Saddle Rails

[Drake]

Do saddle rails vary in diameter ?
Just fitted a new saddle,and although everything is tight,the new saddle rocks up and down slightly . It's as though the rails are sligjhtly different in diameter . Can't measure the old saddle rails . . s/h bike came without saddle .
Qustion is . . are there purpose made shims to overcome this problem,or is it homemade shims made from a coke can .
The saddle post is a micro adjust type .

[bikepacker]

All my saddles have rails that are 7mm. Don't know if this is the industry standard.

[gaz]

Apart from a few sprung Brooks models that have multiple rails I've not come across anything with rails that doesn't fit a modern micro-adjust type post.

Is there any possibility of some grit, swarf or such-like trapped in the assembly? Could the seat-post clamp itself be bent or otherwise damaged?

Take apart, clean, examine thoroughly, and reassemble.

[531colin]

Some budget micro-adjust seatposts have steel plates that clamp the saddle rails...I have seen these bent in bikes that have had a hard life.
Better quality seatposts have the same parts in cast alloy

[reohn2]

Some budget micro-adjust seatposts have steel plates that clamp the saddle rails...I have seen these bent in bikes that have had a hard life.
Better quality seatposts have the same parts in cast alloy

+1
I've seen alu ones bent too(old SR or Kalloy?)

[Drake]

Apart from a few sprung Brooks models that have multiple rails I've not come across anything with rails that doesn't fit a modern micro-adjust type post.

Is there any possibility of some grit, swarf or such-like trapped in the assembly? Could the seat-post clamp itself be bent or otherwise damaged?

Take apart, clean, examine thoroughly, and reassemble.

Done as you all suggested . There does appear to be wear in the alloy locating grooves on the stem . The clamping plate is made of steel and although i can't be 100% certain,it does look slightly out of shape .
Have just tried new saddle on a spare post and it fits perfectly .
I did wonder if this saddle,(which was by no means expensive)had odd size rails . Obviously not .
Trip to the LBS tomorrow will be required . Manythanks men .

[The Mechanic]

FWIW some seat posts come with a note saying that they are for 7mm rails only. I have always assumed that means that some are a slightly different diameter. A couple of years ago I bought a Revell bike from circa 1983. The saddle was a plastic covered affair marked "Maddison". The rails on that were too close together to fit a mordern post but they did fit the post that came with the bike.

[edocaster]

Just to resurrect this thread, I have a similar problem with a 2nd hand Brooks saddle, which seems to have 6.8mm rails. Not a huge amount of difference from the 7mm standard, but it's causing my seat clamp to 'bottom out' on itself. The clamp seems to be cast alloy, and not bent as far as I can tell.

Is it OK for me to just shim the rails with a couple of layers of aluminium Coke can?

[CREPELLO]

Just to resurrect this thread, I have a similar problem with a 2nd hand Brooks saddle, which seems to have 6.8mm rails. Not a huge amount of difference from the 7mm standard, but it's causing my seat clamp to 'bottom out' on itself. The clamp seems to be cast alloy, and not bent as far as I can tell.

Is it OK for me to just shim the rails with a couple of layers of aluminium Coke can?

You could try it and see, but in my experience soft alu shims can distort and lead to ceaseless creaking - especially fiddly little shims required for saddle rails.

If the clamp had enough metal to it, I would be consider filing away a little at the clamp. It's much better to do a job without shims if you can help it.

[edocaster]

My mistake - the clamp isn't bottoming out, it only looked like it due to overlaping ridges. It's a 'radiussed top' post, and has a tendency to slide and tilt back. Have torqued it up tight so hopefully won't move.