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DEALING WITH THE HIDDEN EVILS OF UNSPRUNG WEIGHT

August 26, 2013
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Stick your arms straight out. Rapidly lower them to your sides and back up again. Do it ten times. Do it fast. Now grab a 10-pound weight in each hand. Repeat. Feel the difference?

Guess what? It takes serious effort to imitate the up-and-down motion of a hawk with weight on each arm. If you are human (as opposed to avian), your arms moved slower and responded sluggishly when changing direction. Additionally, the strain of holding the weights could be felt in your shoulders. There is no doubt that you could move your arms quicker and with less stress if you  didn’t have any extra weight attached to them.

Now, imagine that your arms are your bike’s swingarm. The 10-pound weight mimics the mass of your bike’s wheels. Your arm is the swingarm, your shoulder socket is the swingarm pivot and your muscles are the shock absorbers. The increased muscle strain and sluggish reaction time that your body felt can be extrapolated over to the mechanical parts of your bike’s suspension. Extra weight on moving parts is a big no-no.

Extra weight that isn’t supported by the springs is called unsprung weight. Like ying, yang and boing, there are three kinds of weight on a motocross bike: sprung, unsprung and hybrid.

Sprung weight: The complete chassis, including all the parts that are suspended above the suspension’s springs, is 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.

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. The answer is complicated, but it does have a definitive 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 sprung and unsprung weight. Thus, the weight of the springs is different from sprung and unsprung weight; it is referred  to as “hybrid weight.”

The discussion of whether the springs are sprung or unsprung leads us to question the status of the fork and shock fluids. Since the shock fluid is housed in the body of the shock absorber (and the shock body is bolted to the frame), it is sprung weight. It’s not as clear-cut with the front fork oil. More oil is used in the fork than the shock, and most of it resides down in the bottom of the fork legs. This fluid moves with the unsprung fork legs, brake parts, axle and front wheel?thus it is unsprung. The saving grace of fork oil is that it gets splashed around in the upper tube assemblies, lessening the amount that is actually unsprung. It’s best to presume that one-half of the fork oil is hybrid weight.

While on the subject of hybrid weight, the weight of the front brake hose is split between sprung and unsprung weight. Most of the rear hose is unsprung weight.

NEGATIVE EFFECTS OF WEIGHT BELOW THE SPRINGS

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 in 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.

It goes without saying that 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.

Thankfully, unsprung weight doesn’t play as large of a role on high-amplitude impacts like spaced-out whoops and landings from jumps, because those are singular motions (not repetitive).

Lighter tires, tubes, rotors, hubs, spokes, rims and sprockets reduce the gyroscopic effect of the spinning wheels. With less gyro, the bike steers lighter, handles better and accelerates harder. This is something that even the most oblivious test rider can feel.

Have you ever wondered why the shock body and fork stanchions are mounted on 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 remove the cast sliders away from the wheel.

If you are interested in reducing unsprung weight, you should know that some kinds of unsprung weight are worse than others. For example, the further away from the swingarm pivot the weight is located, the faster and farther it moves and the quicker it’s forced to change direction. You need to concentrate on lessening unsprung weight that is at the furthest end of the fulcrum arm. Conversely, the closer the weight is to the swingarm pivot, the less it affects suspension performance.

With this information, you can see why the rear wheel, rear tire, rear axle and rear brake have the greatest impact on unsprung weight. Although the front and rear suspension are functionally different, the negative effects of unsprung weight are equally debilitating.

The manufacturers do all they can to lessen unsprung weight?within reason and budgets. Thus you see 32-spoke rear wheels (instead of the traditional 36 spokes), minimalist hubs (so small that it is hard for aftermarket hubs to beat their weight), swiss-cheese brake rotors (mated to miniature brake pads), and one-piece cast swingarms (replacing welded together designs). But, in truth, unsprung weight isn’t as much of a concern when the bike rolls off the showroom floor?because the bike is what it is. In this era of incredibly fat machines like the KX450F, 450SXF and RM-Z450, most of the burden of reducing unsprung weight is passed down to the owner.

In the name of perceived performance, many riders start adding aftermarket stuff on their bikes in the places where it hurts most. A beefier chain guide is unsprung. Disc guards are unsprung. Heavier aftermarket wheels are unsprung. Heavy-duty tubes are unsprung. Steel/aluminum combo sprockets are unsprung. Some of these parts are necessary to address issues of concern in durability or reliability, but the weight of the parts that attach below the springs needs to be carefully calculated.

Factory teams spend beaucoup bucks to reduce unsprung weight. Most works bikes have magnesium hubs, titanium linkage bolts, Ti axles, aluminum spoke nipples, tapered  and butted spokes, and magnesium (or beryllium) brake calipers. They would have coated aluminum rotors if they were AMA legal.

WHAT CAN YOU DO AT HOME?

So what can you do at home? Start paying attention to how much unsprung parts weigh. Compare everything you replace with the weight of the stock parts. Keep the big picture in mind. An ounce here and an ounce there can eventually add up to a one-pound reduction in unsprung weight. Here is a quick hit list:

Tires: Start with your tire choices. Unsprung weight is most easily influenced by tire weight. Tires vary in weight by phenomenal amounts. Competing tires can have as much as a one-pound difference in weight (and that is a major difference when it starts moving up and down over every ripple in the dirt).

Tubes: It is possible to take ounces off the unsprung weight by using lightweight tubes. While heavy riders and rocky tracks might require a heavy tube, many riders can get away with thinner tubes. There are also natural rubber tubes from Moose and Michelin that don’t necessarily weigh a whole lot more than stock, but offer more flat protection.

Sprockets: If you’re not replacing it with the stock part or stock size, compare the weight of the replacement part directly against the OEM sprocket.

Chain: Not only are chains heavy, but they are also the worst kind of rotating mass.

Rim locks:
If your bike runs two rim locks in the back, lose one and save several ounces.

Spoke nipples: If your bike doesn’t have aluminum spoke nipples, consider switching to them.

Brake rotors: Aftermarket brake rotors come in lots of shapes, sizes and weights. Go for the lightest one.

Titanium: It’s possible to cut 2 to 3 pounds of unsprung weight (and spend upward of $2000) by using titanium axles and linkage bolts. Most MXA test riders are wary of titanium front axles. We don’t think that they are as good at resisting flex as steel front axles. Titanium rear axles are a very good idea.

Shock springs: One of the most expensive Ti components is the shock spring. It’s an easy way to cut almost 2 pounds off the suspension. Keep in mind that half of that savings is split with the sprung chassis weight. Additional advantages of a titanium spring are quicker reaction time and less vibration.

Chain guides: A carbon chain guide assembly can save up to a 1/4 pound of unsprung weight. Since it’s positioned farther outboard from the pivot, a carbon guide can benefit suspension action more than a Ti shock spring and linkage bolts combined.

When you put your bike on a weight-reduction 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.

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