Sturmey Archer over locknut spacing
So this has been puzzling me and I was wondering if anyone here has any arcane knowledge on why SA ends up with seemingly odd-ball numbers for the over locknut dimensions for their hubs.
3 speed alloy hub, S-RF3:
175mm axle = 127mm O.L.D.
163mm axle = 117mm O.L.D.
148mm axle = 108mm O.L.D.
At first I thought they trying to provide 2-3 mm of space so you can add a chain tensioner or somesuch. But every chain tensioner I know of (with the exception of the MKS tensioners) go on the outside of the dropouts/track ends, so that wouldn't make any sense.
Then there's their 5 speed hub. S-RF5:
175mm axle = 119 O.L.D.
148mm axle = 111 O.L.D.
This contradicts the dimensions from their 3-speed hub. I know we're just talking about 1 to 3mm, but if you're designing a hub to fit in a 120mm dropout spacing, for example, why not make the O.L.D. 120mm? What am I missing?
Is it possible that SA is trying to accommodate dropouts made from material of different widths? As you probably know, dropout widths vary considerably, with some dropouts on cheap bikes almost about as thin as common sheet metal. There used to be anti-rotation washers of different widths, not sure if there still are.
Last edited by wim; 02-01-2013 at 08:05 AM.
Back in the day you used to be able to put a freewheel on an SA hub - there were 3 & 4 spd units and of course that would require a wider OLD. We even kludged a 5 spd freewheel but there were only a few threads showing on the axle so you had to weld a bar across the front of the bolt-on derailleur hanger and use that to keep the axle from moving when you pedaled hard.
Originally Posted by Love Commander
The current differences in axle length are probably just what wim suggested - supplying axles to deal with different frame spacing.
I can't make sense of that personally. For example, 117 is no known standard that I know, but roughly equal to 120. But that 120 (or any other) is measured inside-to-inside on the dropout, or OLD on the axle, so dropout thickness has no bearing on the measurement.
Originally Posted by Kerry Irons
I believe they spec the minimum for that particular hub/axle, with the understanding that because they use threaded axles, spinning off the locknut and adding spacing washers to hit the desired frame dimension is trivial. They also give the outer dimension to answer to wim's and your thought - how much axle thread will be left after that's all accounted.
That is, I don't think the question was so much about axle length than it was the unusual OLD specs.
A good habit is as hard to break as a bad one..
That's exactly my thinking. 127 corresponds to 130, 117 to 120, and 108 to old school 110 spacing for the 3-speed (not that I've ever seen that particular spacing actually available for purchase anywhere).
Originally Posted by danl1
Just seems strange/interesting to me. I know that in the end, it's not going to have a functional impact*. But I also figured there had to be some kooky time-worn reason that SA went with those specs instead of going straight for industry standard, especially now that they've switched owners and have had a chance to standardize designs and such.
Thanks for the responses, so far.
*entertaining the idea of tossing a 3-speed on my Surly Steamroller** (120mm spacing).
**mostly because I have the itch to build a new set of wheels.
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