Definitely more grip/less sand than last year. The good thing about running Thurs/Friday this year will be that the line will be established and good rubber will be laid down. The bad thing is that the line will be established and it will be very DIRTY off line.

 

Hey, only on day one ........and that would be Geoff Clark.

Dave

 

Yes, a properly designed wing can make significant DF at ~40mph.

For example, the wing on my car, in a wind tunnel, should make around 100-120# of DF. On the car with the dirty air coming off the roof of the car it probably makes 50-70% of that number.

Now in a car like ours you want to make the rear end loose so it rotates easily in slower corners. Thing is, at higher speeds you start to get squirrley and the lack of rear traction becomes your limiting factor. The wings make exponential downforce vs. speed, so the faster you go the faster the downforce ramps up. So, even thought a particular wing may only make 30# @20mph and 100# @40mph, it will make 700# @100mph. So, the faster you go the more stable the car gets. . .

With our cars it is possible to get TOO MUCH rear downforce, then the car will just understeer and push all over the place because the front tires have less traction than the rear. That's it in a nutshell.

EVOlutionary

 

Saw geoff(sp) in topeka just cruising so pulled him over.. hopefully I can make it there since its only a couple minutes away but since im in school and all idk if i will be able to make it.. looks awsome though

 

That doesn't mean it's exponential. The resistance, and therefore the downforce, grows with the _square_ of speed.

d

 

Thanks overview of the basics Evolutionary, but donour is right, downforce increases as a function of the square of speed. My thought is that wings do not make a significant difference @ true auto-x speeds. There is always the exception to that, as certain regions run higher speed auto-x events than others or what is "officially" stated in the rule book. In these scenarios the effects of a wing will be greater.

With a wing positioned with such a high AOA the car is going to be producing significant drag but again at low speeds that is not a significant issue. And its not like we have long straights in auto-x where it's limiting our top speed by much. Also the full effects of a wing are seen when a car is traveling in a straight line.

Let's say it sees 70% of a 100lb of downforce = 70lb of downforce when going straight.

When a car is cornering the downforce drastically reduces, how much depends on the speed of the car which depends on the sharpness of the corner. From my experience "High speed" auto-x corners then to be around 45ish mph and low speed tend to be 10-15ish mph, depending on course design. Why does the downforce significantly reduce? Because the surface area the air is traveling through to generate the downforce greatly reduces. The air is now going at an angle over the wing which greatly reduces it's "effective" surface area. How much "effective" surface area will depend on the corner and the sideways angle the air has to travle over the wing. A sweeper will give you more and sharp corners significantly less.

Let's say during a sweeper the effective surface area get reduced by 1/2. As we know downforce is a fuction of speed, surface area, AOA, etc ... This should reduce the available downforce by 1/2 it's straight line value.

So we then get 0.5 x 70 lbs = 35 lbs of downforce during sweeper that is if we were going at the same speed as a straight line. But that isn't the case as we have to slow down for a corner. And since downforce is a function of the square of speed, every minute drop in speed will significantly reduce the amount of available downforce. So the 35 lbs of downforce would still greatly reduce as the speeds have dropped down.

Eg: This is not the exact aero formula but only serves as an example to show how much difference just a 1mph drop can make ...
4mph^2 = 16 lbs
3mph^2 = 9 lbs

So let's say the 35 lbs drops down significantly due to a reduction in speed to say 18lbs of down force (intelligent guess) total for the rear ... In the grand scheme of things, you could get more than that with your spare tire in the trunk.

See where I'm getting at with this ... Since auto-x is a constant state of cornering at relatively slow speeds the full effect of the wing is never realized. If your cornering speeds are in excess of 40+ mph like on a race track then it makes sense to tweak the rear wing to increase grip. That is why I just don't think wings make a significant impact @ auto-x speeds and is not worth the effort ... but I also realize ever little bit counts. Thoughts?

 

You might just want to ask those with huge wings on their Evos if they put them on their cars for looks only, or if there is a "functional" reason.

It would be interesting to see what kind of answers you get.

 

I have probably the largest wing on any autocross EVO in the country. It is there for a reason. You can read my thread on EVO Aerodynamics Review (or something like that) for my reasoning.

You could also ask Eric Stemler for his input. Last year he had probably the largest wing every mounted on any evo in the world (maybe not, but it was pretty big, and it was there for a reason)

EVOlutionary

 

I agree that the downforce is almost useless, but I think what is missing is the drag analysis. Lets just say that the wing is making at least as much drag as it is downforce at the angle it's at (yes i know that is probably still conservative because it's most certainly stalled so the drag is likely to be much higher then the downforce). So you put 70lbs of drag on a ~3 foot lever arm (about the CG, not the trunk lid). What you get is a significant moment applied to the car, which is reacted by increased force in the rear tires and decreased force in the front tires. Is this moment going to increase your peak cornering force? of course not. It will change the balance of the car in a speed dependent manner. As Evolutionary said, this allows the car to be setup statically with more oversteer with out having the back end get all out of shape in the higherspeed stuff, aka slaloms. If you put a wing on your car and didn't make any suspension changes to accomodate the change in balance, you will be slower. Assuming your car was balanced before you put the wing on.

 

Thanks for the correcting my mathematical terminology. Sorry, haven't had a math class in a while . Isn't exponential growth the same things as growing at "the square" of something.

Basically what I was trying to say is that you will gain more DF going from 50-60 than you will going from 20-30.

EVOlutionary

 

Just a question - do you think that Vic Sias and Bob Tunnell have huge wings on their BMW's for looks? Do you think that without the added traction they would be as sompetitive as they are?

Have you seen George Bowland's AMod car? The fastest autocross car in the country? It looks like a little sprint car with huge multi-level wings in the front and rear.

Properly designed wing systems DO work. You make as much grip as possible in one end (whichever one is more "grip limited" and then use the other end to tune the traction balance of the car.

 

Another thing to add that isn't quite as apparent. The effect the wing has on driver confidence can not be neglected for most of us mere mortals. A car that gains stability as speed increases goes a long way towards keeping the driver from chickening out on some course elements that otherwise would scare the crap out of you. So while the wing may not overall improve the performance of the car it can help improve the performance of the driver.

 

I believe Steve's comment is in regards to the ST* rules that state
So if you're using a wing in ST*, no, no they don't provide significant down force. wink wink

 

If you look around, the cars that do well with big wings are the RWD high HP cars. What is the limit for putting down power in a SM BMW? Corner exit, throttle on oversteer. What does a big rear wing do to the car? high speed understeer. So the easy answer is they absolutely wouldn't be as competitive without the big wings. However I can guarantee that it isn't because the wing is increasing their steady state grip level. I'm sure they do 1.3-1.4g's lateral just like the rest of us.


where you don' t see big wings on competitive cars is on cars that have corner exit understeer (stu evos???) like sts civics, and pretty much anything FWD.

 

*scratches head*