SundayDriver

All the literature (For example, Milliken & Milliken which is pretty much the bible when it comes to this stuff) suggests that approximately 1/2 of the torsion bar weight goes to each category.

If I understand your question about lowering the car, when the body hits the ground the springs and torsion bars are supporting zero weight.

If I follow where you are going...
However, that is not relevent to sprung/unsprung mass. We confuse ourselves since we use pounds for both weight and mass but they are not the same thing. It happens that 1 pound (mass) = 1 pound (weight) under 1.0 g of gravity. But again, they are not the same. If you put 100 lb-mass on a scale it weighs 100 lbs. If you lift it to read 50 lbs weight, it still has 100 lbs of mass. This is what is happening with the suspension. What matters is the mass, not what weight you have on a scale or how much weight a spring is carrying.

SundayDriver

I just realized that I probably misunderstood your question about lowering the car. If you meant what weight when sitting at normal ride height, here is my answer...

It depends on where you measure. Let's use a torsion bar, but the same applied to a spring.
At the body end of the torsion bar, it supports just the car weight which you called sprung without torsion bars. So the bars support 2000 llbs.
But at the other end of the torsion bars, they also have to support the weight of the bars so the answer is 2010 lbs.

Assuming this is a linear function, then the average would be to add in 1/2 of the weight of the bars.
This also applies to any other linkage that has connection to the body.

shiners780

I think our bone of contention has been the use of the words mass vs weight. I initially used the word weight because most of the material I have read regarding this issue (sprung/unsprung) has used the word weight, sometimes inaccurately interchanged with the word mass. As you have stated, they clearly are not the same thing, and mass is the more accurate and useful value when used in this situation.

Given our car example above, sprung weight and sprung mass can both be assigned a value, but those values are not the same. IMO, the sprung weight value is 2010#, whereas the sprung mass value is approx. 2005#. Referring to the initial question regarding whether the weight of the torsion bar is considered sprung weight, I still contend the answer is yes. However, as you point out, the mass of the torsion bar is divided between both sprung and unsprung mass.

Here is my line of thinking...

To determine the sprung weight: The total weight supported by the torsion bar is 2010#. That value includes the weight of the torsion bar itself. This measurement is taken at the outer end of the torsion bar while the car is sitting at normal ride height, since that is the only point on the torsion bar that is actually supporting the weight. The sprung weight is therefore 2010#.

To determine the sprung mass: The total mass supported by the torsion bar is also 2010#. That value includes the mass of the torsion bar itself. However, the measurement of mass is taken along the entire length of the torsion bar since the bar twists along it's length to support that mass. Unlike the support of the sprung weight which uses only a single point of measurement, the measurement of mass uses the entire length of the torsion bar. Therefore, the entire length of the torsion bar is the measurement point used to determine the sprung mass. As you stated, the mass of the bars themselves (10#) would then be divided between both sprung and unsprung mass. Sprung mass is approx. 2005#, and unsprung mass is approx. 105#.

Using this logic, the mass of the torsion bars is divided between sprung and unsprung mass; the weight of the torsion bars is considered sprung weight.

SundayDriver

This thread has gotten me to really think about sprung/unsprung mass. My conclusion is that the concept is pretty meaningless with modern cars. I am kind of writing this as I try to think this out, so ride along and help me see what I am missing here.

1) As long as we are at 1 standard gravity and not playing games with scales and where weight is sitting, mass and weight are the same thing for this discussion.

2) The concept of unsprung mass/weight is outdated (badly). It was developed when there was essentially one suspension design - solid axles. Now a solid axle is pretty much limited to one degree of motion and in a simple model we can ignore everything except both tires moving in sync. In other words, no rotation in any axis. From there we have a model that is very close to the classic mass-spring-damper from physics 101. In this model, it does not much matter where we add or subtract a pound. If we add unsprung weight, it makes the axle (suspension) harder to control. It has a pretty equal effect whether we do that at the center of the axle or the end.

3) Modern suspensions have little in common with the above model. There is rotation around pickup points. If we add a pound at the pickup point axis of an A arm, it has very little effect compared to adding that same mass/weight at the tire. It is much harder for the shocks to control the weight that has been added at the wheel vs. more inboard in an double A arm design.

4) Yet, a measure of unsprung weight or mass does not account for this difference. This became clear when I started to think about torsion bars. I am pretty sure that 1/2 of the weight of the torsion bar is unsprung. However, it has almost no effect because it rotates a very small amount vs. other parts that travel up and down.

Seems to me that a meaningful measure would have to be equivalent mass at the shock, rather than total mass or weight. That does not negate the value of reducing unsprung mass in a given design, but it is clear that 2 designs with identical unsprung and sprung mass will not behave the same.

So there are my thoughts, though I have no clue what they are good for. This seems to be more of a mental exercise than anything of really practical value.

shiners780

Absolutely! A discussion in a different forum got me thinking, which resulted in my initial posting. Even though the practical application is absent, you gotta admit it's a fun exercise (not to mention a terrific learning experience).

SundayDriver

I agree and it really got me thinking about aspects of suspension design I had never considered.