Re: [CR]Re: Campagnolo SR 39T chainring?

Example: Framebuilding:Restoration

Date: Sun, 24 Oct 2004 16:10:24 -0700
From: Steve Maas <>
Subject: Re: [CR]Re: Campagnolo SR 39T chainring?
References: <>
In-Reply-To: <>

You can prove this pretty easily with a little math. The diameter of any 144 BCD chainring, to the outside of the mounting bolts' lip, is 156 mm. No part of the chain can come within this circle, or it interferes with the mounting bolts.

A 39-tooth chainring, with half-inch pitch, has a diameter, to the center of the chain links, of 158 mm. However, the width of the plates in the link can be as much as 10 mm (largest of several chains I just measured). So, the diameter to the inside edge of the chain, when it fits on the chainring, is ~148 mm. This will interfere badly with the mounting bolts.

It's simply impossible to have a 39T chainring with 144 BCD bolt circle. This is precisely why Campagnolo, and others, eventually switched to smaller BCDs.

I have one 41T ring, and it's really marginal; the chain actually does not quite clear the chainring bolts. However, the interference is small, so it does work.

Steve Maas Long Beach California

> writes:
>><< Check this out:
>>I did not think this was possible. 41T was as small as I have heard of or
> seen. Even with that size you must relieve the crankarm for chain clearance.So
> I wrote to the seller, and he said: This 39T chainring was produced by
> Campagnolo solely for professional teams, as a 'granny gear' for the big Alpine
> mountain stages. It was never introduced to the retail market. Its 84.6mm across.