Mate has asked me to give his boardman team a makeover, including the usual wider bars, shorter stem and dropper.

The dropper however is a 30.9 whilst the seat tube is 31.6

On fitting using a shim I’m a bit concerned that the seat tube top tube joint is quite far below the seat collar and as such the shim doesn’t extend past the top tube.

Pics below, however on looking at the original post the min insertion line is exactly the same as the shim, so maybe not going to be an issue.

So what do folks think. Its not me whos going to die, but then again I’d feel slightly bad if my mate did. I guess one other option is buying another shim and using half of thaat below the existing one? unless anyone knows of a shim longer than 10 cm

Does the frame show a minimum insertion? Does the seat post minimum insert less than the shim?

no marking on frame. Min insertion on original post same as shim exactly

ignore the minimum insert on the posts – it’s the frame that counts.

If the seat post/shim combo is not at least a couple of cm below the junction then don’t do it would be my approach.

ignore the minimum insert on the posts – it’s the frame that counts.

in this case certainly.

As above. It might suck but really in this case a shim is likely to lead to needing a frame replacement, and thats more costly than selling his dropper and buying another in the right size.

hmmm..I suspected this may be the case!

hes on a budget and the post is an old one of mine I’ve given him. Replacing not an option!

What about a longer shim? or buying another and curring off the top so I can run one further down the seat tube..ie past the joint. WOuld that work?

Replacing not an option!

Swap?

A longer shim ((140mm or something like that) would work, as would buying another shim, cutting off at the shoulder and bonding it to the bottom part of the seatpost, after first fitting the upper shim, of course.

The only shims that I’ve seen longer than 100mm are the Syntace ones, as used in Litevilles, but they only make 34.9 -31.6 and 34.9 – 30.9 as far as I’m aware.

So the minimum post insertion is 100mm? Then the problem is the same with or without a shim. A longer shim will not offer any structural advantage.

It looks like the post and the shim are too short for that particular frame.

the problem is the same with or without a shim.

you’re allowed to insert a post further in than the min insert point though. Which is why it’s the frame requirement that is more important than the post one. The post min insert isn’t irrelevant but it only applies to the post – you need to use the frame min insert as an override.

So the minimum post insertion is 100mm? Then the problem is the same with or without a shim. A longer shim will not offer any structural advantage.

It looks like the post and the shim are too short for that particular frame.

Surey if the shim was say 150mm then that would go well past the joint. The post is easily long enough.

If I could find a shim that length then why wouldn’t it provide structural support?

As for swapping…bit reluctant to do that as I know this one actually works, and thats rare amongst second hand dropper posts! Its a lst option should no other solutions present themselves!

So the minimum post insertion is 100mm? Then the problem is the same with or without a shim. A longer shim will not offer any structural advantage.

It looks like the post and the shim are too short for that particular frame.

The post may be inserted past the minimum line though, we don’t know and a longer shim would offer a structural advantage to the frame.

buying another and cutting off the top so I can run one further down the seat tube..ie past the joint. Would that work?

This. It’ll cost next to nothing and will fix the problem. Use some superglue or similar to bond the lower bit of shim to the post before inserting into the frame.

remember, it’s a combination of minimum insertions

Minimum insertion mark on post shows you the amount of post that needs to be supported for the post to be within spec*

Minimum insertion on frame is the amount of post that needs to be inside the frame for the frame to be within spec**

The two may or may not overlap…

* using the load case that the post was designed around
** using the load case that the frame was designed around

It is still possible to be outside of either of those specs for example:

– if you are massively heavier then they accounted for
– if the post is waaay longer than the frame designer accounted for
– if there is some super wacky slack seat angle going on
etc.

As Andy R above says a longer shim would be better, and in most cases*** will be OK, but some manufacturers can be funny about the use of shims in general as there may be additional concerns like tolerance of shim size, clamping force required etc. that can mean there are still issues or things moving and flexing in ways that are not ideal.

*** doesn’t help for situations where a seat-tube is internally butted or tapered and bigger diameter lower down meaning that no matter how much post is in the frame it isn’t supported past the butting anyway.

Radical option. Cut 15mm off the frame. Seat clamp will still fit. Has the bonus of letting you drop the saddle even lower

I’d trust it. In days of yore, some posts only came in 3 sizes, and the intermediate ones were made up with shims (X Lite? USE? I think?) and there weren’t seat posts cracking all over the shop. But I’ll give you that this was in the days where TTs often met at the top of the ST…

It’s 0.35 of a mm either way, any small movement/flex will easily be supported further down the tube by the post meeting the side. I’d wager many frames aren’t constant diameter all the way down the ST anyway.

I’m not saying I’m right, I’m just saying that I personally would be fine with it.

Sorry, I read that as the OP was only inserting the post 100mm.

I’m going to point out that the ST ‘mast’ looks braced as well 🙂

For what it would cost the two shim system is the way to go – and leave the lower shim as long as the i.d. of the seat tube will permit.

There’s a brace from the top tube which imo changes the advice from the usual “overlap with top tube junction”.

While gluing a shim sounds reasonable, check the seat tube actually supports the seat post that far down, the seat tube may be shimmed already!

I’d be tempted to cut the st down but resale is then over.

Keep the bars and stem for when they come back in to fashion.

just called my local bike shop to see if they had another shim

their advice…dont double up on shims, will creak and fall apart 🙁

A one pint beer can, cut the top and bottom off, split up the middle. Use and much width-wise as you need so you don’t get too much overlap (as it’ll be too hard to get in, baby).

My guess is your lbs is thinking you mean one shim inside another

If you are going to use a shim it needs to be a good fit and constant along the length it needs to be, so no ‘2 shims one installed further down’. If you can make one out of beer can then great, but bear in mind its movement at the bottom of the post (ie. only being properly supported near the top) that will kill the frame. I have a frame that cracked because the seat tube was slightly oval so the bottom of the post could move, meant the seat tube was not being assisted by the post so was being loaded more than intended.

For clarity, when i say bottom of the post, i mean where it is supported to below the top-tube join. Doesnt need to be supported right to the bottom of the post itself, just to a place below the tt join.

What’s the point of the seatpost brace if you are going to enforce a min. insertion past the main junction?

As a temporary measure I have put cheap shims all the way down the tube.
I can’t see any reason that wouldn’t work.

What’s the point of the seatpost brace if you are going to enforce a min. insertion past the main junction?

There is not really any point in the extension at all, save the fact many people wouldnt buy it if it were not there as the bike would be ‘too small’ based on the measured seat tube length. Also someone might thing it doesnt look as good.

What seat post are you using?

its a ks i900 dropper

ive never seen a min-insertion marked on the frame, it might be the user manual for the bike, this may be available in the manufacturers web archive…
or email them and ask what min-insertion is, it might be the same as the post…

or it could be ala comencial and be below the main junction with the top tube

I think USE make their Shims, you could maybe ask if they can make an extra long one?

USE would do this. they make custom stuff if you ask.

Not sure if it would work but I’ve a spares repairs 31.6 i950 (edit might a 900 even) in the garage you’re welcome to to see if you can swap the outer/internals etc over?

STATO – Member
If you are going to use a shim it needs to be a good fit and constant along the length it needs to be, so no ‘2 shims one installed further down’.

Assuming that the seat tube has a constant i.d. and, for argument’s sake, 150mm of seatpost is inserted in the frame could you explain why, from an engineering point of view, two shims occupying the full 150mm will act any differently to one shim 150mm long?

It’s not as if the shims are reinforcing the frame but rather the longer the shim, the less the seatpost is able to flex within the frame, which is what ultimately might lead to frame failure.

Min insertion mark on a frame… 🙄

It will be fine.
Go ride.
You’ll spend all day arguing with some of the experts on here.

Christ, when I think of some of the bodges I’ve done over the years. 🙂

Min insertion mark on a frame…

There may not be a mark, there rarely is, but there will/should be mention in the manual.

If you’ve ever had to deal with warranty returns you’d know how common it is, some manufacturers are more lenient that others though…

Assuming that the seat tube has a constant i.d. and, for argument’s sake, 150mm of seatpost is inserted in the frame could you explain why, from an engineering point of view, two shims occupying the full 150mm will act any differently to one shim 150mm long?

You would be correct in saying two shims occupying the full 150mm would be ok. Which is what i was trying to clarify with “a shim it needs to be a good fit and constant along the length it needs to be”, perhaps i should have said ‘any shimming’. Previous comments on using 2 shims suggested one at the bottom, which does not imply a fully supported post (i.e. a possible gap between shims) so possible bending force on the upper part of the seat tube due to the post bending in the middle. Obviously this assumes a reasonable gap between them to allow the bending, maybe just a 10mm gap might actually be ok, suppose it depends where bending might happen, but the safest answer is a one piece shim and then you are sure. As i said, i had a frame crack due to post movement so just trying to avoid that issue for anyone else as it can be an expensive issue (if not injury).

Min insertion mark on a frame…

Its not that uncommon.

It will be fine.
Go ride.
You’ll spend all day arguing with some of the experts on here.

Christ, when I think of some of the bodges I’ve done over the years. Ive done a lot of quick shady bodges and ridden things that were stupidly unsafe, id not recommend it to others though, its not my neck or wallet on the line.

Cut an inch off that shim and bond it to the bottom of the seatpost. Make sure you keep the top bit of the shim on the post as you do this. Then insert it and carry on.

having had a boardman Fs frame crack around the collar whilst using a shim and dropper, i’d make sure the post was fully supported next time.

Min insertion mark on frame

I’ve seen a small hole drilled into seat tube and a manual that said “make sure seatpost is visible through inspection hole”