Sprung vs. unsprung weight


#1

I would be very grateful if you could settle a “debate” between two friends. Which adds more to acceleration…reducing sprung, or unsprung weight?



I’m writing to you after reviewing many useless / incomplete / vague answers were found “online”.



Can you help, and provide an answer that will at least determine the winner between sprung and unsprung? I’m looking only for one or the other, not a formula that would apply to every car / situation.



Thanks,

Bob




#2

Only if traction was a problem with acceleration and the sprung or unsprung weight aided traction would there be a difference. In general, reducing weight of any type aids acceleration. Now if your question was “what aids handling” that is completely different.


#3

Sprung and unsprung weight affect handling and ride control. This ratio has no effect on acceleration, which is a pure mass / horsepower thing, unless you are accelerating at such a high rate that traction becomes an issue…But even then the unsprung weight ratio makes little or no difference in the rate of acceleration…


#4

thanks, but please re-read the question…let me re-phrase it another way…which increases acceleration more / greater margin, a reduction of weight of sprung or unsprung weight?


#5

thanks for the reply, but isn’t the lighter car faster, if horsepower, make, model, etc. the same?


#6

Yes, lighter is faster. But sprung and unsprung makes no difference, it’s still weight that must be accelerated regardless if it’s sprung or not.

How do YOU define “unsprung weight”??


#7

Perhaps he means “with or without the trailer attached”:slight_smile:


#8

Your question and comments are un clear. Sprung weight is the part of the car supported by the suspension (springs) un sprung weight is the weight below the suspension (tires, brakes, hubs etc) A car handles better if more of the weight is sprung, supported by the suspension. If you have two cars of the same weight, with one having more sprung weight than the other, assuming equal engines, the acceleration would be the same because the engine has to push the same weight. The car with more un sprung weight will handle better in turns.


#9

All things being equal, it won’t make any difference on a smooth surface. On a rough surface, a lot of unsprung weight will tend to float or hop over the valleys between the bumps where less unsprung weight will tend to be pushed down into the valleys between the bumps more quickly, regaining traction quicker.

Now this is also speed related. At takeoff, you are not moving over the bumps very fast, so again it doesn’t make much difference. But as speed increases, you have better traction with less unsprung weight.

There are a lot of other factors involved here too, this is really only important when all other traction issues have been optimized. Correct shock valving and spring rates have far more affect on traction than a few pounds difference in unsprung weight. The amount of rubber meeting the road has the biggest affect, but tires with a larger contact area usually weigh more, so they have more unsprung weight. The traction gained by the larger contact area trumps the unsprung weight.


#10

There is no answer. I’m sure there may be a difference, but it will be specific to individual cars. Between cars, it will go one way for one car and the other way for another car.

The answers will not be standard for all cars. If there were an answer, who would believe it? The wrong debater or the right debater?

There was a lot of debating when the Titanic was designed and built. It didn’t help the Titanic enough to make the ultimate difference. Galileo would say that both vehicles would hit the ground at the same time.

Watch out for debating. Politicians who would try to win a debate might succeed in putting themselves in an impossible position. The two friends have an advantage; they’re starting from an impossible position.


#11

Consumer Reports (yes, let’s throw them into this debate) was quite critical of the Buicks made before 1961. These Buicks had an enclosed driveshaft (torque tube drive) that added to the unsprung weight. The unsprung weight did affect the ride and handling. However, it straight line acceleration, the Buick Century was one of the faster accelerating cars made.


#12

wellll, Actually, since a lot of unsprung weight involves rotating parts (brake rotors, wheels, tires), if you shave weight off of those areas, you will accelerate faster than you would if you shaved equivalent weight out of sprung-weight areas. Not only are you saving weight, but you’re reducing the amount of mass the engine has to spin.

Of course, reduce it too much and your handling will suffer as a result.


#13

If the unsprung weight reduction is from a part that is is not rotating (if it’s in an axle housing, etc) the reduction will have no difference in acceleration to a reduction in sprung weight.

If the unsprung weight reduction is from a rotating part, it will have an added benefit to acceleration from not having to increase the rotational inertia of that part. So lightweight wheels have both the benefit of reduced weight and reduced rotational inertia.

If the sprung weight reduction if from a rotating part (typically the flywheel), it will also have both the benefit of reduced weight and reduced rotational inertia.


#14

The difference in increased acceleration if your weight savings is from sprung or unsprung weight is in the same catagory as getting better mileage with the headlamps on or off,purely an academic exercise.

I will say that shaving a few pounds off a VW flywheel really increased the throttle response of the engine,that is, it reved much quicker.


#15

Shadowfax has it right. Given cars of equal weight, the car with the least rotating weight will have faster acceleration unless traction is a problem.
Look at it this way, both cars have the same weight to accelerate forward but the one car has to overcome the extra rotational weight to accelerate to a higher rpm.
That is why on the early hotrods with flathead fords of modest power one of the most important modifications was to lighten the stock flywheel. The loss of total weight was insignificant, but the loss of rotational mass was very significant.


#16

It was explained to me one time in a service school that this has more to do with ride and handling rather than acceleration.
My unscientific opinion would be that any acceleration differences would be so small as to be near non-existent.

Some older Subarus (FF1s) Audis, etc. used inboard brakes to move that weight inwards and those cars were certainly not designed for performance purposes.


#17

Let me add a couple of things here. texases made a good point about the rotational mass. Again you are facing a compromise. You can reduce the unsprung weight some by going to very expensive ultralight wheels, that is assuming you want to keep the same size wheels. You could also reduce the unsprung weight by using smaller wheels and tires, but then yo face a traction problem. Now the cure is worse than the problem.

The main reason that racers/tuners want less unsprung weight is for high speed cornering on less than perfect surfaces. The greater the unsprung mass (as opposed to weight), the greater the momentum of that mass at speed. When the wheel hits a bump, the greater the unsprung mass, the longer it tends to stay airborne.

Unsprung mass is only a part of the total equation. Everything has to work in balance to optimize the handling of a vehicle. Changing only one part of the design often results in poorer handling. If you are going to modify a suspension system, I would recommend that you get a complete system that is engineered to meet your needs. Mixing off the shelf, aftermarket parts and not knowing how to optimize the alignment settings will no doubt reduce the performance of your vehicle, not enhance it.


#18

The OP long gone, makes no difference, these threads take on a life of their own…people saying the same thing over and over…


#19

Yep, another ‘drive by posting’.