Why no single sided swing arms in the bicycle world? They’ve been adopted by a fair number of manufacturers in the high-performance motorcycle world–certainly not uniformly, but a fair number nonetheless. The advantages proposed there include reducing unsprung weight, giving easier access to the rear tire, reducing total bike weight, and a host of related benefits. In the mountain bike world, we have the Lefty in front, but why no single sided swing arms in back?
I used my Google-fu to do some searching…there are a couple TT bike designs that use single sided chain stays to minimize drag (including at least one current bike). In the mountain bike world, though, there’s not much.
There’s this bike:
Which is a single-sided, downhill oriented prototype built years ago by Millyard racing. A few details are available here. But there hasn’t been any further development of that bike since 2008. Incidentally, it was an offspring of this bike:
That bike, as you might note, has a double-sided swing arm, with one side housing a completely enclosed chain.
There is also this bike:
Which was developed as a research project as more fully described on the builder’s website here, and as briefly discussed on MTBR here. That bike is from 2009, and again apparently fell off the face of the earth. In looking at that design, it is intriguing, but also a bit scary.
I’m not saying I could do better…but I am saying I wouldn’t ride a bike with a single sided swing arm (SSS) that was that weakly built…with a mitered butt weld right at the edge of the tire.
The rear hub isn’t confidence inspiring, either.
It is an interesting design. Because of the linkage he used, he had to use a jackshaft and twin chain design. That does improve ground clearance and eliminate a tensioner, at the expense of greater complexity.
Why am I pondering this? Because this is a bike blog. Those who can, do. Those who cannot, blog.
What could be the advantages of a SSS? Well, if you put the SSS on the non-drive side, you’d have no chainslap issues, no chainsuck, amazing mud and debris clearance, ease of using whatever form of drive you wanted (belt/chain). You would have to use internal geared hubs to have any rear shifting ability. Assuming that you worked out the tensioning issue (maybe an overbuilt version of the Salsa Alternator, with quick release for adjustability), it would be really easy to pop the chain off and pop the wheel off for repair or replacement. It would also be really easy to swap out rear cogs.
In a more advanced setting, if you had a bike with a single sided fork up front and an SSS in the rear, you could have wheels that were built around hollow hubs that were splined. The front ‘outer’ wheel hub would fit onto an inner hub that would spin freely, and the rear ‘outer’ hub would fit onto an wheel hub that was set up with a traditional pawl setup for power in one direction and free spinning in the other. You could have the front and rear be totally interchangeable then–simply release whatever mechanism holds the outer hub to the inner hub, slide the outer hub off the inner hub’s splines, and voila. If you were doing endurance racing around a short-ish track, you could easily stock a spare wheel/tire/outer hub, and swap them out quickly if needed.
I think what is potentially more interesting about this idea (the splined inner/outer hub) is that you could have the brake disc mounted to the inner hub…so it would stay on the bike if you pulled the wheel. That means you could have a wheel release that is completely lateral–no need for dropouts to allow you to slide the brake disc out of the caliper, no need to worry about caliper clearance/alignment with wheel removal.
If you built a SSS the other way, and put the SSS on the drive-side, you could run any traditional drivetrain, and swapping out or removing the rear wheel wouldn’t require chain removal at all. The brake disc and the cassette could either be affixed to a splined inner hub (as described above), or could be set up to simply interface into the side of a one-piece hub with interlocking teeth–nothing too complicated.
Thinking about that concept, from a fatbike perspective, if you can live with either an offset rear wheel (a la Pugsley or Moonlander) or a dished rear wheel, you could run tires as wide as you wanted. If you’re building a FS fatbike, you could design the swing arm with spacers that could be inserted to space the swing arm over for fat tire clearance, or removed to bring the swing arm in for regular MTB tire clearance, and have a multi-purpose bike. If you wanted the ultimate in mud/sand/snow drivetrain, you could do an enclosed shaft-drive SSS (or single sided hardtail).
One of the reasons it hasn’t caught on hugely in the motorcycle world is because, in the world of high-performance bikes, there is a ton of strain put on the rear wheel. Obviously, for mountain bikes powered by us mere mortals, those strains are greatly reduced. There would be some added complexities, but I’m curious why it seems this concept hasn’t been explored more.
It’s just food for thought on a Friday.