slip because they are lightly serrated by how the post is machined by design intent.
HTH.
One thing I forgot to mention is that I have cut the seatpost down by about three or four inches (it is still in under the "Max" range). Could this be a problem?
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Old 05-05-08, 11:28 AM
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Originally Posted by grahny View Post
My thomson slipped like mad in my Ti frame. I used some tacx carbon assembly paste I had laying around and it hasn't budged since.
Interesting, don't you have a Lynskey? This is happening on my Lynskey...
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Old 05-05-08, 11:34 AM
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Originally Posted by powpow View Post
One thing I forgot to mention is that I have cut the seatpost down by about three or four inches (it is still in under the "Max" range). Could this be a problem?
As long as the seatpost goes far enough into the frame that it goes beyond the top tube, you should be ok. Don't think it would affect slipping, but.... From LH Thomson's site:

Q - My Thomson Seatpost hits my rear suspension. Can I cut the end of the seatpost off? How should I cut it?
A - Warning: Once a seatpost has been cut off, the Max Line is No Longer Valid! This can lead to dangerous situations where the seatpost is not inserted into the frame far enough. Damage to the bicycle, seatpost, and/or injury to the rider could result.

Cut the post using a hacksaw. We do not recommend using a pipe cutter. Pipe cutters mar the finish, and are difficult to use on the Thomson seatpost. Be sure to round the corner where the seatpost has been cut using a file or a deburring tool. Mark a new max line on your seatpost that is as far away from the end of your post as the old max line used to be. If the seatpost has been modified, the warranty is at our discretion.

Originally Posted by powpow View Post
Interesting, don't you have a Lynskey? This is happening on my Lynskey...
Yup, it does seem odd. I think there's "Ti prep" out there too, but since the carbon paste worked, I haven't looked further into it.
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Old 05-05-08, 12:37 PM
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Originally Posted by powpow View Post
One thing I forgot to mention is that I have cut the seatpost down by about three or four inches (it is still in under the "Max" range). Could this be a problem?
Shouldn't influence it. Retention of a seat post is due to localized pressure per unit area of the "collar" on the seat tube...and little to due with insertion depth...which distributes force down the tube for support removing torque from the collar locally. There are all kinds of strategies for eliminating seat post slippage. You always want to start with the least invasive method. Light sanding of the post under the collar will generally be all you need. Hairspray will further increase friction. Some use a small wedge that increases local stress. I strongly urge against this approach because point loading the post has not only consequences to the post but to the seat tube collar as well...particularly acute with CF due to its notch sensitivity.
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Old 05-05-08, 12:58 PM
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Originally Posted by Campag4life View Post
Shouldn't influence it. Retention of a seat post is due to localized pressure per unit area of the "collar" on the seat tube...and little to due with insertion depth...which distributes force down the tube for support removing torque from the collar locally. There are all kinds of strategies for eliminating seat post slippage. You always want to start with the least invasive method. Light sanding of the post under the collar will generally be all you need. Hairspray will further increase friction. Some use a small wedge that increases local stress. I strongly urge against this approach because point loading the post has not only consequences to the post but to the seat tube collar as well...particularly acute with CF due to its notch sensitivity.
Thanks, I'll try sanding.
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Old 05-05-08, 02:09 PM
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Get a rear reflector with collar to fit your seat post. Tighten the bicycle's frame collar on your seat post as usual. Slide the reflector collar down until it rests on top of the regular seat post collar. Tighten the reflector's collar. It will add enough gripping power to keep your seat post from slipping. It also marks the height of your seat post so you can remove it and put it back to the same position with ease.
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