Motorcycle suspension has a variety of roles to play. While rider comfort is one of the roles, the most important role is actually keeping the tyres in contact with the ground surface. As the ground surface is rarely perfectly smooth, this means wheels need to travel “up and down” relative to the rest of the motorcycle to stay in contact with the surface. This comes back to the laws of inertia. The bike with its mass and velocity will not “track” to the bumps and so the suspension alters the distance between the road and the bike’s sprung mass.
After my own spectacularly underwhelming career as a motorcycle racer, my interest in motor-sport (and in particular motorcycle road-racing) has remained. Club level racing is a great way of witnessing the enormous difference in talent from the A-graders down to the “also rans”. I have attended club-days as a spectator and they provide an insight that watching top-level competition does not provide.
One such revelation was the difference between suspension that has been set up well, and suspension that has not. The event that stands out in my mind was at Victoria’s Broadford circuit. At the end of the front straight was a particularly bumpy entry into turn one. The riders would be braking heavily for Turn one, across a series of “ripples” that had formed in the track. Watching the bikes at either end of the field (i.e. the fastest riders and the slowest riders) revealed vastly different “behaviour” from the bikes: The fastest bikes were smooth and composed across the bumps. The slowest, jumped and skipped across the bumps. If you watched the two and were asked which ones looked faster, you would have probably guessed the wrong way around. Put simply, the slowest riders were trying far harder to control their bikes, whilst the fastest riders didn’t look to be expending hardly any effort at all!
There is obviously more to being good at motorcycle racing than having decent suspension, but the difference was striking. Bad suspension fails to keep in contact with the road surface. A wheel in the air, has no grip. Good suspension also relays the “feel” of the road surface through to the rider. The feedback given allows a rider the ability to judge the grip levels available.
Motorcycle suspension has an extra limitation applied to it, that cars do not have: Motorcycles lean. If you hit a 25mm high bump (approximately one inch) when travelling in a straight line, the suspension needs to compress 25mm to avoid the bump being transferred through to the rest of the motorcycle. Once the motorcycle leans over, compressing the suspension changes both the vertical distance between bike and ground, as well as the horizontal distance.
By recalling year nine trigonometry, we see that if you hit a 25mm bump at a 45 degree lean angle, the suspension now has to travel 29.4 mm. Increase the lean angle to 50 degrees (from vertical), and the distance the suspension needs to travel increases to 33.6 mm. (And you used to complain that you’d never use the stuff you learnt in maths in real life!)
On a lot of roads, the notion of a bump only being 25mm high is laughable. In the real world, motorcycle suspension does not prevent some of the bump being transferred to the rest of the motorcycle. Some transference is necessary or else the rider cannot feel what the bike is doing. Bump (or “shock”) absorption occurs not only through the motorcycle suspension, but through the flexing of the tyres and indeed the motorcycle chassis. If I recall correctly, in the early 90s, a lot of race bike manufacturers would make their frames with as little “flex” as possible in an attempt to improve the handling of their machines. They reasoned that such an approach worked for car racing. However, they found that some chassis flex (in certain directions) actually improved the handling of their bikes.
In terms of components used for motorcycle suspension there are some fairly common approaches and a few that are less common. Most modern sports-bikes consist of two front telescopic forks with internal springs controlling the front wheel and a single rear shock absorber with an external spring, controlling the rear wheel. This is a fairly-standard evolutionary design, but by no means the only one. Like most aspects of motorcycle design, this system is a compromise and alternative systems offer alternative advantages and disadvantages. But that is a story for another time.