Free-Spinning Hubcap Physics

Factual Questions
1

This question come straight from my 9 year old son. Lately we have been seeing more and more free spinning hubcaps (obviously, I’m too L7 to know the right name). I see them on souped up Japanese and Korean cars mostly. These chrome odditites look like regular faux rims, but are apparently attached to the wheel’s hub with a central bearing that allows the thing to spin independently of the tire’s rotation. So at a stop, the tires appear to continue to rotate. Conversely, from a long stop, the car appears to move without the tire rotating. It is an neat effect, much like neon was before it became so prevalent. The last one I saw was actually a two part jobber(they might all have been like this): one chrome hubcap was seated on the tire’s rim as usual, its metal was embossed to look like a fancy tire rim; the second, centrally attached to the first, was embossed as the first was, but had windows in the metal to allow some visual interplay between the two parts.

So my son’s question(paraphrased down from the original long winded one):

Do those use more energy than an equivalent arrangement with the outer one not free spinning?

Any ideas?

2

Very slightly more energy is consumed by losses due to frictional heating in the bearings, but not enough to measurably impact your gasoline consumption.

3

Not neccessarily depending on if the test is city or highway. As they say YMMV :smiley:

In the real world I’ll agree with Q.E.D. that it would probably take more energy for the reasons stated but let’s try a few though experiments. The mass of a typical hubcap is so low compared to the wheel and tire that any difference would be minute. Make them from heavy flywheels and you might get some different results depending on conditions.

If you stipulate an ultra low friction bearing it will take less energy to accelerate the car to a specific speed than if the flywheel/hubcap were fixed to the wheel. A moving car has two kinds of K.E., linear from the whole mass going down the road and rotational from the spinning bits. If you start with your car on black ice you can rev the engine all you want and only spin the wheels. It takes some mergy to spin the wheels. If the hubcaps are free spinning they will stay stationary and not require any energy to rotate.

Let the ice melt to the blacktop and we can get moving. At first the hubcaps won’t spin but they will slowly come up to speed from bearing friction and boundary layer air between the cap and rotating wheel. At a steady state the whole system will require more energy as Q.E.D. said.

When you stop you need to turn the K.E. of the whole linear moving mass and rotational K.E. from the wheels and tires into heat via the brakes. They will not need to convert K.E. from the hubcaps which continue to spin.

4

Thanks QED and Padeye,

I guess what I see from your posts is that in general this system will use more energy than the equivalent fused system due to the resistance of the bearing. After sleeping on it last night I have a followup: Does that mean that the resistance to rotation offered by air does not figure in? At a long-held steady speed of 60 MPH for example, the rims both rotate at exactly the same speed? If not, would this not imply that a faster rotating, eg. the fused configuration, would use more energy? I know I’m missing something as you answers were so quick and in agreement. Maybe something about that boundary layer of air?

5

Well, there’s a couple ways to look at this.

First, from a practical point of view, the spinning cap is more complex, and thus likely noticeably heavier, than an ordinary fixed hub cap. So having to accelerate that extra mass, both linearly (in the direction of vehicle travel) and rotationally, as well as the increasing tire hysteresis, takes extra energy, period.

However, it’s a more interesting question if we stipulate that the total mass of the cap assembly is the same, whether it spins or not, so let’s so stipulate.

Now, imagine two car trips: one long, and one short. Over the long trip, the spinning hub will eventually match speeds with the wheel. However, during the period of time where the hub and wheel speeds are mismatched, there will be (unrecoverable) frictional losses in the bearing, so it does take some minute amount of extra energy (the amount dumped into the bearing) to get to the point where the speeds are matched.

For the short trip however (a “short trip” could be the distance between two stop signs, say), the spinning hub never gets up to full speed before you’re stopped again. Thus, you don’t need quite as much energy as you would have with a solid cap. For the shortest trips, where this energy savings overbalances the friction loss, you come out ahead.

For the air resistance thing, I think you could (as Padeye does) simply lump the energy loss due to the air in with the bearing frictional losses. The airflow is probably such that it would tend to push the hub to match speeds with the wheel (because the airflow is heavily influenced by the spinning tire), and thus energy loss to the air would be greater in the rotating-hub scenario. My WAG would be that the losses to the air would be smaller, but still same order of magnitude, as the frictional bearing losses. This last bit’s pretty tenuous, because it likely depends on exact geometries.

So, to recap: If the assembly weighs no more than a solid assembly, and it’s got a low-friction bearing, and you do a lot of stop-and-go driving, the you come out ahead.

6

just a side note: in the hiphop world and rim culture they are referred to as sprewell spinners or just sprewells after basketball player latrell sprewell who showcased them on his episode of MTV cribs