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Science and Stuff

When a wheel is rotating inside a typical road vehicle/enclosed wheel arch (as opposed to open-wheel Indy/F1 type car), there are crazy air pressure regions fore and aft of the spinning mass.  Since I don’t have an accurate CFD model of the FR-S on a rolling surface, I will hypothesize that the FR-S is similar to most other enclosed wheel road cars and follows somewhat loosely the generalized pressure regions presented in in Milliken’s Race Car Vehicle Dynamics and Katz’s Race Car Aerodynamics.  I simplified the generalization in Figure 1, so that we could visualize what we were attempting to do. As a wheel rotates forward, negative pressure (blue) is created pretty much throughout the entire fender well—the air flow follows in the direction of tire rotation (you can attribute this to the basic fluid mechanics principle of the no-slip condition). We have the greatest positive pressure zone directly in front of the tire, and the lowest negative pressure directly behind the tire.  The only problem is the pressures vary wildly depending on where you are inside the fender well.

Figure 1: Over Simplified Air Flow Pressure regions in an enclosed wheel arch.

Without having to trigger any nightmare flashbacks to your required semester of Fluids, basic need-to-know is that in the blue regions where the negative pressures vary around the wheel, this means that the air velocity also varies at those specific points (Bernoulli’s equation of pressure coefficient). When there is varying air pressure with differing air vectors, this leads to pressure drag. Which sucks (both figuratively and literally).  In order to meet our first requirement for inducing a pressure change, functionality calls for a vent or opening.  We decided to place a gradual swept opening directly behind the front wheel and have the opening go as high up as possible.  Since the negative pressures follows the tire rotation, the more air pressure we can extract directly behind the wheel nearest to the ground, will help reduce the amount of air pressure as we go higher up the fender well.

Figure 2: Computer Render of our Aero Fender Concept, Side View.

Working in collaboration with Victory Function USA, we came up with several different renderings and ultimately decided on the design as shown in Figures 2 and 3.  To assist with meeting Requirement 2, we wanted to do a conservative wide flare above the tire to allow full range of wheel travel for steering and dampening.  The “straight edge” design cue that connects the front bumper to the door line was both an aesthetic decision (Requirement 6) as well as function– airflow over the top of the fender when leaving the edge would help create a vortex to assist with more inner fender well air extraction.

Figure 3: Computer Render of our Aero Fender Concept, Closer

Plug Development

Once Mike approved the design renders, we brought the car in for on-vehicle shaping.  The FR-S/BRZ chassis has some, let’s call them, “unique frame features”.  The OEM fender and wheel well liner were removed and we got a chance to look at what we were dealing with in terms of wheel and tire clearance and our physical limitations on shape.

The FR-S/BRZ frame has a stamping seam on the underside of the front frame, directly underneath the triangular arch support.

1. Mike Kojima says:

Yes, Cheston designed them.