The swingarm is an important part on a motorcycle. Both imposing and aesthetic, it must withstand immense efforts while having strong flexion capacities to promote grip. Swingarms have undergone some major changes over the past few decades, the most notable being the move from classic to inverted designs and the change in material: from aluminum to carbon. Let's focus on a few details that are important.
The invasion of carbon swingarms has already been dealt with in one of our files. But we have also seen the position of the swingarms evolve on the MotoGP prototypes: they were located above the axis of the swingarm (classic position) and are now below this same axis (inverted position).
This design allows for a shock absorber placed lower, and therefore frees up space under the saddle to place the fuel tank. Another desired goal is to increase torsional rigidity, and the best weight/torsional rigidity ratio is obtained with a closed box of good size, located close to the axis of the swinging arm, and on which support two lateral triangles.
Finally, by concentrating the material towards the axis of the swinging arm rather than towards the axis of the wheel, this arrangement also makes it possible to minimize the impact of the swinging arm on the unsprung mass.
The Yamaha swingarm at the end of 2019
All eyes in the paddock were on Yamaha's carbon swingarm when it appeared during the second half of the 2019 season. It was the first one they used in racing and it was also mated to a new exhaust.
Carbon is an incredibly practical material for these kinds of parts. Indeed, carbon parts are manufactured by superimposing thin layers of carbon fiber. It is this layering process that makes it a practical material for these kinds of parts because it means that one can tailor the strength of the part simply by adding more layers in an area, or even by using a type of weave different carbon!
When Yamaha tried their carbon swingarm, they found that the reduced weight made the rear end more precise under acceleration and the bike felt easier when changing angles.
The Ducati swingarm at the end of 2019
Ducati was the first factory to unanimously use carbon swingarms in the modern era. They have had many versions, all with pros and cons of their own.
One detail about the Ducati swingarm is to notice how far the lowest part of the swingarm goes down. One of the major differences between conventional and inverted swingarms is that inverted swingarms make it easier to control twist because the flex line is below the pivot point of the swingarm axle.
Twisting of the swingarms is undesirable, at high angles. Indeed, if a swinging arm were to twist, it would change the angle of the rear wheel in relation to the front, which would cause problems when turning!
Thus, MotoGP turned to inverted designs as motorcycle speeds and swingarm forces increased over the years. Additionally, because they are stiffer, less material is required to achieve the same characteristics, so those that are inverted can be lighter than conventional swingarms.
Suzuki is one of two factories that has not developed a carbon swingarm
Aluminum is a heavier material than carbon and is not as strong. However, that doesn't mean Suzuki's swingarm isn't as strong as its competitors. What is certain is that the GSX-RR has incredible cornering potential. Jack Miller said frequently in 2019 that he couldn't imagine how fast and stable the Suzuki could be in corners.
With Suzuki's inline 4-cylinder engine and perhaps a more conventional swingarm than their competitors' carbon ones, it appears they have found a compromise that they are happy with for now, and which is likely to earn !
Aprilia also does not use a carbon swingarm
Aprilia is the other factory that does not use a carbon swingarm. In 2019, however, they tested several versions of their aluminum swingarm.
Finding the perfect swingarm is a tricky business, vertical force must be balanced with lateral force with flexion to reduce swingarm twist.
Vertical resistance is extremely important. The swingarm must be strong enough to withstand the incredible forces that pass through it during very hard acceleration. And we must not forget braking...
Lateral strength is crucial. The swingarm needs to be strong enough for the bike to turn at an incredibly high speed, but it also needs to allow just the right amount of lateral flex so that the swingarm can help the suspension absorb shock and the tires find adhesion. Not enough flex and there will be a lot of feedback from the bumps and the tire will work harder, causing it to overheat. Too much flex and the bike will feel spongy and unstable. The tire will also overheat.
The carbon swingarm of the Honda RC213V is fascinating
This is one that we rarely have the opportunity to observe up close because of the electric blankets that Honda uses (they are very enveloping). During 2020 pre-season testing, Ducati was experimenting with a new swingarm with a different weave. It would be interesting to see the surface of Honda's swingarm to see if there is anything really different compared to other factories.
Another detail in the Honda swingarm is the small rear brake caliper cooling duct that runs along the bottom of the swingarm. It directs air under the swingarm and directs it back to the caliper. An insignificant detail for the average person, but which allows the cooling performance of the caliper to be increased.
Here is KTM's carbon swingarm, which appeared in early 2019
Pol Espargaró immediately adopted it, saying that it favors changing angles while preserving the tires. He was so positive about it that as soon as he tried it, he had it fitted to both of his prototypes and no longer used the aluminum parts.
Some companies work on 3D printed swing arms, maybe we will see them one day in MotoGP!
