Strake distance on production bases cars

[Fifty]

I've been attempting to read everything I can, I have a few books on order...

It seems most discussion is about rear diffuser angle, convex vs concave, wide throat vs not, distance past the chassis, etc etc...

What I'm wondering, with out any cfd or any real computer skills to speak of, what is a good starting point for strake spacing, short vs long, number of strakes etc....

This is on a Subaru that is going to do some
Land speed runs. So low drag is key to the down force.

Also, a lot of what I've read is the key to these diffusers is to try to run the angle of the flow for the air to go into the flow coming up off the bottom of any wing... However what happens if your not running a wing?

[Just_a_fan]

Why do you need downforce for a land speed run? Just to give some stability?

[Fifty]

Yeah, as much down force that you can get that's not super draggy! The faster you go, the less you weigh, and the road (or runway) is not with out imperfections.

Also, there are a lot of 100-120 mph crashes due to lack of traction when shifting and then also at the moment after crossing the lights and people jump off the throttle, weight forward, and then spin. Or on the brakes, go light in the rear and spin.

Or just when cruising down the track and get light and loose and their is a traction variance and a spin.


Also, once the land speed run is done, the car will become a track day fun car. So I'm not completely sacrificing the future just for the land speed run (it will hurt me here and there, but this is just for fun, not for any sort of real competition points etc. I have a goal of 205 mph)

[Just_a_fan]

Sounds like you need just enough to counter the natural lift that comes with all production road cars. Have you thought about trying a small spoiler/ gurney on the rear edge of the tail? That can have a powerful effect with surprisingly little drag penalty.

By strakes, do you mean the "dive planes" on the front corners:

[Fifty]

Awe crud. I just realized that my post made perfect sense to me...however didn't contain any details to make it clear to anyone else. Sorry about that!

The car has a small "wing" from the factory. It's a hatch, so it's more of a drag reducing extension. I am running a gurney flap on that.

What I am going to do is build a front diffuser, that extends about 12 inches from the front (I found a pressure map of the cars front) and then also build a flat bottom.

As the flat bottom heads to the rear, I want to build a rear diffuser. I will have the exhaust split and exit up in the rear bumper allowing the diffuser to rise quite signifucantly.

What I am looking to figure out, is the number, location etc of the strakes on a rear diffuser.

[Cold Fussion]

By strakes, do you mean the "dive planes" on the front corners:

He's talking about the vortex generating strakes inside the diffuser.

[Fifty]

The first McBeath book showed today, and I am reading it now.

This is not my car:


But I am going to cut the bumper up like this. Also, instead of a single exhaust, I am going to split it and have it tuck up and exit above and out board of the two reflectors in the black part of the bumper.

Here is a picture of what the bumper originally /currently looks like:



So I need to still build the flat bottom with some gates to direct flow etc, and then sort out the rear diffuser. Curved or flat (going up) and then how far out past the bumper if at all, and the stakes...

[Cold Fussion]

What other books are you getting? The Joeseph Katz book's are excellent. Carrol Smith's 'Tune to Win' gives a good practical understanding as well.

[Fifty]

The Katz book is the other new one.the rest I've had for years. Stacks and stacks, suspension, tune, drive, composites etc etc wtc

I've played with "aero" before, but nothing as detrimental as a car that is going over 150
And up into the 200's...

[Just_a_fan]

By strakes, do you mean the "dive planes" on the front corners:

He's talking about the vortex generating strakes inside the diffuser.

I know that now.

The strakes aren't necessarily about vortex generation, they also act to ensure that separation in one part of the diffuser doesn't affect the whole diffuser.

To the OP, there are unlikely to be any "rules of thumb" for this: you will need to assess the diffuser and strakes either by CFD or track time. Flow viz would be useful if you can make/obtain some.