Re: [Bulk] Re: Half-step on Sturmey, was [CR]How to build up this Hetchins?

Date: Mon, 15 Sep 2008 12:42:37 -0500
To: Dmitry Yaitskov <dima@rogers.com>, classicrendezvous@bikelist.org
From: "Mark Stonich" <mark@bikesmithdesign.com>
Subject: Re: [Bulk] Re: Half-step on Sturmey, was [CR]How to build up this Hetchins?
In-Reply-To: <1262235490.20080915123431@rogers.com>
References: <48CE3D89.9060804@verizon.net>

At 11:34 AM 9/15/2008, Dmitry Yaitskov wrote:
>As I noted in my other post (but forgot to mention in the original
>one), this bike has the "FM" model, with 19/22 sprockets combo, and a
>44t chainring, which, if my math is right, gives 2 almost duplicate
>gears, and is too low overall. Advice on how to make the gearing more
>useful will be very much appreciated.

19/22 is a perfect split for the ratios on an AW, but would suck with an FM. With the close 3 ratios at the top, it makes sense to just use the 2nd cog to extend the range, not try for 1/2 step.

I was using 18/24 on my FM with a Huret Svelto. http://bikesmithdesign.com/MyBikes/rra/18-24-svelto.jpg When I figured out that I should have a 21 as my main cog I decided that a 2nd cog big enough to be useful might overload the fragile looking low gear. If the range is sufficient for your terrain, the four ratios of an FM with one cog are sweetly spaced.

16/22 would give you a nice spread. 16 22 --------- 84 61 H 74 54 N 64 46 L 49 36 B

BTW If you have an alloy shell you don't want very low gears on an FM unless you are a lightweight or a bit underpowered. The low gear pawls work in slots cut into the shell, unlike the AW/FW/S5 where are ramps in a continuous band. http://bikesmithdesign.com/SA/fm-fw-left-ball-cup.jpg A steel shell adequately supports these (for lack of a better word) crenellations. The old, thin, alloy shells flex enough that the pawls can bust these off http://bikesmithdesign.com/SA/fm-alloy-oops.jpg

I have a spare alloy FM shell, but I'll be sneaking FW guts into it.

Another potential problem; Make sure you have enough threads engaged on the right end of the axle. The derailleur claw eliminates about 3 threads. I've seen several FM axles with the threads stripped off where someone tried to get by with 4-5 threads.

Sheldon Brown came up with a brilliant (no surprise there) solution. From a posting to GentlemanCyclist shortly before his passing.

"On my 63-speed OTB and my 54 speed homebuilt tandem, I used a countersink bit to make a conical recess into the adaptor claw. Any derailer with an adaptor claw can be made to work.

I then modified the Sturdley-Armchair right axle nut to have a conical end that fit into the recess. (This was done in high-tech fashion: I chucked the axle nut into my drill press and held a file against the corners of the nut.)"

This would give you about 4 -5 more threads of engagement compared to an unmodified claw and a thin,hardened washer.

Mark Stonich;
     BikeSmith Design & Fabrication
       5349 Elliot Ave S. - Minneapolis. MN 55417
            Ph. (612) 824-2372 http://bikesmithdesign.com
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