small seatpost

[stewie911]

I bought a seatpost that is 1mm(I think) to small in diameter. I can fasten it to my frame, with trouble.

 

Is there something I can wrap around the seatpost to make it larger as I am afraid I'll damage the frame of my mtb?

 

[dollar]

Beware - Many frames have been stuffed up - scrapped because of exactly this. Spend the money and get exactly the correct diameter seatpost! You have been warned 

[droo]

What size seatpost do you need? Road or mountain? You can either swap it out, buy a new one, or shim it. But it is better to have the right size post. Just don't ride it unless you've sorted it out.

[dollar]

You could shim it but I dont recommend it - the shim slips, someone else adjusts the saddle etc etc. Get the correct size.

[Johan Bornman]

I don't understand how a frame can be ruined because of shimming.

 

1mm is quite a bit to shim but it can be done. Go to an engineering supply show and buy shim stock - it is a thin sheet of  brass. Cut strips and wrap it around, like you say.

 

Or, go to the bottle-store and buy several cans of imported beer. Fosters, Boddingtons or the like. These are sold in aluminium cans, not steel.

 

Cut shimstock from this using just an ordinary pair of scissors.

 

It's a pain to do but if you really don't want to buy a new post, do it, there is no danger attached other than a cut finger.

 

[Minion]

If you don't feel like making your own, you can buy seatpost shims from CRC:

http://www.chainreactioncycles.com/Categories.aspx?CategoryID=215

 

[stewie911]

I like the "buy some beer" method. Think I'll give that a try thanks.

[dollar]

Ah - my new china Johan Was wondering what happened to you!

 

Where did I say that a frame can be ruined because of shimming? I said I dont recommend it. Why - because if it's not done correctly then you do run the risk of damaging the frame. Also somebody may forget about the shim at some later stage.

 

Also I worry about aluminium/brass/aluminium together. Thing called galvanic action - just add a bit of water. I guess that you could use a aluminium blik.

 

Personally if I have a bike worth a few grand then I would just get the correct post.

 

[Johan Bornman]

Ah - my new china Johan Was wondering what happened to you!

Where did I say that a frame can be ruined because of shimming? I said I dont recommend it. Why - because if it's not done correctly then you do run the risk of damaging the frame. Also somebody may forget about the shim at some later stage.

 

Right in the second post where you warned that many frames have been ruined - stuffed up' date=' as you put it, from shimming. Or is that Zim Dollar speaking and not you?

 

You still haven't told us how on earth one can possibly "stuff up" a frame by shimming. I can't imagine how...tell us please.

 

 

Also I worry about aluminium/brass/aluminium together. Thing called galvanic action - just add a bit of water. I guess that you could use a aluminium blik.

 

I can think of two dozen places on my bike where different metals meet - brass, alu and stainless in my rims; titanium, steel and alu in my brake calipers; steel and alu in my seatpost; every bearing sits inside aluminium; my pedals have steel axles that reside in alu cranks etc etc etc.

 

Galvanic action is strangely absent in all of those. It's not as dire or as prevalent as you think. I've shimmed many a handlebar and seatpost, all successfully, none stuck...touch wood.

 

Personally if I have a bike worth a few grand then I would just get the correct post.

 

Well, that's just you. The OP wanted to know about shimming and we're telling him about it. It's safe, cheap and very doable.

 

Stop handing out dire warnings.

Stop drinking pink drinks.

And remember, I'm watching you!

 

 

[dollar]

Greetings again, I am pleased to see that you are on top form tonight, and I trust that you are in good health.

 

At the risk of repeating myself, where exactly in the second post (or the first) do I say that shimming stuffs up a frame??? - can I quote myself -

"You could shim it but I dont recommend it - the shim slips, someone else adjusts the saddle etc etc. Get the correct size." I am not going to argue about the galvanic action - because I have not actually seen it in a seatpost. BUT do you have brass and aluminium (two reactive metals) touching on your bike? I am not talking about the bearings and all the other stuff - steel is not the problem. Galvanic action with steel is not much of an issue. I have however experienced a number of stuck seatposts - that's why I said that I "worry". Any finally I have actually seen a frame that was indirectly damaged by a shim - the owner removed the post for some reason. He then replaced the post and cracked the frame when he tightened the bolt. He only found out afterwards that it was in fact shimmed by the previous owner - albeit not very well.

[dollar]

Sorry - I already said finally, but one more obvious thing that I forgot to mention about shimming.

 

THE LENGTH OF THE SHIM BECOMES YOUR DE_FACTO INSERTION LENGTH.

 

So, if you have a piece of shim 30mm long that you use at the clamp so that you can make it lekker tight, then you actually have a 30mm insertion length! The bottom bit of the stem has a mm or so of clearance cause its not shimmed and it moves around in the tube as it gets loaded from the side - something that happens on a MTB? Where is all the load concentrated?

 

 

 

[Johan Bornman]

This theory assumes two things:

 

1) That the shim is several millimeters thick and

2) That there is no flex in the system at all.

 

You'll find that the seat collar and seat post on most bikes have a different diameter, with the collar being slightly smaller. Flex in the seat tube and seat post sees to it that the post touches the frame at its lowest point under load, effectively making the insertion lengh whatever the lenght of the post inside the tube is.

 

We're talking shimming here of 26.8mm posts to 27.2 etc - a shim of 0.4mm in total or 0.2mm on each side. No system is so rigid that the effective lenght is the depth of the collar.

[dollar]

I agree .2mm either side is nothing to worry about. I would be a touch concerned if it was more than half a mil either side though - or if I did not actually know what it was - I had not measured it because I dont have a vernier or mike.