Single Sided Swingarm: Why Not?

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.


15 thoughts on “Single Sided Swingarm: Why Not?

  1. The reason many,many long travel bike builders use multiple chains is because of chain growth and pedal feedback, which when working with 7″+ of travel, can be a problem. Using an idler wheel or a setup like the one mentioned allows the builders to place the cogs in a location that keeps your feet from getting kicked back. It also allows the use of an IGH in the center of the bike, so there is less unsprung weight and no derail to rip off on the wheel.

    quick 30sec synopsis, need to clock in. Long travel DH bike suspension designs are a huge topic, and drastically different then XC suspension.

    • Yeah, I understand that. And with the proper design, if the linkage mirrors the pulley locations, you can have no change in pulley distances (or resulting chain tension) over the course of the suspension travel. Or, like Chupacabra, you can have great tire and ground clearance.

  2. Consider that despite Cannondale’s claims of performance benefits the Lefty has never really taken off. I can’t recall the last time I saw one in the wild on a ride.

    My dualsport motorcycle uses a dual sided swing arm and a two legged fork.

    If there was one area I could get improved on my FS MTB it wouldn’t be a single sided swing arm it would be keeping the same performance and dropping the price tag from $5K down to something more affordable like $3K.

    safe riding,


  3. From what I’ve read, single-sided swingarms on motorbikes are actually heavier than double-sided due to having to be stiffer – doesn’t actually reduce unsprung weight. Sometimes traditional designs are still better-suited for the job 🙂

    • I’ve not seen that…comparing two iterations of the same cycle, the single sided successor almost always has a lighter rear triangle, from what I recall of my motorcycle days.

  4. Thanks for posting up my bike.(the second one) My website is
    Its funny when you look back on your old work and think, Yeah I coulda done a little more there.

    I was in college when I designed and made that puppy. Just a prototype for proof of concept. It was never meant to race DH. It was just as they say, because I could do it.

    I’ve learned a lot since then and look forward to reviving the concept soon.

  5. This is quite an old topic, but I was looking into single sided hub assemblies, I was wondering if a bolt thru hub would take a single sided assembly (force wise), providing the axle is strong enough.

    • Ahh yes, a post from my formative years.

      Depending on the strength/size of the bolt you spec, yes, I’m sure it could. That said, I’ve come to believe (over time) that the benefits of a single-sided swing arm are greatly outweighed by the detriments (either being flexy, or being unduly heavy).

      • Great answer.

        A stock bike wheel will not do. Even a 20mm thru axle on a front hub is not enough. I had to make my own axle for the prototype showed above that I made in college because it would not have been strong enough and definitely would not have been stiff enough. I’m planning on implementing a bb shell axle perhaps for it this time to maintain stock bike components, its just a question of how to standardize a larger bore for the axle to go into the hub. What I did on my custom axle was to press out the bearings that are in the wheel hub as they are not necessary which makes the ID of the hub much larger and then you can use a larger axle.

        I don’t know where you get your empirical data from Lawfarm, but that’s pretty spurious reasoning when there arent enough single sided bike swingarms designed and made to even count on a Simpson’s hand.

        I think people don’t have the gall to do it and the big guys won’t because its not cost effective. Doesnt mean it can’t be done. If Ducati, BMW, Honda and KTM do it on their moto’s that are designed to do a lot more than a mtn bike is, I can’t see why it isn’t possible. It’s not like people don’t care how much theyre moto’s way.

        Check out my latest on my website, The Blonde Bomber on . I’m looking to collaborate on a single sided concept with some people I’m in talks with right now. We aim to find out why you should do a single sided swingarm.

        Because its super badass.

      • I agree–it could be super badass. I look forward to following your project. Didn’t mean to hate on it–wasn’t aware you were still working on it. I’ll be curious if you can maintain the same rigidity without adding weight. If you could, the other potential benefits of single-sided swing arm could be pretty fantastic.

      • I don’t promise to not add any weight hah. That is a lot to ask for, but it can be reasonable.

        Sorry to come down on you like that, but I like definitive statements.

        Those benefits are the other reason I’m still pursuing this concept. Plus you gotta have something different then anyone else.

      • I’m not looking ultimate mtb lightness, but I wanted to explore the concept as a form of bicycle-art for a city bike, I have a workshop with metal machines, and want to build a frame out of square tubes (in a 45 degree angle).
        thanks for the replies

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s