onespeedbiker

Well I'm doing something I always joke about others doing; asking for information after I've already bought the item. I have a bike, 853 steel with a Nitto polished alloy seatpost that I just can't get to stay put. The size is spot on, even went to the trouble of increasing the size of the seat tube a 1/10mm to get the next size up to fit. I have only used one clamp which is just a straight forward clamp. If I tighten the clamp as tight as I dare with a 5mm wrench, it will hold for a couple of rides, but it will then slip again. I've read great reviews about the Thomson, but never really asked a forum about it. Well I bought one and I'm off on a ride to test it out. It directs one to use 25 inch pounds! Half what I have been using with the other seat clamp! But Thomson says their patent design needs nothing more. Anyway wish me luck and I would appreciate in real world experiences with the Thomson clamp and I'll followup with a post afterwards.

FBinNY 

Any clamp will hold if ll is right. Before anything else, check that both the clamp ears and the top sides of the slot don't touch each other when tight. Once there's contct across the gap, the clamp and/or the seat tube cannot close farther and hold.

I hope you didn't complicate things by enlarging the seat tube ID since there's not that much material in an 853 tube, and if you've reamed it oversize, you're stuck between a rock and a hard place. You can't go down, and going larger to the next stock seatpost size may not be an option.

If there's nothing keeping the tube from pinching (ears or slot don't touch) and you only need a bit more hold, you can get it by using coarse lapping compound on the post in the clamping zone (top 2" of set tube). The grit in the LP will bite into both the post and seat tube, locking them together at very low clamping forces.

well biked

Thomson

FBinNY 

The best clamp in the world won't help if there's an underlying problem that isn't solved first.

fietsbob

went from Generic one on my Bike Friday to a Surly Constrictor,
https://surlybikes.com/parts/constrictor
but when it still slipped .
rather than Strip out the threads cut in the Aluminum, over torquing it,
(I did crank down on the bolt)
[no max torque data in packaging]

I clamped a 27.2mm Ahrens Wise Cracker around the(thudbuster) seat post itself ,
https://www.ahrensbicycles.com/Bottle-Opener.htm
it's sitting on top of the Surly Constrictor, now... No more slipping..

This, will do the same, minus the beverage Opener https://salsacycles.com/components/post-lock


I did something Similar on my Brompton. took the lip off a QR seatpost clamp,
and so it will open easily to fold with both QR levers..

well biked

My post was just a friendly spell check for onespeedbiker.

onespeedbiker

Point taken !

onespeedbiker

Okay, this has a long back story; first I know how a seat clamp works and that the top sides of the slot require a gap to tighten. The bicycle is a 1999 Schwinn Peleton. The OEM seatpost was a 27.0 Titec that has a bead blast texture and seemed to stay put in the seat tube. My problems started when I upgraded to a Nitto 27.0. While the two seatposts measured the same, the Nitto would not stay put; I additionally noticed that both seatposts tended to rattle around in the seat tube. I don't know if this was a design flaw or not, but considering the amount of post the Peleton/Circut seatpost problems have generated, at the very least there is a tolerance issue. The seat tube narrows from 31.8 at the bottom to 30mm at the top, the ID of the seat tube would seem to allow 27.2mm seatpost, but there is a bottle neck where the top tube and stays are welded which I assume is the reason the frame was spec'd 27.0mm seatpost. Anyway, numerous posters have claimed that a 27.2 will fit. I have an old Campy NR 27.2 that needed some re-surfacing; I sanded and polished the post leaving it just under 27.2 and it fit that Schwinn seat tube like a glove (no rattling). I then bought a 27.2 Nitto which couldn't get by the bottle neck at the welds. I did some light filing and then used a brake cylinder hone to clean up the bottle neck area, but it was not enough for the seatpost to fit. I then sanded down the lower portion of the seatpost until it fit without binding. The seatpost now fits snugly, but still slips occasionally, which is why I suspect the clamp may have been part of the problem.

Anyway, I have returned from my ride and the seatpost stayed solid with the Thomson clamp. With the old clamp, even though it was tightened with twice the torque of the Thompson, I could still make the seatpost turn slightly if I applied some twisting force to the saddle; this doesn't happen with the Thomson, so I am now convinced that all seatpost clamps are not created equal and hopefully the seatpost issues are done with this bike.

FBinNY 

Since you've solved your problem, further discussion is moot. If you have more slippage in the future, we can revisit it and try to resolve the issue. Otherwise enjoy he bike.

fietsbob

Re Thomson, I only saw pictures from one side, the bolt head.. are those also threads cut directly into the aluminum?


those external dimensions?, is the inside Bore 27.2? and how far down?

onespeedbiker

No, the bolt threads into a barrel nut that is inserted through the bottom.

Yes those are external dimensions and yes the internal bore is 27.2 except in the area of the top tube weld (about 29mm down), where it narrows to 27.1mm. There the seat tube extends 22mm beyond the top tube with a rear compression slot; the bore is slightly flared to an ID of 27.3mm however the seatpost clamp is 29.8 and it compresses bore to 27.2. The seat tube narrows to 30mm about 60mm below the top tube is 29mm, which again is the approx with of the bottleneck. When inserting a slightly undersize 27.2 seatpost, the post will slide in the top, but bind a little at the bottleneck for about 29mm; once past the bottleneck however, the post will continue down the bore without any additional binding beyond the initial resistance from the bottleneck.

onespeedbiker

And if the glove don't fit you must acquit. Wouldn't it be nice if life was that simple.