Unsprung weight vs. Sprung weight -- a discussion

[Marcus-SanDiego]

This topic often results in a good discussion -- though can, at times, become contentious because of the ratios that people assign to sprung: unsprung.

I've seen ratios that range from 2:1 to 10:1. I've seen people claim that the reduction of unsprung weight results in HP gains (wrong; but it does free up resources that can be dedicated to rotating mass).

Additionally, there are two things that people talk about when it comes to unsprung weight. One of them, however, should not be dealt with when discussing unsprung weight: acceleration (and braking). Rotational mass is relevant here. Unsprung weight is not.

Meanwhile, unsprung weight is relevant to the discussion of grip -- especially as it pertains to uneven driving surfaces.

If I'm wrong about dividing this discussion into two separate issues, please let me know.

I'm hoping we have a physics geek on the site who has a good understanding of this stuff -- and who can explain it in lay terms.

Discuss.

[ZHP-FTW]

These are probably mistaken for the same idea or principle when they really are completely separate. If someone can find a informative link to the two principles and post them here that would be awesome. I will look as well.

[danewilson77]

Rotational Un-Sprung Mass: Some of the un-sprung and semi-sprung weight is rotational and some is fixed. The wheels, brake rotors, and hubs are rotational. It takes a lot more energy to accelerate the wheels in a forward direction AND in a spinning motion than would if you were just carrying them in your trunk. Also when you get up to speed, the mass of the wheels creates a gyroscopic force, which has to be overpowered when you change direction and speed. These spinning forces are largely why lighter wheels can make such a noticeable performance impact. Mass towards the center has a lessor impact where mass towards the edge of the rim and tire has a greater impact. This can be VERY important when determining how much energy it takes to accelerate a certain wheel, but because of the complexity, I have never seen a wheel retailer market such specifications. It is possible that if a 30lbs wheels mass was centered near the hub and a 20lbs wheel center of mass was near the rim… the 30lbs wheel may require less energy to accelerate.

Non-Rotating Un-sprung Mass: The non-rotating unsprung and semi-sprung parts are the brake calipers, control arm, and other suspension parts. These parts are not rotating, but they are moving as you drive over bumps in the road. When you hit a bump that pushes tire up it moves the wheel and suspension parts. Ideally the wheel would move up so quickly as to match the contour of the bump, but since the un-sprung wheel and suspension parts has mass, the bump needs to exert energy to accelerate these parts up. If these parts were to have less mass, then your suspension would ride up the bump better and most importantly down the other side quicker. This would give you a better tire contact patch, better road holding, and a smoother ride over the bump, but lightening up the non-rotational mass probably will not give you a noticeable improvement in acceleration.

If you significantly change your unsprung weight, it will also be a very good idea to change your spring rates and dampening (bump and rebound) rates of your shocks. In the real world application, I do not think anybody is selling anything for the E46 that is so specialized, but some people with adjustable shocks do have an opportunity for some tuning!

[MasterC17]

Would love to know the exact ratio for Unsprung to Sprung weight - for wheels at least. Although the 135's are great looking wheels they are just too heavy to be used frequently in autocross/track application. I'd love to pick up a set of ARC-8's. Also, would the offset be partial in determining whether the weight was closer to the hub or the edge of the rim?

[RITmusic2k]

Dane, that's an excellent breakdown of the topic; well thought-out and clearly worded. Bravo.

Master C17, all other things being equal, a wheel with a different offset will not affect its rotational moment of inertia, though I suppose it's possible to have a miniscule effect along the "vertical" axis of rotation of a front wheel (that is to say, the axis around which the wheel is steered)... but I suspect that difference would be immeasurably small compared to the obvious change in steering feel due to the differences in steering geometry.