THE DIFFERENCE OF SPRUNG Vs. UNSPRUNG WEIGHT
OpenRoad
Sprung Vs. Unsprung Weight
I was RECENTLY asked to explain the difference between sprung and unsprung weight. If we’ve been deluding ourselves thinking everyone had a clear idea what those terms meant, let me try to clear up some of the confusion.
In simplified form, what we call unsprung weight are the wheels and everything directly connected to them— moving in response to the road—and sprung weight is made of the components attached to the chassis, including the rider.
Without suspension, our motorcycles would leave the ground over rough roads, making control difficult and comfort impossible. But even with suspension, the unsprung weight in the wheels and their attachments still wants to leave the ground and bounce down rough roads.
We have two ways to control this force. One is suspension travel, and the greater the travel provided (given equally effective damping), the softer the springs we can use to control this bouncing mass— improving comfort. Two, we must also make the unsprung weight as light as possible in relation to the sprung weight so that the chassis’ inertia resists being disturbed and works to keep the wheels in contact with the road—improving control.
Notice that it’s the intended use that determines the appropriate suspension design. Dragstrips are smooth enough that suspension is barely necessary, while offroad use is the most demanding scenario. Everything else falls between these two extremes. Some cruisers can give an acceptable ride quality with little more than 2" of travel, while less than a foot might be inadequate on a supercross track.
Because the ratio of sprung-to-unsprung weight is critical, the weight of the whole motorcycle becomes significant. If a cruiser weighs 700 lbs. and has 80 lbs. of unsprung weight (entirely possible), this ratio is 80/620 or 1:7.75. But to achieve this same ratio on a dirt bike that weighs just 250 lbs., the unsprung weight must be no more than 32.26 lbs.—almost impossible. Thus, in addition to longtravel suspension, we see aluminum axles and almost dainty ultralight brake calipers and discs fitted to high-end dirtbikes.
And while the concept of reducing a motorcycle’s weight to improve its performance has been around forever, notice that where the weight is removed makes a big difference. For instance, if we remove 20 lbs. of unsprung weight from our hypothetical 700-lb. cruiser (perhaps with carbon fiber wheels), its ratio would be 60/620 or 1:10.3, a huge improvement. But if we were to take the 20 lbs. from the sprung weight, the ratio would be 80/600, or 1:7.5, slightly worse than stock. And although a bike’s theoretical performance is based on its power-to-weight ratio (the same in both examples), in fact, the necessary traction to use all that performance may depend on how well the tire is able to maintain contact with the road, in which case the better sprung/unsprung ratio will also have a performance advantage.
It’s easy to see most of the components that make up unsprung weight: the tire/ wheel, axle, brake components, fender, cables and hoses, etc. But, a portion of the suspension system itself can also be considered unsprung weight. For instance, we want the heavier end of the shock absorber(s) attached to the chassis, and if any springs are progressively wound or have a variable rate, we want the most closely spaced coils at the top. Because these coils will bind first in compression, we want them positioned where they become sprung weight at that point.
Rear unsprung weight will usually be much greater than at the front. Although the brake disc and its caliper are typically smaller and lighter, and a rear fender may not be attached to the swingarm, the wider rear wheel and tire are much heavier, and in addition you’ll have a final drive system. If it’s a chain, it will also need a shock damper in the rear hub (rubber damping blocks and vanes), while a belt drive usually won’t have a shock damper as the belt itself is slightly elastic, so a belt drive can be lighter than a chain drive. But, sadly for those who favor its clean appearance, shaft drive systems significantly increase rear unsprung weight, as the bevel or hypoid gears are heavy steel, need a steel driveshaft, additional bearings and are enclosed in a gear case with lubricating oil inside.
The swingarm is another special case. Clearly the axle end should be considered unsprung and the chassis pivot end sprung, but where do you draw the line? Physics tells us that it’s the swingarm’s mean pivot radius or half its length, not at its center of gravity.
While manufacturers typically go to great pains to minimize unsprung weight, you will still see us complain about it on occasion. There are two common situations: One is when a cruiser maker designs its wheels purely for style, without concern for their weight. Combine very heavy wheels with even slightly rough roads and a tremendous hammering can pass through stylishly short-travel suspension to beat up the rider and passenger. The other is when large adventure bikes with shaft drive are used off-road. Although their suspensions may have sufficient travel to handle sizeable bumps, the rear suspension still may not be able to follow washboard surfaces. Because the rear swingarm can be considered a kind of pendulum (imagine hanging the bike from the front axle with the rear suspension disconnected to visualize this) it has a natural frequency, and when cyclical bumps excite this frequency, the suspension is overwhelmed. The resulting hammering sensation is what we call axle tramp. The heavier this unsprung weight, the more violent the tramping. To deal with it, we’ve found that riding the rear brake slightly while accelerating over such surfaces can help to keep the rear end under better control, but it can’t eliminate the underlying physics.
Popular OEM techniques to reduce unsprung weight include larger diameter hollow front axles, brake discs mounted to the middle of cast wheel spokes, eliminating the weight of separate disc carriers and allowing a lighter hub structure, reducing the number of mounting pins on brake carriers, and reducing the thickness of wheel rims, discs and even tires. Lighter wheels also reduce gyroscopic inertia, a very important subject in itself, but too complicated to include here....