Slipping seatpost

[MikeDee]

You can try a different seatpost. There are slight size differences, even though they are supposed to be the same size. Happened to me.

[Samuel D]

This problem has persisted, even after recently changing the seatpost to a Nitto S83.

The seatpost slips down over a period of weeks in tiny increments as I ride over the biggest bumps. This leaves vertical scratch marks at the rear of the post:



A few months ago I disassembled the clamp completely, cleaned it (even using a die to chase the threads), and then lubricated the threads with grease. Not much improvement if any was noticed.

Then I increased the torque on the clamp bolt to 8 Nm and eventually 9 Nm (from the specified 7 Nm). It still slipped in the long run.

The only thing I haven’t tried is installing the seatpost dry, for fear of it seizing.

However, these bikes aren't "known for" slipping posts....with the clamp undone, you shouldn't be able to wobble the post....if you can, it could be undersize, when it will work its way down.....lashings of grease will make it an "airtight" fit even if its a bit slack.

The seatpost does rock slightly when installed dry without the clamp; lashings of grease may have hidden that with the Ritchey.

The seatpost is 27.20 mm according to my Vernier calliper (a Mitutoyo that goes down to 0.05 mm). The internal diameter of the seat tube is harder to measure but seems to be closer to 27.25 mm in most orientations.

Is another clamp likely to exert significantly more force? I don’t want to use two clamps or another ugly method.

(The current Spa-branded clamp operates on this principle to avoid bending the bolt; perhaps the Spa demo bikes that 531colin mentions above use something else.)

[chris_suffolk]

I had a similar issue, which I solved by making a shim (from an old coke can) which sits inside the clamp. No issues since, and my clamp says tighten to 10nm which is pretty tight

[Bmblbzzz]

I've had a problem with a slipping seat post recently. In my case I'm pretty sure the problem is a small gouge on the side of the seat post from an attempted theft, so I've bought another post, which I'm just about to fit. Ironic I should see this thread now, as the new post is a Ritchey Classic!

[pwa]

I had a similar issue, which I solved by making a shim (from an old coke can) which sits inside the clamp. No issues since, and my clamp says tighten to 10nm which is pretty tight

If the post is so slack you can get a coke can shim in, surely the post is just the wrong size in the first place. But if your fix works, it works.

[Samuel D]

My seat tube is about 27.25 mm or at most 27.30 mm in a couple of spots (the tube has some superficial rust and it’s hard to measure accurately with callipers, not least because the slot cut into the back of it may interfere with the diameter). This makes me doubt I could squeeze in a Coke-can shim, at least all the way around the circumference.

Ironic I should see this thread now, as the new post is a Ritchey Classic!

I changed my post for other reasons and don’t think the Ritchey can be blamed. It behaved identically to the Nitto that I’ve measured at 27.2 mm.

[Brucey]

This problem has persisted, even after recently changing the seatpost to a Nitto S83....

IME the thing that causes the slippage is not that the seat pin is able to slide lengthwise too easily within the clamped region, it is that the seat pin is a bad enough fit that it is able to rock slightly within the clamped region, and this amounts to slow slippage in normal use.

Some possible solutions to this

1) ream the frame to match the exact measured sixe of a 27.4mm seat pin
2) ream the frame round (if it hasn't been done then the clamp region is nearly always deformed so that the seat tube won't accept a seat pin of 'exact size') and then machine a 27.4mm seat pin to fit perfectly
3) find some other means of stopping the bottom of the seat pin from being able to move back and forth.

The last of these is easiest to try, for example

a) you can secure a strip of shim stock (or coke can) to the base of the seat pin using a cork or something. The strip is folded back along the length of the seat pin, at the front or the rear. The strip should stay put when the seat pin is fitted, but it may come adrift when it is pulled out. The strip can be thick enough to reduce or prevent the rocking. Even if it just slows the movement it will show that you are on the right track.

b) if you have a seat pin that is over-length you can massage the bottom of it with a hammer to make it slightly oval, and therefore a better fit in the seat tube (front to back). This may be enough to prevent rocking.

I favour option 2 above, in that a seat tube which accepts a full 27.4mm seat pin is very (too?) thin walled.

BTW I think that chris's solution is to put the shim between a removable clamp and the outside of the seat tube. If the clamp is a bad fit on the outside of the seat tube this can be a good solution, as without it the pressure of the clamp is not evenly distributed.

cheers

[Peejay56]

Try this stuff....

https://www.syntace.com/index.cfm?pid=3&pk=1347

Worked a treat for me.

Pete.

[chris_suffolk]

If the post is so slack you can get a coke can shim in, surely the post is just the wrong size in the first place. But if your fix works, it works.

The post is tight in the down tube, it was the clamp wouldn't do up tight enough before the ends of the jaws met, without holding it tight enough. A shim inside the clamp gives just that extra bit of space to get tight before the jaws touch.

[chris_suffolk]

BTW I think that chris's solution is to put the shim between a removable clamp and the outside of the seat tube. If the clamp is a bad fit on the outside of the seat tube this can be a good solution, as without it the pressure of the clamp is not evenly distributed.

Exactly....

[pwa]

If the post is so slack you can get a coke can shim in, surely the post is just the wrong size in the first place. But if your fix works, it works.

The post is tight in the down tube, it was the clamp wouldn't do up tight enough before the ends of the jaws met, without holding it tight enough. A shim inside the clamp gives just that extra bit of space to get tight before the jaws touch.

Ah. I see.

[pwa]

I've been impressed by my Thomson collar. A bit dearer than most, but it secures the post with low torque (using paste) and is really well made.https://www.evanscycles.com/thomson-sea ... gJAnvD_BwE

[De Sisti]

I had a slipping seatpost on my Spa Audax a few years ago (which totally spoilt the Upper Thames Audax).
As I entered Chipping Norton I found a hardware store and purchased a jubilee clip, which sorted out the
problem. It's still on the bike to this day.

[Mick F]

Grease/lubrication is for moving parts. So, by all means, if you want to make your seatpost move within your seat tube ... lubricate it.

Mine have ALWAYS been lubricated.
Not just the bike I have now, but always on all bikes.

It's comparatively recently that I bought some copper grease, but Mercian has general purpose grease on it, and always has done since brand new. Same seatpost too.

[Samuel D]

Thanks for the ideas, all.

I have a few books including Zinn’s that say to use grease.

I’ve just taken the clamp apart again. The more I look at it, the more I think it’s a dud.



Photo 1. Note height of top face of cylinder nut:




Photo 2. Note height of lower face of cylinder nut by comparison. This cylinder nut is symmetrical so should be roughly flush on both sides:




Photo 3. The reason for this disparity is that the hole through the clamp for the bolt was drilled askew:




Photo 4:




Not only is the bolt hole askew, it’s not in a straight line across both sides of the clamp. It’s as if it was drilled with the clamp slightly distorted. As a result, the bolt is constantly trying to push the left half in photo 3 upward while pushing the right half downward. Over time the steel screw has managed to cut a shallow thread into the aluminium clamp at the red arrow in photo 4 (and another in the hole on the other side of the clamp).

This makes the bolt appear to screw into both halves. Only when torque is high enough to bend the clamp in sheer do these partial threads give up their hold of the bolt and allow the clamp to clamp. However, who can tell what happens when the clamp is on the bicycle, with friction against the seat tube resisting sheer between the two sides?

Does the panel think I should try another clamp before taking more extreme measures?

De Sisti: is this the same clamp that came with your Spa Audax?

I've been impressed by my Thomson collar. A bit dearer than most, but it secures the post with low torque (using paste) and is really well made.https://www.evanscycles.com/thomson-sea ... gJAnvD_BwE

Well made, but to a design that bends the bolt!