Bicycle Mechanics - Plastic shims - shortcomings?

I work on a lot of bikes for friends and neighbor kids as well as keeping my own fleet mobile.

There is always a call for shims because it is often difficult to find exact matches for seat posts and handlebar stems.

Naturally, I don't have time to run to the LBS to get a manufactured shim, so I have to make my own. I used to use thin steel sheet metal for shim stock, but it is prone to rust and is difficult to work with.

Recently, I have switched to plastic shim stock. I am almost embarassed to say, but I make the shim stock from plastic cola bottles.

It is easy to cut, easy to modify, easy to work with, and seems to work very well.

Am I missing something critical? Does anybody know of some shortcoming of such plastic shim stock? Am I creating an impending disaster? Why don't I see more plastic shim stock on the market?

Servus!

Well Mike, I'm going by my limited knowledge of metallurgy here.

The problem I see is that the plastic will succumb to sideward pressure forces over time, causing the plastic to wear and compress and consequently no longer shim worth a darn. Metal also is subject to these forces, but compresses in almost immeasurably small amounts.

My advice is to wait and see. Give these plastic shims some time and check them later for the amount of wear they receive over a, say 6 month period. I don't think the plastic will last that long, but I have been wrong in the past. Just ask RangerGirl!

Why dont you use Al beer cans ?

Originally posted by MichaelW
Why dont you use Al beer cans ?

Two reasons for not using aluminum beer cans:

1) Aluminum and steel eventually freeze together.

2) Aluminum cans are so thin that it takes too many shims and becomes difficult to work with.

If the soda-pop shims don't work out, you might obtain some Delrin stock at a plastic supply company and try cutting shims out of that. Delrin is very durable (for plastic) and is used for those gears you often see in things like fans. I became acquainted with Delrin when I built a harpsichord in 1970. Modern harpsichords often use Delrin for the jacks and plectra.

Originally posted by Ranger
The problem I see is that the plastic will succumb to sideward pressure forces over time, causing the plastic to wear and compress and consequently no longer shim worth a darn. Metal also is subject to these forces, but compresses in almost immeasurably small amounts.


I agree w/ Ranger's assessment, I don't think the plastic will hold up in the long run.

BTW, commercial seatpost shims I have seen are aluminum as well as the seatposts themselves. Use a little anti-sieze or plain old grease to prevent freezing.

A good source for small cheap sheets of metal (aluminum, copper, etc.) is McMaster-Carr (http://www.mcmaster.com/). This place is a do it yourselfer's dream. Every nut and bolt imaginable, tools, hardware, raw materials etc - there's even a section for Shim Stock. If you neeed it, they probably have it and for a good price. Generally no minimums and they sell direct to the public.

Originally posted by riderx

BTW, commercial seatpost shims I have seen are aluminum as well as the seatposts themselves

Yes, I have seen commercially sold aluminum shims. I won't use them with steel - greased or not greased.

Did you ever try to get an aluminum seat post that had united with the steel post tube? Before it is done (and all or part of the bike is destroyed), you will be swearing in three new foreign languages.

Thanks for the suggestion for shim material supplier, by the way. Looks promising.

Originally posted by mike
Did you ever try to get an aluminum seat post that had united with the steel post tube? Before it is done (and all or part of the bike is destroyed), you will be swearing in three new foreign languages.

Yes, I have an old steel frame hanging on the wall w/ a alum. seatpost cold welded in it. :irritated: But, since I've been religious about using grease or anti-sieze, I haven't had problems. Do you use aluminum seatposts in your steel frames? Just curious, because this is what most seatposts are made of these days.

Having studied plastics in College and worked extensively with it in the Ski and other industries, I don't think you have anything to worry about, for the application and type of plastic you're using.

Originally posted by pat5319
Having studied plastics in College and worked extensively with it in the Ski and other industries, I don't think you have anything to worry about, for the application and type of plastic you're using.

Thanks, Pat. So far so good!

Funny, I was just dealing with a shim issue this weekend. I was mounting stuff for commuting on my handlebars (lights, computer) and I found that cutting pieces of old tubes made a perfect shim for mounting stuff on handlebars. The squishiness of the rubber provided just the perfect bite to mount stuff securely. I wouldn't recommend this for mounting seatposts and stems though!

Originally posted by Ellen
I found that cutting pieces of old tubes made a perfect shim for mounting stuff on handlebars. The squishiness of the rubber provided just the perfect bite to mount stuff securely.

Nice discovery, Ellen.

There are a lot of uses for old inner tubes.

For the uses you discovered, you may find rubber tape to be much easier to use because it stays in place while you mount the hardware. You can get rubber tape at most hardware stores for about $1.50 per roll.