Sprung weight v. Unsprung weight. What's the difference?

pbs911 · pbs911's Garage

I hear alot about a difference between sprung weight and unsprung weight, the latter being less desirable if you have a choice. What differentiates the two and what are the effects?

Dutchfly · Dutchfly's Garage

Sprung weight vs. unsprung weight has everything to do with the way your suspension is set up. Or if you like it, the way your wheel is attached to your car.

Your wheel is fixed via an axle that is held in position by control arms a shock absorber and a spring to the body of you car.

When you look at the spring, its task is to absorb any dynamics of the body. This can be a pothole in the road surface or the body moving forward (nose down) when you're braking.

The shock absorbs the wiggling movement of the spring, so the wheel doesn't bounce around. Basically the word "shock absorber" implies that the shock absorber absorbs shocks... This is not correct. The spring absorbs shocks. The shock absorber dampens the wobbling movement of the spring. So a better name for a shock absorber is movement dampener.

Back to the weight issue. Unsprung weight consists of the wheel and tire, the control arms, and part of the axle (driven or non-driven). Sprung weight consists of all the rest of the car's mass.

So, eg. the heavier a wheel gets the higher your unsprung weight is, the more difficulty the shock will have to keep the tire to the road. Less unsprung weight improves the function of the shock and spring to keep the tire levelled, so you have improved road handling.

PT

Quote:

Originally posted by pbs911
I hear alot about a difference between sprung weight and unsprung weight, the latter being less desirable if you have a choice. What differentiates the two and what are the effects?

silverc4s · silverc4s's Garage

Well, I'd say that overall, weight of any kind is undesireable in a car.

Unsprung weight is that material at the four corners of the car (generally) that is not supported by the suspension elements.

This includes wheels, tires, some fractional part of the suspension, along with the axles, brakes, hubs, etc. A reduction in this weight is generally regarded as helpful in keeping the tire patch in compliance with the road surface, and less dramatically subjected to physical forces acting upon it.

Carrying these ideas to ridiculous extemes caused folks to dream up such things as inboard brakes, and to have GOOD ideas like alloy wheels (and alloy wheel nuts).

Enigma03 · 10-28-2002, 11:56 AM

I heard from someone that the weight outside the spring is also multiplied by a factor of around 7? or so....I'm not sure where I heard this but quite postive it was from listening to the race car drivers that hung around with my father and it seemed to be a big thing about reducing it...as it affected handling and speed.

Jim T · 10-28-2002, 11:56 AM

Unsprung weight is weight that cannot be transferred/redistributed by the car's suspension.

Why is it important? Imagine you are driving down a slightly bumpy road - your head is moving, but not a lot, because it is highly sprung, by the suspension, the seats, your body, etc.

Now imagine the road wheel and the a-arms. They are undergoing dramatic movement. Because they are in direct contact with the road, with no suspension to be able to absorb or transfer forces. Because of this, their weight is extremely important.

The wheel/tire assembly is the major unsprung weight. It also has a gyroscopic effect - the less weight on that, the better for handling - the gyroscope tends to want to go in a straight line.

To take it to the extreme, if your wheels weighed 500 lbs each (total of 2000 lbs), on a vehicle that weighed a total of 3000 lbs, it would be impossible to control that vehicle at speeds in any situation other than a straight line on a very smooth road. Any minor bumps would send the car airborne very easily, and it would be very resistant to turning.

That's the extreme, but the concept is the same.

I've read that an Indy car engineer would rather add 5 lbs of sprung weight to a car than 1 lb of unsprung weight.

Dutchfly · Dutchfly's Garage

Quote:

Originally posted by Enigma03
I heard from someone that the weight outside the spring is also multiplied by a factor of around 7? or so....I'm not sure where I heard this but quite postive it was from listening to the race car drivers that hung around with my father and it seemed to be a big thing about reducing it...as it affected handling and speed.

BETTER HANDLING = MORE SPEED

Enigma03 · 10-28-2002, 12:05 PM

Quote:

BETTER HANDLING = MORE SPEED

Not in a straight line tho...handling dosen't play that much to going straight(at least I wouldn't think so)....for refrence tho the racing I grew up in was dirt track oval racing...and they way the turn is basically a slide instead of a turn

...and it meant a lot to be able to get the car sideways really fast but under control.

E

HawgRyder · HawgRyder's Garage

Another way of looking at it....the lighter the wheel/tire/brake combo...the more likely that the tire will stay in contact with the road.
The higher the weight...the longer it takes to come back down after it has hit a bump.
Hard to steer going into a fast corner if your wheels are off the ground!!
Bob

Dutchfly · Dutchfly's Garage

Driving any Porsche in a straight line would be a shame! These cars are built to race around corners! No dragsters, no ovals.... Good lord!

And I still think that better handling gets you more speed on a straight line as well. The bumpier, the better! Think of what a Cayenne is able to do.....

Quote:

Originally posted by Enigma03

Not in a straight line tho...handling dosen't play that much to going straight(at least I wouldn't think so)....for refrence tho the racing I grew up in was dirt track oval racing...and they way the turn is basically a slide instead of a turn ...and it meant a lot to be able to get the car sideways really fast but under control.

E

Dutchfly · Dutchfly's Garage

GENAU!

Quote:

Originally posted by HawgRyder
Another way of looking at it....the lighter the wheel/tire/brake combo...the more likely that the tire will stay in contact with the road.
The higher the weight...the longer it takes to come back down after it has hit a bump.
Hard to steer going into a fast corner if your wheels are off the ground!!
Bob

island911 · 10-28-2002, 12:30 PM

Quote:

Originally posted by Dutchfly
GENAU!

Translations:
GENAU! - German for "Exactly"

dampens/dampeners - to get wet . . often misused when "damps" /"damper"(slow down, deaden) is intended. ¹
e.g.; Rain put a damper on our picnic plans, as it would lead to a dampened picnic blanket.

1 Also used to annoy engineers.

Dutchfly · Dutchfly's Garage

Sie kennen sich aus!

Quote:

Originally posted by island911
Translations:
GENAU! - German for "Exactly"

dampens/dampeners - to get wet . . often misused when "damps" /"damper"(slow down, deaden) is intended. ¹
e.g.; Rain put a damper on our picnic plans, as it would lead to a dampened picnic blanket.

1 Also used to annoy engineers.

island911 · 10-28-2002, 12:53 PM

Paul, all I can add to the above input, is; the amount of unsprung weight you can "afford" is dependent on how much sprung weight you have. For example, a Rolls can have some fairly big unsprung weight, relative to a 911, and still keep it's wheels well planted.
With your ever lightening 911, you will want to keep this all in check. (read: no big reds and turbo twists for you)

RallyJon · 10-28-2002, 01:42 PM

The rotational inertia effects are pretty significant.

This guy put together a spreadsheet to calculate the effect of putting larger/heavier wheels and tires on a car.
http://www.siscom.net/~welter/racing/rotation.html

To summarize, an additional 5 lbs per corner of wheel/tire weight (20 lbs total) has an equivalent effect on acceleration as 50-80lbs of weight in the car.

Wavey · 10-28-2002, 02:21 PM

A good way to visualize the effect of unsprung weight is to imagine yourself holding a lightweight basketball and moving it around, changing direction rapidly, paying attention to how this actionb makes your body move. Then imagine the same thing with the same ball filled with concrete. The more unsprung weight there is, the more it's movement upsets the chassis.

Bill Verburg · Bill Verburg's Garage

RallyJon, that's interesting. A few years ago I did a similar ss but focused on hp to move extra tire weight as well as hp to move additional chassis weight. But came to somewhat similar conclusions. I was able to verify some of the #s from dyno comparisons, unfortunately the tire weight and radius were both changed at the same time somewhat obscuring the results. From the math the more meaningful # is the tire radius.