Let's say I wanted to build a challenge car with lots of aero. (So effective at autox speeds). But I don't want to change the car's appearance too much. What can/should I do?
Let's say I'm ok with a splitter sticking out a bit in front of the bumper, but not ok with a giant plywood 3 element wing on the nose.
Flat bottom? Splitter? Diffuser? Ground effect tunnels? Tunnels with wings inside?
All of the above. To get significant downforce at autocross speeds you need to do one of tow things:
go BIG with the components you add (bigger does in fact mean better with low speed aero)
AND/OR
go complicated with something like the sucker cars that have been used in the past or active aero.
For both options, to get the effects you are asking for, they need to be removable or tunable or they will kill your drag time with increased drag.
The net benefit of downforce can be measured for autocross, but it is small. Imagine you add 1000 pounds of downforce (how is another matter). For a 3000 pound car you have just increased grip by approximately 33%. Very roughly that should reduce your autocross time by somewhat less than that 33% (drag still works at autocross speeds too). On a typical 50second autocross run, that would land you at roughly 35 seconds. That is a very crude approximation and is not likley to actually be achievable. more likely the time will drop by half of the grip increase, which puts you closer to 42 seconds. Now do the same sort of calculations with something more realistic like 100 pounds of downforce/grip gain. The expected autocross time will be ~98% of the baseline autocross time or 49.2 seconds.
0.8 seconds sounds like a lot and it is in typical competition. You will need to learn to drive the car differently to take advantage of the extra grip and learn the course in six runs or less (at the challenge). I will admit that I am not the best autocrosser due to my lack of trust in the ultimate grip and use my runs as shakedowns for the pro drivers. I can routinely count on multiple seconds of time gain with a pro driver in a car they have never turned a lap in over my time.
If you are a regular, competitive, autocrosser, then adding downforce may give you what you are after, but it may be small. It will require some work and testing to make sure it is tuned and operational and you need to make it adjustable enough to not hurt the rest of the competition (challenge specific).
In reply to stafford1500 :
Steve you're the pro on this, so this is an honest question...
Wouldn't adding 1000 lbs of grip also need a helluva lot of additional horsepower to overcome the drag?
In reply to SV reX :
Not at typical autocross speeds. The gearing reduction and resulting torque multiplication in low gears will be enough to overcome the ~300 pounds of extra drag with that 1000 pounds of downforce. Dont expect to go much faster than 60-70mph though. We could work the numbers and figure out the maximum downforce required to guarantee the tires never have a chance of slipping up to 60mph. Anything beyond that is unused effort.
The real problem with that much downforce is the spring loads. The tire become the springs at some point because the springs have to be so stiff. Tires do a weird thing at high loads where the grip can start to fall off, not because the rubber stops gripping but because the tires mechanical reactions are pushed beyond design limits for total load (car plus aero). If you thnk aero is tough to wrap your head around start digging into tire performance prediction. We have more tire guys than aero guys at work.
In reply to stafford1500 :
Interesting. Thanks!
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