By Doug Westly | Safety Editor
IMPORTANT NOTICE: Ultimately, the safety of motorcycle riders and their passengers is their own responsibility. Nothing presented in the column supersedes, negates or relieves a motorcyclist and/or passenger from assumption of personal responsibility for their actions and safety.
Suspension
When you first started to ride, chances are you didn’t think much about how you were connected to the wheels of your motorcycle. You knew there were forks up front and a shock or shocks in the back, and that they helped smooth out the ride. Then as you became a more experienced rider, maybe you began to appreciate these bike components. However, have you ever taken the time to really consider the part your motorcycle’s suspension plays in the dynamics of riding, and what it means to your safety?
Let’s start by defining the motorcycle suspension for our discussion. Here we’re discussing the front forks, rear shocks, their components and any other bike systems that might contribute to handling the gravitational and inertial forces impacting the motorcycle dynamics as you ride. In other words, the pieces of the bike that control the forward, rearward, up and down motions of the bike while in motion.
Among wheeled vehicles, motorcycles and bicycles are unique. As multiple wheel, single track vehicles, the physics affecting the machine in motion are different from multi-track vehicles. This is primarily due to one factor…lean. At any speed greater than a walk, a motorcycle has to lean in order to turn. This is so the bike and ride can balance the forces of gravity, inertia, friction and ground support during the maneuver. You can actually calculate the amount of lean a motorcycle needs using the laws of circular motion. It goes something like this:
Lean angle = v2/gr, where v is the forward speed, r is the radius of the turn and g is the acceleration of gravity. There are various other factors, such as compensation for width of modern tires, center of gravity shift on the motorcycle during the turn, etc., but you get the picture. Lean angle is critical to a motorcycle and we can control it (hopefully) given any set of defined factors.
Next, we have to recognize that the laws of physics mean that anytime we provide a control input to the motorcycle, there will be a reaction by the machine to that input. In terms of suspension, that means that every time we accelerate, we are effectively taking the weight of the motorcycle and shifting it to the rear, placing more load on the rear shock(s). Every time we roll off the throttle and/or apply the brakes, the slowing of the motorcycle shifts the weight to the front, increasing the load on the front shocks (and telelever link on BMWs, etc).
Finally, suspension is designed by the manufacturers to absorb shock transmitted from contact between the road and tires, respond to lean angle and inertial forces, support the weight of the motorcycle on the wheels/tires, etc. Your suspension is working very hard, all the time.
OK, so how does all of this impact safety? Here are just a few thoughts:
When you corner and lean the motorcycle, the suspension is taking unequal loads on either side of the bike. This is particularly true of the front suspension, which by its nature (to include single-sided front suspension systems like the RADD(tm) suspension on Yamaha’s GTS1000, sold in 1993-1994) carries two forks, one on either side of the wheel. This means the front forks need to be in good condition and working as designed to absorb and control these unequal load forces.
When you accelerate or decelerate, the motorcycle’s weight shifts backwards or forwards, respectively. The suspension has to control these inertial weight shifts, allowing the rider to remain in control. This includes not only the initial weight shift, but the return of the weight to its static position after the maneuver.
To figure all this out, here are some terms and simple definitions that you’ve probably already heard:
Pre-load: All motorcycle suspension springs front and rear, are designed to be under compression, even when fully extended. Pre-load is the amount of spring compression once the weight of the rider(s) and any other elements are added to the bike, while at rest.
Sag: Sag is the difference in compression between the springs being totally extended (while still under compression) and supporting the weight of the bike and rider. This element may or may not be adjustable, given any particular suspension system. On adjustable systems, pre-load and sag can be set to avoid “bottoming out”, wherein the springs/shocks compress to their physical limits.
Damping: Damping is the system control for the rate at which springs/shocks compress. Again, some systems allow for external adjustment of the damping rate.
Rebound: Rebound is the rate/force with which springs extend when the weight is subsequently loaded from them.
You can start to see why good suspension systems and components are expensive. As motorcycle performance and demands increase, higher performing suspension systems are needed to keep these forces under control. At the highest performance levels (road racing, MX racing, trials riding, etc.), suspension systems are one of the most critical machine elements. The bottom line is that for our suspension system to work effectively, it must be able to cope with the forces we subject it to when we ride.
If we overload the bike, we increase the pre-load, perhaps beyond design limits, or beyond the current optimal adjustment. This decreases the amount of available spring compression and suspension travel, and can change the damping rate. We could bottom out the suspension under extremely dynamic riding conditions, transmitting excessive shock to the bike and rider, upsetting balance, traction (friction coefficients), etc. If we load the bike more heavily than usual, can we adjust the suspension settings to compensate for the additional weight? It depends on the bike and the adjustability (or not) of the system.
If we don’t keep the suspension system in good operating condition (front fork oil leaks, etc.), then we degrade the suspension performance, again potentially contributing to control failure.
If we exceed the performance capabilities of the suspension, (overloading, excessive control and/or inertial input) then the system may again not be able to effectively respond, causing control failure.
A good rider understands the critical nature of her or his motorcycle’s suspension. A very good rider will be able to sense/feel the suspension system as it works, while the bike is in motion. This is particularly critical when dynamic forces are being applied to the bike (accelerating, decelerating, turning, etc.).
If you haven’t given much thought to your bike’s suspension in the past, why not? Motorcycles are dynamic machines. Your bike responds to your input, but can only do that within limits. As the rider, you need to understand your motorcycle’s systems, capabilities and limitations. You need to keep the system in good operating condition. The more you know about your bike, the safer you (and your passenger, if you carry one) will be!
Ride Smooth, Ride Safe!
Doug Westly
