I was playing around with some numbers and found some interesting results when it comes to downforce. In my opinion, for anyone interested in really increasing the track potency of their 2 or 3, aero work should be on the list.

Here's why (I'm sure many are already aware of how much of an impact real downforce makes, but I found it interesting seeing in numbers what a comparable "street" car can do with aero).

Differences between a track day car with a serious downforce package and one without seem to be as big as the difference between the best street tires and toyos or the difference between toyos and hoosiers. GT buyers spend all kinds of money on a GT3 or GT2 as a great track day car because of great suspension/brakes/engine/tranny, but the stock downforce leaves a lot to be desired.

The new Viper ACR claims 1000 pounds of downforce at 150 mph. This should be around 444 pounds at 100 mph or 217 pounds at 70 mph.

What this means for handling is that you should have more grip. It's easy to figure out how much more because how much grip you have is proportional to the force on the tire. If you add the downforce to the car's weight and divide by the weight you can see what percentage more grip you'd have.

At 70 mph you'd have 6.4% more grip or you'd be able to pull 1.064 G's instead of 1

At 100 mph you'd have 13.1% more grip or you'd be able to pull 1.131 G's
instead of 1

As we all know, these are huge increases in lateral grip

By comparison a GT2 makes only 57 pounds of downforce at 100 mph instead of 444 and only 26.8 pounds of downforce at 70 mph instead of 217.

 

keith the only problems with trying to get more aero from street cars

1. who is really going to tack dive planes on the front bumper?
2. flat bottoms are pretty hard to install, find, maintain
3. ride height is a big factor- don't see too many street cars with RSR ride height...

 

Yes, all good points.

1. Me.

2 and 3.
Race cars can do things that street cars can't or more importantly things that you wouldn't want to do because of curbs (ride height). However, besides style, I don't see anything on the ACR that makes it less streetable. The ACR does have underbody goodies, but I think a lot comes from just the dive plates/splitter/wing.

I guess my point is that there are (large) gains to be made without sacrifice besides looks. This doesn't even have to be worked out by the factory. A good example is the aftermarket lotus aero by ReVerie. It seems very well engineered and adds 300 pounds of downforce at 100 mph with only splitter/dive plates/wing. Surely the GT2 can do better than 57 pounds. I'm sure a splitter that you could design to attach at the track along with an only slightly larger wing adjusted to bring a higher angle of attack at the track would bring that number to 157.

Reading some old posts, I think you're familiar with Jack Olsen's aero work. He's done a lot and I think it has helped get that relatively low powered street 1972 911 to 2:01s at Thill and 1:29s at Willow. However he can easily run with just a ducktail and no front splitter for the street. It seems to me like for some people aero could be fun to tinker with like suspension is for everyone who tracks their GT cars.

 

Jack's car

 

Downforce comes at a cost - drag and thus highway mpg, right? So there may be business reasons why road cars like the GT2 have limited aero.

There's talk of "active bodywork" coming on future generations of cars.

 

The problem with 911´s is the engine placement as always, you cannot really do anything good underneath the rear of the car which is the single most important place to work with when it comes to aero..

 

Keith,

I also have heard the ACR Viper is downforce limited to 160mph. Even so I watched it run 2:02's @ VIR on Michelin PS2's (Viper Spec which are a softer compound than regular PS2's which was confirmed by Michelin) during the 2008 OLOA. Mark Davia's 996TT pulled the Viper out of then Oak Tree up the hill but was slower overall.

Peter

 

making a "track nose " isn't a bad idea...chin spoiler and dive planes would be really easy to add 100-250lbs of downforce and anti-lift to the front end.

triss- you can have anupward exiting flat underbody on a 911 with a bit of rake just like the 997RSR. you'd probably not want to run it in traffic or on aircooled cars however as heat soaking would be bad. RSRs can generate 2.5 sustained G through their aero and michelin super slicks.

 

Yargk

you have assumed a linear relationship with downforce and G's. I don't know and it may indeed be a proper assumption. However I would guess that the friction coefficent of the tires alone would decrease that.

Do you know the case?

 

To a good approximation the frictional force provided by the tires is a linear function of the downward force which is downforce + the weight of the car.

If you assume that with no downforce you can pull 1 G in a corner then the coefficient of friction is 1.

G = (Frictional force)/(weight of car) = [Coefficient of friction * (weight + downforce)]/weight = coefficient of friction *downforce/weight + coefficient of friction*(weight/weight) = 1 + downforce/weight

G = 1 + downforce/weight is the relation I used above.

However, you're right to suspect that this is not a perfect relationship. In fact frictional force is not exactly a linear function of the downward force. It's less than linear. This is why lighter cars or cars with wider wheels can corner better than heavy cars or cars with narrow tires. If you make a car lighter, you're getting less grip per tire, but not as much less as the car is light so you can actually corner harder. So the grip provided by downforce is slightly less than above, but I think the correction is very small for the numbers given.

 

keith, be honest- you took that answer straight out of the skip barber handbook!

in laymans terms sans equations...

downforce affects lateral G by providing an increase in relative tyre contact patches which therefore allows the vehicle to corner harder. ultimately- the relationship between cornering speed and lateral G involves the radius of the turn as well...the faster and tighter the turn, the more stress it places on the suspension and tires. there is an actual mathematical constant but it's not really necessary unless you're running computer simulations to discover exactly how fast corners should be taken (that's how they calculate theoretical speeds in F1)

the effect of aero downforce is negated by poor ride height tuning as well as poor shocks and springs.

the stiffer the car is, the more effective the actual aero is- and therefore the more the contact patch increases, and lateral G potential increases. "aero center" is also important because you can affectively unbalance the car like the Gt1 did a few years ago and make it do a wheelie and have it become undriveable...

so- slam your ride down to the ground, throw slicks on, and crank up both the rear wing and block as much air from going under the nose as possible! ever notice how modern corvettes have a totally vertical air dam under their bumper? anti-lift is another component...

so allegreto- to answer your question- downforce is only as linear as your springs are stiff. if ride height is effectively constant, the downforce will more effectively translate into increasing lateral G's. its difficult to tune sometimes though, because for sports racers or formula cars that are so dependent on stiff chassis and aero to help at high speeds- they skid and understeer terribly below 70 or 80mph (the speed that aero really begins to kick in)

phew I'm out of breath.

 

phew, I'm out of brain cells

 

OK, but it seems to me that in the real world;

1) The tire design will only allow "so much" grip before the frictional forces are overwhelmed by the lateral forces
2) The tire grips the road, but the road also grips the tire. Also there are electrostatic forces between tire and road. Do all those forces repond linearly, or nearly so, to downforce?
3) Tires have different coefficients on different surfaces. Is that also linear?

I guess what I'm asking is; you have described a theorhetical relationship, how well does that translate in the real world. I'm not asking about shocks and rebound etc, just if real-world factors influence the relationship or it pretty much holds up across a broad operating range.

 

Interesting thread.

So if we take the OP original theory that 57 lbs. of downforce is far too little for a car like the GT2, then what would the increase downforce numbers be if you were to put on a cup splitter and an RS wing? Those are the factory pieces available at this point, uless you go with a GT3 rear cup wing.

If you go to any POC or PCA TT or club race, most everyone has the double element wings on the rear of the car that were fabricated in someones garage out of fiberglass.

Chris Walord should be able to chime in on this discussion.

 

Nah, it's just freshman physics, if it matches skip barber, maybe I did it right.