Coming Unglued

By David L. Hough

My personal motorcycling hero from the good old days was Mike "the Bike" Hailwood. Mike is no longer with us, but in his prime he could ride a motorcycle around the tricky Isle of Man circuit faster than anyone else, back in the 1970's. He knew the 37-mile circuit like the back of his hand, and rode every race consistently, smoothly, and in absolutely in control, without every hanging a knee off the bike. It was like Mike the Bike was glued onto the saddle.

Well, sports fans, times have changed. Today, when you see roadracers in action, they all hang off the bike towards the inside of turns. They hang off so far they actually slide their knees on the pavement, which is why they have plastic "pucks" on the outside of their leathers. You may have wondered whether hanging off is something the street rider should even consider.

em>(above: Even large touring and sport-touring bikes can benefit from hanging off. It's important to feel what your bike is trying to tell you.)

"What's so important about hanging off?"

"Is hanging off a real advantage, or just a show-off gimmick?"

"Even if hanging off is an advantage on the track, is it a skill I should use on the road?"

In a nutshell, if you're riding a sportbike similar to those racetrack machines, hanging off in turns stabilizes the bike and reduces steering effort, while helping maintain leanover clearance. If that's all you need to know about hanging off, skip the rest of this and go riding. But if you're curious about why it works, or you're a little paranoid about trying it, get your head on tight, and stay with us.

One major difference between Mike Hailwood's style and that of today's roadracers is that motorcycles have been improved. Let's consider tires, for instance. Back in the 70's, motorcycle tires generally had round cross sections. Today's motorcycle tires are typically much fatter and wider, with oval-shaped profiles. Tire profiles and compounds contribute greatly to what happens as the bike is leaned over into turns. Not only do tire compounds provide better wear and better traction, but wide, low profiles have some advantages. First, the wider, flatter footprint puts more rubber on the road, and bridges across surface problems such as grooves or cracks. Second, an oval profile results in more consistent engine RPM as the bike leans over.

Tire Profile vs. Drag

One characteristic of a wider profile tire is that the contact ring shifts further towards the sidewall than a narrow tire as the wheel leans over (Figure 1). What's important about that? Well, it effects steering. The front wheel is being pushed down the road by its axles, but the tire is dragging backward down at the contact ring, due to it's rolling friction. As the bike leans over, the tire contact ring moves farther and farther from the centerline of the bike, so the drag on the tire has more torque to pull the wheel towards the turn. In other words, the tire's drag steers the wheel more towards the turn as the bike is leaned over.

em>(above: Figure 1. The greater the lean angle, the more the front wheel steers itself towards the turn.)

Be aware that a two-wheeler balances by countersteering. To lean the bike right, you momentarily steer the front wheel left, which forces the motorcycle to lean ("roll") towards the right. So, in a left turn, if the front tire steers itself more towards the left, the effect is that the motorcycle wants to roll itself upright, back into a straight line again. (figure 2)

(above: Figure 2. If the front tire steers itself more towards the turn, the effect is that the motorcycle wants to steer itself upright.)

To keep the motorcycle leaned over and continuing around the turn, the rider must maintain pressure on the low grip. Letting up on that low grip at the curve exit allows the front wheel to steer itself a little tighter, rolling the bike back vertical again. But you may have noticed that sometimes the bike seems to hold a lean with very little pressure on the low grip, and sometimes you've got to push hard on the low grip, or even pull on the low grip to maintain the same lean angle. What's going on? Why isn't steering effort the same all the time?

Part of the answer is road camber, the slant of the road in curves. Some roads slant toward the curve (positive camber), some slant away from the curve ("off-camber") Consider a bike leaned over to the same angle in three different curves, one with positive camber, one with the pavement level, and one off-camber. Notice that the lean angle of the bikes is the same in all three turns. (Figure 3)

em>(above: Figure 3. In a positive-camber turn, steering should feel very neutral. In a level turn, you may have to push on the low grip to maintain the same curving line. In the off-camber curve, a much stronger push on the low grip will be needed to hold the same line.)

In the positive-camber turn, the contact ring will be close to the bike centerline, so side drag from the tire will be minimal, and steering should feel very neutral. In a level turn, there will be a moderate amount of offset drag, requiring more push on the low grip to maintain the same curving line. In the off-camber curve, the tire will drag much more towards the inside, requiring a much stronger push on the low grip to hold the same line.

Changing Road Camber

If you think about how roads snake up and down hills and around corners, it should be obvious that road camber is constantly changing. Even on a straight road, the surface may stagger from a left slant to a right slant, and back again. Most of those twisty two-lane roads we like to ride have a crowned center to allow rainwater to run off, so where you position the bike in the lane makes a difference in the camber under your tires at the moment. If you follow our suggested "delayed apex" line, your tires will cross different cambers at different parts of the curve. That helps explain why the feedback from your handlebar grips seems to change from one moment to the next.

Back in the "good old days", road camber didn't have as much effect, because those old round-profile motorcycle tires didn't cause as much of a sideways shift in the position of the contact ring as the bike leaned over. There can't be much side drag on a 2.75-18": or 3.25-19" tire. Dunlop even designed a "Trigonic" front tire that had a triangular cross section more like a V than an O. The idea was to have maximum tire contact with the bike leaned over. But one result was that the contact ring remained close to the bike centerline right up to about 40 or 45-degree lean angles. And with the harder rubber compounds available in those days, riders had to be pretty nervy to lean the bike over that far. Those few riders who were adequately nervy discovered that when the tire suddenly made full contact on the flattish side of it's tread, a bike could make some surprising changes of direction, or produce some scary wobbles.