Speed Secrets: Dynamic Friction

By Ross Bentley

March 26, 2014

You are probably already a very good driver. If you're someone who reads this newsletter, you're also pretty technical and well-informed on theory. So you probably know about the friction circle, the challenges of trail braking, the string between the bottom of the steering wheel and the brake pedal, and the excitement of getting light over the crest into the Corkscrew at Laguna Seca.

Well, forget it.
I'm here to tell you about a kind of friction you've never heard about. I'm selling a witchdoctor's potion of a magical art. I'm letting you into a secret club. And I'm making up a phrase: dynamic friction.

I think of classic bubble-gum adhesive traction as being "static" friction - the tire compound is stuck to the track surface until the rotation of the wheel un-sticks it. Molecularly, it's a static relationship. By definition, this relationship describes the edge of the friction circle. But "dynamic" friction is happening when the relationship between the compound and the surface is already in motion. You're sliding, but you're still gripping. You're doing a burnout, or you're already on your way to the wall, or you're trying to imitate Bo Duke. You're channelling Fangio and Moss in the early days, when a little slip angle was the fastest way around a track. This is the area in which rally drivers, drifters, and stuntmen work.

Dynamic friction is the very thing that most racers are trying to avoid. But I'm here to tell you that this state can be your friend - that just slightly over the limit is a great place to be.

You already know that more load on the contact patch of a tire, whether it's from downforce or weight transfer, gives you a larger friction circle for that tire. The less-well-known thing is that additional load gives you more and modulate-able friction even when you're beyond the limits of the friction circle. Skiers and snowboarders know this intuitively - slide down the hill with your edge perpendicular to the slope and rock your weight from side to side across the fall line, and the end of your contact patch that gets the extra load, slips less.

This is exactly how traditional rally driving in All-Wheel-Drive cars happens. Trail brake into the corner and turn in to set the car sliding, then wide-open throttle to "claw in" to the apex with the front wheels pointing straight ahead. The sliding and spinning of the tires cleans the ball-bearing gravel or loose dirt off the surface. You use your right foot on the throttle and left foot on the brake like the rudder of an airplane: a little less throttle and more brake to transfer the weight forward to turn into the apex, then a little less brake and a little more throttle to transfer weight to the rear, straighten out, and track out at exit. Done properly, it's a beautiful movement, but it can be hard to master, especially for good track racers because the whole point is to throw the car over the edge of the envelope and then chase it with the weight transfer.

Doing the same thing with a 2WD car, whether it's front-drive or rear-drive, adds a layer to your on-the-fly friction calculus. Controlling wheelspin and lockup changes the amount of friction at each end by pushing those contact patches further away from the friction circle, rather than by any weight transfer. So the traditional drift technique is to initiate a slide by locking up the rear wheels with an auxiliary handbrake (which is real exciting when drifting a Viper at 90mph looking at the outside wall on a banking) and then modulating the rear traction with wheelspin. Most people don't realize that top drifters are often modulating the throttle trying to match road speed or dipping the clutch to bring some traction back and control the angle of the car, keeping it right on the limit of maximum opposite lock.

The highest application of this art, I feel, is in front-wheel-drive rallying. Here you have a constant interplay between weight transfer while sliding and overlap of left-foot-braking with throttle that's controlling both your front wheel speed and your rear-wheel lockup in a modulated way with both pedals. Left-foot trail-brake the car into the corner, then play the throttle and the brake against each other, not only for weight transfer, but also to dance each end of the car closer or further away from the friction circle to control the angle of the car.

What if your interest isn't in rallying, or drifting, or just behaving like a hooligan? How can you use these rally techniques on the track? Well, first, you should practice beyond the slip limit as much as possible, so that you understand where it is in all circumstances. Rain, for example, doesn't scare you when you know when you're going to start to slide.

Second, you should know, and if possible get a feeling for, how you can control a car when already sliding. While the weather is grim, you should look for every snowy parking lot you can find. Then translate that to drifting the Corkscrew in the rain. Rain doesn't scare you when you know how you're going to slide.

And third, and most importantly, you should learn this from rally drivers: never give up. Just because you're sliding doesn't mean you're in the weeds. And even when you're in the weeds, you're not in the trees yet.

And even when you're in the trees, you may yet be able to save it. We've taken down a tree or two and still won the rally (try to go in backwards, by the way; unless you're in a 911, the important bits are at the rear).

And always remember this: as long as your tires are touching some kind of ground, there's usually something you can do with them. Even, famously, in the dirt on 8A at Laguna Seca. 
- Andrew Comrie-Picard

Website: www.acpracing.com
Exerpted from Ross Bentley’s Speed Secrets WeeklyFor more tips and additional articles on the art and science of racing, click here to subscribe.  
Also be sure to check out Ross Bentley's book, Ultimate Speed Secrets: The Complete Guide to High-Performance and Race Driving
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