jimcroisdale wrote: ↑
Tue Jul 10, 2018 10:32 am
Given a "Cockroach with Flat Bottom" shape, what are the implications of the flat floor? Would it give any downforce without a diffuser? Would, say 30mm ride height be significantly different to a 100mm ride height? What difference would it make?
Yes, even with a prefectly horizonal floor and no diffuser, it will produce some downforce.
For example, some high pressure air that accumulates at the tip of your nosecone will spill under the floor while decompressing. Judgind by the shape of your nosecone, there's a chance that airflow will spiral into little vortices. Also, low pressure at the sides of the bodyword might suck some air out of the floor, like a diffuser would do. The end result being some downforce.
As e36jon pointed out, there is more friction closer to the ground due to viscous effects. That doesn't only produce drag, but increases pressure too (producing lift).
But let's not forget about Ground Effect, which is an incredibly efficient way of producing downforce. That effect heavily depends upon ride height. 100mm height might give you about a lot less downforce than 30mm.
But, as mentioned before, you don't want to get "too close" to the ground due to the prevalence of viscous effects.
"Too close" depends on how performing your undertray is. The less performing, the higher you want it to be. In other words, there's a sweet point (in terms of ride height) between Ground Effect and Viscous Effects, and that sweet point will be closer to the ground if your undertray is potentially able to produce a lot of downforce.
All in all, I think it would be good for your car (if you actually need that downforce) to have a flat floor as long as you can find the sweet spot. You could increase downforce or manage its centre of pressure by raking your car by some fractions of a degree. I would also tend to use a diffuser because that's the only way to produce a sensible amount of downforce for your driver to feel more confident around the corners; flat floor in ground effect + diffuser is one of the most efficient ways of producing downforce, and you would't have to worry too much about the mentioned viscous effects even at 30mm.