Re: [CR] Tandem timing rings, was CIRQUE - stuff I will have for sale

(Example: Framebuilding:Brazing Technique)

Content-class: urn:content-classes:message
Date: Mon, 31 May 2010 15:49:05 -0700
In-Reply-To: <4C025386.9030409@verizon.net>
Thread-Topic: [CR] Tandem timing rings, was CIRQUE - stuff I will have for sale
thread-index: Acr/79l2Uk5VSLwiQPKfxpop8mw48QBHxgJA
References: <4C025386.9030409@verizon.net>
From: "Mark Bulgier" <Mark@bulgier.net>
To: <hsachs@alumni.rice.edu>, <charliepedal@verizon.net>, "Classic Rendezvous" <classicrendezvous@bikelist.org>
Subject: Re: [CR] Tandem timing rings, was CIRQUE - stuff I will have for sale


Harvey Sachs wrote [about timing rings with an odd number of teeth]:
> [snip] they may not work if the tandem currently has rings with an
> even number of teeth. Back in the day [...] I discovered this the
> hard way. [...] The eccentric didn't have enough throw to adjust
> properly for the odd-number rings; the boom tube length had
> been set for evens. Really. Moral: if you've got even, stay with
> even. And vice versa.

Harvey is right, that's safe advice. On the other hand, switching from even to odd or vice-versa might work fine, especially on newer tandems.

If the eccentricity ("E") of the eccentric BB insert is over 1/2", then odd or even cogs will work, _without_ a half-link in the chain, regardless of the boom tube length. The reason for that 1/2" figure, in case it isn't obvious, is that it's the length of one chain link. You adjust the overall chain length in increments of 1" (two pins), but the eccentric is tightening the upper and lower runs of the chain at the same time, so 1/2" of adjustment is all you need.

Sadly, many older are rather stupidly designed IMO, with inadequate E. The badly-designed ones I've seen don't even typically use a smaller outer shell; they just give up E for no reason, by having the threaded hole for the BB cups not be all the way over to one side in the insert. The Haden eccentrics I used in the 70s to early 80s were like that -- infuriating because making them right is no harder, and there is no downside whatsoever to having adequate E.

My old custom frame design spreadsheet had an input cell for E, and would calculate if you were in the danger zone of needing odd-numbered tandem timing chain rings, based on the calculated bottom tube length. I would always tweak the design to get a bottom tube well into the even-numbered rings range, because those are so much more available. Now that there are eccentric inserts available with E > 0.5", there's no need to worry about "odd" or "even" boom tube lengths anymore.

If you want to measure E on a particular eccentric, measure from the BB threads to the outer edge of the insert, at the narrowest part and at the widest part, and subtract.

Now, there may be another reason to stick with even-numbered timing chain rings. The great Sheldon Brown contended that the chain will last longer if both timing rings have an even number. I haven't tried to test to see if he's right, but his argument makes sense to me. Go to the following URL http://www.sheldonbrown.com/synchain.html then scroll about halfway down to "Odd or Even".

Mark Bulgier
Seattle, WA USA