Track Design and Aerodynamic Lift?

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SlartyBartFast
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Track Design and Aerodynamic Lift?

Post by SlartyBartFast »

SuperDave has made the following claim/hypothesis a number of times:
SuperDave wrote:As far as I can tell (all other things being equal) there's essentially no difference between edge guiding and center guiding or at least the differences are 10th order effects. EXCEPT, there's one big difference. The solid center guide typical of wood tracks acts as an air spoiler under the car. This prevents the car from generating 'lift' and allows the COG to be placed further back in the car. SuperTrack, our plastic track, which uses edge guiding obviously does not have this spoiler. But not commonly recognized, neither to most aluminum tracks since they do not use a solid center guide but rather two narrow center rails with a large valley in between. (An argument could be made that the narrow rails enhance the lift by keeping the high pressure air from leaking out the side.)
I have contended at least once that the idea of aerodynamic lift being produced is unlikely at best. The lift experienced by cars at the transition from curve to flat, IMO, is more likely due to the inertia/momentum of the cars. Not aerodynamics. Therefore, IMO, extra lift shown on super tracks is due to a more abrupt radius change in the transition. This is the same phenomenon that so many complained about on older Best Tracks with the tight transition bend.

So can anyone test the theory? Time to put the wind tunnel to use.
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gpraceman
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Re: Track Design and Aerodynamic Lift?

Post by gpraceman »

SlartyBartFast wrote:So can anyone test the theory? Time to put the wind tunnel to use.
That would be an interesting experiment. I too am skeptical of the lift issue. Sure, I believe that a PWD car can generate some lift, but enough for the car to want to come out of its lane is where I have my doubts.

Measuring drag is relatively easy, but measuring lift is not. The car must be restrained in the airflow without affecting the lift measurement. I think the only realistic way would be to just test lift on the front end of the car (if lift is causing a problem, then it is likely to be more of an issue with the front end anyways). The rear wheels would have to be attached to the bottom of the wind tunnel. Then you can have a thin thread coming down from the center of the car, between the two front wheels. The thread will be attached to a small weight which is sitting on a scale (one that measures to 0.01 ounces or less). With no airflow, the thread must not have any slack. As the air speed increases, then the lift of the front end would be the difference in scale measurements (flow vs. no flow).

The cross-section of the car would matter, so it should be wing shaped to maximize the potential for lift.

Can the front wheels be made to lift off of the bottom of the wind tunnel?
If so, just how light does the front end need to be? Can it be done with the COG forward of the rear axles? Can it be done with a 5oz car?

IMO the most likely scenario of lift being a problem is with a car that has a very light front end due to the COG being behind the rear axles.
Last edited by gpraceman on Mon Apr 30, 2007 12:47 pm, edited 1 time in total.
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SlartyBartFast
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Re: Track Design and Aerodynamic Lift?

Post by SlartyBartFast »

gpraceman wrote:Measuring drag is relatively easy, but measuring lift is not.
Sure about that? Any method that measures drag that leaves the wheels to move adds the error of axle and wheel frictions to the drag measurement.

Here’s one setup: http://www.aerorocket.com/offer.html
See "Drag (CD) and Lift (CL) Coefficients"

But for my suggested testing, I’d go simpler. Could probably ball park it by simply tying down the back wheels and pointing a blower at the front end. We only want to see if the front end will lift.

Turn up the blower until the front end visibly lifts. Need a wind speed reading to see if the velocity is attainable by a PWD car down a track.

Then add a center guide, do the same. See if there’s a difference.
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Re: Track Design and Aerodynamic Lift?

Post by gpraceman »

SlartyBartFast wrote:Sure about that? Any method that measures drag that leaves the wheels to move adds the error of axle and wheel frictions to the drag measurement.

Here’s one setup: http://www.aerorocket.com/offer.html
See "Drag (CD) and Lift (CL) Coefficients"
To me, their rig to measure both lift and drag is not as simple as the rig I proposed. If you want a real measure of lift, then you need to mount the car so it can be suspended off of the bottom of the tunnel, like they show, which is a more complicated rig. It is also not as realistic as having a "track" beneath the car.
SlartyBartFast wrote:But for my suggested testing, I’d go simpler. Could probably ball park it by simply tying down the back wheels and pointing a blower at the front end. We only want to see if the front end will lift.

Turn up the blower until the front end visibly lifts. Need a wind speed reading to see if the velocity is attainable by a PWD car down a track.

Then add a center guide, do the same. See if there’s a difference.
If the tester has the ability to measure the air flow speed, then I can see doing that. I still would be interested to see if it would be possible to get the front end to lift at all with the COG forward of the rear axles and the air flow at a realistic speed.
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Re: Track Design and Aerodynamic Lift?

Post by SuperDave »

While you're testing, be sure you test for both orientations of the car. That is, front wheels nearer the bumper (as shown on the box as I recall) and rear wheels nearer the bumper (where the COG can be further back and the front real light. You don't need to lift the car, just the front end and not very far.
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Re: Track Design and Aerodynamic Lift?

Post by Cory »

SuperDave wrote:While you're testing, be sure you test for both orientations of the car. That is, front wheels nearer the bumper (as shown on the box as I recall) and rear wheels nearer the bumper (where the COG can be further back and the front real light. You don't need to lift the car, just the front end and not very far.
This is anecdotal, but it makes sense to me.

The one really serious wheelie that I saw during competition was a "car on the box" configuration, with the wheels toward the front.

Absolute COG location is not the key here. COG location relative to the rear axle is the key. Putting the wheels forward means it's easier to put nearly half the weight behind the rear axle and, IMO, this is what can cause a wheelie when you hit a bump or something.

In the case I witnessed, a bump is exactly what happened. The car looked just like one of those high speed racing boats when it catches some air. This car managed to come back down to the track and finished the race, but two things seemed obvious in this isolated example: (1) The air had lifted the front of the car, and (2) it slowed the car down.
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