Rotary Hub

Over the last 18 months ​I’ve been running a couple of modified Sturmey Archer three speed hubs – one on my mountain bike and another on my commuter. As I’ve explained elsewhere, I initially considered two different Sturmey Archer hubs as a starting point – the S-RK3 and the RS-RK3. I plumbed for the former given the wider range of shifter options and the simpler clutch mechanism, but it is this latter aspect that has given me some cause for concern. The clutch mechanism is operated by what’s known as a toggle-chain that enters the hub through its hollow axle, which includes cut-outs that allow the axle key to slide backwards and forwards. So despite the addition of sealed cartridge bearings in place of the standard cup and cone bearings, the hub still remains vulnerable to the elements through this hollow axle (even if it is a relatively small hole that is mostly filled with the toggle-chain).
 
My modified hub on the mountain bike, following well over a year of riding (and hosing down), was showing signs of water ingress that I discovered when I decided to strip it down to see how things were going. Nothing too serious, but the grease used inside the hub is thin (almost like a thick oil, known as a semi-fluid grease) and is relatively easily washed away from surfaces. This had resulted in some small patches of corrosion within the hub, even if functionally it was fine. Still, definitely room for improvement.
 
The alternative RS-RK3 hub’s clutch mechanism replaces the toggle chain and hollow axle with a cam that is operated by a rotary mechanism (externally similar to those used on Shimano Alfine hubs for example). This approach has a couple of benefits. Firstly, all of the hubs moving parts are placed between the two dropouts (unlike hubs with a toggle chain, where this chain sits outside of the drive side drop-out), better protecting it from damage. Secondly, the hub has the potential to be far better sealed simply due to having a solid rather than hollow axle.
 
So far, so good. However, as well as being a bit more complex, the rotary hubs have been pretty difficult things to get hold of (at least for me living in the U.K.). The usual outlets don’t list them, with only certain versions of the five speed hub seemingly being on offer. So I rang the UK distributor who told me that the European importer (based in the Netherlands) do keep a stock of rotary three speed hubs, but only the RS-RF3. That small change in nomenclature means this particular hub doesn’t come with a disc rotor mount. This was a problem because, as far as I’ve been concerned, all other forms of brake have been obsolete to me for quite some time for the type of riding that I do.
 
One of the big benefits of Sturmey Archer hubs (certainly when compared with other hub gears made by companies also beginning with an S) is the availability and inter-compatibility of spare parts. I happened to have an old, unmodified S-RK3 (toggle, disc) lying around and wondered whether the hub shell (including disc rotor mount) and planet cage assembly from this could replace those on a RS-RF3 (rotary, non-disc) to create the hub that I was after. I took the chance and ordered the rotary hub.
 
(One thing to note is that, as standard, the shifter that the RS-RF3 comes with isn't the thumb shifter that I was after. However, I was allowed to swap this at no cost when I purchased it.)
 
With both hubs stripped down and cleaned up I set to work making my mongrel. My guesswork paid off and the parts from the two hubs played together just fine. The biggest problem has been the Over Locknut Dimension (O.L.D. or the width of the hub between the dropouts) and the axle length. As standard the rotary hub has an O.L.D. of 129mm rather than the 135mm that I required for my frame (and which the toggle chain hubs come with). Sticking the wider hub shell from the toggle chain hub on to the rotary hub takes this up to 137mm. Initially I wasn’t too fussed by this as most frames can flex a little. However, when I added the disc rotor it became clear that the spacing of this was now sufficiently out to prevent it aligning with the caliper.
 
The centreline of the hub can’t be adjusted like it can with the toggle chain hubs by simply adding / removing locknuts on either end of the axle due to the fixed requirements of the rotary mechanism at the drive-side end of the hub. Instead, I needed to add around an extra 4mm to the O.L.D. to get the brake to work and achieved this by placing the non-drive-side, non-turn washer on the inside, rather than the outside of the dropout. Unlike simply adding extra washers, this had the added benefit of reducing the need to accommodate more parts on an axle designed for a shorter O.L.D. So the brake rotor was now in the right place, but I’ve got a less-than-ideal 141mm O.L.D.
 
With everything in place I built the hub into a wheel and coaxed it into the dropouts. It fits, but requires a little persuasion. Initially I’m sticking with the original cup and cone bearings to check that the hub works and to identify any teething problems (such as the aforementioned brake rotor alignment). If everything continues to work well then I’ll hopefully be getting the hub parts machined to accept cartridge bearings early in the New Year. Longer term I’d like to try again at getting hold of an RS-RK3 in the U.K. as my starting point to avoid the issues described above, but the current hub should still prove a useful stepping stone at determining whether the four sets of loose bearings (up from three in the toggle-chain hubs) can be replaced with cartridge bearings.
 
Hopefully I’ll be back with an update in the not-too-distant future.