Toadmeister

I'm a Clydesdale about 270#. 27.2mm stock Carbon seatpost, Janis Renegade, kept sliding down on me after 3500+ miles. Post is visibly gouged around the clamp bolt area from sliding. No cracks I could see. After 3500+ miles I'm not surprised and I've put on weight this year unfortunately.

Tried carbon paste. Tried an aluminum shim from a beer can. Kept tighten more and more until I sheared the bolt! Never cracked the carbon post however.



Onto a new Carbon post, Whiskey #7. A little sliding down at first, don't want to over torque again. Applied carbon paste. Riding indoors on a trainer now winter is here.



So dang. Other than loosing more weight and not leaning back that puts my full body weight on the rear post, what to try?

chaadster

That’s a tricky one, and I don’t have a lot of experience with slipping posts, but it has happened to me in years past. It was so long ago, I don’t recall the specifics or the fix, but it was also pre-CF, so probably not relevant anyway.

First I’d suggest cleaning out the seattube thoroughly and reinstalling with fresh CF grip compound. That’s probably free to do, and it’s quick and easy, so try that if you have not.

Because it was all good for 3.5k miles, it kind of sounds like the post clamp has failed you, perhaps by deforming and stretching. I’m not aware of a common fix, but you could ring a framebuilder and see if it’s possible to cut off the integrated clamp and have an extension welded on which uses a collar. Between the work and the paint situation, though, that could well be impractical in terms of cost.

Maybe you could switch post material, and try aluminum or titanium in hopes a little more friction helps, but if you have enough gap to get a can in there, that’s probably a real problem and the harbinger of a new frame purchase (or new bike).

Sorry Idon’t have any real solid suggestion, and best luck to you.

jaxgtr

yep, I feel you brother....I had the same issue on a steel frame I owned once. those little bolts can be over torqued and snapped. Like Chaadster said, I used CF paste to get some additional grip. I was also lucky in that I had a little bit of room at the top above the bolt that allowed me to put a frame collar on it, but the cf paste really did a nice job.

trailangel

You could put another clamp on the seatpost right above the frame clamp to keep it from sliding down.
I personally would switch to alloy seatpost with 25mm setback, use fricton paste and also knurl the seatpost (with a punch)where the clamp works on it.
You need that setback with that saddle.

cyclist2000

I put a rear light and clamp on the seat post, right above the seatpost color so it won't slip down anymore.

banana jam 

Toad, you're too big for carbon posts. Measure the id diameter of your seat tube, and find the right size sp to fit it. Something is amiss. Use a good set of calipers and pedal your can off literally!

Toadmeister

I actually did use a simple hose clamp for a while this summer. Helped for a while to get me through the summer but things eventually got worse again and when the bolt snapped I abandoned it and got the new post with the new seat bolt.

I too feel I need the setback, the Whiskey one is 18mm but I’d like more.

Toadmeister

This might be reality...

I did caliper it and I got the right sizes. It is a tight fit.

Took the post off tonight and cleaned everything off with rubbing alcohol, including the inside of the seat tube about 2” deep that I could reach, which was a little greasy from frame saver. Going to let it dry overnight and fit up with fresh carbon paste tommarow.

I also fingered the inside of the seat tube hole (don’t worry, I’m experienced) and it could use some filling down in a spot, there seems to be a bit of a ridge by the seat clamp corresponding with scratching/scraping on the seat post.

Toadmeister

Steel frame, but I noticed the inside of that seat tube and clamp area to have a bit of a ridge (I fingered it!), maybe that’s deformation. That little bit of ridge inside is scratching/scraping the seat tube and contributing to the problem I think. I’m going to file it down a bit and smooth it out.

Yes, It would be possible to trim off at the seat clamp, heck I could grind it myself and add a separate aftermarket clamp as a last resort. Here’s a better pic of that area.







fingering the hole

chaadster

That first closeup pic of the seat clamp seems to show that ridge,which also looks like a stepped diameter...almost like the collar was brazed on too high up the tube, which is very strange. If the step is an illusion and it is just a ridge, that too would be enough to compromise purchase on the post and create an undesirable stress riser on the seatpost, which seems particlarly risky using a CF post.

If a ridge and you can sand, I suppose it’s fairly low risk of damage (to the frame) if your careful, ut it may be worth having an experienced tech take a look at it and make an assessment. If there is any chance of getting the producer, Jamis, to cover warranty repair or replacement, you’ll be flushing thadown the toilet taking a file to it. I saw that 853 sticker; not an inexpensive frame.

banana jam 

A shop has a proper seat tube reamer or use a honing device from the auto parts store
https://www.northerntool.com/shop/to...B&gclsrc=aw.ds

get that seat tube cleaned up. Prior to any seatpost installation. And alloy is your friend not carbon in this instance. Ritchey Comp comes to mind.

a_d_a_m

I wonder, if'n a fella went with a regular metal seatpost, if a flaring tool might be able to be used to flare the unclamped portion of the tube to prevent it from sliding further down? Just enough to make the OD of the pipe prevent it from going below the clamp?

Alternately (and this might compromise the strength of the seatpost), drllling a small hole through the seatpost and just sliding a grade-8 bolt through the hole...?

(I acknowledge that both of these are sorta redneck engineering solutions to a problem)

Toadmeister

Great discussion here and I appreciate everyone's input.

After yesterday's cleaning, I went in and filed down some burs/notches in the slot pictured below. May not be apparent in the pics, but that's what my finder can feel. Not quite sharp enough to cut the skin but it's there. I made it smooth. The rest of the Seat tube ID is pretty smooth despite the braising of the collar there.

Toadmeister

I also confirmed the OD of the seat post and ID of the seat tube again. Nominally they match (yes I can vary a little bit but total not more than +/- 0.1 depending on how I measure).

I added the carbon paste and inserted it again, a snug fit for sure but not overtight or loose. I tightened her up again and will ride a little tonight to see if it helped.

chaadster

Nice work, and hopefully it works. Keep us posted.

Toadmeister

Had nice winter weather weather in Iowa this weekend so got outside in the Fat Bike!

finally tried out the cleaned up seat post interface tonight, it was better but still slid down a little over 20 min. Bummer, I don’t think it’s gonna get any better short of me loosing 20lbs. Entirely possible over the winter when I tend to focus on weight loss.

the only time I noticed the post sliding down was when I leaned back and put my full weight on the seat, this really doesn’t happen when I’m riding on the road or trail.

chaadster

Man, that’s a dog. I really think that clamp design is weird, with the pinch bolt sitting above the clamp zone.

Rather than rationalize living with it— it’s definitely going to slip riding outside— I’d recommend contacting Jamis, or directly through your LBS if possible, and see if they can offer any solution. It may even be a known issue they’re rectifying...who knows.

Toadmeister

I think I will contact Jamis and pick their brain some.

I rode again the other night and no sliding, good. Might be over-torquing the retention hardware tho.

gutter007

Have you tried filing/sanding/scoring the post up a bit?

It may be worth trying with you steel/aluminum posts. I now that scoring those a bit will cause zero net damage.

My guess is that there is not enough contact with your post, and the rest of the tube. The more friction the better. Scoring/sanding the area near the clamp will increase the friction there and help the grip compound on the rest of the tube.

Just a couple of different ideas to throw on the pile.

masi61

The replacement "CR-MO" seatpost bolt looks good. Just curious if the slot pinches in at all when you tighten the bolt to snug the Whisky post in place. Also, one other question pertaining to seatpost height. It appears you are quite tall and you have the post up near the limit line. Is this true? How much material is in the frame and is there enough post that it extends fully below the intersection of the seat tube and the top tube?

Toadmeister

I do a very light scuff on the seat post, I don't want to remove too much of the resin surface. The carbon paste sort of has a similar purpose.

Toadmeister

Yes, I'm 6'4" tall. There is still about 8" in the fame I believe but didn't measure. It is a tall post.

The slot does pinch in some but not as much as I'd like, which is part of the problem. Something I want to give Jamis feedback on.

masi61

a

Did you grease the threads on the CR-MO seatpost bolt and are there still threads visible when it is tight to your satisfaction? Some post bolts "bottom out" and stop snugging up the slot because of this. If this is the case, you could get the next size smaller of bolt since you are attempting more tightness. If the slot is pinched in at the top creating more of an inverted "V" shape to the slot you might need a very small shim to take up any undersize slop. Or you could use one of those adjustable reamers and actually enlarge the bore all the way up to a 27.4 and put a proper Thomson post in there.

tallbikeman

Weird problem. Hope you find an answer and tell us.

tallbikeman

A couple more thoughts to add to the pile. Check for microscopic cracks in the seatpost tube and the carbon seatpost. Carbon is a matrix of resin and carbon fiber. The resin is either epoxy resin or something similar. It has very low strength. It is easy to damage carbon parts with tightening practices for steel or aluminum parts. I've read that carbon parts have much different (usually lower) torque specifications than steel or aluminum. If the carbon seatpost has delaminated inside due to high compression from the seat post clamp it might just be bending inward and thus slipping. Interested to hear of a fix for this problem. If you have this same problem with an aluminum post then I would check that frame for cracks. I ride two steel frames with almost identical seatpost clamps and aluminum posts. They are solid. I weigh 250lbs and sit fairly upright.