Rally Car Aerodynamics - Side Slipping through the Air
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In this video, we’re analyzing how aerodynamics impact the performance of rally cars.
Surprisingly enough, in many ways the aerodynamics of rally cars are far more complex than in Formula One, where the cars enjoy some of the world’s smoothest and grippiest bits of tarmac. That’s quite a big contrast to rally cars, where driving in a straight line without wheel spin, drifting or jumping is a rare occasion.
So, where do you start if you want to optimize the aerodynamic performance? The hard reality is that you’ll have to look at multiple scenario's and strike a delicate balance between aerodynamic performance in each of those.
Let's take this 2016 Skoda Rally car in normal setup (regular ride height) and straight-line driving at 40 m/s as the reference. As shown by these red clouds, which indicate where a drag-increasing wake is created, this clearly isn’t the most streamlined car out there:
The rooftop air intake disturbs the air to such an extent that in its wake the center of the rear wing loses pressure and performance. And those wheel arches cause a lot of separation of the airflow at the sides of the car.
But enough on theoretical straight-line driving – it almost never happens, it’s time to start jumping & drifting!
If we increase the car's ride height by 15cm, which is not uncommon just before or during a jump, then the drag on the car increases by 16% and the downforce is cut in half! As you can imagine, losing downforce and grip just before take-off can be quite tricky for the launch. And once you’re flying, you’re slowing down quite a lot because of the drag!
Massive changes in ride height like this are one of the main reasons it’s difficult to boost rally car performance by using underfloor aerodynamics, diffusers and so on: the downforce they generate is extremely sensitive to changes in ride height. And perhaps even more important, that change of downforce can be more pronounced at the front or the rear, changing the aerodynamic balance and thus the handling & stability of the car.
Next, let's look at drifting. Obviously, there's enough debate going on already as to whether you'll be faster or not when drifting. When it comes to aerodynamics, the drag on this car increased by 43% at a moderate drift angle of 15°! That would slow you down quite a lot. But there's good news as well:
The entire car starts acting like a giant wind shield, with the wind hitting the outer side flank. This generates around 100kg of lateral force pushing the car back into the corner.
And, interestingly, the wake of the rooftop air intake is no longer affecting the rear wing, which is now happily creating downforce, so it seems that this car really comes together in the corners.
Note: the 3D model of the rally car used in this video was a public one from hum3D.com and is not an official model from Skoda: it serves mainly to highlight general rally car challenges and not to make official claims to the performance of this car in question.
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