Understanding Unsprung Weight & Hardtail Effect

For anyone interested in how unsprung weight affects a bike here's part of a conversation I read...........

QUESTION
Dear Fred,
I hear people talking about unsprung weight and how important it is on a motorcycle, but I do not know what it is. How is it measured? What are its benefits of unsprung weight? Can a local guy like me feel the difference? If so, what can I do to get more of it?

ANSWER

First and foremost, you don’t want more of it. Here is a quick test that you can do in the comfort of your living room to see how unsprung weight affects a motorcycle. 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 bird with weight on each arm. If you are human (as opposed to avian), your arms moved slower and responded sluggishly when changing direction while holding the weights. 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 your arms are your bike’s swingarm. The 10-pound weight mimics the mass of your bike’s wheel. 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. There are three kinds of weight on a 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, and rider. 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 toward 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 — otherwise 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.

Have you ever wondered why the shock body and fork stanchions are mounted on top of new, high performance units, instead of on the bottom like they were back in the 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 stanchions away from the wheel.

Factory racing 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.

HARDTAIL
In light of the facts stated above, anyone considering making their bike a hardtail should consider this - since the rear of hardtail bike has no rear shocks. springs, or other suspension parts that would provide sprung weight, all the weight on the rear wheel is undesirable unsprung weight. That is why hardtail bikes do not handle bumps, braking, etc. as well as bikes that have a sprung rear suspension. Ed

And the price your spine pays for looking 'cool'.


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