There are many aspects that affect the selection of suspension used by GP teams: track conditions, tyres used, pilot feedback, and engineering calculations. Though the suspension may seem a simple component at first it is in fact particularly complex. The front fork alone is made up of over 300 parts. In MotoGP the suspension is supplied by Öhlins, a company whose production is based entirely on these components.
The four most important terms when referring to suspension are: Springs, preload, compression, and extension or rebound. A perfect equilibrium is necessary between all these concepts and elements for the motorbike to behave as we want, adapting to needs of the rider.
Springs are components that support the suspended weight of the body. They need a certain force to be compressed which depends on its stiffness. So, the selection of a specific spring will depend on what the rider needs for each track. Springs are categorised by the force needed for its compression which is measured in Newtons/millimetre. Typical MotoGP springs are in the range of 8 to 12 N/mm.
Stiffer springs offer higher support when braking and compress more slowly. The trade-off is that they make the motorbike more critical during leaning and there is higher tyre wear.
On the other hand, softer springs offer lower stability during braking and the spring can reach full compression in hard brakingwith the dangers this entails. Nevertheless, the motorbike will be easier to control when leaning and soft brakes are usually preferred when it rains on the circuit.
To carry out the spring adjustment, it is enough to simply open the front fork lids to take out the component and insert a different one. The operation takes under two minutes and allows the insertion of springs with different stiffness's in each tube to achieve a more precise adjustment. In the case of the rear shock absorber, the process is somewhat longer and it's usually necessary to have a complete module ready to install.
We call preload the compression exerted on the spring while the assembly is at rest. In other words, it's compressed slightly to avoid it reaching its full extension. By increasing the preload, the system will require more force to compress, as such, the suspension will take longer to sink. We can also vary the preload to modify the height of the vehicle by changing the distance covered by the shock-absorber, which for MotoGP is 130 mm that not particularly ample.
The preload adjustment varies between certain limits, like any other element, so, if we're going to exceed them, it's better to change the springs to stiffer or softer ones and aim for higher precision.
During the GP we can see technicians carrying out this adjustment in the box, using spanners in the top part of the fork and turning one way or another depending whether they want to increase or decrease the preload.
Compression and extension
Once we have the springs we want and the preload adjusted to our needs, we can go ahead with the compression and extension adjustments. The suspension reacts by compressing when we come across a bump, in the fork when braking, or in the rear shock-absorber when accelerating. Once the suspension is compressed, the spring will always tend to recover its original position starting the process of extension or rebound. Depending on the adjustment we're using, the extension will be faster or slower.
The shock-absorber has an internal fluid that goes through some valves when the assembly compresses. By means of these valves we can determine the compression and extension adjustment, if we close them somewhat the fluid will pass more slowly, thus slowing the process. If we open them, we achieve the opposite result. The adjustment is so precise that different speeds can be established for compression and extension.
The suitable speed must permit the tyre to be in constant contact with the floor, maximising the adherence of the rubber. It also has an impact when it comes to accelerating, braking, and changing the support of the motorbike. As such, a correct cycle of compression and extension is essential.
To the rider's taste
The most important thing when taking decisions with respect to suspension is that it adapts to the needs of the rider. There are no calculated criteria of effectiveness, and two riders can, on the same motorbike, get the same times with different suspension configurations. Everything depends on the style of driving, skidding more or less, different sensations during braking...