Fluorescent Pallor

Fluorescent pallor

TDC

Kevin Cameron

WHEN A POWERFUL MOTORCYCLE IS accelerating in first gear, tension in the chain is large. For 600 pounds of motorcycle, fuel and rider, accelerating hard with the front wheel light, there can be very close to 2000 pounds of chain-pull force. The most important effect of this tension, of course, is that it accelerates the motorcycle.

It has other effects. Because the upper run of the chain is at an angle to the plane of the swingarm, it exerts a downward force on it, tending to extend the suspension and lift the back of the machine. This angle is of the order of 7 to 10 degrees, and the resulting lift force is large, like 200-350 pounds. This force, which we can call the chain-pull tangent force, is responsible for the fact that motorcycles don’t squat down in back when they accelerate, as cars do. In cars, because the acceleration force is delivered at pavement level but the vehicle’s center of mass is a foot or two above that, there is a torque tending to rotate the car rearward on its suspension. With only its suspension springs to resist this rearward weight transfer, the car squats in the rear as it accelerates.

The same forces act on an accelerating motorcycle, but the existence of the chain-pull tangent force opposes the squat. Tangent force is often large enough to actually make the rear suspension top out, to fully extended.

For many years, this effect made little difference to anyone. Motorcycles were considered very sporty if they had 50 horsepower, and suspensions were stiff and their travel was short. Lift and squat were abstract concepts with no application to motorcycles.

Then in the 1970s, long-travel suspension appeared, with much softer springing. This meant that motorcycles could and did move a lot more on their suspensions. In the 1980s, the 100-hp motorcycle became common, so lower-gear chain-pull force increased greatly. Adjustable suspensions appeared. With these, you could change front or rear ride height, spring preload and damping force.

Riders quickly discovered that they could decrease steering-head angle by tilting bikes forward on their suspension-resulting in what Doug Chandler has called “the stinkbug look.” This not only speeded up the steering, but also slightly increased the weight carried by the front wheel, valuable when trying to steer while accelerating hard out of a corner. It’s easy, given the physics of the motorcycle, to be in a cornering situation in which the harder you open the throttle, the lighter the front wheel gets, and the less grip it has, resulting in the machine not holding line. You have the choice of either accelerating less hard to make the bike steer, or of running wide, possibly off the road. Neither option is attractive. Therefore racers tended to lower the front and raise the rear.

This good work could all be undone in an instant if the bike squatted during acceleration. Rear ride height would decrease, weight would be transferred off the front wheel, which would then steer less well and the bike would again run wide on corner exit.

It took a while to figure out that there was some range of swingarm-pivot locations on motorcycles. In general they were high, causing the chain to run close to contact with the top of the swingarm when fully extended. But there were also machines with lower pivots, like Yamaha’s TZ500. Now people realized that the higher the pivot, the greater the chain-tension tangent forcei.e. anti-squat-and vice-versa. People began to experiment with adjustable swingarm-pivot heights. When everything was just right, squat was canceled and the front end retained enough weight to steer rather than push.

Yet almost anything could interfere with this just-right-ness. If you change the front sprocket, you’ve changed the chain-pull force. Any change to either sprocket’s diameter raises or lowers one end of the chain, thereby changing its angle to the swingarm, and the resulting anti-squat force. Same for changes in rear ride height. All this transforms the just-right-ness into a will-o-the-wisp. At a given event, the setup might be good, but the normal changes necessary at another track could destroy that goodness.

On some machines, the pivot height can’t be changed because the pivot passes through the engine cases. Then anti-squat becomes the job of a stiff spring and/or restricted suspension travel, both of which can be harmful to tire grip. Once a good setup is achieved, people want to avoid changing it, so in some cases, gear-ratio changes have been made inside the transmission itself to avoid changing sprockets.

Now along comes the Buell Firebolt, with its fixed rear-axle position, its toothbelt final drive and its drive tension control by means of an idler wheel. This motorcycle’s novel design suggests a lot of things.

First, the thinness of a belt allows it to run closer to the front of the swingarm than a chain can. This offers more latitude in pivot location. Second, the presence of the tensioner suggests once again the idea of positively controlling the angle of chain or belt to the swingarm, rather than leaving it to the happenstance of sprocket size and suspension position. I’m wondering when some enterprising race team will stop fiddling with secondrate tricks like excessively stiff springs and eccentric swingarm pivot shafts, and actually make the chain angle be exactly what they want it to be-at all times and in all conditions. How difficult would it be to put an idler above the chain, to keep its angle to the swingarm from decreasing as the suspension compresses? Or even to control the height of such an idler with a linkage connected to the suspension?

The normal fate of ideas such as this is the thought that, “If this idea were any good, someone would already have tried it. Nobody’s using it, so therefore it doesn’t work. I’ll forget about it.” This kind of thinking has the added advantage of keeping the weekends free. We can all go to the lake rather than risking fluorescent pallor in the shop, trying to solve weird problems.