MuSTANK Reader
12/29/22 12:13 p.m.

I looked at the aero section and didn't find any answers. This may well not be the correct area either, but since so much of The Challenge has to do with autocross, I thought that I would give it a try.

Simple question I guess. Does aero/wings/air dams/splitters/spoilers actually make a difference at autocross speeds? Our local autocross has a twenty foot straight at the start and a thirty foot at the finish with twistys and turns that pretty much require that the cars be driven at paring lot speeds.  The cars pretty much don't get over thirty/forty miles an hour.

I've seem many, many autocross cars on-line with extensive aero attached and I REALLY love that look, bit it would be expensive to buy and expensive to manufacture if unnecessary. I'm hoping to build my '58 Fiat 600 into something for The Challenge and would very much like to justify adding Abarth-style hill climb aero to the body, just not really sure that I agree that it's needed for such a low speed event as autocross.

Any help or answers would be a benefit right now. Thanks—

sleepyhead the buffalo
sleepyhead the buffalo GRM+ Memberand PowerDork
12/29/22 12:24 p.m.

site has a big impact on "is aero worth it".  At your site, it sounds like not.

At the SoloNats site, and GRM Challenge site... speeds can top 60+mph.

also, "aero" can work at speeds as low as 5mph.  Recall:

Lift = 0.5*Density*(speed^2)*Area*Coefficient

so, if you reduce speed, you have to make up for it in "shape" (I.e. coefficient) or area.

there's also the Cheaparral / McMurty approach of "power augmented downforce"... with more than (their) one way to go about doing that.

Another thing to keep in mind:   
increased mechanical grip means you can corner at higher speeds, which means the aero can be more effective, which makes more grip, which means you can go faster... until the tire runs out of grip.  But, that can be a difficult driving experience to develop and then trust.

MuSTANK Reader
12/29/22 1:25 p.m.

"also, "aero" can work at speeds as low as 5mph."

WHOA, I had no idea that aero would make a difference at walking speeds! ! 

Thanks for the information, looks like I won't bother with the whole Abarth thing after all.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
12/29/22 1:33 p.m.

To make aero work at slower speeds, it has to be bigger and more aggressive.  Which is basically what Sleepyhead said but a little more distilled :)


sleepyhead the buffalo
sleepyhead the buffalo GRM+ Memberand PowerDork
12/29/22 1:49 p.m.

yeah, I might have said that a bit ineloquently...

"you can make downforce, at 5mph."  I don't know I'd go so far as saying that it would necessarily 'make a difference' that slow, though.

100lb.downforce / (0.5*.002377*(7.3 ft/sec ^ 2) ) = 1566 sqft

a "maximum" Cl you could probably try to achieve is around '5'.  So, you'd need ~300sqft of "downforce creating area" to pull that off.




maybe I should raise that from 5mph to 15mph?

That'd be about 35sqft required.

Byrneon27 GRM+ Memberand Reader
12/30/22 7:34 a.m.

In my extensive "country engineer" testing of autocross aero the functionality increases with tire and horsepower. The wing elements need to be very aggressive and by nature drag heavy, you need power to pull 'em through the air. 

GameboyRMH GRM+ Memberand MegaDork
12/31/22 4:25 p.m.

Quick example of aero developed for autocross:

MuSTANK Reader
1/1/23 1:51 p.m.

We had a car like that compete at the autocross at the Horse Neck Beach parking lot a while ago. Very well build and as interesting as can be to see all of the intricacies that went into the build ! 


nocones GRM+ Memberand PowerDork
1/1/23 2:44 p.m.

I put back yard engineered "full aero" on the LMP360.   

Does it work at autox?  I dunno I was the fastest $2000 car despite being on street tires.  I will say this about it.  At low speeds in full mechnical grip the car is biased to oversteer.  As speed increases even at Autox speeds the car transitions to nuetral.   At nationals at the top of Autox cornering Speeds the car started to have a sniff of understeer.  My aero is very rear biased.  

At the post challenge firm trackday it absolutely worked.  The car felt very different in the 70mph corners then it did in the 30-40 mph corners.  

Also I removed the second element of the rear wing and the entire front wing for a drag pass and the car went 4mph faster trap speed, but was less stable when lift/coast at the finish line.  With the full aero on it was rock stable but that changed with the aero change.   Those runs where within 20 minutes of each other and the launch felt similar for both so I don't believe it was track evolution that caused the change.  

MuSTANK Reader
1/2/23 2:14 p.m.

In reply to nocones :

Thanks for the real world information ! ! As I mentioned , I would just LOVE to fit the Fiat with some serious Abarth-style hill climb aero, even if I end up making my own or modifying some existing fiberglass pieces, (abarth repros are out of range) your post is encouraging for sure. To this end I traded the dune buggy body for some Mullholand Look VW pieces. Sort of a fiberglass for fiberglass trade. I read that Abarth used to keep the rear deck lids open to act as a rear "wing" and also keep the engines cooler. My little shell came with a semi-trashed rear lid, kind of hoping that the VW parts can be made to work and have an eventual affect. The basic Mulholland Look rear wing and front air dam are shown in the advertising photos below. 


AnthonyGS (Forum Supporter)
AnthonyGS (Forum Supporter) GRM+ Memberand UberDork
1/2/23 10:05 p.m.

Yes it does work at autocross speeds.


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