What needs to be tuned/adjusted when adding aero for autox ?

Hi everyone,

I'm considering adding a spoiler, like the one shown below, to my Miata for autox this year. I'm looking for your help & advice on what tuning should I'll need to do to get the most benefit from this. My questions are in 2 areas: what adjustments will be needed on the car? and what is the best way to tell if an adjustment it helping or hurting?

On the 1st question (adjustments on the car), I'm thinking of three possible areas. The angle of the spoiler, shock damping/rebound, and spring rate. Are there other possible adjustment you can think of or suggest ?

On the 2nd question (how to tell if an adjustment helps/hurts) I don't have any good ideas. How can I tell if the angle of the spoiler is too steep or too shallow?

I'd appreciate any advice or suggestions that you have!

Thanks,

-- Pete

I have no experience on this, but without data acquisition, you may have to only go by autocross times and feel. You can also adjust tire pressure (of course) if it changes rear grip too much for your driving style. Of course, the angle of the spoiler needs to be pretty steep because of the slow speeds of autocross.

Mark Donahue says "Skidpad testing for the win."

Start by looking at CSP cars, you'll see some angles to start with. And keep increasing the angle until your times start to increase.

In reply to PeteD: Seems to me...extra downforce on the rear should be balanced by extra downforce up front. In any event, downforce is like extra weight, as far as the springs are concerned, which would create a need for increased spring rate, to keep the downforce from using up your spring travel. Stiffer springs will call for more damping, and perhaps more tire pressure.

Adjustment to the driver would be the first thing.

@Keith Tanner: good idea! I'll try that!

@Danny Shields: yes, I'm also worried that added rear down force, by itself, will make the car understeer.

iceracer wrote: Adjustment to the driver would be the first thing.

iceracer, I don't get what you mean, can you explain a bit more?

-- Pete

PeteD wrote: @Keith Tanner: good idea! I'll try that! @Danny Shields: yes, I'm also worried that added rear down force, by itself, will make the car understeer.

iceracer wrote: Adjustment to the driver would be the first thing.

iceracer, I don't get what you mean, can you explain a bit more? -- Pete

Improve driver skill before adding aero aids.

Check out our podcast with Steve Stafford. We talk a bit about non-wing autocross aero. Long story short, you can indeed make significant downforce at autocross speeds with Street-Prepared level aero. But you have to think about both ends at once. A properly designed rear spoiler should be paired with a properly designed front spoiler and (ideally) splitter to control the extremely turbulent air at the front of the car.

As for angle on the rear, it will generally be quite steep. You're basically creating downforce through adding drag leverage, not necessarily by redirecting the airflow. You're essentially using that rear spoiler to create a localized high-pressure zone over the trunk.