Traction.

It is the vehicular equivalent of oxygen: without it, one quickly finds oneself in a ditch, useless to the world at large. And owing several thousand dollars to men whose services border on extortion.

Or, one may find oneself sliding backwards downhill at alarming speed toward a four-ton pick-up truck crowded with a dozen bermuda-shorted tourists.

The Caribbean paradise of St. John has an unfortunate combination of 45-degree switchbacks, intense rains, and rental cars with bald tires. However, if you want an object lesson in traction, you couldn't ask for a better demonstration. It's even better than spinning one's tires in a futile attempt to climb an ice-slicked New England hill, because in St. John the rain will soon pass and you'll be on the beach before you can say "Where's the goddam snowplow when you need it?"

If you don't crash into the tourist taxi, that is.

Cars were once known as "horseless carriages." But one major difference between the car and the carriage is the source of traction. Carriages are tugged by horses, each with four hooves capable of finding a safe piece of earth from which to push off. But cars push themselves with their wheels. Think about the size of a tire's contact patch (where it touches the pavement) versus the size of a horse's hoof: a horse's hoof is slightly smaller than a tire's contact patch. But each horse is distributing one measly horsepower through its hooves.

A modern car with a typical engine has between 130-200 horepower. And it's sending that power to only two wheels, with a total contact patch smaller than four horse hooves. Trouble MUST ensue.

Even in the earliest days of the automobile, car makers struggled to increase traction: instead of steel-rimmed, wood-spoke wheels (like carriages), car makers used rubber-coated wheels. They replaced the steel band around the wooden wheel with a layer of the blessed sticky stuff. These wheels were the same shape as the old ones: the contact patch was tiny, but at least it was made of rubber. Cars tottered around in high heels with sticky soles. It was better than nothing, but you still wouldn't want to sprint down a country road

The next innovation--possibly the greatest advance in traction ever made--was the switch to air-filled tires in the 1910s and 1920s. Now the tire could flatten on the bottom, increasing the contact patch from a few fingers-widths to a couple hands. The tires were still pretty narrow, but it's a lot easier to run in Converse All-Stars than stilettos.

By this time, cars could easily have 30 or 40 horsepower. Power still outpaced the contact patch by a factor of 10. Over the next three decades, tire technology stagnated. But engines got bigger.

Then, one glorious day, the sun rose faster and brighter in the sky. The birds sang “Halleluljah,” but no one could hear them over the roar of Hemis and big-block Chevys: it was the dawn of the muscle car. Even a Chevy Impala, the Camry of its day, could put in a decent show at the drag strip before toting the kids down to the diner for burgers and chocolate shakes.

In the now-desperate quest to double or triple the contact patch, car makers adopted the tubeless tire (a.k.a, "steel-belted radial"). No longer donught-shaped like bicycle tires, they gained their now-familiar rectangular cross-section in the 1960's. They also lost their endearing habit of exploding in sharp turns.

I always thought the term "classic" was a misnomer for old cars, but that's another subject....

Alas, the steel-belted radial seems to be a plateau for tire technology. The only major improvements have been in size: tires have gotten taller and wider, helping to increase the contact patch. A 14-inch tire used to be high-performance, but today not even a Civic would be caught dead in the automotive equivalent of moonboots.

This plateau in tire technology left manufacturers in a bind: where to go from here to increase the contact patch? The answer was simple: more tires! In fact, an egocentric genius named Ettore Bugatti pioneered four-wheel drive back in the 1910s. He wanted his racing cars to wear Nikes while his competitors still wobbled in five-inch heels. This idea didn't catch on, due to its mind-blowing expense and unreliability. But the appeal of doubling the contact patch remained strong. Like computers and bathyspheres, the technology needed some time to catch up to the concept.

The first wide-scale use was, of course, the WWII Jeep. Now we'll have to get a little technical, but this will help explain why I nearly sent a dozen happy tourists off a rain-slicked cliff and into the ocean: the system in these Jeeps (and still in use on the modern Jeep Wrangler) has a big flaw. It lacks a front differential. A differential is a complicated doohickey that sits in the middle of an axle and allows the left and right wheels to spin at different speeds. (First invented, btw, by the Chinese who thought that the emperor's cabin on his carriage should always point south. But that's another story...)

In normal mode, only the Jeep’s rear wheels receive power. If you engage the front wheels, they receive power, but must always spin at an identical speed due to the lack of a differential. When you're going straight, all is well. Alas, this system does not cope well with that other element of driving: the turn. As you turn, the outside wheel has to cover a larger distance, so it needs to spin faster. But it’s stuck rotating at the same speed as the inside wheel. So it gets yanked along by the inside wheel, skipping over the pavement. This causes stress on the axle and damage to the tire, and the car literally hops.

So if you’re driving a rented Jeep Wrangler on a nice asphalt road, perhaps in the Caribbean paradise of St. John, you would not have the front wheels engaged. If you come upon a steep, wet hill, you have to decide: do I stop traffic to put the Jeep in neutral and engage the front wheels? Or do I assume that being on-road means that 4wd would be overkill, and just keep going up the hill?

If you said keep going, you’re wrong.

After nearly knocking the tourist taxi off the road, I halted the Jeep, engaged the front wheels and made it up the hill.

If the Jeep had a front differential, I could have just left the front wheels engaged the whole time, doubling my contact patch and avoiding a minor heart attack.

[A note, should this blog ever be read by Jeep enthusiasts: yes, it does actually have a front differential. But it’s so limited that the car still suffers from serious hopping and skipping in corners.]

Now, I’ll spare you a lot of the drama of how we got from nasty old Jeeps to modern Subarus, where all the tires receive power all the time. The short story is that the Japanese added a simple type of front differential, and made the whole thing fairly cheap and reliable. (To be fair, Audi led the way, but the importance of “cheap and reliable” never sank in with them.)

Back to the contact patch: nearly all of today’s high-performance cars have big steel-belted radial tires and some type of four-wheel drive. But the contact patch is still too small, because their engines push out 300 or 400 horsepower. It’s all-too-easy to overwhelm their little patches of rubber, spin the tires, and slide around.

But it is FUN.