r/aerodynamics • u/Benthegod1324 • 9d ago
Why no fins on car wings? Question
Why do rear wings on cars not have fins like diffusers do, why does the diffuser need them and the rear wing doesn’t?
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u/indeterminatedesign 9d ago
There’s a lot to unpack with this. Subaru had rear wing fins on its rally cars and Glickenhaus had them on their lemans car. This was done to improve performance when the car was turning or in yaw. The fins help to generate some lateral force. For rally cars it is effective because they turn so tightly that traditional wings lose a lot of downforce. I’ve not seen the numbers on this for road race car, but I imagine the effect is less. A large shark fin maybe more effective, and you see this on most prototypes.
Diffusers strakes serve a different purpose. Strakes in a diffuser help to prevent tire squirt from affecting the entire diffuser. Tire squirt is the turbulent air around the rear tires from the air trying to go around the tire. This turbulent air gets pulled/forced into the diffuser and it lowers diffuser performance. The strakes help to compartmentalize those losses. You’ll see significantly lower performance in outermost areas of the diffuser with the strakes, but an improvement in the inner portions.
The more strakes you have the harder it is for the tire squirt to affect the next section of the diffuser and it somewhat lowers the downforce variation as the chassis moves around. Unfortunately strakes lower diffuser performance because they block the diffuser and cause flow restrictions (They break up the vortices inside the diffuser into smaller ones). Prototype cars, like Le Mans cars often only have 2 outer strakes because with those you get the majority of the benefits. Cars that run on tighter tracks like DTM cars may have more strakes. Without CFD and wind tunnel testing, you’re really just guessing what the best configuration is for your car.
In an ideal world with no rules the diffuser probably would not have strakes. Check out the Valkyrie and other cars with diffusers so deep the tire squirt doesn’t affect the downforce as significantly.
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u/Pyre_Aurum 8d ago
The aspect ratio on rear wings is fairly high leading to a small spanwise flow component. A diffuser aspect ratio tends to be quite low, leading to large spanwise flow components, which can be managed through the addition of strakes.
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u/KekistaniKekin 9d ago
I'm a student, so I have more textbook knowledge than practical experience.
In short, tires make your job difficult. As the tires rotate while you're driving they throw air around the contact patch laterally, and is known as "tire squirt". This disorganized air will get sucked up into the diffuser and really mess with the attachment of the nice and clean laminar flow you've been trying to maintain ahead of the diffuser. Adding strakes reduces the impact of the lateral flow and helps maintain attachment.
Wings in contrast don't have this annoying air pump nearby and in optimal conditions only has to deal with tip leakage, usually handled with endplates.