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The sliding seat post....

Old 07-10-19, 07:51 AM
  #1  
Jeremiesmith77
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The sliding seat post....

Hello, I am 6'5", 315 lb and ride about 200 miles a week. Now that I have really strong 36 spoke wheels (no broken spokes in 2000 miles since April!), my only major problem is a seat post that slides down on me no matter how much I tighten the clamp. I have even broken a few of the seat clamp bolts trying to tighten it up as much as possible but it always slides down on me.

Typically, my seat post will slide down 2-3 inches during a 20 mile ride, which is no big deal on mile 15 mile commute to work as I can adjust it before each ride. But, it is annoying to have to readjust it 3-4 times during a long century ride, especially because I fear breaking the bolt in frustration while being far from civilization. This past Sunday, I rode the last hour of a 125 mile ride with the seat too low because I felt the bolt was getting weak and was afraid of breaking it. Finishing the end of a very long ride with my leg extension out of position really did a number on my hamstrings...

A friend of mine mentioned that he heard there was a paste that helped with this but he did not know what it is. Any suggestions?

Last edited by Jeremiesmith77; 07-10-19 at 07:55 AM.
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Old 07-10-19, 08:06 AM
  #2  
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wonder if you would get a better response in the Bicycle Mechanics sub-forum. those techies are always finding a new way to free up a "stuck" seat post, so maybe they know a way to get yours stuck!
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Old 07-10-19, 08:41 AM
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I have this problem with my hardtail MTB, especially on a rough trail ride. Fortunately it has a removable seatpost clamp, so I purchased some extras that have a heavier clamp bolt. They are unbranded but look at lot like what origin8 has always sold. This works better, and I have spares. My next option is to just try a different post, but I haven't taken the time to find a long enough one and I'm not going to pay over $100 for new one. That might be your best option if you have an integrated post clamp.
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Old 07-10-19, 08:58 AM
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Jon T
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Maybe an automotive hose clamp placed around the seatpost itself above the seat tube ?
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Old 07-10-19, 09:02 AM
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I had to replace a seat tube once, although my symptoms weren't as dramatic as yours. First I tried a heavy duty clamp (Surly, perhaps?) Finally I laid a straight edge along the tube, and could see where I'd squeezed the tube so hard it had narrowed where it needed to be clamped.
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Old 07-10-19, 09:53 AM
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https://www.amazon.com/Park-Tool-SAC...59232224&psc=1
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Old 07-10-19, 10:07 AM
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there are double seat post clamps.. Lower, grips the frame 31.8, (1.25") like a typical post clamp,

Upper grips the seat post itself 27.2 mm the common road bike seat post tube size..


another option: maybe mass produced frame seat tube reaming, a bit loose, for the seat post, in it..

getting an over-sized post and a careful (adjustable) hand reaming gets it fitting precisely ,
then the effective clamping force needed is lessened..








....
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Old 07-10-19, 10:22 AM
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jadocs
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Is it a carbon seat post? If so you need to get some carbon assembly paste and put it on.
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Old 07-10-19, 11:33 AM
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Originally Posted by Jeremiesmith77 View Post
Hello, I am 6'5", 315 lb and ride about 200 miles a week. Now that I have really strong 36 spoke wheels (no broken spokes in 2000 miles since April!), my only major problem is a seat post that slides down on me no matter how much I tighten the clamp. I have even broken a few of the seat clamp bolts trying to tighten it up as much as possible but it always slides down on me.

Typically, my seat post will slide down 2-3 inches during a 20 mile ride, which is no big deal on mile 15 mile commute to work as I can adjust it before each ride. But, it is annoying to have to readjust it 3-4 times during a long century ride, especially because I fear breaking the bolt in frustration while being far from civilization. This past Sunday, I rode the last hour of a 125 mile ride with the seat too low because I felt the bolt was getting weak and was afraid of breaking it. Finishing the end of a very long ride with my leg extension out of position really did a number on my hamstrings...

A friend of mine mentioned that he heard there was a paste that helped with this but he did not know what it is. Any suggestions?
That is a bummer that your post keeps slipping down, a real bummer! You're not the only one though, this problem is much more prevalent than you might think.

Could you tell us a few things about your bike? : 1) what kind of frame is it. Is it steel, aluminum, carbon fiber or titanium? 2) what is the material of your seatpost, aluminum or carbon? 3) What is the diameter of your seatpost, 27.2 or some other diameter? 4) what kind of seat collar do you have?

If you have access to a metric dial caliper, accurate to 0.02 mm, check the outer dimension of your seatpost down to 0.1mm tolerance. Do the same with the seat tube bore with the seat collar removed if it is the external type. Check the diameter front to back and side to side to get an idea of the bore is consistent. Check the slot that is cut in the back of the seat tube and see if the slot sides are parallel or pinched. If pinched, this could be an indication that your seatpost is undersized. This would explain you breaking bolts.

I have found that some brands of seatpost (generic ones usually) are 0.02 to 0.1 mm undersized. This can be enough to create slippage. It is possible to ream out a frame one size up. This is what I had to do with my aluminum bike. It was slipping all the time with the 27.2 mm post so we (my LBS mechanic actually, I just watched and learned...) used a bore reaming tool with cutting oil to remove a small amount of material and frequently checked our work with a caliper until I was just under 27.4 mm. I now use a Thomson 27.4 mm post with a new Salsa seat collar. I grease my post in the frame and I have never had another slippage issue with that bike, ever. I am often over 200 so I am definitely a clyde.

Last edited by masi61; 07-10-19 at 11:39 AM.
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Old 07-10-19, 12:16 PM
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Use a piece of thin wall plastic tube, perhaps ABS or PVC that fits over the seat post the right length as a spacer.

Also as mentioned you may find a heavy duty seatpost clamp.
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Old 07-10-19, 01:31 PM
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Carbon grip paste is not limited to carbon. You can use it on Alu post too as far as I know. That should solve your problem.
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Old 07-10-19, 06:45 PM
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You may want to buy a slightly larger tube. If you have a set of calipers you can measure the down tube opening. I did that once on a pawn store bike I bought. Made a big difference.
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Old 07-10-19, 07:23 PM
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If your clamp is of good quality, if it is, true some carbon paste (not really paste but adds a texture surface to two smooth parts) or a shim.

If your clamp keeps needed to tighten, Id upgrade the clamp to a salsa, hope, niner, for $20-30 and be done with the problem. A quality cnc milled clamp is really strong compared to what the OEM uses in most cases.
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Old 07-10-19, 10:06 PM
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Originally Posted by fietsbob View Post
there are double seat post clamps.. Lower, grips the frame 31.8, (1.25") like a typical post clamp,

Upper grips the seat post itself 27.2 mm the common road bike seat post tube size..
This ^^ along with some carbon paste as mentioned a few times as well. My first carbon bike was a Felt F4. It came with the double clamp and never had any slip (nor did I on my previous alu bike). My current steed (Canyon CFSL) I have used carbon paste on the post, but I also had to change the post out for more setback. I went to a 3T post that has a rough surface around the lower part and zero slippage there either
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Old 07-11-19, 06:35 PM
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This is exactly what I needed. Picked some up at the LBS then rode 40 miles without even a millimeter of slippage. Thanks to all that weighed in!
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Old 07-13-19, 06:38 PM
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Is it the original seat post? I've worked for bike co-ops and have rehabed quite a few bikes over the years. Every once in a while you will find that someone has replaced a seat post with one that seemed "close enough". There are numerous seatpost sizes available and some are very close in size. If you don't have a good caliper, find a bike shop or other mechanic who has one and measure both the tube and the post. Don't try it with a tape measure as you are unlikely to get an accurate reading.

In lieu of a new seat post, you might be able to use a shim to snug things up. A clamp above the seat tube is another option to keep your post from sliding down, but it doesn't solve the problem of a loose post turning too easily which can be a nuisance.
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