MXA TECH SPEC: WHAT MAKES UNSPRUNG WEIGHT SO IMPORTANT?

There are three kinds of weight on a motocross bike: sprung, unsprung and hybrid.

Sprung weight: All the parts that are suspended above the suspension’s springs are considered “sprung weight.” That includes the frame, engine, plastic, controls, saddle, upper fork tubes and shock body. It’s called sprung weight because springs are used to hold it up and to isolate it from the bumps that the wheels hit.

Unsprung weight: Unsprung weight is the mass that hangs below the springs (fork legs, hubs, brake calipers, rotors, spokes, rims, swingarm, linkage, tires, tubes and rim locks). Unsprung weight is more closely connected to the ground than the parts above it, because when you hit a bump, unsprung components compress into the chassis to absorb the impact (and, of course, rebound back after the impact).

Hybrid weight: One of the conundrums of calculating sprung and unsprung weight is the question of whether the shock and fork springs are one or the other. Here is the answer. While the chassis floats on the shock and fork springs as if the coils were part of the sprung mechanism, the wheels push into the springs as if they were part of the unsprung weight. Thus, the weight of the springs is split 50/50 between being sprung and unsprung. Thus, the weight of the springs is different from sprung and unsprung weight‚ and is referred to as “hybrid weight.”

Your bike’s suspension is constantly moving. If it’s not compressing, it’s rebounding. At the top and bottom of the suspension’s stroke‚ whether fully bottomed, topped out or somewhere between, the wheels must change direction as quickly as possible. The less weight hanging from the suspension components, the easier it is for the mass of the wheels to stop and accelerate in the opposite direction. It’s no secret that the quicker the wheel reacts, the better the suspension will be‚ because the next bump is coming faster than the mechanism can respond to.

The less unsprung weight, the less strain will be placed on the pivots, springs and dampers. The lower the unsprung weight, the quicker the suspension reacts to bumps. The improvement is most significant over repetitious, high-frequency bumps, like braking and acceleration bumps. Why? The more weight you load on the wheels, the more the shock absorber has to struggle to bring the mass to a stop. As the damper struggles with the weight, the suspension packs, the wheels kick over the bumps and the bike becomes a handful.

Lighter tires, tubes, rotors, hubs, spokes, rims, wheel spacers, axles, axle nuts and sprockets also reduce the gyroscopic effect of the spinning wheels. With less gyro, the bike steers lighter, handles better and accelerates harder. Have you ever wondered why the shock body and fork stanchions are mounted on the top of the units instead of on the bottom like they were back in the early 1970s? The designers turn the shock and forks upside down to change the weight of the heaviest parts of the suspension into sprung weight. On the shock, this leaves only the shock shaft and spring collar below the spring. As for the forks, upside-down forks move the cast sliders away from the wheel.

What can you do at home to reduced unsprung weight? Start paying attention to how much unsprung parts weigh. Keep the big picture in mind. An ounce here and an ounce there can eventually add up to a 1-pound reduction in unsprung weight. When you put your bike on a weight-savings plan, it is important that you remember the 6-to-1 rule. It is said that a 1-pound reduction in unsprung weight is equal to a 6-pound reduction in sprung weight. Thus, if you’re going to spend money on lightweight hardware, concentrate on unsprung weight.