How effective is a flat floor on your car?
Track guys love them, and manufacturers are putting more effort into smoothing their car's underbodies, but what do they actually do?
Lately there's been a big push for improved aerodynamics on both road and track cars, with both performance and economy gains on the table. While the most obvious changes to cars occur where we can see them, the most significant gains are actually had where we can't see; the undertray. Up until about 10-15 years ago, many manufacturers simply didn't care about the underbody of the car, leaving everything exposed. This is great as it allows heaps of cooling for components like the engine, transmission and exhaust, as well as being cost effective and allowing for easy maintenance. However, it's about the worst thing you can do aerodynamically, as the disruptions to the airflow under the car both increase drag and decrease downforce.
A factory underbody. Aerodynamicists have nightmares about this stuff.
For the purposes of today's experiment, I've run a virtual wind tunnel (CFD) simulation on an NC MX5 Miata, where I've just mocked up a basic underbody, and then skinned it with a flat floor. For more details on the model and the results, check the video out at the bottom!
Without a flat floor there is significant flow separation under the car, and the air slows down (deeper blue). This is bad for both downforce and drag.
Without the smooth undertray on, the flow is somewhat less than ideal (look at the pictures and video). We've got high pressure regions in front of obstacles and protrusions creating drag, and flow through the engine bay forcing separation. This drops the air velocity under the whole undertray. As high velocity typically equals low pressure, this means we end up with a higher pressure on the total underbody (lift). By adding the flat floor we dropped our drag force at 180km/h by 15kg, and our downforce increased by 50kg. That's a sizeable gain for only adding a flat floor and not performing any other modifications. And the flat floor geometry I've added isn't even close to optimised for performance, so you can really see why manufacturers are starting to fit heaps of plastic panels on the undersides of their cars to smoothen it all out.
With a smooth underbody, the air stays attached and high speed, improving aero performance.
So, by adding a smooth undertray to your car, it's basically a win for either track or street applications, all you need to do is sort out the cooling. For more information and a more detailed explanation, check out the video below!
Dr Kyle Forster is a qualified Aerodynamicist, race car engineer, and the man behind JKF Aero, a firm that offers a variety of aerodynamic consultancy services for racing purposes. If you have any questions for Kyle or have any suggestions for future videos, drop them in the comments below!