Undertray and Diffuser Question

[Works]

Hi there. New here.

Stumbled on this site while looking for answers to various questions I had. Signed up immediately, as this appears to be a very knowledgeable place to be.

Im busy designing a track day car. No rules, no regs. Been reading a lot on diffusers, rake etc, with lots of conflicting ideas being thrown around.
I was hoping I could get some sound advice on this design. Ive pretty much set up my CAD files so that I can easily change the length and profile of the splitter, rake and rear diffuser.

I probably have gone way too extreme on the diffuser for example, but was curious as to what you aerodynamics guys feedback would be on this.

[Works]

Dimensions in (mm)

[Works]

[Works]

Oh, and just a further note: there will be side skirts, which are not yet done.

[RicME85]

Im no expert but from what I have learnt doing the KVRC/MVRC competitions, the end of the diffuser goes up far too aggressively. You would be better moving the second kick towards the initial kick of the diffuser to lessen the severity.

[Works]

Hi RicME85

Thanks for the reply.

More like this ?



Moving A to something more like B.
Also, how does C affect things?
Should it ideally be parallel to B, or is tapering it off towards the body just fine?

[rjsa]

Even B is too steep IMO. C should be vertical.

EDIT: The flat part forward should end where your car's CG would be. That'll be the approximate center of pressure.

[RicME85]

Agreed

[Works]

That's a great ref ... Thanks guys ... Will post up revision ...

[Paul]

If you really want to look for limits there, I'd make the angle adjustable, mount a camera and do some test runs with old-fashioned flow-viz:

[Works]

@Paul, ha ha yes That would be a good exercise ... natures windtunnel ... but still got to build this thing ...

Guys, heres a revision based off feedback:











Any thoughts??

[Works]

[Nickel]

I think that ferrari routes it's exhaust into that middle space. I assume if you're not putting anything there in your design, it would be best to keep this part profiled as the rest of your diffuser. Someone else can correct me if I'm wrong/

[e36jon]

The new layout looks more 'right'. Congrats.

Looking at the bottom plan view (two posts up) it seems like the channel + strakes may converge / pinch midways through. Air being elastic nothing will break, but, I believe the ideal situation is to expand evenly going towards the rear. You can double-check your reality by making some cross-sections along the path and measuring cross-sectional area, and it should be a constant increase... (Not well explained but hopefully you get the idea.). The 'fix' if it is indeed needed will probably involve narrowing the beginning of the transaxle cover somewhat.

Looking forward to seeing the progress!

Jon

PS: Loved seeing the Miata with the aero test tufts on it. Not a car I associate with aero too much...

[Works]

I think that ferrari routes it's exhaust into that middle space. I assume if you're not putting anything there in your design, it would be best to keep this part profiled as the rest of your diffuser. Someone else can correct me if I'm wrong/

Yup, good point, but I actually need that space for a pretty substantial diff, so its worked out pretty well just aping the whole thing ...

@e36jon >> If I understand you correctly, you're saying that the four distance values (in magenta) should be equal throughout the entire length of the fins?