Seatpost sinking during cycle

[Albagord]

Looking for advice: Fitted a SR SUNTOUR New SP12-NCX Suspension SeatPost 30.9 X 350MM to my Cube Pro-reaction 500 two years ago. It was a straight swop out for the Cube seat post measuring 30-9mm. I noticed that at the end of the ride that the Suntour seat post had sunk by over an inch passed the mark that I had put on the post as the ideal adjustment. Have checked the outer diameter of the Suntour post with digital calipers and it measures a 30mm. I would need to get a 0.9mm shim made or buy another suspension seat post. Looked online and suggestions for fix vary from spraying it with womens' hair spray, fibre grease, woughing seat post with file and cut a cola can as a shim. QR clamp would not fully close so replaced with seat post appropriate allan screw headed bolt and nut but still not cured it.

 

[steve_sordy]

From the sound of it, the seat tube is designed for 30.9 and your seat post was supposed to be 30.9 also, but is actually 30.0. If you had noticed two years ago when you fitted the seat post, you could have sent it back on the basis that they sold you the wrong size. But I doubt that you'd get away with that now. Might be worth a try though.

If that fails, or you no longer have the proof of purchase etc then shims are the solution, there is no need to mess about with files or hair spray.

Google "seat post shim 30.9 to 30.0" and have a look around. You will need to have a really good look because I just had a quick look and couldn't find anything quickly. I suspect that the problem is that the seat post is 30.0mm. I had one of those and it was on an American bike from 2010, so maybe it's not a common size anymore.

If you can't find a manufactured seat post shim of the correct dimensions then you need to get creative.

The difference in diameter is 0.9mm so the gap and hence the shim will be 0.45mm thick. That is only 0.0177" thick and that is not a lot at all. Try measuring the thickness of cans to see if you can make up a combination that is thick enough. Go for aluminium or plastic not steel as it will rust.

Here's another idea. The thickness of sheet metal is specified by gauge. As it happens 26 gauge is 0.018", which is only three 10,000's of an inch thicker (ie nothing to worry about). So go buy some 26-gauge aluminium sheet and make your own shim. Make it 100mm long and X wide. For X see below

The external circumference of the 30mm seat post is Pi x 30mm = 3.1417 x 30 = 94.25mm.

You need a gap, so make it 90mm (=X). ie Cut a piece 90mm x 100mm from your 26-gauge sheet.

Then bend it around your 30mm seat post. That will be easier said than done, so unless you have some craft skills and the tools to go with it, I'd get someone else who has to do it for you. You would also be better off using some 30mm steel bar instead of your seat post to bend it around.

Another idea if you are desperate.

Find a shim that is of the correct thickness (or maybe slightly smaller) and as close to the diameter size you need as you can find. Then cut off the flange at the top. You will then be able to use this as it is the flange that stops the shim from adopting a different diameter. In use, you risk losing the shim down the seat tube (that was what the flange is for), but you will be able to fish it out with a coat hanger bent at the end.

 

[peteandsoo]

cut up a coke can, should do the trick

 

[Albagord]

From the sound of it, the seat tube is designed for 30.9 and your seat post was supposed to be 30.9 also, but is actually 30.0. If you had noticed two years ago when you fitted the seat post, you could have sent it back on the basis that they sold you the wrong size. But I doubt that you'd get away with that now. Might be worth a try though.

If that fails, or you no longer have the proof of purchase etc then shims are the solution, there is no need to mess about with files or hair spray.

Google "seat post shim 30.9 to 30.0" and have a look around. You will need to have a really good look because I just had a quick look and couldn't find anything quickly. I suspect that the problem is that the seat post is 30.0mm. I had one of those and it was on an American bike from 2010, so maybe it's not a common size anymore.

If you can't find a manufactured seat post shim of the correct dimensions then you need to get creative.

The difference in diameter is 0.9mm so the gap and hence the shim will be 0.45mm thick. That is only 0.0177" thick and that is not a lot at all. Try measuring the thickness of cans to see if you can make up a combination that is thick enough. Go for aluminium or plastic not steel as it will rust.

Here's another idea. The thickness of sheet metal is specified by gauge. As it happens 26 gauge is 0.018", which is only three 10,000's of an inch thicker (ie nothing to worry about). So go buy some 26-gauge aluminium sheet and make your own shim. Make it 100mm long and X wide. For X see below

The external circumference of the 30mm seat post is Pi x 30mm = 3.1417 x 30 = 94.25mm.

You need a gap, so make it 90mm (=X). ie Cut a piece 90mm x 100mm from your 26-gauge sheet.

Then bend it around your 30mm seat post. That will be easier said than done, so unless you have some craft skills and the tools to go with it, I'd get someone else who has to do it for you. You would also be better off using some 30mm steel bar instead of your seat post to bend it around.

Another idea if you are desperate.

Find a shim that is of the correct thickness (or maybe slightly smaller) and as close to the diameter size you need as you can find. Then cut off the flange at the top. You will then be able to use this as it is the flange that stops the shim from adopting a different diameter. In use, you risk losing the shim down the seat tube (that was what the flange is for), but you will be able to fish it out with a coat hanger bent at the end.

Thanks Steve, you certainly know your stuff in that comprehensive answer. Checked Suntour OD again and it is 30.9mm. I was wrong in my original calculation, maybe due to age and eye sight. Got another 30.9mm from a friend and it is the same issue, an oversized tube on the bike frame, fractional but oversized. Friend who knows a bike mechanic said that the mechanic has seen the same issue on bikes up to £10k. Going to try a hard wood insert in the bike frame cut so I achieve my seat height and put the pillar in with a fibre paste. If that fails then I will resort to your suggestion. Thanks again for taking time to answer my query.

 

[steve_sordy]

I had thought that suggesting you check your measurement would have been patronising, so I chickened out.

@peteandsoo It looks like coke cans ARE the answer after all!

 

[theremotejuggernaut]

QR clamp would not fully close so replaced with seat post appropriate allan screw headed bolt and nut but still not cured it.

Did you for a nut and bolt to the qr seatpost?

Have you tried a normal, bolt up seat post clamp?

 

[Albagord]

Did you for a nut and bolt to the qr seatpost?

Have you tried a normal, bolt up seat post clamp?

Yes tried bolt and nut, Did not make any difference.

 

[steve_sordy]

If the seat post is 30.9 and the seat tube is 30.9, then you should not have a problem. Is the bolt that tightens the seat clamp across the slot at the top of the seat tube? If not then move it so that the bolt is across the slot. That should do the trick.

Some fibre gel like this should help as well, worth a try!

https://www.chainreactioncycles.com/finish-line-fiber-grip-assembly-gel/rp-prod30012?gs=1&sku=sku93292&utm_source=google&utm_term=&utm_campaign=&utm_medium=base&gclid=Cj0KCQjww4-hBhCtARIsAC9gR3ajqszoDIksPZbtQnCuhcYVzhae8DhXjZ2w96r8zn_4Oy0x0q043J8aAnt_EALw_wcB&gclsrc=aw.ds

 

[Albagord]

If the seat post is 30.9 and the seat tube is 30.9, then you should not have a problem. Is the bolt that tightens the seat clamp across the slot at the top of the seat tube? If not then move it so that the bolt is across the slot. That should do the trick.

Some fibre gel like this should help as well, worth a try!

https://www.chainreactioncycles.com/finish-line-fiber-grip-assembly-gel/rp-prod30012?gs=1&sku=sku93292&utm_source=google&utm_term=&utm_campaign=&utm_medium=base&gclid=Cj0KCQjww4-hBhCtARIsAC9gR3ajqszoDIksPZbtQnCuhcYVzhae8DhXjZ2w96r8zn_4Oy0x0q043J8aAnt_EALw_wcB&gclsrc=aw.ds

Seems an oversized tube on the bike frame. Spoke to a friend who is a bike mechanic and seems he's seen this even on bikes up to £10k. Bit of wood inserted in bike frame to suit measured height of seat pillar and some fibre gel as you suggested has fixed the issue. Thank you.

 

[steve_sordy]

.................. Bit of wood inserted in bike frame to suit .................

Just out of interest, what sort of wood and where did you put it? I can only think of two options:
1) Something really thin to act as a shim.
2) Something smaller than the seat tube diameter dropped down the seat tube to prevent the seat post from dropping too far.

Both of these are what I would call Heath Robinson solutions. They will work, but they are neither durable nor "nice".

Please surprise me with a third solution that is both durable and "nice".

 

[Albagord]

Just out of interest, what sort of wood and where did you put it? I can only think of two options:
1) Something really thin to act as a shim.
2) Something smaller than the seat tube diameter dropped down the seat tube to prevent the seat post from dropping too far.

Both of these are what I would call Heath Robinson solutions. They will work, but they are neither durable nor "nice".

Please surprise me with a third solution that is both durable and "nice".

Steve cut a bit of mahogany hard wood. It can take the punishment.

 

[Binhill1]

Looking for advice: Fitted a SR SUNTOUR New SP12-NCX Suspension SeatPost 30.9 X 350MM to my Cube Pro-reaction 500 two years ago. It was a straight swop out for the Cube seat post measuring 30-9mm. I noticed that at the end of the ride that the Suntour seat post had sunk by over an inch passed the mark that I had put on the post as the ideal adjustment. Have checked the outer diameter of the Suntour post with digital calipers and it measures a 30mm. I would need to get a 0.9mm shim made or buy another suspension seat post. Looked online and suggestions for fix vary from spraying it with womens' hair spray, fibre grease, woughing seat post with file and cut a cola can as a shim. QR clamp would not fully close so replaced with seat post appropriate allan screw headed bolt and nut but still not cured it. View attachment 110316​

Am I missing something OR did you say you fitted it two years ago . Mine noo it's a. SETARDAY NIGHT.

 

[cappuccino34]

I'd be tempted to get an adjustable reamer down the seat tube and inch or so and just take a smidge out of the tube to
a: square up the bore
b: create a small step in the tube so that your shim bush is much less likely to slip down inside the seat tube because retrieving it could be almost impossible if it does.