Of all the impulses that seize and shake men in their late teens and early twenties like dogs worrying the nonexistent marrow out of rawhide bones, surely the urge to ignore and evade the advice of one's own father is among the strongest. I remember it well; the sullen occasional rage at being lectured on topics ranging from investment strategies to whether or not it was foolhardy to drive through a mountain range on vaguely foggy mornings. It seemed to me sometimes that I would be lifted off the planet by my impotent fury, boosted through the stratosphere rocket-like into an escape velocity beyond which I would never, ever, ever again hear that I was an idiot for ordering hotel room service or buying a soda at the airport.
I was not surprised, therefore, when a friend of mine wrote me, regarding his newly-licensed, college-aged son: "He also doesn't believe that going downhill or on a wet surface (especially new rain) has a significant effect on braking distance." Well, Matt, I sympathize with your impulse to ignore what sounds like a thousand old wives' tales rapid-firing from your dad's mouth as you are just trying to drive somewhere in a vaguely companionable silence, but take it from me: on this subject, the old man's got a point, one that applies to racetracks and roads equally, and I'll explain why.
The actual and precise methods by which cars "grip" the road can be difficult for even the classically-educated to understand; words like "viscoelastic" and "hysteresis" appear early in the journey to comprehension and are quickly joined by various equations along the way. But for the purposes of this discussion, all we need to know is that the harder you press on a tire, the more cornering and braking grip you will have.
You can get this grip by putting weight on top of a tire. Ask anybody who drives a pickup truck in the winter: without a load over the wheels, you ain't goin' nowhere. Weight comes with its own set of inertial problems, which is why city buses don't corner as well as Corvettes, but that's a discussion for another time.
It's also possible to increase grip by putting an aerodynamic load on the wheels. That's what the wings on Formula 1 cars do: they press down on the tires, increasing their grip. Most car enthusiasts understand this, even if we tend to misunderstand it in a way that gets us very excited about aerodynamically insignificant and aesthetically questionable spoilers.
So we've established that pressure on the tire increases grip. This is where the idea of grade-related traction comes into play. We all learned in high-school physics that there is an equal and opposite reaction to every force. Let's say you're driving along a flat road and you come to a hill. As your car starts to climb the hill, there is force exerted upwards through the tires by the road. Otherwise the car would just go right through the hill, the way it would if the hill were made of paper or soap bubbles. And there is an equal force pressing downwards from the car into the hill, courtesy of gravity, otherwise the car would just fly away into the air.
If you're a scientist, that paragraph probably just made you convulse in agitation, but it's accurate enough for the purposes of this discussion. The point is that there is additional force pressing on the tire compared to what you'd get on a flat road. And since traction in a tire is a function of pressure, there is more traction going uphill than there is on a flat road.
Experienced racers know this: banked turns have grip. And most of us understand it on some intrinsic level. What's harder to understand is why we lose grip going downhill. The quick answer is that it's just the opposite of going uphill. As the road drops away from the car, pressure on the tires decreases, reducing grip. It might only be a reduction of five percent, or even one percent, but when you're operating on the edge of tire traction, as you do on a racetrack, that small percentage can be vitally important.
Most of the time on the road, we have plenty of extra grip on hand, so going downhill doesn't matter. But when you mix in other factors that reduce traction, like ice or rain, it's easy to find yourself in a situation where you are closer to the limit of the tires than you realize.
I learned this lesson in my very first NASA race with my own car, back in 2008. It was an April morning at Mid-Ohio, raining like hell. They waved the green flag and we headed downhill towards "The Esses." I was right at the threshold of braking when I was very slightly bumped by the car behind me. I mean slightly. It wasn't enough to knock paint off the bumper. But it was enough to push me past the already low limits of a rain-soaked downhill straight, and I went nose-first into "China Beach". That's right: in my first race as a car owner (as opposed to a car renter or arrive-and-driver) I didn't make it past the first turn. I assure you that I approached that corner with respect every single time afterwards, particularly when it was raining.
Elsewhere on this website you will read that one of our editors crashed a car during our Performance Car Of The Year testing. Having arrived at that corner immediately after the event, I was struck by just how steeply downhill it was. Like roller-coaster downhill. The kind of road grade that steals grip just as surely as oil or coolant on the racing surface. It set off alarm bells in my head, even though I'd been forewarned about the situation and I was driving through at school-zone speeds. I was reminded of Laguna Seca's Corkscrew. Not a place you should plan on getting much steering done until you reach the bottom. This sort of thing matters more than we would like it to, even in the age of R-compound tires, near-perfect stability-control systems, and outstanding driver training methods.
It's always a drag for a young man to find out that he is wrong and his father is right. So I take no pleasure, Matt, in telling you that your dad is totally on point about the dangers of driving downhill. But I can't help but chuckle a bit. Because you're going to find out as you age that your dad was right about all sorts of things. The same way my dad was. In fact, the older I get, the smarter the old man seems to be. Except when he tells me to sell my motorcycles and turn in my race licenses. He's totally wrong about that. At least for now.
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.
Jack Baruth is a writer and competitor who has earned podiums in more than fifteen different classes and sanctions of automotive and cycling competition, in both amateur and professional capacities, as well as an enthusiastic hobbyist musician and audiophile who owns hundreds of musical instruments and audio systems. His work has appeared in Bicycling, Cycle World, Road & Track, WIRED, Wheels Weekly, EVO Malaysia, Esquire, and many other publications. His original design for a guitar, the Melody Burner, has been played by Billy Gibbons, Sheryl Crow, and others.
