Wheel Aerodynamics

[Callum]

I seeking assistance in developing the lowest lift possible for a car with exposed front wheels. Local vehicle certification requirements mandate the use of wheels cover (mud guards) and I think this may actually make the wheel lift, due to aerodynamic forces, worse. Anyone know how to improve this situation?
Some ideas; vents in top of wheel cover, vortex generators on top of cover, a flow triping fence, etc

all advice welcome, advice supported by results or papers prefered.

 

[GregLocock]

Well, without doing the maths the first thing I'd say is that mudguards will probably improve things a bit, since the velocity will be halved at each interface.

Anyway Mr Milliken in "Race Car Vehicle Dynamics" has some thoughts on the subject, and comes to the surprising conclusion that there isn't /that/ much difference in net lift between a stationary and a rolling wheel. If so then it seems arguable that there isn't much difference between a fully guarded wheel (ie a stationary wheel) and a rotating wheel. Therefore cover as much of the wheel as possible.

Katz also wrote some SAE papers on the subject. 860218, 890600 and possibly 890601 may be relevant. Or his book, which has an obvious title that I can't find. or his other book "Race Car Aerodynamics: Designing for Speed"





Cheers

Greg Locock

 

[TVP]

Vents in the top of the wheel cover will definitely reduce lift, a la current vehicles in Sportscar racing (LeMans type cars). The Katz book that GregLocock mentioned is quite good. A quick search of the technical papers on the SAE website will give you the latest info on aerodynamics of wheels.

 

[patprimmer]

I imagine that the mudguard is a motor cycle type, that follows the circumference of the tyre closely.
I expect that if it is bought forward to below the axle, this will eliminate a scoop effect. With no wind tunnel data, I would guess that pressure would build up in front of the tyre, causing air to flow between the tyre and the guard, causing lift and drag. I expect that the highest pressure points would be near the axle height were the tyre face is vertical, and near the ground, due to ground effects. I also guess that the lowest pressure might be about half way between, or slightly lower than half way.

Vents near the top and down the trailing edge must help reduce lift, but will most probably increase drag especially if they are designed to generate a down force.

Also, an upturned lip to the top of the top vent should generate some downforce.

Wether or not these vents render the mudguard useless as a mudguard is another story. Regards
pat