unsprung weight vs. sprung weight

[johnc]

The suspension of an automobile must provide the following:

 

1. Compliance allowing the tires to follow road imperfections.

2. Isolate the chassis from the vertical accelerations caused by road imperfections.

3. Control of the attitude of the tire to the road surface.

 

Basically an automotive suspenstion must isolate the sprung mass from the unsprung mass to achieve the three main objectives above. The less mass the automotive suspension has to contend with (both unsprung and sprung) the faster it can respond (the suspension is said to work at a higher frequency.)

 

Since the unsprung mass (wheels, tires, hubs, etc.) must respond to higher force and higher frequency accelerations than the sprung mass (because of spring isolation) then we can understand why reducing 1 pound of unsprung mass is the same as reducing 4 pounds of sprung mass from a handling/braking standpoint. If we look at bump compliance as just one aspect we can see how a suspension that reacts quicker will leave the tire more in contact with the road, thus affording more grip.

 

As for a 240Z, I don't know the exact unsprung weight numbers but its fairly low compared to a number of other vehicles of the era. When calculating the number you need to include 1/2 of the weight of the strut, shock, and spring. The driveshaft and differential are sprung weight and 1/2 of the halfshafts are unsprung weight.

 

The raw unsprung weight number is not as important as making efforts to reduce that number. To determine spring rates you can either ask someone or start with Carrol Smith's "Tune to Win", in particular, Chapter 6 - "Rates and Rate Control - Springs and Anti-Roll Bars."

[Guest Anonymous]

Couple of questions to help decide spring rates.

 

How much of a 240z's weight is unsprung? Anybody know? Assuming say, a total weight of 2400lbs, how much of that is actually sprung?

 

Also not clear on why reducing the unsprung weight is so important. I understand the effects of rotational weight on acceleration and braking, but what else am I missing?

 

Are the springs and struts considered unsprung? Probably. The rearend and driveshaft too, or a fraction of the driveshaft as somewhere in that line the weight transfers to sprung when it gets to the transmission.

Any calculations? Estimates?