Rep Power: 381

some one might want to sticky this somewhere.


Ok. Boys and girls. To end this debate, and shed some actual fact on this subject. The magazine Grassroots Motorsports a well-known and respected Racing magazine did an article that tested how Sprung and unsprung weight affected a cars performance. They did 2 tests, one on acceleration, and the other on handling. This is what they found.


1. Weight Vs Acceleration.

To find out what a set of lightweight wheels would be worth in terms of speed and elapsed time, eight-pound steel discs were attached to a set of 36-pound wheels in an effort to bring then to the weight (and moment of inertia) of the wheels they were replacing. (note: base line wheels were 17X9 with tires weighed 54 LBS Vs test wheels of the same size and tire that weighed 34LBS)

The car was also equipped with removable ballast bolted to the roll cage at about the car’s center of gravity. This meant adding or removing ballast had no effect on the weight transfer or front to rear distribution.

The car was initially ballasted to bring the weight with fuel, driver and wheel discs to 2000 lbs. Before the second set of runs, the four eight pound steel discs were removed. To keep the car’s weight consistent at 2000 pounds, 32 pounds of ballast were added.

For the third test, the 32 Lbs of extra ballast were removed giving us a 1968 LBS car. For the final test the discs were placed back on the wheels, while more internal ballast was removed until the car matched the times produced by the lightweight wheels.

Our testing revealed a few facts. First, the wheels, even on this somewhat low-powered test car, had a very measurable effect.

Fitting the lightweight wheels and restoring the weight as internal ballast showed that the reduced wheel moment of inertia alone was worth more then half a tenth on the quarter mile. At the end of the quarter mile, this translated into a lead of 6.7 feet over the base like vehicle. Dropping the weight from the car by 32 pounds (1968 pounds total) while running the lightweight wheels further reduced the elapsed time by a solid tenth, resulting in a 13.1 foot lead over our baseline runs.

Now how much weight would the car have to lose to match the gains seen by just the lightweight wheels? With the wheel discs reinstalled, we had to drop the car’s sprung weight by 60 pounds to match the performance gains seen by losing the 32 pounds of rotating weight.

What we can say here is that weight removed from the wheels as at least a 70% more positive effect on performance than weight removed from the sprung part of the vehicle.

Baseline: heavy wheels 2000-pound car
average E.T.:14.7303 sec
average speed:88.98 mph

Light wheels 2000 pound car
average E.T. 14.6973 sec
average speed: 89.17 mph
improvements over baseline: 0.051 sec, .019 mph, 6.7 feet

Light wheels 1968 pound car
average E.T.: 14.6303 sec
average speed: 89.52 mph
improvement over baseline: 0.1000 sec, 0.54 mph, 13.1 feet.

Heavy wheels 1940 pound car
Average E.T: 14.6307
Average speed: 89.65 mph
Improvents over baseline: 0.996 sec., .67 mph, 13.1 feet.

 
Rep Power: 331

interesting facts, but not exactly new news. Good post though.

 
Rep Power: 311

And here I was thinking that my 17" heavy azz wheels really slowed me down... phew! Good post thanks.

 
Rep Power: 381

It appears that the only time you would see the difference is if you were really in to racing. I autocross, so even .001 can make a difference as to wheather you win or loose. there is also a difference on how the car handles through turns, when dealing with rotational mass and Unsprung weight. (Lighter is better). If you are in to drag racing or street racing, then any advantage you can get is well worth it, even if it only gives you a .1 sec boost. at the track that can be all thats needed to win.

 
Rep Power: 0

good post