Hi I was wondering if anyone can help me,

I’ve got a Kona Explosif 2014 650B.

The seat post size is supposed to be 31.6mm and I’ve got a Thomson 31.6mm seat post, but the Thomson seat post slides around in the frame, because it isn’t big enough (regardless of how tight the clamp is screwed).

The original seat post that came with the bike has a shim. The shim just about fits on the original seat post although it is really rigid and gets stuck. I can’t adjust the original seat post when it’s in the frame with the shim on it, because the shim won’t stay in the frame.

I’ve tried putting the shim on the Thomson seat post but that is even more rigid, and the Thomson seat post won’t fit into the frame when it has the shim on it.

I think the shim may be meant to be fixed into the frame, but it’s come out.

I don’t really understand what I should be doing.

Can anyone help?

You could glue the shim into the frame. Might not last long.

You could send it all back.

If you can measure the seat tube you could buy one of the right thickness, hopefully better machined than the on you have, albeit the machining error may be elsewhere.

Glueing the shim into the frame seems like the best option. I guess it depends how often you need to adjust/remove the seatpost though.

Thomson seatposts have a bit of a reputation for being a bit skinny. I’d try another make.

Thomson seatposts have a bit of a reputation for being a bit skinny

You mean machined to decent tolerances?

Are you positive the thomson is 31.6? Not 30.9?

Was the bike/frame new when you bought it?

Also, as said, double check the size of the new post just in case.

As above, measure both seat posts carefully. I wouldn’t glue the shim to the frame.

I believe the frame has a lifetime warranty, so if the original purchaser may be worth contacting your lbs/the distributor to enquiry?

I’ve had a similar issue with a steel frame that should have had a shim pemanently mounted but for reasons it wasn’t.

There’s actually quite a bit of spring in post shims, it seems.

What finally worked for me was cleaning the inside of the tube and shim, then reinserting with a bit of carbogrip, and jamming a known correct cheap post in, clamping it tight with a tiny dab of grease and ignoring it for a couple of weeks. Cheap post got heavily scuffed inserting. Now happy with a lev.

Have you tried tightening the clamp up with just the thomson in? You may have bent the top of the seat tube (without shim) over and made it effectively undersize. You may need to use some force to “reform” the top of the seat tube properly before you go any further. Pictures would help.

FWIW i have a couple of (ancient) Ti frames both of which include a bonded in aluminium shim to take the seat tube down to a reasonable size for a seat post. The third and fourth frame in the set (both now sadly and terminally dead) both had shims that came out annoyingly regularly. On the plus side, the stuck shims gave me a very handy guide as to how far into the frame the seatpost needed inserting…….

The frame is supposed to be 31.6mm seatpost size, but whether that’s with ot without a bonded in shim, only someone who owns one (or Kona themselves) can say, I suppose.
As the post is a Thomson it will have the diameter etched on it, so that’s easy to check. If you have a micrometer, vernier or digital calipers then you can double check this too.
What thickness is the shim that you have? For comparison, a Coke can is .004″ (0.1mm) thick, so you can always cut a strip from one to use as a feeler gauge, just so you have some idea what size difference you’re looking at.

I’ve made shims from Coke cans, to go from 27.0mm post to 27.2mm frame. I could adjust the post with it fitted, as I rolled a lip over the top of the seatpost which was then held in place with the seatpost clamp.

Thanks for your replies. To respond to the points made:

Yes I bought the bike (again, that is Kona Explosif 2014 650B) from new. The Thomson seat post (Thomson Elite Inline Seat Post) is definitely 31.6mm (x 410mm long).

I’ve tried tightening the clamp with just the Thomson seat post in (that is, both the original clamp, and a Hope Quick Release (sz 34.9) clamp that I bought), but the seat post isn’t tight enough to stay in place on its own.

I’m not sure what the thickness of the shim is, but when I put the Thomson into the shim it gets jammed very rigidly, and I had to use a hammer and screwdriver to separate them (causing obvious unwanted scratching etc. to the Thomson). Even if the shim was bonded into the frame as it should be, I wonder if a Thomson could actually be used with this frame?

Equally, when I put the original seat post which came with the bike into the shim, it gets pretty rigid – they will together go into the frame, but the shim stays stuck to the seat post so no adjustments are possible (I’ve made this point already, I think).

Annoying thing I am going to Cannock Chase tomorrow but I’m probably going to have to hire a bike if I can’t get this sorted tonight (unlikely, although I may try my hand at Andy R’s solution with a coke can tonight).

I’ll have a look at contacting Kona or CycleStore where I bought the bike from.

Cheers again for all your inputs.

Firstly Thomson do sometimes have a rep for being “small”.
However, I had a Thomson that was fine in a Spesh with a Hope seat clamp, but when swapped over to my Giant, it would slowly slide down.
I replaced the hope clamp with a Salsa Flip off seat clamp and it doesn’t move now.

The Thomson seat post (Thomson Elite Inline Seat Post) is definitely 31.6mm (x 410mm long).[/I]

Do you mean you’ve actually measured it, or just what is printed on the post? If the latter, measure it and then the seattube (inner).

Aldi are selling digital calipers for £10.

If you haven’t done so already, I’d take the shim out again and properly clean it up, also the inside of the seat-tube and remove any corrosion and grit that could be enough to tighten the fit of the seatpost. It doesn’t take much to do that.

Got similar problem with my 30.9 thomson dropper in 31.6 ti frame+ stupid shim. Keeps slipping even with carbon grip paste and with it feeling really tight in the frame. Bought a hope seat clamp and tightened to 6nm, if that doesn’t work will have to get a new post.

I’d recommend a different clamp from the Hope ones, as per my comments upthread.

Hope QRs benefit greatly from a drop of oil in the brass cam – makes a huge difference to clamping force, but yep, not on anyone’s list of most functional seat QR ever.

Salsa is the traditional suggestion for ‘something better’ although I’m usually fine with Thomson clamps for plain clamp.

If you reread a few of the above posts, get the cheapest 31.6 post you can – ugly and SH ideal, put the shim back in, smear a little grease on the post and force it in – properly twisting and thumping like you expect a scratched to hell post. Clamp it and walk away for as many days as you can. When you’ve taken the post back out, you’ll probably find the shim pops out a little but basically stays in place. Without bonding it that’s as good as it’ll get. You should have no issues with your Thomson post now, hopefully.