seatpost slipping: suggestions?

[azrider]

Originally Posted by David Kirk

very good. Now we are getting somewhere.

A post labeled "27.2" is meant to fit into a seat tube that is 27.2 I.D. and it is not supposed to actually measure 27.2.....it is a nominal measurement. A 27.2 post will not fit into a 27.2 hole and it needs to be just a bit smaller.

The actual diameter of a 27.2 post should be 27.15 so it looks like your post is right on.

This is of course good to know but it doesn't answer the problem of your properly sized post moving in your frame and this leads me to my follow up question - when you slip the post in does it take some work to get it in? Do you need to push and twist a bit to get it in or does it just slip or fall in?

dave

Oh I definitely have to work it side to side to get it to fit.

[Black Dog]

Originally Posted by azrider

Oh I definitely have to work it side to side to get it to fit.

How far in does the post slip in before it meets some resistance? You need to get your calipers on the ID of the seat tube.

[David Kirk]

Originally Posted by azrider

Oh I definitely have to work it side to side to get it to fit.

Cool - so it sounds like the seat post is the right size and that the fit to the seat tube is certainly in the ballpark. I don't suspect the length of the slot being the issue as Serotta built many thousands of those bikes and slipping posts are not in any way common.

So in my mind it boils down to two possibilities -

- I would check that the M6 bolt has clean threads and that it's straight. Remove the bolt and wipe it clean to be sure that the threads look good and the same from right under the head to the end. If it is any way suspect put a new one in. Also give it a spin to be sure that it's straight. Many of the original quality bolts were lost and replaced with bolts that have the hardness of butter and these bolts will bend when fully tightened. If it's bent it will not clamp as it should. After typing all of this I think the best plan would be to go to your local Ace Hardware and spend 75 cents on a new M6 stainless bolt. It can't hurt. Grease the threads and the underside of the head well and try it.

- the other possibility is is that you are simply not tightening it enough. We never spec'd a torque value back in the day and I have no idea just how to tell you just how tight it should be. That said i would try it a bit tighter and see what you get.


dave

[Fatty]

If the post has grease at this point pull it out, throw some sand on the greased post and reinsert. Bet a dollar to a donut it won't slip.

[cmbicycles]

If you pull the bolt all the way out, do the bolt's threads run the full length of the bolt or do they stop short of the bolt head? Is the bolt itself, and under the head greased?

[Doug Fattic]

Originally Posted by azrider

Hey DFattic. Sorry for not responding earlier, and I appreciate what you had recommended, but once I read the part about taking a file to the frame I got a little too nervous. I have since measured the slot and it's rougly 1.10"

soooo......................

Well you are right on the edge of having the slot too short. In my 40+ years of building and painting frames, I've repaired a number of classic European frames made poorly but loved by their owners. Some of them had sloopily done seat lug slots that often resulted in damage to the seat lug ears (trying to tighten the lug enough to hold a slipping post) and perhaps the seat tube cracking below the slot. Sometimes this problem was related to the use of cheap stamped lugs that did not have enough material for a long enough slot. Just last week I brazed a short section of tube below the slot on a quickly made (but sentimental to the owner) bike boom era frame. By extending the slot and reaming and honing to the right size, a seatpost will now hold in the frame with less bolt tightening force. These experiences are what has led me to check slot length if there is some slipping problem. On my personal frame I only extended the length of my slot 1/8 of an inch and it made all the difference in the world. No matter how hard I tightened my seat lug when it was an inch long, it would still slip. After the surgery it held solid with much less force. This knowledge is why I kept insisting on checking your slot length. Of course there are all sorts of variables that contribute to what is the necessary slot length.

The file you would want to use is an American Pattern 4" round file with either a bastard or 2nd cut. They probably don't have it at your local hardware store. A
6" round file is pretty commonly available and you can use just the tip of it to extend the slot. Of course you can take it to your local frame builder too if this kind of work makes you nervous.

[T-Crush]

My CSi had the same issue. Gradual slip over a 100 mile ride. I tried three different posts, all measured between 27.14 and 27.16 at the relevant points (using Helios dial calipers - I put myself through college and grad school as a machinist). I tried paste, grit and even rock climber's chalk. Some helped, but none solved the issue. This solved the problem once and for all.

http://landlordscycling.com/2012/01/...uill-seatpost/

Between the clamp and the quill, both tightened to spec, it's been rock solid for years.

[Peter P.]

Interesting thing I found while on the forum, reading another thread.

Doug Fattic and Dave Kirk have already suggested the length of the seat tube slot can be too short, inhibiting clamping force.

Well, Tom Ritchey mentions in a display of the Road Logic frame that the seat tube slot is longer, to deal with carbon seatposts.

Can anyone measure the slot on their recent vintage Road Logic and tell us what the length is?

[azrider]

Originally Posted by T-Crush

My CSi had the same issue. Gradual slip over a 100 mile ride. I tried three different posts, all measured between 27.14 and 27.16 at the relevant points (using Helios dial calipers - I put myself through college and grad school as a machinist). I tried paste, grit and even rock climber's chalk. Some helped, but none solved the issue. This solved the problem once and for all.

http://landlordscycling.com/2012/01/...uill-seatpost/

Between the clamp and the quill, both tightened to spec, it's been rock solid for years.

THat thing is genius.....how the heck do you order one?

[Kontact]

Originally Posted by azrider

THat thing is genius.....how the heck do you order one?

A time machine. You'll have to find a used one.

[Kontact]

A 1 1/8" slot minimum is an observation from another frame. But the OP bike has a fastback seat cluster, which locates the bolt lower from the top than a more classic cluster:



So 1 1/8" might not be sufficient given that the bolt is 1/4" lower on the OP bike.

I'd lower the slot.

[bikingshearer]

Originally Posted by Ken C

I had the same problem with a cheap steel commuter frame a Pake C'Mute.

I use carbon paste on an aluminum post and now it does not slip without having to result in excessive torque on the binder bolt.

I had the same problem with my old (1967) Schwinn Paramount frame (now the baddest ass commuter ever). Carbon paste plus a very small soda can shim did the trick for me. I tried the shim first, and added the paste when the shim alone didn't quiiiiiite get it done.

With the benefit of hindsight, I'd say try the paste first. If that doesn't work, add the shim (and you'll be surprised by how narrow the shim will be, but using snips on a soda can is easy-peasy-lemon-squeasy). If that doesn't work, do Old Spud's knurling thing. If that doesn't work, get a different bike.

[Kontact]

Originally Posted by bikingshearer

I had the same problem with my old (1967) Schwinn Paramount frame (now the baddest ass commuter ever). Carbon paste plus a very small soda can shim did the trick for me. I tried the shim first, and added the paste when the shim alone didn't quiiiiiite get it done.

With the benefit of hindsight, I'd say try the paste first. If that doesn't work, add the shim (and you'll be surprised by how narrow the shim will be, but using snips on a soda can is easy-peasy-lemon-squeasy). If that doesn't work, do Old Spud's knurling thing. If that doesn't work, get a different bike.

I can't speak for 1967, but many later Paramounts used 27.4 seat posts.

[bikingshearer]

Originally Posted by Kontact

I can't speak for 1967, but many later Paramounts used 27.4 seat posts.

Good thought, and I tried a 27.4 - no chance. I even found and tried a 27.3 - again, no chance, at least not without an unacceptable amount of grinding. So I was left with paste-plus-shim. Not exactly the most elegant solution, but it's working.

I think 27.4's didn't come in for at least another 10 years, maybe more, but I'm not certain. I do know that any Paramount I have seen and could check from the late 60s to mid 70s had a 27.2 post.