Race handling theory question
 

I am a novice/intermediate DE driver with my 944T, it has a full cage, coilovers, double-adjust shocks, solid bushings, and is currently running BFG R1 tires. I was looking at my traces from my lap timer and noticed that I occasionally pull greater than 1 G in tighter corners but not in the higher speed corners which I initially attributed to fear on my part but then it raised the following question in my mind -

I have assumed that if your car makes no downforce and has no lift (not a safe assumption I realize) and if the track is not banked or off-camber (also not a safe assumption) that the car can pull the same number of Gs at 60mph on a shorter radius as it can at 120mph on a longer radius.

Is this generally held to be true or is there something about wheel/tire rotational speed that would affect this? Thanks in advance if you know.

more speed would transfer more weight on the faster sweeper than a tighter slower i would think.

Weight transfer is a result of the G's.

At higher speed, tires have less lateral grip due to centrifugal force, and the time allowed for the rubber to bond to the molecules of the road. There is less damping force in the tires at speed, as well. At low speed, the slip angle is greater so there is less efficient generation of lateral force.

There will be an optimal point somewhere in the middle radii.

The aerodynamic forces will dominate, this discussion, though.

Flieger, when you say that, "At higher speed, tires have less lateral grip due to centrifugal force ..." do you mean that the tire carcass deforms due to the centrifugal force due to wheel rotational speed and becomes less efficient? - this seems plausible. I'm not sure if I buy the bonding time argument but it's something to think about.

The tire will grow more in the center of the tread due to the centrifugal force- like how a dragster tire gets taller.

The molecules in the tire must deform and mechanically lock into micro-cavities in the road. There is also n adhesion force due to various inter-molecular forces such as van der Waals forces / dspersion forces. There is less time for the tire rubber to stop, deform, and bond using these two mechanisms when at speed.

These effects are dependent on tire construction, sidewall stiffness, width, and very much the rubber compound. Sticky race tires have more of the adhesive bonding which is part of the reason they wear out so fast.

The relative camber of the tire to the road is more efficient at generating lateral force than is tire slip angle (steering input) when under lots of load near the tires' limits. This makes the track camber and your suspension kinematics very important.

So, for those who race - assuming Flieger's well thought out explanation is correct - how much of a reduction in the g's you are pulling do you see at speed? Obviously any observation is going to be highly dependent on your aero set-up and I'm mostly interested in cars with no real downforce. I'm not sure how much more lift there is at 120 compared with 60mph and how much it affects my lateral G potential but it would be interesting to have some idea.

Your answer will depend on the speeds. You want 120 mph and 60 mph turns? 30 mph would likely be less than higher speed due to the work the front tires have to do at that high steering angle.

Maybe someone can do some actual skidpad work out at the Streets of Willow here in California- lots of guys do track days there.

I don't have specific numbers anymore, but if you plotted speed vs. g's on my data you would have definitely seen this same thing. As speed goes up, Max G's go down.

Thanks, the real reason for this post is that I thought to myself "if you just get the nerve to go into turn 1 at 135 mph and keep the loading similar to what you have in the slower corners you'll save seconds" but then I realized I was probably not thinking clearly.

Interesting question!
I looked at some data from a fast lap at Willow Springs.
Unfortunately, I just have a hard copy at this location and can't manipulate the data to find actual values, but looking at the graph, it looks like the slower corner pulls slightly more Gs.
Turn #2, the avg. speed is 106 mph
Turn #8, the avg. is 141 mph
The lateral Gs look to be slightly greater in Turn #2.
But there are so many other factors in play here (e.g. turn radius, bank angle, turn #2 happens to be a climbing turn where #8 is pretty level, wind direction affecting downforce etc.). So, not sure my example means anything at all :)
http://forums.pelicanparts.com/uploa...1284437866.jpg

I think the physics of mass and inertia play a significant role in this discussion as well.

With my car, higher speed means greater g's in the turn (comparing a 100 mph corner to a 40 mph corner) and the difference is due to down force. I get up to 1.4-1.5 g's in high speed turns and IIRC 1.0-1.2 in the slower turns.

Keep "in mind" G-forces are working on your brain as much as your tires! ;)

Tom W - what are you driving that has that kind of downforce - fairly significant it would sound. BYprodriver - unfortunately it would appear that the G-forces work more on my brain than for my tires.

Time to jump in.

We think of this "higher speed / lower G's" problem as a balancing act between mechanical grip, aerodynamics, and driver confidence (speaking particularly of cars that do not generate net downforce).

In the lower speed corners, 2 parts of the balancing act are most prominent: the tires' mechanical grip, as described very nicely by Flieger; and higher driver confidence (since failure at slower speeds typically does not carry with it the same punishment as that from higher speeds.) Combined, these two aspects produce good G forces.

In the higher speed corners, all 3 parts are now in play, but things have changed: mechanical grip in not as secure, since the pavement and the tire no longer have as much time to "bond"; the driver's confidence is somewhat more tenuous from the punishment factor mentioned above; AND aero is now working against us in the form of LIFT. Most cars experience plenty of lift on high speed straights, but imagine multiplying that lift by 1.5X or 2X as the car turns in yaw!

So when Chris says he consistantly generates more G forces in the lower speed turns, it has nothing to do with any lack of skill or daring at high speed. It has everything to do with the car's own capability, which changes dynamically as speed increases.

And yes, one can overcome much of this high speed diminished capability with true aerodynamic DOWNFORCE - that's what it's for.

Ed

Or, if wings are not your thing, just give the car a Scandinavian flick and powerslide around the turn. All the G force will be in the longitudinal direction on a G meter, but it gets the job done. ;)

I think trying to isolate the tire mechanical grip versus speed is tricky. It would be interesting to examine how the greater steering/slip angles at low speeds would compare to the decreased contact patch and bond time at high speed. The tire is most efficient at generating lateral force at some intermediate slip angle, temperature, load, camber, etc. so there will be some 'optimal' speed and turn radius where purely mecanical grip is maximized. This can never really be found since one cannot run the car in a vacuum. :)

We wholeheartedly agree - very tricky. And one of the primary factors effecting comparison is that most cars do not "weigh" as much at high speed - at least in the vertical plane.

Ed

funny, about a month ago I was at a road course with my new Grand Sport Vette that has a G meter on the HUD and noticed I just couldn't pull the Gs in the high speed turn at the end of the straight and was getting rather ticked off trying to figure out what I was doing wrong (first time I have had any sort of G meter to pay attention to). Wasn't like the tires weren't howling at me.

I too attributed it to lift as Vettes exhibit lift without a good splitter on the front, and the factory spoiler on the rear is somewhat smallish so not much help there. I wasn't about to attribute it to my driving :D :p In all seriousness, the answers in this post are what I have determined to be the causes as well, especially the lift IMHO. I think with an aftermarket ZR1 splitter, side skirts and a rear wing or taller spoiler I could get similar Gs, but it takes some huevos to drive faster and faster "into the downforce" thru a turn to see if the DF picks up or lets go based on Ed's mention of consequences at speed.

Time v. tire adhesion seems crazy to me, the incremental delta t that a particular section of tire is actually in contact with the ground at any sort of track speeds, "slow" or fast would have to be down in the milliseconds of time so I am surprised that would be a possible reason, but I am not saying it isn't... just surprised it has that much of a noticeable affect. I would think lift and subsequent reduction of normal force at the tire contact patch would be the biggest reason for reduced friction capacity at the tire...

I agree that lift is the over-riding factor here, but the original poster was posing a theoretical question looking at mechanical grip in isolation. I do also agree that the tire adhesion vs. time is a small difference, but I argue that it is still a difference in a hypothetical smooth, level, turn with no air.

What's the latest on the Focke-Wulf 914?

A number of years ago, I read an interview with Alex Zanardi, who was describing working on setup at Indy. His primary engineer (whose name escapes me now) casually mentioned that, now that Alex was laying down consistantly fast laps, they were going to "really trim out" the car.

Zanardi couldn't believe it, pleading his case that he felt he was inches from crashing several times on each lap. The engineer proceeded confidently with his plan, reducing wing angles, etc., and then said, in essence, "There, now go out and drive really fast!"

To Zanardi's amazement (and relief), lap times fell, AND the car felt more controlled.

So, fellow Wizards, try plotting this one: By reducing the extra DRAG that slightly steeper wing angles (higher downforce) generate, the car will go faster, thereby generating - uh - higher downforce. Where are our huevos when we need them most?