Yargk

Friction isn't actually my area of expertise so I'll fall back on empirical evidence instead of looking up the theory. (reading your post again, I think this is what you want). It seems to me that if you take a car and reduce the weight by 10%, then the cornering grip is actually not changed much. If it doesn't change at all then this implies a linear relationship (reduced pull of the mass of the car in the corner exactly balances the reduced frictional force from less downward force). If the grip goes up if you reduce weight then the relationship is not linear, but if grip only goes up a little then it's very close to linear. Downforce is just adding more down force on the tires without adding weight to the car so to me the above gives evidence that a linear relationship is very accurate over a good range of downforce (say 0-10% of the weight of the car for fun).

I'm sure at higher amounts of downforce the real world deviates from this linear relationship. Again using the heavy/light car analogy, imagine taking a 3000 pound car, adding 1000 pounds and not making the tires any wider. Of course you'll have a lot less grip and this is evidence of a departure from a linear relationship. Basically, if you push down harder on a tire it will grip more. At first this is linear, but later you'll get decreasing returns.

Paraphrasing wikipedia, apparently the simple linear relationship that I've been using is "Coulomb" friction (although I've read elsewhere that Leonardo da Vinci actually found the relationship in certain specific instances a few hundred years earlier). This formula assumes that there is only a small portion of the two surfaces actually in contact and this contributes to the friction (think of rough surfaces touching, only the peaks touch). As you increase the normal force, this area of contact increases linearly. But of course this can only be approximately linear.

Yargk

CJ:

You bring up a good point. If your car isn't stiff then large amounts of downforce will throw things off and you can put the suspension in a regime of decreased performance. However, even with a perfectly stiff suspension, you'll get less than linear returns from BIG downforce just due to the nature of the frictional force vs. normal force.

allegretto

I think you're getting at the point I was interested in which I'll restate thusly;

OK assuming the tires are not the limiting factor I think there are forces that do not increase linearly. But putting that aside, I think there are real world limits on a tire too. So while the equation seems to indicate "x", the real world outcome is "x-y" where y is the limit of the tire + non-linear components of the tire/road interface.

chris walrod

Trouble is DF is not simply proportional to speed. Its not a direct relationship and one would have to look at an aero map of this Viper to determine what DF levels correlate with what speed. Sadly its not a simple thing, aero work.

The biggest roadblock in making a street car aero worthy, so to speak, is ride height control and values and the inability to use real tunnels under the car.

Ground effect racecars are setup to run as low as the driver will tolerate, literally. Too low, butts get hot from heat generated being on the skids too much.

I feel the largest gains any street car will really benefit from is aero drag reduction. When I say street car, I mean something that can be driven daily. Jack does a great job at configuring his black beauty in adding wings and floor bolt on bits for track events.

In the end, for a street car, all we can do is flatten the bottom of the car the best we can (Porsche does a good job at this straight from the factory), spring up the car the most you can tolerate and run it low.

Wings on the back of cars are generally to negate aero lift, not so much create real downforce. Wings, I mean factory type of stuff, not the monster wicked stuff we see on a lot of track only cars.

More downforce is more drag.

C.J. Ichiban

yay at least I got something right! thanks for chiming in chris.

allegreto- tires can handle a lot of punishment in terms of lateral g's if they have the maximum vertical force that yargk was referring to and the least amount of weight trying to change directions... hence GT1 and GT2 ALMS cars can hit peak lateral Gs around 3 and sustain 2.5 just because are essentially super modified, stiff and lighter weight street type cars (rather than radicals, swift formula cars, sportsracers, etc)

allegretto

CJ-

you've touched on the kernel of my interest. with the lower weight, big wing and low front, flat bottom to the Cup I had an interest in how much this all helps me increase my G's. MOTEC says "a lot!!". but I know I'm nowhere near the cars idealized limits. so far I've just been toying with it, getting used to things happening much faster and quicker.

next year I hope to be able to get down to optimizing the platform for me. I just wasn't sure how far the tires would take me. there is little warning in this car compared to street cars...

NJ-GT

The only aero differences between the Viper ACR and the SRT-10 Coupe are the rear wing, the front chin spoiler and the dive planes.

Notice the chin spoiler and dive planes in the Cup S, and the bigger and taller rear wing. Porsche's ACR treatment for the GT3 Cup.

Even though the extra drag will reduce speed on the straights, the higher cornering speeds will produce lower lap times.

These changes can be done to a dedicated race car, I don't see how a street driven car can survive with such low ride height and chin spoiler. But to CJ's point, bolt-on parts for just the track day is not a bad idea.

va122

If I remember my physics correctly the relationship is not linear as drag increases to the third power relative to speed. Please don't ask me to derive it.

I personally, have the cup ft spoiler, RS nose with the radiator cant and duct and the gurney lip from the 997 and the 8 deg shims on my RS wing and I'm looking for a wicker (gurney lip for the top wing) All easy changes to do. Just shake the money stick at the car.

Yargk

I just used the square relationship as an approximation.

summary for all the stuff above:

at low downforce, cornering G linear in (downforce/weight) i think

downforce roughly proportional to the square of the speed so if you go twice as fast you'll get 4 times the downforce

Drag FORCE also proportional to the square of the speed. However power is force*velocity so with another factor of speed you get power used to overcome drag is proportional to speed to the third power.

Yargk

When thinking about tire limits I believe that a 3000 pound car with 1000 pounds of downforce cornering at 1.33 Gs puts the same stress on a tire as a 4000 pound car with zero downforce pulling 1 G. So certainly the tires would hold up to our modified high downforce track day car. As CJ pointed out, they can actually hold up to a lot more.

allegretto

would seem to me that a 3000lb car with 1000lb downforce would put exactly the same stress as a 4000lb car with no downforce. force is force

BTW, nice to see someone understand what "power" is. perhaps many of you here know this, but you'd be amazed by the discussions I've had with folks who think it's about "force".

C.J. Ichiban

allegreto, keith and rad- you're getting dangerously close to advice that is no longer free...I'll be in touch when we're ready to get some 'help' to you

Yargk

I was actually going to say the same thing to you after reading one of your responses in the 7gt3.2 thread. It's very refreshing.

I think people understand that power is related to top speed, but they get hung up on the F=ma thing for acceleration. To show that power is also what matters for acceleration I tried explaining it once on the board by showing that force at the wheels is due to gearing and torque. The gearing is effected by what rpm you make the torque at. So then it turns out force at the wheels is proportional to rpmXtorque. What's that... POWER.

Yargk

insite

downforce is a funny thing. with regard to grip, here's a layman's explanation on how it changes with aero (yes, engineers, i know newtons are force and kg's are weight, but we are on earth, so i will use kg's as force for clarity).

imagine a car that weighs 1000kg and has no downforce. assume it can corner at 1G. this means that the car can withstand 1000kg of lateral force before it loses grip.

now let's add 500kg of downforce. now the total weight on the tires is 1500kg. my tires are still good for 1G, but 1G of grip is now 1500kg (my total vertical load). guess what? the car itself still only weighs 1000kg; laterally, i am still only controlling 1000kg. i have just increased the lateral force required to break the car loose by 50%, which means i can increase my corner speed. it would appear to an accelerometer in the car that i am able to pull 1.5 lateral G's. the tires are still only pulling 1.0G relative to the vertical load applied to them. make sense?

there is a caveat: tires become less efficient when over or under loaded. what this means is that if a tire is capable of 1.0G while loaded with 300kg, this does NOT mean that i still get 1.0G while loaded with 1000kg; it will get something less. there is an optimal pressure load for any given compound. deviate from this optimal load over or under and grip deviates from peak. race tire manufacturers usually provide graphs that show lateral grip as a function of vertical load; these can be used to choose the best size / compound for the chassis / aero package.