there are 2 racecars: both weigh 2000 lbs. and are the same shape. however, one has extremely heavy (steel) tires/wheels but a light body and chasis. the other is the opposite; heavy body/chasis but VERY light (aluminum) wheels.
which will accelerate/brake faster, or are they the same?
(if you can think of a detail i left out, just assume everything else equal, for the sake of brevity…example, location of weight on the car will be lower to the ground on the heavy wheel car, thus affecting the accel. … IGNORE THIS b/c it is negligible!!!)
The one with the lighter wheels will do everything better. Both have to accelerate/decelerate the same amount of weight, but they also have to change the rotational speed of the wheels. Heavier wheels makes this more difficult.
No cite, but I’ve read a lot on topics like this. Any book on performance modifications for racing cars should touch on the benefits of lighter wheels.
The car with the lightweight wheels is faster. Part of the reason is the car has less rotating mass to overcome, but the main reason has to do with something that’s not so obvious called “unsprung weight”.
The portions of the car that move up and down with the wheels, such as the tires, axles, and brakes, are not isolated from the road by the springs. These parts are considered unsprung weight. Parts of the car that are supported by the springs, such as the bodywork, engine, frame, and driver, are considered sprung weight. Parts that are attached to the frame at one end and the wheels at the other, like the suspension control arms, shocks, and the springs themselves, are called partially-sprung weight. There are ways to calculate the exact proportion of sprung and unsprung weight of these parts, and this ratio depends on the particulars of the geometrys involved.
The ratio of the sprung to unsprung weight is a very important factor in the handling of the car. The car with a high sprung to unsprung ratio (in our case, the light wheeled car) will have much better grip and generally ride better. This is because the relatively heavier body of the car is not disturbed as much by the light wheels bouncing up and down over bumps and other irregularities in the road surface.
In a race car, unsprung weight is critical and there are various tricks used to keep this to a minimum and shift some unsprung weight to the chassis, where it becomes sprung weight. For example, shock absorbers are usually mounted “upside down” when compared to a road car so only the relatively light piston rod is moving with the wheels, rather than the heavier shock body. Some cars have even gone so far as to have inboard brakes, moving the heavy brake calipers and disks from the wheels to the differential, where they act on the driveshafts (this configuration has problems with cooling, so you don’t see it too much except in some open-wheeled cars).
Excellent post, Jet Jaguar, but (correct me if I am wrong) the sprung/unsprung ratio has more of an impact on handling than on acceleration. I believe the performance benefits come from rotational inertia effects that ski mentions. Reducing the weight of the tires is roughly equivelent to using a lighter flywheel.
I know this is a little off topic from the poster’s hypothetical question, but in the real world there is also generally a tradeoff between wheel weight and rim size. Larger rims allow for wider tires with stiffer sidewalls which can improve both acceleration and cornering even though the rims may weigh more. That said, it is important to find the true harmonious balance between the various elements.
Reducing wheel weight is always a good thing as long as you do not reduce size, but it also is an expensive modification if all you want is acceleration.
That’s true to some extent, it will affect cornering more than acceleration/braking. But in real-world conditions, roads are never perfectly glass-smooth. There will always be bumps, undulations, and other irregularities in the road surface, and the car with a better sprung/unsprung ratio will “hook up” better and have more traction, so it can accelerate and brake a little bit harder. It will also effect the way the chassis settles from the weight transfer.