cut extra 'slot' in frame's seat tube so seat binder will pinch more?
 

i've a Cooker Max Fatbike (made by Charge, I believe UK), aluminum frame. seatpost is 27.2, the clamp is 31.8. I've measure both, accurate.

I put on a green alloy seatpost clamp as it looked better (i now... i know..) 31.8 size (measured). Torque wrenched it to 7 nm, seat slipped. torqued it to 8 nm, post slipped. I lined inside of the clamp with thin strip of aluminum cut from beer can. torqued again to 7 nm, slipped, and then when i torqued to 8nm the clamp snapped.

so i bought another, different brand, thicker looking green alloy clamp and the exact same think happened.

Now i'm wondering, the seat tube of the frame has a single 'slot' cut in the front just above the down tube i imagine to help the 'squeeze' occur. But since aluminum is so stiff, i'm wondering if i could get some more 'squeeze flex' of the seat tube if i cut an additional slot, in the back.

thoughts?

if all of the parts are sized correctly, the aluminum should barely need to deflect to clamp the post. Cutting the slot further down likely won't hurt anything but I can't see it helping. What likely will help is some grip paste typically meant for clamping carbon fiber parts. It will offer the same benefits for aluminum, though. You could also consider adding a second clamp on the seat post itself to prevent slippage. I have seen one piece clamps that do that, too. You aren't alone with this issue.

That said, you may want to learn to unweight the bike when you hit bumps in the road. Not only will it help with seat post slippage, your wheels and butt will thank you as well.

How long is the existing slot? And as jj951 said, if all were well chosen dimensionally how were you able to insert a shim without spreading the slot? Agree too with grip paste being the next try. Andy.

Make sure that no part of the clamp or slot is closed to where both sides touch when tight. If so, it means your post is undersized.

Then make sure that the slot in the tube and clamp line up because this produces the best clamping action. Or if the clamps don't line up, make sure to use grease between the clamp band and seat tube.

Then make sure the pinch bolt threads are greased.

If all is right, then tighten the bolt until you feel the rate that the torque ramps up increasing suddenly, plus about 1/8 turn. Follow by a test ride.

If all else fails, use a traction agent in lieu of the grease on the post.

Is the only changed part the clamp? Does it work with the original clamp without being overtorqued?

No caliper needed, the frame is the gage that counts. Just as there's variation between posts (stamped with the same size) there are differences between frame seat tube IDs that are claimed to otherwise be xx.x. Since it's the frame that "can't" be changed use it as the final determinate. Andy.

i believe all parts are sized correctly, have measure with a quality caliper and tried several 27.2 posts with the same result. i've heard of this grip paste before, could you respond with a URL link so i could see a brand or manufacturer's product?

everything seems to measure fine. the shim i made was about 1/4 inch wide and 2 inches long, i placed it between the seat collar and the frame. it definitely made the collar 'grab' the frame better, as the clamp bolt became tight with fewer turns, but it seems like the clamp bolt can't squeeze the frame much.

unweighting the bike with bumps etc might work but it's not really the solution i'm looking for. the posts slips even when i do an easy road ride for maybe 10 miles. i weigh 180.

it does work with he original clamp, a big burly steel one with, i think an m8 bolt. i've put that one back on for the time being, but hoping to change it. i know, i know, i'm a 'pretty bike *****' i like toying with the rigs with colored aluminum parts to bling it up. i know it's stupid, but so am i so it's perfect match.

Finish Line: http://www.amazon.com/Finish-Line-Ca.../dp/B018FT9HOW

Park Tool: http://www.amazon.com/Park-Tool-Supe.../dp/B00AZ2NY2Y

If the post is slipping on an easy 10 mile ride at your weight, I am inclined to think that something is 't quite right. Without inspecting the parts myself it is hard to say. Hopefully one of the grip paste options above does the trick.

Since it seems the post held with the original clamp, we can rule out size/tolerance issues, and - probably - the need for grip paste.

I suspect that the real issue isn't the hardware, but the OP himself and his reliance on a torque wrench. He's letting reliance on an assumption about a torque spec keep him from tightening the clamp tight enough to do it's job.

The relationship between torque spec and the bolt tension which produces clamping power is based on the pitch and diameter of the bolt, or the ramp angle. Finer threads on larger bolts have shallower ramp angles and will clamp tighter at the same torque, so the tight torque for one clamp may well be inadequate with another.

Other factors include whether the clamp was oriented with the slot aligned as I suggested earlier or not, and whether the OP has a carbon fiber post.

If you can get a shim(Coors?) between the clamp and the frame, the clamp is oversize. Go to the next smaller clamp.

I might have said the same thing but he claims he snapped at clamp at 8 Nm. Perhaps he had a bum clamp or a cheap torque wrench that didn't click over at right torque. More torque isn't always the answer, though.

If he jammed the shim in there, the clamp most likely spread a bit to accommodate the extra thickness. But, if the shim fit easily, then I would agree that he has the wrong size clamp. And the wrong size clamp would explain the slippage, too.

Some clamps are if a design with a blind hole in the nut. I have seen instances where the bolt will bottom out inside and adding more tightening torque only binds the bolt and does not clamp any further. Just one thing to look for.
Good luck.

I Resolved a slowly slipping seat post by adding a second clamp band wrapping directly around the seat post itself.

for my Brompton it is a second QR band , that I removed the lip edge, so it would slip upside down over the seatpost.

not concours d elegance look but a Jubilee/radiator hose clamp will do .. used one to make an old Office chair work again .


there are seatpost sizing shims , machined in various OD with the ID in 27.2, and 25.4 ..

USE UK made some with a 25.0 ID with Various OD , then another series with a 27.2 ID for frames with bigger..

they made suspension seat posts in 25 & 27.2 .. I used the 25 to use the seatposts from my AlAn again with steel frames, later.

Whenever I read a post like the OP's about a slipping post or other odd problem, I think that the person has to ask himself the first of the 4 questions (mechanic's version).

"How is my bike different from all the other bikes?". Tens of thousands of similar bikes don't have this problem, yet this one does. There has to be something different, and looking for that difference is the first step in solving the problem.

First, do no harm. Try a surly clamp, beefy and strong, not sure about colors.

Surly new stainless band clamp includes the barrel nut of the aluminum ones

Some of you might not like this, but when I have had this problem, everything the right size, and the seatpost still slowly creeps down, I knurled the seatpost. Take slightly pointed punch and made 6-8 light hits on the seatpost around the diameter where the band or pinchbolt tightens.

That's the elegant answer...and good imo. Mine, when I was doing some trial fitting of a different saddle, didn't want to disturb the current setup, and only had a slightly undersized spare seatpost on hand was to wrap the post with a layer of duct tape. :)

No, I didn't leave it like that.

A few questions:

How tight is the seat post when you slide it in? Is it loose? On my bike, I have to push or pull fairly hard, and wiggle it a little, since the fit is quite tight.

How is the fit of the collar clamp on the frame with the bolt untightened? Did you fit the beer can shim between the clamp and the frame? That seems very odd. Like trailangel said, that seems oversized. On my bike, I can turn the collar quite easily with the bolt loose, but once the bolt is even slightly tightened, the collar won't turn at all.

Did you buy the bike locally? Do they have any more to compare? (Or was it working correctly with the original clamp?)


Carbon assembly paste: I have a tube of Tacx Carbon Assembly Compound. It's a pink gel with plastic(?) grit in it. Spread some on the seat post, then slide it in. I even use it on aluminum stems with aluminum bars, and on saddle rails. I don't need to clamp nearly as tight. Good stuff.

Most bike stores have small packets now, for handing out when they sell an all-carbon bike. But I've used my tube fairly often over the 5 years (!) that I've had it. There's other brands than Tacx, too.

Slot cutting:
Don't cut it. That could eventually start a crack propagating and wreck the frame. The factory slots have a smooth, rounded bottom instead of the sharp saw cut. It shouldn't be needed.

We had this problem on my wifes factory new Raleigh. It turned out the slot was very narrow and the sides of the slot touched when tightening and before the seat was tight. We fixed it by grinding the slot a bit widr so the clamp could close to a slightly smaller diameter, not with another slot. I vite this is a good candidate for the source..