Quote:
Originally Posted by Otto
When designing front wheel deflectors, you might want to consult the Hucho book: In that there is a pic of a car (Calibra, perhaps) in a wind tunnel, and it shows that centerline airflow deflects at about a 60 degree angle outboard toward the front tires. So, the wheel deflectors should be canted inboard correspondingly, i.e., pigeon-toed so as to meet the incidence of the air as it actually impacts the front wheels.
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I remember when I first started looking at the designs of factory deflectors I was puzzled by their "pigeon-toed" placement, and wondered if it was because of what you found in the Hucho book; oncoming air flow will be directed outward as it nears the nose of the car because there will be greater pressure underneath the car than alongside it. Since then I had come to think, based on what others were saying, that the placement inward of the tire profile was to divert air from being sucked up into the low pressure wheel wells and creating turbulence as it cycles in and out. But if the airflow is angled outward, esp. as much as your windtunnel picture suggests, there would be even more air, and more turbulence, created by air interacting with the wheel well cavity, as well as with the inward face of the tires/wheels!
So, it seems to me that the placement of the front tire deflectors should be as I had planned, but with less concern to cover the outer side of the front tire profile (as this could actually increase the wake by diverting air further out from the sides of the car than it would otherwise travel), and with the shape of the deflectors pointed at an inward angle toward the front, instead of facing directly forward.
The McLaren SLR has front aero-shaped deflectors that exhibit this design strategy. Also, the deflectors on the current Civic and new Insight (they use the same pieces) seem to bear out the outward-facing flow phenomenon as they use a dual-deflector design (see attachment) that seems to first deflect as much of that outward-facing flow as early as possible without creating a lot of high pressure build up underneath the front of the car, and then deflect the remaining air away from the tires/wheel wells. The aerodynamicists seem to be trying to divert air w/o adding drag by doing so in a two-tiered fashion, much like some of the barge board designs on F1 cars in the last several years.
Given all this, why are factory deflectors always flat, forward-facing tabs? My guess is that these are cheap, require less fine-tuning in the windtunnel, and allow air to be diverted to either side of them equally so that a car will be more stable in crosswinds or when traveling through the wake of semis. Personally, I will go with a more 'optimized' and sculpted shape for my deflectors because it's more challenging and has potentially greater benefit, and I'll live with more instability on the highway if that results.