tiago

Hi,

I just got a child bike seat on freecycle.co.uk, but it requires the bike's seatube to be over 32 mm in diameter to attach. Unfortunately the old steel Raleigh I am using has a seat tube with about 28 mm. Any ideas for producing a shim this thick? Or buy one if this exists? Has anyone tried bent aluminium plates, duct tape or any other materials? As you may have guessed the result has to be solid but doesn't have to be pretty.

Cheers,
Tiago

Camilo

I dont understand how you could "shim" a too-small seat tube to accomodate a larger seat post. Shims go the other way, reducing.

Maybe I'm not understanding what you'er asking.

tiago

The idea is to put some hard material 2 mm thick around the seat tube making it thicker, and then attach the seat clamp (does anyone recommend brass?). It is this type of seat:
https://dsp.imageg.net/graphics/produ...10769p275w.jpg

t.

thomson

Cut some shims from an aluminum can

twoflats

I get my shim stock (aluminum or SS tubing) from https://www.onlinemetals.com/.

You'll need to get a very accurate measurement of the O.D. (outside diameter) of your seat tube. Then buy a piece of tubing (6061 aluminum is easiest to work with) that has an I.D. (inside diameter) that is equal to your seat tube O.D. and an O.D. that matches what your child seat clamp requires. You can get onlinemetals to cut a short piece for you so that all you need to do is cut the tubing (in half) along its length, and you have a perfect shim. You'll also want a file (or dremel grinding bit) to knock any sharp edges off of the shim before you use it.

good luck.

norwood

I assume you mean "seatpost" instead of seat tube. And this is a "jump seat" type of child carrier correct? A product like that usually comes with an assortment of shims as obviously not every bike is not going to have the same size seatpost dia. Lacking these, you will have to some of you own or have a bike shop do so. A machine shop may be able to help you also. Aluminum would work best.

tiago

Thanks for the suggestions. I will try to find an alu tube supplier in the UK or with p&p+tax this will cost me too much. The jump seat attaches to the bike's seat tube which should be stronger than the seat post. This is a used (and free) child seat so it didn't come with extra fittings.

t.

DannoXYZ 

There are already existing 2mm shims that will work. No need to re-invent the wheel. Just call around some bike-shops and ask for some 2mm thick aluminium handlebar shims. These come in split half-circles and exact ID/OD won't matter. As you tighten the clamp over them, they'll spread and squeeze them to fit around the tubing perfectly. It's really the 2mm thickness that's important.

tiago

Thanks Danno, that looks ideal!