Bicycle Mechanics - Seatpost sliding.
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10-18-10, 04:23 PM
OH GOSH am I frustrated with this. I've tighened the clamp on my motobecane messenger for the seatpost as tight as I can and it just keeps sliding after maybe 1 minute. I've tried it all. Rubbing chalk on the post, putting duct tape above where I want my seat post atl. Is this a seatpost problem or a clamp problem? I am just too annoyed with this problem...
Step one, look at the ears where the clamping bolt goes. If they touch in the back, you'll never get the post tight.
Usually clamping ears that touch when tightened mean that the post is too small and you need to find the right diameter post. Sometimes the ears touch even with the right post, and if so, check for cracking at the base. Sometimes the ears are badly formed, or bent from over-tightening in the past. If you only need a bit more, you can file away the inside back to give them more room to close.
If you have a replaceable clamp, and the ears touch, check the post size, clamp size, and if both are right, buy a new clamp.
Finally, to get decent clamping pressure make sure the hardware is oiled or greased so it turns smoothly,. but if all checks out 100% the last remedy is to use some coarse lapping compound, or carbon assembly on the post in the 1" or so that's under the clamp. That will provide some maechanical engagement through the grit biting both the post and frame and giving you greater grip at lower clamping force.
10-18-10, 04:52 PM
If your seat post is the right size for your frame, it should not slide. I would first make SURE it is not smaller than the frame is designed for. Even .2mm can make it difficult or impossible to tighten properly. Look down inside the seat tube with the seat post removed, and see if the top of the tube is pulled inward. If it is pulled in even slightly, this could be an indication that you may be trying in vain to tighten the tube on a post that is too small. Another giveaway is if you remove the clamp, then slide the post in about half its length, and try to move the post rotationally. If it has any movement at the bottom, with the top acting as a pivot, this shows the top of the tube may be pushed in, while attempting to tighten.
You may have to do a little research to find out what size seat post your frame is made for, but then you can get one that fits, and it should tighten properly.
10-18-10, 05:10 PM
they may have bored out the frame a little oversize, ,try beer can shims to fill in the extra space.
sounds like BD ships stuff that may have challenges dealers usually sort out,
but they are bypassed, definition of direct.
like getting a guitar from musicians friend, they handle un opened cartons , period.
I cured a slipping seat post on my folding bike , by adding a second clamp-band just big enough to go around the seatpost.
It sits on top of the frames QR.
10-18-10, 06:21 PM
Is it the stock seatpost? If not, I'd guess it's the wrong size before you do anything else, try one size bigger.
10-18-10, 06:40 PM
try replacing the binder bolt. I had a seatpost that wouldn't tighten and it turned out that the binder bolt was damaged and stopped turning before the clamp was tight enough. after getting a new one, make sure to grease the threads.
10-18-10, 08:30 PM
10-18-10, 10:01 PM
According to the pictures at the BD website the Messanger uses a separate seat clamp collar. It is also the style where the 5 mm bolt runs into a steel round bar nut that sits in a hole. So I'm inclined to think that cyclist2000 may be on the right track with thoughts of a damaged or not tapped far enough thread. Or if the round insert nut has no hole behind it to allow any excess bolt to fit through perhaps the bolt is bottoming out before sufficient clamping pressure is gained.
Looneydude. Turn the bolt out counting how many turns it takes to remove it. Slip the collar off the bike and then turn the screw back in that many turns and then keep going. If you can't easily turn the screw another 2 or 3 turns then something is very wrong and you'll have to study things up to determine what is at fault and correct as needed. Just don't turn the screw in far enough to bend the collar closed. If it comes to that take the screw and the little round steel nut out of the collar and try it. While it is out ensure that there is a hole that allows for the screw coming out the other side of the nut and that it doesn't bind. in that hole. You may need to open it up wiht a drill if it does bind.
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