There is an important relationship between 3 things here.
1) Weight of axle, tires, wheels etc. - UNsprung weight
2) Weight of the rest of the vehicle - Sprung weight
3) Strength of the springs.
You have to consider the springs THREE different ways.
1) Support of the vehicle ABOVE the springs
2) Restraining factor on remaining portion of the veh BELOW the springs.
3) Downward force exerted by the spring.
It is actually the percentage of unsprung weight to total vehicle weight that is important as long as the springs are properly rated for the load. Vehicle control is at its ultimate when each wheel exerts the same amount force (weight) on the ground.
Go to each extreme. 100% sprung vs 100% unsprung. 4,000 pound vehicle equally divided on each tire (1,000 pounds per tire).
100% SPRUNG. A vehicle whose wheels, tires and axle weigh nothing. The strength of the springs is adjusted so that each spring has good flexability when 1,000 pounds is sitting on it. One wheel drops into a hole. Our goal is to keep 1,000 pounds on each wheel at its contact with the ground. On this 100% sprung vehicle what causes the wheel to go downward into the hole. Can't be the weight of the axle, because it weighs nothing. The tension on the spring which is created by the weight it is holding up forces the wheel downward. So the wheel does not DROP into the hole, it is PUSHED into the hole by the tension on the spring.
100% UNSPRUNG. In this case the springs will do nothing because there is no weight on top of them to compress them. Each wheel still has 1,000 pounds on it. Each wheel will follow the contours of the ground just like the 100% Sprung vehicle, but what happens to the portion of the vehicle above the springs? Our magic vehicle still has a body, seats, engine and all. It just weighs nothing. Since there is no weight above the springs, there is no spring action. When a wheel drops into a hole, all that force is transmitted to the portion of the vehicle above the springs. It is just as if the axles were solidly mounted with no spring action at all, so if the body of the vehicle does not twist, the wheel will not drop. Either that or the wheel on the opposite corner will raise up off the ground. Again, even in this case when ALL the weight is below the springs, it isn't the axle weight that causes the wheel to drop into the hole. It is the spring action pushing it down, and in this case since there is no spring tension and no spring action, the wheel does not follow the contours of the ground.
As our mythical vehicle proceeds from 100% UNsprung toward 100% Sprung, the closer it gets to 100% sprung, the more evenly the downward force is exerted at each wheel WHEN UNEVEN TERRAIN IS EXPERIENCED and the less effect the uneven terrain has on the sprung portion of the vehicle.
A guess on my part, but a graph of the effect of going from unsprung to sprung would probably start off at a small incline then increase until it got into the 75% or so sprung range where the incline would start to flatten until it reached 100% sprung. If this is the case, changing the axle weight of a fairly well balanced vehicle would have some effect, but not a great deal of effect unless the weight change was substantial, because the sprung weight, body etc., is in the upper portion percentage wise of the total weight - the area where the curve is in its flattening stage at the top of the graph.
In the expample that CJDave gives - the tractor/trailer - removing the trailer puts the sprung weight way down the graph where not only does each pound have more of an effect individually, it is also way down in an area where the strength of the springs needed to support the vehicle when fully loaded causes the axles to act as if they are solidly mounted to the frame with no spring action at all. This is a good example of not only too much unsprung weight, but also springs that are not properly rated for the work load. If you took that same tractor, reduced the strength of the springs to work properly WITHOUT the load of the trailer, the handling would be much improved. Newer tractors have accomplished this by using air bag suspension instead of springs. The bags are aired up to a certain HEIGHT. More weight equals more air pressure to attain the proper height - less weight, less air pressure which results in a much better ride both loaded and empty and better handling characteristics. The practicle effect is "adjustable spring strength".
Final thought: As speed increases, balance becomes more critical. Balance is less noticable in an engine running at 1,000 rpms than at 10,000 rpms. An out of balance engine might run for years at 1,000 rpms and seconds at 10,000 rpms before it shakes itself to pieces. Using the same logic, a vehicle traveling at high speed is much more vunerable to the "out of balance" condition created with high percentages of unsprung wight than it is at low speed. REASONABLE changes in unsprung weight will probably have very little noticable effect on slow moving off road vehicles. Large changes in unsprung weight would have a noticable effect on body lean since the weight of the axles would tend to pull the vehicle toward the distended wheel more so than a lighter axle and tire wrould, but this gets into the area where the springs are no longer acting as support for the weight above them and instead are acting as a restraining device to prevent the wheel from destending any further - again having the characteristics of a solidly mounted axle. When the springs begin to restrain the axle instead of pushing it down, the traction and handling characteristics begin to sharply decline.
Doug '97 TJ
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