pakossa

Have a steel MTB frame, aluminum post (26.8). Even when I tighten the thing as hard as possible -- yes, I know you're not supposed to do that! -- it still slips. (I put "frame saver" in it before building it up . . . could that have anything to do with it?) I'm thinking perhaps spraying something in the seat tube, or widening that gap in the seat tube with a file. Would either of those work? Or is there something better to try?

Shimagnolo

I use this on metal posts & frames, even though it is advertised for CF parts.
It has a gritty component that improves the friction between parts so you don't need to overtighten the clamp bolt.
And yes, I would wipe out the framesaver in the area of the clamp.

https://www.tacx.com/en/products/tool...embly-compound

gyozadude

Hopefully you don't have some hidden crack in the seat post binder clamp, but that can sometimes be the problem if you're tightening down really hard, and the gap has disappeared in the back of the seat lug where the binder bolt is.

Hopefully that's not the case.

These are what I'd check for:
1. Is the seat post inserted deeply enough? It should be inserted beyond any "max height" markings
2. Is the seat post diameter accurate? Use some accurate calipers and measure. It might surprise you how many seat posts are actually off. I have two lower-cost Kalloy posts both marked 27.0, and one is 26.89 averaged around where the binder bolt would clamp onto it for my seat height, while the other was 27.06 or something. The fatter one fits more snug and has a few scratches now on it, but the narrower one has a hard time gripping. I use the fatter one.
3. Is the seat post surface treated with something? I made the mistake on a vintage bike once of polishing out the scratches in a seat post and this smoothed out the surface and made the diameter narrower. Suddenly, the seat binder bolt was bottoming out and the seat post wouldn't get a good grip. There was still a gap between the ears on the seat lug, so I found a 2mm aluminum washer and stuck that on the right hand side of the binder bolt and then it gave me just a bit more grip. The steel is holding, and I don't over tighten, and I use a fine sharpie marker on the base of the seat post and make a fine line above the seat lug to make sure the post is still holding (I weigh a lot) and it's holding. And yes, I still grease my seat posts like normal.
4. Perhaps you will need to measure the ID of the seat tube, and find that you need 27.0? In some rare cases, the actual seat post is 26.9 but marked 26.8, and your seat tube was reamed for 27.0.

JohnDThompson 

First, are you certain you have the right size post? If the sides of the slot in the seat tube touch, or even simply aren't parallel, that's a sign that your post is too small.

FBinNY 

+1
The slot is a key indicator of an undersized seatpost. The tube had the right ID when the slot was cut, so when clamped the slot should close only a tiny bit at the top. The next thing to check is the ears the bolt passes through (whether they're part of a clamp, or part of the frame). If they touch in the back, odds are there's a crack at the base of one (or the post is too small).

There are things, like using assembly paste, you can do to reduce slippage, but the fact is you shouldn't have slippage in the first place, so put some time and effort in finding the cause, so you fix the right problem.

dscheidt

Assumes facts not in evidence. It's the way to bet, but it's not a sure bet.

FBinNY 

It's not a sure bet, but it's a bet that anyone who knew how bikes were made would make willingly.

The slot distortion is a good measure of how far undersized the post is. If the slot pinches in by 1mm, then the post diameter is .3mm smaller than the tube, meaning the post is the wrong size.

JohnDThompson 

We can hope that the OP will report back on this.

fietsbob

As original parts & 27.2, post ,bike friday, Thud buster post, stock generic binder
gave way to a Surly/QBP 'constrictor' .. but still slipped.. up near stripping threads tight
so I got one of the Ahrens 'wise crackers' in 27.2, and clamped it directly on the seat post itself.

and the combination worked..

did something Similar on my Brompton, though the band directly around the seatpost,
was a de-lipped QR one of, appropriately, larger diameter,
and to fold the bike , both QR levers get opened.

Shimagnolo

I wasn't sure what you were talking about, so I googled it.
You are talking about the one next to the bottom of this page, correct?

https://www.ahrensbicycles.com/Bottle-Opener.htm

Yo Spiff

I've also got a bike with a slipping post. I know I have the correct sized post, but I think the real problem is that the post may be at the narrower end of the spec while the frame may be a touch wide. I originally tried the paste and it didn't help a bit. Busted two binder bolts due to overtightening, then stole one off a project bike in my garage. This one doesn't break. I roughed up the lower end of the seatpost so it grabs the tube a bit better. Very little slippage now (maybe .5mm over a 30+ mile ride), but I bought another seatpost of a different make on Ebay to see if it fits tighter. Maybe some of my ideas can help in your case as well.

A search on this forum for "slipping seatpost" will yield numerous other threads with some good suggestions in them.

fietsbob

Yes.

Mike had a 1.125" clamp on one, too. another bike, it went on the steerer tube..
a locking Spacer.. in effect..

achoo

OK, hasn't been mentioned yet, so:

If you have one, LOSE THE QUICK-RELEASE SEATPOST CLAMP!

Now that's out of the way, this could also be a solution:



And FWIW, since the OP is talking about taking a file to the slot in the seat tube, I'm betting on an undersized post. Because saying he might take a file to it tells me that slot is distorted an awful lot.

FBinNY 

The first step to solving a problem is to diagnose it properly.

Under normal circumstances, properly sized metal posts in metal frames DO NOT SLIP, and even are greased and still do not slip. So there's something wrong.

The OPs reference to widening the slot with a file implies that the top may be touching such that it cannot be tightened farther. That in turn clearly indicates that the seatpost is of the wrong diameter for that frame, and from the extent of excess narrowing of the slot, I wouldn't be at all surprised if the correct seatpost were as large as 27.2mm.

In any case, if the slot is narrowing more than 1mm or so, the OP should confirm the correct post diameter (trial and error with larger posts) before trying to fix what may not be broken.

pakossa

OK, guys, I checked: With the clamp REALLY tight, there is a 3 - 4 mm gap in the slot, with no "distortion." I then tried a 27.2 post, but that is about the same size as the seat tube of the frame, so it must be 26.8. Guess I'll try the carbon compound stuff first.

wrk101

They also make a 27.0 seat post.

FBinNY 

True, but it's an uncommon size. In any case, a non-tapered slot when clamped is a reliable indicator that the right post is in there.

Something is preventing the tube or clamp from clamping effectively. It could be bad threads, of a QR cam that doesn't wok right. This kind of thing usually takes 20 seconds to diagnose when seeing it in the flesh, but is difficult via the net.

The OPs best bet is to see if there's a local bike co-op, where an experienced person can put his eyes and hands to it.

Raging_Bulls

Same problem here. Brand new high-end scandium MTB frame vs. KCNC Ti-Pro Lite post. The frame is perfect, but the post is actually 27.1mm and ribbed, which decreases the contact patch even more. I have a regular (no particular brand) seat post for it too, but that also requires a lot of force to secure. Turns out it's 27.15mm.
Would one of those dual clamps solve this? I'm thinking a normal amount of force on the lower part of the clamp and as much as necessary on the top part (the post itself). I'll also be putting tape around the entire part that's inside the frame, to minimize sideways play once it's mounted securely.

Funnily enough, I had the same problem with anoher Ti Pro Lite post on another frame. Turns out that one was 31.5 rather than 31.6mm. You'd almost start to wonder if certain brands are doing this on purpose.

FBinNY 

Half a millimeter, or even a full millimeter shouldn't, by itself cause clamping issues. While carbon posts are prone to slippage, partly because their surface is slippery, and partly because they don't tolerate hard clamping well, metal posts shouldn't slip.

You need to have a sense of perspective and history. Posts have been clamped effectively for decades despite sloppier tolerances than today. Also many of these were "slippery" chrome plated steel, and clamped effectively even when greased.

So when I hear of problems holding a metal post in a metal frame, my first reaction is what's wrong? Poor clamp design, poor hardware, or some detail that was omitted.

Case in point, sometime back a friend came to me with a problem clamping his carbon post using the clamp reversed so the clamp slot was 180° opposed to the slot in the tube (common practice with cbn posts).

No matter how hard he tightened the bolt he couldn't hold the post. Turns out that he installed the clamp dry over the seat tube, and so the clamp couldn't slide around the tube and equalize the clamping force. That friction meant that he was essentially trying to squeeze the solid tube in the area of the clamp's slot (which is impossible). We took it apart, greased the clamp, and presto, problem solved.

Shimagnolo

Now that is interesting.
I didn't know there was supposed to be grease under the clamp.
My Calfee's post occasionally slips.
I'm going to take the clamp off and check this.

FBinNY 

I don't know about "supposed to" but id the clamp and seat tube slots aren't aligned, the clamp can't work unless it slips over the seat tube. That should become obvious as soon as you note that the tube closes by pinching the slot, which is bridged by a solid section of clamp. If you want slippage, grease can go a long way toward allowing it to happen. If the frame maker says don't use grease, you might try a silicone based grease.

Slippage between clamp and frame is less important when the slots align, but can't hurt.

Meanwhile, clamp design may also be a factor. On a well designed clamp, the ears will uniformly pull the clamp ring into tension, but many clamps I've looked at the ear to ring transition area is so weak, that the pinch bolt does a better job bending the ears than tensioning the ring.

Seat post slippage is one of those "Damascus Steel" issues I see in the bike world, where things that worked for a century no longer do, and no one seems to be willing to research where along the way they made a wrong turn.

Raging_Bulls

My main concern is the rotation of the post really. It doesn't drop anymore once I apply twice the recommended force (or more), but keeps rotating for quite a while longer.

I have he tube's slot at the front and the clamp's open end at the back indeed. However I just applied grease on the clamp after reading FBinNY's post and turned the clamp around as well. Even after using all my force to tighten the bolt, I don't notice any improvement. Only after I put extra leverage on the Allen key, I can tighten it enough to stop the seat post from moving about.

Perhaps I could sand away the seatpost's ano right where the clamp is, but I doubt it'll make much difference. Because of the finely ribbed surface of the post, there's barely any contact surface between it and the tube anyway. Perhaps that's the cause of the entire problem?
Or maybe I could roughen up the inside of the seat tube a bit as well, for that extra bit of resistance.

Oh, and I did check for dirt, oil or grease, but didn't find anything.

FBinNY 

It could be the design of the clamp itself. Take a look at the clamps in the top row here. The design of the Salsa and Thomson clamps ensure that screw force on the ears pulls the band well, but the Sunlite's design doesn't. As the bolt is tensioned, the ears bend, mis-aligning the threads and binding the screw instead of evenly tensioning the band.

If your clamp looks like the Sunlite, you might try a different clamp.

Raging_Bulls

You might be on to something there. Thanks for pointing that out, never would have thought of that myself.



Unfortunately I don't have any other seat clamps in that size here, so I'll need to order one. Might as well go for a dual one then, just to be sure it's solved. Otherwise I can keep ordering clamps.

FBinNY 

Yes, your clamp is most definitely one of poor design. When the bolt it tightened the ears bend and most of the force goes to pushing the corners in rather than tensioning the band and pulling the seat tube tighter. Clamps of this type are probably the single biggest cause of carbon seatpost failures.