Yahoo Autos There's Danger on the Far Side Of 'Tire Mountain'
So here's the setup: Two weeks ago, I drove a brand-new, tuned-and-prepared BMW M2 at Mid-Ohio. I'd been hoping for an opportunity to drive the M2 at Mid-Ohio ever since our four-way BMW test at the Thermal Club in Southern California earlier this year, so when the M2's owner contacted me about taking his car for a spin, it took me all of three seconds to accept the invitation.
The nice people at the Buckeye Chapter of the BMWCCA allowed me to sneak in for a session in exchange for my promise to say something nice about their program. I actually have quite a few nice things to say about their program, from the generally safe driving behavior I witnessed on-track to the low car count per session. But the nicest thing about the event was that I had a chance to drive a brand-new M2, so let's get back to that.
I drove first, with the owner in the right seat. I took a series of conservative laps, making sure to preserve the car, the tires, and the brakes. Then we swapped and the owner drove. He was a fearless young man, to put it mildly, and he nearly lost the M2 in Thunder Valley during our out lap. He was constantly on the edge of tire grip, sliding around, making last-minute corrections. Definitely the balls-out hero type.
Now here's the odd thing: Our lap times were almost identical. If you'd been in the back seat with me, you'd have sworn that we were just taking a Sunday drive, albeit one where we hit 145mph on Mid-O's back straight. If you'd taken that same seat with the owner during his stint, you'd have either thrown up or passed out.
If you're an experienced track rat, you're no doubt thinking, "Well, Jack's lines must have been better." But I'm here to tell you that my lines around Mid-Ohio are not that great for getting a low lap time. Almost all of my time around that track has been in a club race situation, so my default line is more defensive than it is fast. I think the car's owner actually had a better line than I did.
If the owner was driving on the limit, and he had a better line, why wasn't he any faster? The answer was actually staring us in the face, kind of. The M2 has the ability to display tire temperatures on the center screen, and those tire temperatures were noticeably lower when I was driving. That was not by accident. I was on what I like to think of as "the safe side of the mountain." My M2-owning friend was on the dangerous side. I'll explain.
We can start with the fact that nobody appears to truly understand how tires generate lateral grip. We have a general idea, and there are a variety of theories that are generally reflected in real-world results, but any truly scientific discussion of lateral grip involves some genuine uncertainty. Nevertheless, there are some hard-and-fast rules on which we can rely.
The rule that we'll discuss here is the rule that governs the relationship between tire slip angle and lateral grip. When we're driving on the street, we generally enjoy a direct-ratio relationship between the two. If you're driving a Camry around your neighborhood at 25mph, turning the wheel X amount gets you X amount of grip, and turning the wheel 2X, or twice as much, gets you 2X worth of grip. This relationship holds true until we get nearly to the maximum lateral grip of the tire. At that point, turning the wheel X more gets us a little bit less than X more grip.
Finally, we hit the magic point where we are getting the absolute maximum grip out of the tire. So what happens when we turn the wheel more? Well, the grip decreases a little bit. If we turn the wheel even more, the grip will decrease a bit more, and so on and so forth until we've sailed nose-first into a trackside barrier. You get the idea.
This diagram may help. As you can see, there's a peak grip, and there's a slope on both sides. Think of it as a mountain. We can call it "Tire Mountain".
An utterly perfect driver-if there is such a thing-would always steer the car at the maximum possible grip. In the real world, most of us are lucky to get to the 95 percent grip level. But note that the 95 percent grip level can be found on either side of the 100 percent grip level. So let's imagine two different 95 percent drivers for a minute.
Driver A is a little cautious. He always stays on the safe side of 100 percent. But he's not always exactly at 95 percent. When he errs on the side of caution, he's going to get less than 95 percent of the available grip. When he gets a little crazy, he will be closer to 100 percent. His laps are very smooth and predictable.
Driver B is a little wild. He always stays on the dangerous side of 100 percent. When he errs on the side of caution, he backs up towards 100 percent. When he pushes it too far, he gets less than 95 percent grip. His laps are full of screeching tires and lurid slides.
In the case of the M2 at Mid-Ohio, I was Driver A and the owner was Driver B. So we were getting the same kind of grip in two very different ways. And that's why his tire temperatures were higher than mine. He was consistently pushing the tires past their maximum, generating a lot of heat. I was keeping the tires cool.
Which one of us was right? That's not an easy question to answer. He was certainly having more fun than I was. He was pushing the limits of the car, which was a learning experience for him. Keep in mind, too, that the M2 belonged to him, so he had far more carte blanche to wreck it than I did.
As a club racer, I'm kind of trained to drive on the safe side of Tire Mountain. The reason for that is that racing is unpredictable and you never know when you might need to crank your steering wheel a little bit more in the middle of a turn to avoid a spinning car. You can't be sure that there won't be dirt or gravel past the apex from somebody else's off-track maneuver. There's also the fact that tires live longer when you keep their operating temperature lower. As a racer, you're always managing your tire temperature, saving a little bit in reserve in case you need to make a big move on the car ahead or defend against a pass.
With that said, if you're going to race you'd better know how to handle the car when it gets to the other side of Tire Mountain and starts sliding around. And you can't learn how to handle the car in those situations without actually putting yourself in those situations. It's a Catch-22 of sorts; the less stupid stuff you do in a race car, the less prepared you are to correct stupid stuff when it happens.
If you spend any amount of time around trackday driver's education you'll hear the phrase "Smooth is fast" thrown around until you're sick of it. I'm here to tell you that the "Smooth is fast" guys live on the safe side of Tire Mountain. You can stay there for your whole career, if you like. But the party's on the other side. So take a trip there, when you can safely do so. Just make sure you're prepared for the consequences.
Born in Brooklyn but banished to Ohio, Jack Baruth has won races on four different kinds of bicycles and in seven different kinds of cars. Everything he writes should probably come with a trigger warning. His column, Avoidable Contact, runs twice a week.
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