A car's dependence on aerodynamic grip

[wrigs]

Could one imagine a car that could go through a certain corner at 100 km/h but not 50 km/h, due to it's high dependence on aerodynamic grip instead of mechanical grip?

[wesley123]

yes i could

[speedsense]

Ummmm- a car with high downforce and little mechanical grip, who we going to get to drive such a car? LOL....

[wrigs]

Ummmm- a car with high downforce and little mechanical grip, who we going to get to drive such a car? LOL....

I'd do it. You know, for science.

[hollus]

I read from Niki Lauda that when he was running the "Fan Car" (1976 I think), if it would lack grip in a corner, you could simply take it faster the next time around and it would grip better!
It had more to do with engine revs than the cars' speed though, as the speed of the fan was dependent on engine revs.
Now... this reminds me of blown diffusers...
The Bulls took turn 9 in Barcelona at full blast this year. It would be interesting to know whether they could have made it "just lifting", or then they would have had to brake like everybody else?

[bettonracing]

Not to mention that driving the car slowly will not maintain the heat required in the tyres (and possibly brakes)...

Some semi-pro drivers of aero cars complain about this "too slow for an aero car" phenomenon. EuroBOSS, DSR & drivers moving up into LeMans prototypes come to mind.

Regards,

H. Kurt Betton

[RacingManiac]

The whole Top Gear episode with Richard Hammond trying to drive the Renault F1 car comes to mind. He needs to go faster to get the wing to work, to put heat into tire and brake harder for the brake to work. Going slower doesn't do any of those thing and as a result he can't go faster because there are no heat in the tire, no downforce, and no brake.....

I think if you plot the cornering speed vs performance of the aero downforce, the tire performance for vertical load under 1G, and ultimate tire grip, there is probably a transition point where the aero load overtakes the tire grip and both when they ultimately being limited to the ultimate grip.

[philgarraway]

Yes a car does rely alot now on the grip that the aerodynamics produce over mechanical grip. As the car travels faster, more air flow over the body work of pushes the car into the ground. Great examples of this are, if you look at the Red Bull's from Japan practices this year you will notice that they take turns 2 to 7 on full throttle, compare this to the HRT's they are on 1/2 - 3/4 throttle, the HRT's have alot less down force and therefore can't control, when watching the in-car shots of the HRT drivers you can clearly see them struggle with the car as they fight for more grip. Red Bull have had one of the best cars for some years this year because of the aero packages that Adrian Newey designed in the off-season, hence there success this year.

[Ciro Pabón]

Could one imagine a car that could go through a certain corner at 100 km/h but not 50 km/h, due to it's high dependence on aerodynamic grip instead of mechanical grip?

Is this a trick question?

Excuse me for interrupting the chitchat about fan cars (how appropriate!) and aerodynamic flow but, at first sight, that would be a very funny corner...

At 50 km/hr and with a 1.5 friction coefficient (a very reasonable one, by the way) that corner should have a radius of 13 meters. Ridiculous. Even the corner to enter my garage has a larger radius (altough I concede it has a large radius for a garage).

I have never designed such a "racing corner" in my life. It's not a corner: an intersection has larger minimum radii.

Simple: even a truck cannot go around that curve because of geometric reasons...



The minimum radius in a parking garage for cars is 7.6 meters (because the rear wheels follow a different path of the frontal wheels and that issue has little relationship with grip: it's a problem of how small is the minimum turning radius of your car with the steering wheel "locked" in one direction).

To give you a general idea, if you consider small cars and try to "squeeze" the radii as much as possible you cannot possibly go lower than 5 meters. Even a Tata Nano (I'm guessing here) has a minimum turning diameter between walls of 9.5 meters, in the name of Enzo Ferrari and Bruce McLaren! (blessed be their souls).

The point is that there are minimum radii to take in account, even if you only consider a curve designed for turning at "zero" speed (let's say, at 1 km/hr)...

Finally, any track is 11 meters wide, minimum (not taking in account infamous tracks like San Marino and its horrible 9 m width). Even if the corner has zero radius (that is, two straight walls joining at the "curve"), by using the width of the track (outside, apex, outside) you can imagine a trajectory with a larger radius at most tracks, ergo, a larger speed, and that doesn't take in account the possibility of sliding laterally a bit or going over the kerbs.

Just imagine yourself saying to Alonso that he cannot take a curve a 50 km/hr. Heck, I can take ANY curve at 50 km/hr and I drive a kart! (and a very old one, with perpetually worn tyres and, as you might know, a kart has no wings or, at least, mine hasn't).

A Barbie pink bike with sparkling paint, a basket and ribbons in the handlebars can go at 40 km/hr... at least if you go downhill! I'm sure even an HRT can do better than that... and I will have mercy and I won't talk about the impossibility of such a large margin like the one mentioned previously: a car at full throttle and another one at half throttle? C'mon, I have been racing, actually, and let me assure you: the margins are smaller.

I can imagine the talking between the engineer and Chandhok: "Karun, Karun, step in a bit, that little girl in the Barbie bike is holding you back! You can go faster than 50, I assure you! The wind tunnel figures don't lie! Karun, those tires are matched perfect and staggered special" (á la Days of Thunder.. ) "you can do better than her, go, go!".

People.... the way to do engineering (this is NOT science) is checking your ballpark figures in your head EVERY time you're confronted with a riddle.

[Jonsson]

Ciro, that was a little too much focus on the corner itself. It is most easily explained in an F1 corner where the cars can't actually go faster than 100 km/h, particularly the Red Bulls, since they are slightly more aerodynamically efficient. You'll very quickly notice that an F1 car, no matter how precisely designed, will be able to take such a corner (as long as it's not banked) at 50 km/h much more easily. The reason being; the downforce produced by aerodynamics has to overcome a centripetal acceleration force towards the radial center of the corner. When speed approaches 100 km/h, air alone is not viscous enough to produce enough downforce to match the increase in centripetal force.

Now, you could always make a car have so much torque, that acceleration through a corner like that is fairly difficult; which was probably the Lauda problem. And torque is more neutralized at higher speeds, so that would make it "feel" easier for the driver. But in terms of "mechanical" grip, or the combination of the car's mass, tyres, and suspension, and the level of downforce already available at 50 km/h, any corner will be more easily conquered at lower speeds.

[Ciro Pabón]

Dear Jonsson: I focus in the radii of the corner to make clear that there are not one corner in this wide world of racing that has to be taken at 50 km/hr. If you beg to differ, please show me one corner with a 13 meter radius... and I'll take back my post.

You need to develop a "good engineering sense", I'd say (I'd beg!). This includes a quick checking of figures in any problem you analyze.

The banking you mention is another good example: you HAVE to understand that in the context of top speeds for a curve, in the circuits of Formula One, banking is like 2 or 3 percent of the centripetal force. Banking is important in ovals, but not in F1. Why are you mentioning it beats me.

The point I am trying to make is that threads speaking about generalities where members don't take the time to check, from the top of their head, the figures involved in any problem, can make their lucubrations to seem kind of ridiculous.

If I'm talking about, I don't know, the force you need to lift a car with a jack to change a tyre AND I say that there is difference between lifting a 2 grams car and a 4 grams car (yes, there is a difference: one weighs twice what the other weighs), somebody HAS to point out that there are no cars in this world that weigh 2 grams, before giving long explanations about the kind of jacks that exist, and somebody has to make clear that you can raise the car with your hand.

[bettonracing]

Ciro,

While your logic certainly does make sense, and is necessary on forums such as these, I think You came across as unusually abrasive on this topic.

Keep in mind that cars such as FSAE & similarly-small cars that do in fact navigate corners 13m & smaller. Some of these cars are aero dependent.

In addition, Monaco track data advertises the sharpest turn at 15m radius (by my calculations using 1.7G & 63kph, the racing line is ~18.5m which implies the actual turn radius is close to the advertised 15m) accomodating F1 cars and their 3100mm+++ wheelbase so it's not too farfetched that cars with smaller turning radii (e.g. ~2600mm wheelbase, greater maximum wheel steering angles, slower speed) should be able to navigate a smaller corner radius.

Whether or not these corners should exist is a different debate, but fact remains 13m radius corners (& smaller) do exist. I believe the OP randomly selected those speeds to ensure his question was understood. Yes, he could have used a more appropriate corner speed (radius), but it's a little less abrasive to mention the lower likelihood of the speeds & return to the intended topic at hand.

Have a beer! Get another bowl of cheerios!

Best Regards,

H. Kurt Betton