There are always options to achieve the hold-up needed and desired on your suspension, but when a rider’s preferences are thrown in the mix, deciphering the best option may not always be easy.
Let’s start with the most intriguing example: spring rate versus damping. This choice is certainly the most interesting because you are trading support from between two completely different elements. Springs are position-sensitive, only caring about its current stroke, while damping is speed-sensitive and its output is dependent on shaft velocity. The balance between these is the foundation of tuning.
Spring force is needed to make the bike sit in an optimum position, and provide clearance and travel for the wheels to move over the bumps. The damping’s primary function is to control the oscillation that the spring will create due to bumps and inertia. There are calculations that can be done to find the balance required to meet the criteria needed for setup, but riders are not robots, they are looking for the correct feel. In most situations, a rider can spring the bike heavy and valve it light to find an increase in comfort, feel in the bars and better tire-to-ground comfort. The downside to this is the oscillation that occurs when you have pushed this balance to its limit.
Your tipping point will be evident as the rider begins making comments that sound more familiar to a sailor rolling through the sea than that of a motocross racer on hard soil. Wallow needs to be controlled through an increase in damping to get the suspension back to a more predictable dynamic position. The best way to balance is through rigorous testing—that is something that REP does regularly.
Next, let’s talk about high-speed versus low-speed damping. Changes in these two areas can be very effective in certain scenarios when overall stiffness is desired, but you have to be aware if the wheels are not moving in obstacles, like the difference between a long jump face under some G-loads versus a small, sharp acceleration bump. You may feel the rear end getting deep into the loaded jump face and realize that you need more damping. But, what is the best way to accomplish this? In the following corner, you may see the rear kicking off some sharp holes out of the corner and think the shock needs to be softer. Well, in this scenario, increasing low-speed compression for the jump face and decreasing high-speed compression for the holes may fix both issues without compromising overall balance.
Knowing the difference between preload and spring rate is a valuable tool. If you were to plot the amount of force the spring creates over the travel at that specific point, that is your effective spring rate. When you increase the spring rate, the slope increases, which means the force is increasing more quickly in respect to travel. As you change the spring rate, you are altering this slope.
Preload, on the other hand, does not change the slope; instead, it shifts and evenly increases the force across the entire travel. Say you are looking for more hold-up from the top of the stroke until the bottom—well, on paper, increasing preload may be the most effective means, because you will stiffen your fork from start to finish. Or, maybe you want your fork to be less stiff at the top of the stroke but have more holdup towards the end of the stroke; increasing the spring rate and decreasing preload may be your answer. Now I say “on paper” because this is the spring’s natural response to the changes made, but the rider may perceive it differently. Many factors are at play here, so it is not uncommon to get a rider response that is completely opposed to the changes made, but that’s why constant testing is so important!
Mark Johnson was an in-house WP suspension technician at Red Bull KTM for 10 years. He has won championships in Supercross, motocross and off-road racing. Now, Mark’s company, Research Engineering Performance or “REP,” is partnered with AEO Powersports.