Seatpost: how loose is acceptable? - Solved

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Tompsk
Posts: 40
Joined: 6 Nov 2014, 9:35am

Seatpost: how loose is acceptable? - Solved

Postby Tompsk » 19 Nov 2017, 6:13pm

I bought a new aluminium frame:

https://www.radialcycles.co.uk/bikes/fr ... frame.html

This states "Seat post size 27.2", however when I insert the correct sized post:

https://www.ribblecycles.co.uk/system-e ... #pid=22919

It is a very loose fit. After inserting 100mm or so of the post it still 'rattles' in the seat tube with a couple of mm sideways play at the very top of the seat post (which is 300mm or so from where the seat post enters the seat tube).

I'm not sure if clamping this seat post into the frame will leave it held only by about 10mm of (now distorted) seat tube which over time may lead to the post creaking and not being safe if it fails at the clamping point. If the seat tube distorts (by bending in at the top) it may then dig into the post. I could try a cola can shim but not sure if the existing gap is big enough using the above post.

On all the other bikes I have ever had the seat post was a snug or tight fit in the seat post, they have never rattled. I've not fitted a post to a brand new frame before so perhaps this is common - with the play going away once the clamp is done up and any slack is taken up?

Options seem to be:
Fit it and see how I get on (may be ok??? but unsure if this will fail or damage either part)
Get a smaller diameter post and shim it (bit hit and miss with ordering seat posts and making home made shims).
Get a 27.4mm post and seeing if that is better, this may not fit at all and seem more difficult to find in my preferred price range (up to £20 or so):

New but a bit pricey http://www.chainreactioncycles.com/thom ... p-prod6066
New, but unavailable https://www.amazon.co.uk/Thomson-Elite- ... B00NINMHCA)
Second hand for 753? http://hilarystone.com/seatposts272.html
Last edited by Tompsk on 6 Dec 2017, 8:54pm, edited 1 time in total.

Brucey
Posts: 27139
Joined: 4 Jan 2012, 6:25pm

Re: Seatpost: how loose is acceptable?

Postby Brucey » 19 Nov 2017, 6:54pm

sounds a bit iffy to me. Worth measuring your seat pin; often one that is labelled '27.2mm' is actually 27.1mm.

A 27.4mm pin may not fit unless you also ream the frame to match.

A bodge-tastic solution is to use a ~20mm wide strip of 0.1mm shim at the front of the seat pin. If the frame has a removable clamping collar, the edge of the shim can be folded over and retained under the collar.

If you do nothing, IME it is very likely that the seat pin will (no matter how tightly it is clamped) work itself and move around under its own steam.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

drossall
Posts: 3941
Joined: 5 Jan 2007, 10:01pm
Location: North Hertfordshire

Re: Seatpost: how loose is acceptable?

Postby drossall » 19 Nov 2017, 7:04pm

Yes, they should be a snug fit. I'm a bit confused by your proposal to get a smaller-diameter post though, because you appear to need a larger one?

You're correct that the Radial site says that you need a 27.2mm post. That's odd, because that's the standard size for 531DB steel tubing. I'd have thought that alloy tubes would take larger posts. And indeed, the review on Cyclist UK says 30.8. Might be best to take the frame along to a shop where you can try some, and buy what fits.

Slightly odd too if, as the Radial site says, the tube has a 27.2 internal diameter, and takes a 31.8 clamp. That's a heck of a thick tube wall, isn't it? Or am I missing something? I've not had many alloy bikes.

Tompsk
Posts: 40
Joined: 6 Nov 2014, 9:35am

Re: Seatpost: how loose is acceptable?

Postby Tompsk » 19 Nov 2017, 10:56pm

drossall wrote:Yes, they should be a snug fit. I'm a bit confused by your proposal to get a smaller-diameter post though, because you appear to need a larger one?

You're correct that the Radial site says that you need a 27.2mm post. That's odd, because that's the standard size for 531DB steel tubing. I'd have thought that alloy tubes would take larger posts. And indeed, the review on Cyclist UK says 30.8. Might be best to take the frame along to a shop where you can try some, and buy what fits.

Slightly odd too if, as the Radial site says, the tube has a 27.2 internal diameter, and takes a 31.8 clamp. That's a heck of a thick tube wall, isn't it? Or am I missing something? I've not had many alloy bikes.


Idea behind getting a slightly smaller diameter post is so a shim can be fitted all the way round it to even out the pressure, the current 'correct' seat post is such a near (but seemingly incorrect) fit the shim may have to be either wafer thin or only go part way round.

The seat post size in the review you linked is for a different (carbon) frame. 27.2mm is a very common seat post size with alloy frames e.g:

http://www.wiggle.co.uk/felt-fr40-road- ... agra-2017/
http://www.wiggle.co.uk/verenti-techniq ... oad-bike-1
https://www.evanscycles.com/focus-cayo- ... e-EV308649

Home made shim or LBS suggestion is the way I may need to go.

Tompsk
Posts: 40
Joined: 6 Nov 2014, 9:35am

Re: Seatpost: how loose is acceptable?

Postby Tompsk » 25 Nov 2017, 12:10pm

Well I've just measured everything up with a digital vernier:

The "27.2mm" seat post is a very consistent 27.15mm wherever I measure along its length - so there or there abouts on specification.

The "27.2mm" seat tube internal diameter is actually 27.45mm all around - at least for the top 5mm which is as far as I can get the fangs of the vernier into. The above 27.2mm seat post seems to rattle equally as it is inserted down the seat tube so I'm guessing the internal diameter of the tube does not change by much as you go down.

I'm going to get a "27.4mm" seat post and see how I go. I've found a silver one in price range - anyone know if a black version (at a similar price) in this rarer diameter is available?

https://thecycleclinic.co.uk/products/k ... increments

(edited for minor typo)

Brucey
Posts: 27139
Joined: 4 Jan 2012, 6:25pm

Re: Seatpost: how loose is acceptable?

Postby Brucey » 25 Nov 2017, 12:27pm

the reason they do this (i.e. supply a seat pin that is nominally about 0.2mm undersize) is so that they can spare themselves from having to ream the seat tube even if it distorts slightly during welding. It is common (and annoying) practice. The distortion, if it is there, will likely be where the welds join the top tube and stays etc to the seat tube.

You will soon find out if your frame needs reaming or not when you try a nominal 27.4mm seat pin in it.

Not sure if black ones in 27.4 are easy to find or not.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Tompsk
Posts: 40
Joined: 6 Nov 2014, 9:35am

Re: Seatpost: how loose is acceptable? - Solved

Postby Tompsk » 6 Dec 2017, 9:01pm

I've now solved the problem (spending an unnecessary extra £20 on my "buy a frame and build a bike from the spares box" project) by buying a 25mm post and a 25 to 27.4mm shim, both from SJS. The shim is sprung wide and so a very tight fit in the tube - not sure if that is going to come out easily. Once in, the shim is the about the correct thickness - the 25mm seat post slides a bit easily in but seems secure once clamped. If I were doing it again I might get a 25.4mm post and get a 25 to 27mm shim for the same effect but where the shim wouldn't be such a tight fit in the seat tube and so easier to extract.

Brucey
Posts: 27139
Joined: 4 Jan 2012, 6:25pm

Re: Seatpost: how loose is acceptable? - Solved

Postby Brucey » 7 Dec 2017, 10:27am

good to hear that you have a resolution to the problem anyway, even if it did cost you £20. I hope the shim is long enough that it doesn't strain the frame too much in bending; many shims could (IMHO) do with being longer.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

De Sisti
Posts: 436
Joined: 17 Jun 2007, 6:03pm

Re: Seatpost: how loose is acceptable? - Solved

Postby De Sisti » 7 Dec 2017, 1:07pm

You could use a jubilee clip to ensure it doesn't slip.