Swingarms and (bumpy) roundabouts

by Simon Hargreaves

Ducati’s Panigale 1199 has a single-sided swingarm. The new 899 has a double sided swingarm. Now that’s a good idea...

It’s intuitive to grip a wheel at the end of each axle to give yourself maximum control of stability and alignment (or, if we’re talking about front wheels, steering). It’s certainly easier – it’s more effort to play wheelbarrows if you only using one arm to hold the barrow wheel. But it also looks cool if you wave with your free hand.

And so it is with swingarms: aesthetics and strength ought to be the prime considerations between single-sided or forked.

Swingarms have many forces to accommodate, but not as many matter as you’d think. An engine tries to bend it using a chain, and bumps in the road try to deflect it. But these either push or pull the swingarm, which make it the suspension’s problem, or try to compress or extend the swingarm along its length – which it’d be good at resisting even it was made of wood.

The forces that mess with a swingarm’s integrity are bending and twisting torques, which either bend the swingarm sideways or act unequally across its width. For example a large wheel, wide tyre, sprocket and brake disc spinning at speed and then turned in one direction sets up an equally large force at 90° to the direction of turn: this is the famous bicycle wheel experiment, where you hold a spinning bike wheel in front of you by its axle, then steer it to the right. You’ll find it also leans to the right, pulling your right arm down and your left arm up, twisting your ‘swingarm’. Furthermore, bumps acting off-centre of a wide tyre going round a corner will try and deflect a swingarm to the side.

Bending and twisting forces generally bend and twist the thing they act on. As it’s important for the rear wheel to point in the same direction as the rest of the bike, it shouldn’t deflect at all (unless you get into the realms of racing, where chassis engineers design swingarms with flex characteristics – strength in some areas, less in others to give riders the ‘feel’ they crave on the limit).

As bike cornering performance increases – faster, steering more quickly, wider tyres – more demands are made on swingarm design and materials. Which is why they’re evolved from steel tubes to box-section steel to box section alloy, then braced alloy and all manner of extrusions, forged and cast sections, banana-shaped, gull-arms etc. And, of course, single-sided swingams.

But a single-sided swingarm has no relative strength advantage. To keep the wheel aligned it has to be more than twice as strong to resist the same forces (because they’re now off-balance). So it’ll either be thicker or require a greater area of material, or be heavily braced internally.

Then it gets heavy, which is why MotoGP bikes don’t use single-sided swingarms. Endurance racers used to because it was easier to swap wheels mid-race. But when it takes around 15 seconds to swap a wheel in a forked swingarm, it’d need a very short race for any disadvantage to be overcome. And short races don’t have pitstops.

Which leaves us with aesthetics, cost and convenience. Single-sided swingarms once looked cool but they’re a bit passé now. And they cost more to manufacture, because there’s more to them. Not to mention single-sided paddock stands are more expensive than conventional stands.

Which is probably what Ducati’s decision to use a forked swingarm is really about: it’s cheaper.