Why We Lost
Why We Lost
Soft science vs. hard facts
KEVIN CAMERON
THE CAR-VERSUS-BIKE QUEStion will always be with us. Either one can be built to extremes to prove a point, but the real interest focuses on comparing realworld, available-in-stores cars and bikes. So when CW's sister publication Car and Driver offered up a test Dodge Viper GTS coupe, we crossed swords with it. This is a 450-horsepower, 3301-pound machine, powered by an aluminum version of the new Mopar V-10 truck engine. Our weapon of choice was a 1997 Yamaha YZF1000R. According to Katella Avenue, this is a 135-horsepower, 437pound package.
Compare power-to-weight ratios. Let’s give the Viper 10 gallons of gas (60 pounds) and a 160-pound driver for a total of 3521 pounds, for a ratio of 7.8 pounds per horsepower. Add 10 percent to the Yamaha’s weight for the Pacific Crossing Factor
(in the time-honored fashion, our testbike weighed 480 pounds on the CW scales), 15 more for half a tank of fuel and 160 pounds for the rider. This gives us 656 pounds and 4.9 pounds per horsepower.
No contest, right? Surprise! As the main story shows, the car beat the bike around the Willow Springs road course by a ponderable 2.5 seconds a lap, and by 23 miles an hour in top speed. The bike, as its power-to-weight ratio would suggest, was the grandmaster of acceleration in 0-60-mph and quarter-mile times.
"Wheelies put an upper limit nn how hard a bike can accelerate; alter the Iront wheel leaves the ground it doesn't matter how much more power you have-it just breaks taillight lenses."
We know that between grand prix cars and grand prix bikes, it’s truly no contest. The cars, with giant, soft-compound tires, pressed down on the road by thousands of pounds of downforce from wings and underbody venturis, can corner at tremendous lateral accelerations of 3 g’s or more. GP bikes, leaning over in comers at 60 degrees from vertical, indicate lateral accelerations of no more than 1.7 g. What this adds up to is that high-performance cars don’t have to slow down as much for comers as bikes do, and if they are given enough power (GP cars are making 750 bhp), even a 200-horsepower bike can’t make up in acceleration what it gives away in comer speed.
While the Viper has no appreciable downforce, its performance shows the same effect: More comer grip equals faster lap times. The Viper’s 0.97-g skidpad cornering acceleration shows it has serious tires and suspension. You will note that a bike has less than a third of its sidewall-to-sidewall tread width on the ground at any one time because of the round profile of its tires. The car, with flat-profile tires, has its full tread width engaged all the time.
Why should it make any difference? The law of friction we learned in high school says contact area makes no difference; drag a brick on its end or its side and the friction is supposed to be the same.
Not for a rubber brick. The more rubber you can stably put down on the road, the more grip you can get. Rubber “keys” to the road’s irregular surface like a gear, molding itself into a detailed mirror image of the pavement. Further, rubber forms molecular bonds to surfaces it touches. Making the contact area larger and softer gives more grip.
A second effect is inflation pressure. If tires were infinitely flexible, their footprint area would be simply vehicle weight divided by inflation pressure. In this model, two vehicles with identical tire-inflation pressures would have equal load per square inch of footprint-and equal grip. But bike tires must typically be inflated harder than car tires, by maybe 20 percent. This reduces footprint area-and grip. Reducing the bike’s tire pressure doesn’t help; it needs that pressure to make its tires hold their shape.
But tires aren’t infinitely flexible, even though tire engineers work very hard to make them so in desired directions. It takes force to bend round tire tread to make it lie flat against the road, and this reduces footprint area. Car tires are cylinders, and bend only in the around-the-tire direction to make a footprint. A bike’s tread surface is curved in two directions-around the tire and across it-and must bend in both directions at once to form a flat footprint. This double stiffness further reduces a bike’s footprint area-and its grip.
There are other interesting differences. The bike, because it has a short wheelbase and relatively high center of gravity, wheelies or stoppies. This puts upper limits on how hard a bike can accelerate or brake; after the front wheel leaves the ground, it doesn’t matter how much more power you have-it just breaks taillight lenses.
A car’s four wide tires take a larger “sample” of the pavement than can a bike’s two narrower, round-profile tires.
The car is less upset by slick or rough pavement for this reason, while a slight dribble of oil on the pavement can have bikes on the ground and sliding.
The bike, being narrower, can take a wider-radius line in any comer, but conversely wastes distance by having to take time to roll over before it can actually begin turning, and roll back upright before it can commence full acceleration.
A more subtle effect is aerodynamics. Bike drag coefficients are poor because air disagrees with our taste. We accept the aerodynamically “dirty” exposed front wheel, the chopped-off fairing and the silly behind-the-front-wheel radiator location of sportbikes, but all these increase drag. A car’s greater length encourages a fuller aero treatment, and its cooling arrangements can be civilized. Typical drag numbers are mid-to-high ,40s for bikes, mid-.30s-to-.40s for production cars. Our testbike was slower than it should have been-its 154-mph average top speed is more typical of hot 600s than of 1000-class machines-but that’s what happened on the day. Another aero effect is gust response; cars suffer less than bikes from wind disturbance.
To distill the argument, cars have traction, bikes have acceleration. In most cases, bikes smoke cars on the street, cars smoke bikes on the track. Chrysler Corp. deserves our admiration for being able to achieve a partial reversal of the usual state of affairs. Mostly, cars and bikes go their separate ways; bike against bike, car vs. car, each group with its own strengths and weaknesses. Now, if we could just find a way to mix & match...
