aerodynamics of wheels

[gtobob]

I've thought a lot about a low profile for the car body, but what about the wheels?
For the skinny modified wheels there's no point, but the stock wheels are approximately a quarter inch wide and a 1 1/8 inches high. For four wheels it represents a fairly big front, yet I don't see any type of wedge/fender treatment for the wheels at WIRL races. Someone else must have pondered this. Is the rotation of the wheel a factor in reducing drag?

As Werner von Braun once said: " one test is worth a thousand opinions." I should try it at a WIRL race but my cars are so slow and I'm testing so many other variables I'm not sure I'll see a subltle difference.
Anyway, I wondered if anyone thought about it.

Bob and Robert,
The Models Boys
"We come in last place...so you don't have to.

[Cam-Car Racing]

I have also thought about the air drag on the wheels, and have built a car that I race at
PDDR that reduces the drag on the rear wheels. I haven't thought of a good way to deal with the front wheels. I haven't been able to determine how much benefit I gain from doing this, but it seems like my fastest cars use this design. You can view the car at
www.p-d-d-r.com. Click on videos, then Nov. stock, and look for a car called Meteor.

[FAST Racing]

gtobob,

I have tried "fairing" both front & rear wheels.

The COG was too far forward. Tried 'lightening' the front by removing as much as I could.

Best I was able to get was 45grams on front axle.

My fastest cars have about 25-30grams on front axle. Sure tracked straight, but got passed
in the last couple of feet before finish.
I think it looks 'racy' ;D Just because it didn't work for
me, doesn't mean it won't! Experiment,ask questions and
build a few cars

[ninjarabbi1997]

Doc Jobe posted about this on his website www.pinewoodderbyphysics.netfirms.com//pwdboard/nfphpbb/viewtopic.php?t=12
"The ordinary kit standard wheels have a drag coefficient of 0.50 which is hard to improve on using vacuum breaker scoops (like a 1/4 circle fender).

Your best bet is to get wheel spin time up to 34 seconds for MU - 0.06." He suggests using his speed kit to do this.

[flyingderby]

I have read some good papers that hold the air and paraphrenalia drag on the cars is not a big issue as they do not go fast enough for long enough to make a difference i.e. that is a wedge compared to a banana shape. While I don't suggest racing your rain gutter sailor down the track, it isn't about aerodynamics is it?

Isn' it all about the wheels, the axle, the lube, the balance and the potential energy?

[A1nogoslo]

flyingderby said:

I have read some good papers that hold the air and paraphrenalia drag on the cars is not a big issue as they do not go fast enough for long enough to make a difference i.e. that is a wedge compared to a banana shape. While I don't suggest racing your rain gutter sailor down the track, it isn't about aerodynamics is it?

Isn' it all about the wheels, the axle, the lube, the balance and the potential energy?

It is definitely in the equation for sure and much greater than you would think...

[ProQuest]

Hey flyingderby:

I'm no physicist, nor am I the resident expert, but I'm happy to throw my 2 cents in.

For the most part I think you are right -- aerodynamics plays a small roll in the performance of pinewood derby cars. I wouldn't go so far as to say aerodynamics is irrelevant, but I consider it to be one of the lesser important factors. Having said that, it is somewhat of a mute point, because of far greater importance is achieving a proper location of the center or gravity or the balance point as you might call it. To achieve the optimal balance point (approximately 1" in front of the rear axles) it is customary to reduce the amount of wood in the front of the chassis and leave considerably more in the rear so that you have sufficient structure within which you can place your weight. As a result, the chassis will usually take on an aerodynamic shape. The trick is to somehow accomplish this low profile aerodynamic shape and at the same time give your car a creative and interesting design -- something I have not been able to do.

There is an outstanding example of style and aerodynamics built by F.A.S.T. Racing on the PWDRacing Board, in the photos section. It is called "Viper." I tried to cut and paste it here, but wasn't able to do it for some reason. Where's my teenager when I need her?

Just my thoughts.

ProQuest/Steve H.

[Gravity Steve]

The best drag coefficient (cd) you would be likely to ever see would be around .35 if you were to be able to cover the whole wheel, and had a really streamlined car. Assuming the car was like the wheels- basically a brick- I used .58 as the cd. Then the difference of ~.2 in the cd is really miniscule, when you consider that the car's maximum theoretical top speed if dropped in a vacuum is just under 11 miles per hour. This appears to be a relatively small effect. I haven't diced it out far enough to determine if it's worth going after, but I haven't bothered. I just made a flat car 1/2" thick and rounded over the front end. I have yet to race this one, though. My wedge with thin wheels was a winner, though, at our company PWD.