thoughts on wheel covers/skirts
 

I was pondering wheel covers and their benefits compared to wheel skirts. I realized that the top of a wheel is going twice as fast as the car in relation to the air. Well if drag is the square of speed increase, then there must be 4 times more drag on the top of the wheel as there is in the middle, and no drag on the bottom. Has anyone used covers and skirts together? Just thinking out loud....

I have used the covers and skirts together. My testing did not compare skirts to covers, I just did each on there own and all my mods together.

But my wheel skirts gave me .4-.8 MPG (best estimate of .5-.6).
My wheel covers gave my 1.6 MPG.

So if I look at the MPG gain of just the back two wheels the covers should =.8MPG. This is more than the wheel skirts give me.

My thought is that the wheel covers also keep air from being pushed out (widening the car- think mist from semi truck on wet day) which is why they gain me more MPG.

I started out with wheel covers and then later installed wheel skirts, leaving the wheel covers in place for over a year under the skirts. I got a mileage boost with the covers, another (larger) increase with the skirts, but no noticable mileage drop when I removed the covers. Once the skirts were in place, the top of the wheel was no longer exposed to the up to twice ground speed air velocity alongside the car, but rather is exposed to a squirrel cage-like rotational airflow inside the wheel well powered by the rotating wheel, so the wheel cover had little effect on total drag from inside the skirted wheel well.

Twice as fast as the speed of the car? Speed of the car (wind) is Vcar=2*pi*r*omega where omega is the angular velocity i.e. rpm. Speed of the rolling surface of the tire relative to its center of rotation is Vrel= r*omega. So a point on the rolling surface of the tire is experience and alternating wind velocity: Vwind = Vcar +Vrel = (2*pi*r + rcos(theta))*omega depending on angle (theta) where it is in the rotation.

Remember there are two kinds of aerodynamic drag: skin friction and form (pressure). Ask yourself: what has more drag, two closely spaced plates moving in opposite direction or one plate moving at speed in a free-stream? When you cover the wheel well you're improving form drag no doubt.. but what have you done to the friction drag? There is now complex flow pattern between the wheel and the wheel well cover. I haven't run the numbers (or done testing) but I do wonder if skirts are worth it.

Most hub caps are like egg beaters chopping their way through the air. But they look good and they provide much needed cooling for brake pads. We can put smooth hub caps on a reduce both the noise (aeroacoustics) and the "egg beater" form drag... but we may need to replace the brake pads much sooner than otherwise (unless we drive a prius that is).

I'm just speculating but I believe if wheel well covers were such a huge benefit in drag reduction, we would be seeing more of them. Its not like the car companies aren't trying. But when average speeds are sub 50 mph they may not be worth it. Just saying.

I don't recall whose signature, but it goes "if you are using the brakes you are doing it all wrong." If you are hypermiling you shouldn't be overheating the brakes so smooth covers should be OK for most of us on this site. They are still open to the air on the inside of the wheel after all.

Bingo!

You should only need to cool the breaks if you use them a lot. I do not. I coast down to 10-15 MPH before breaking. The only time I use my breaks a lot is when I am in part of town where the lights are badly timed or if I am trailing a jerk who stomps on their breaks often.

I understand that... and I have smooth hubcaps on my car but the way my wife drives, (which is not at all uncommon) I would hesitate putting them on hers. Also people who drive in the mountain west have to use their brakes much more so than those living where its level. In my mind its an open issue and I'm currently running the "experiment".

Those people should learn to down shift before they smoke their brakes. I have seen a few cases of smoked/melted brakes. It usually results in a life lost.

Wheel skirts were quite common in days gone by.
They were re-discovered on the Insight (1 gen).
I expect to see more of them in the coming years.

Unfortunately, at least the European fuel efficiency testing is done at unrealistic speeds, so aero mods to improve FE don't pay off in these tests.

Also we've had this big wheel fad going on lately. Its kind of hard to show off your expensive alloy wheels with their "rubber band" tires when they are hidden under a wheel skirt.

I've had no problems with overheating under my four wheel skirts even when coming down a steep (3rd gear climb) mountain road from NC into SC.