Seat post tolerance?

[A.Gent]

Is it possible for seatposts of a slightly different size to the seat tube to fit?

After all, a clamp is needed to keep the posts in place, therefore there must be some tolerance..

Would I be able to fit say, a 31.6mm seat tube in the place of my 30mm tube in my Norco frame, seeign as 30.0 is not a very common size?

 

[Zyphryss]

Would I be able to fit say, a 31.6mm seat tube in the place of my 30mm tube in my Norco frame, seeign as 30.0 is not a very common size?

Go and try

 

[A.Gent]

Go and try

That's the problem...I haven't got any other seatposts to try it. I was wondering if anybody had a definitive answer as to if it would fit or not...

 

[that_stoney_one]

NO.

straight up answer, it wont work, however if your post is smaller than the tube you can buy shims to take up the difference.

 

[fezi]

I'm guessing that 1.6mm is too large a margin for it to fit. I run a 26.8 post in a 27.2 frame.... it works but its not ideal, its only a roadie so I dont care.

 

[A.Gent]

NO.

straight up answer, it wont work, however if your post is smaller than the tube you can buy shims to take up the difference.

Ok thanks, i'll look into it.

I'm guessing that 1.6mm is too large a margin for it to fit. I run a 26.8 post in a 27.2 frame.... it works but its not ideal, its only a roadie so I dont care.

Yeah 0.4 is not much of a margin, so I can see how that would work. Does it slip at all though?

 

[noddy]

no way, that won't work...
seatposts can be slightly smaller (i personaly wouldn't go smaller than 0.4 smaller than recommended) but definatly not larger.

 

[Tomas]

Seriously, exercise some common sense.

A seatpost has a diameter of 30mm and is a snug fit. How did you imagine something 5ish percent bigger fitting in? Really? Was is all that necessary to post on farkin?

fucking school holidays.

 

[fezi]

Yeah 0.4 is not much of a margin, so I can see how that would work. Does it slip at all though?

No slippage. Lube the seat post clamp bolt with a bit of anti-sieze and nip it up nice and tight, like I said its a roadie so I no care if it goes snap. Simple set and forget.

 

[Binaural]

I have made a post .1mm larger fit into a frame but it was hard and slow work, not too recommended. I imagine a seatpost 0.1mm smaller would tend to slip. That's the sort of tolerances you should be thinking about.

 

[Tazed]

Even amongst posts of the same size, there is some variation.
My digital calipers have routinely shown up to 0.2mm difference.
However, that's still small compared to 1.6mm.
So, as mentioned above, forget it.
Go smaller and you can still shim it to fit (I had a 27.2 in a 30.9 shimmed for years, no problem).
But you should always aim for the exact size wherever you can, although I can't recall seeing many posts at 30.0mm, so a shim would expand your selection considerably.
And tell Norco to get their act together!

 

[Grip]

The biggest problem with shims is that because the seat post is effectively only supported at the very top of the seat tube it can move quite easily (back and forth) which can (and often does) cause serious frame damage... especially if you're a tall bloke with the seat post out a fair way.

 

[b_S]

Yeah 0.4 is not much of a margin

0.4 is HUGE!

 

[Tazed]

The biggest problem with shims is that because the seat post is effectively only supported at the very top of the seat tube it can move quite easily (back and forth) which can (and often does) cause serious frame damage... especially if you're a tall bloke with the seat post out a fair way.

The shim should be at least 10cm long, which should cover most of the insertion into the frame in most cases.
If the frame is properly made, then the tolerance between the post and the internal diameter of the tube should be absolutely tiny - to the point where you need a thin layer of grease to overcome friction even moving the post up and down.
That being said, the variations mentioned above also occur in frame tubing, so you need to check it on a frame/post basis, as it's often a bigger gap than that.
But a shim, if used in properly specced situations, shouldn't give you and concerns about damaging the frame. If you get even the slightest rocking, you need to change shims.

 

[BLAKE-2234]

The shim should be at least 10cm long, which should cover most of the insertion into the frame in most cases.
If the frame is properly made, then the tolerance between the post and the internal diameter of the tube should be absolutely tiny - to the point where you need a thin layer of grease to overcome friction even moving the post up and down.
That being said, the variations mentioned above also occur in frame tubing, so you need to check it on a frame/post basis, as it's often a bigger gap than that.
But a shim, if used in properly specced situations, shouldn't give you and concerns about damaging the frame. If you get even the slightest rocking, you need to change shims.

hhhmmm telling grip how to do his job?

 

[dain2772]

hhhmmm telling grip how to do his job?

As they say: Ballsy. Stupid, but ballsy.

Anyhow, Zinn says you can use household items like an aluminium can to make your own homemade shim (if you use the whole can, you should be able to get around 10 cm worth).

 

[Grip]

But a shim, if used in properly specced situations, shouldn't give you and concerns about damaging the frame. If you get even the slightest rocking, you need to change shims.

Hmmmm... actually, no, because nothing you said addresses what I actually said and going by the number of frames we get in that are broken for this very reason I'd suggest there actually is reason for concern.

To totally elliminate seat post rocking you would need a shim that covers just about the entire insertion of seat post and while I certainly haven't seen every shim ever made I'm pretty sure the ones I have seen have all been a fair bit shorter than 10cm while the recommended insertion for most seat posts seems to actually be more than that.

 

[Tazed]

Hmmmm... actually, no, because nothing you said addresses what I actually said and going by the number of frames we get in that are broken for this very reason I'd suggest there actually is reason for concern.

To totally elliminate seat post rocking you would need a shim that covers just about the entire insertion of seat post and while I certainly haven't seen every shim ever made I'm pretty sure the ones I have seen have all been a fair bit shorter than 10cm while the recommended insertion for most seat posts seems to actually be more than that.

Man, people are cagey.
No-one's telling anyone how to do their job, here. You'll notice when I quoted Grip above, there was no comment that anything he wrote was in any way wrong. It was additional information only.
Grip know their stuff, there's no doubting that. But that doesn't mean their experience (and they specialise in repairs, so it's understandable) is the only one.
A shim doesn't have to mean frame destruction.
If you use a shim properly, there's no reason it will damage a frame, that's all. The 2 I've had were 100mm long, one was a stock spec on a Giant.

 

[Grip]

Man, people are cagey.
No-one's telling anyone how to do their job, here. You'll notice when I quoted Grip above, there was no comment that anything he wrote was in any way wrong. It was additional information only.
Grip know their stuff, there's no doubting that. But that doesn't mean their experience (and they specialise in repairs, so it's understandable) is the only one.
A shim doesn't have to mean frame destruction.
If you use a shim properly, there's no reason it will damage a frame, that's all. The 2 I've had were 100mm long, one was a stock spec on a Giant.

Mate, I wasn't meaning to come across as "cagey" (in fact I don't even know what you mean by that word in that context) and I certainly didn't take it that you were trying to tell me my job (someone else said that... but personally there are days when I'd welcome someone telling me what to do and how to do it ), but while your experience with shims has been all good mine has been just about 100% bad (including a LOT of giants broken at their seattubes). And while a shim doesn't HAVE to mean frame destruction I was simply pointing out an overall problem with them being that shimmed seatposts WILL rock (whether or not that rocking is felt by the rider) and that "rocking" seat posts can lead to frame damage... especially if the seat post is out a fair way (increase in leverage).

I seriously should have just shut up. After all... the more frames that break the better off I am

 

[Tazed]

Didn't take it that way from you, mate!
I should also add that the the shim I was using previously was a 100mm USE shim, and would be a great place to start if considering a shim:

"USE shims are made to 100mm length and are longer than other shims on the market. This supports the seatpost inside the frame. All shims are designed with a "top lip" to stop shim sliding into the frame."

Perhaps I should have explained this better above when I was talking about the 100mm bit - there are some 40mm roadie versions out there, and 60mm is normal. If you're lucky, you'll get 80mm in no-name brands. But these USE shims are the best option I have found.

Oh, and on the Giant frame breaking bit, Giant have started adding stickers onto some of their frames (mainly full-suss Maestros) about having a minimum of 80mm insertion (regardless of seatpost minumum insertion line) or risk frame breakage... so shim or no shim, maybe they've been having failures, anyway?