We see all the time now road racers “backing it in” to corners, sometimes with the rear end of the motorcycle out of line with the front just as much as dirt track or supermoto racers. Using engine braking or the rear brake, these riders are skidding the tire just enough that the rear end kicks out a certain amount under braking. Done properly with careful manipulation of the controls, this manoeuvre starts the motorcycle turning before the corner, effectively reducing the arc of the turn that must be completed.
How sideways the bike goes on the entry is determined partly by how much load is on the rear tire, and partly by the amount of slip (how much slower the rear tire is turning that the bike’s actual speed). The wheel’s load depends on how hard the rider is using the front brake to unload the rear end, and while some of the rear braking force can be provided by the rear brake itself, most modern four-stroke race bikes have more than enough engine braking for the desired amount of slip. While a very good rider can modulate the front and rear brakes and the clutch all at the same time for a beautifully controlled slide all the way to the apex of the corner, the goal is to have close to the right amount of skidding provided automatically so that the rider need only fine-tune the action.
One method to accomplish this is to use a slipper clutch, which reduces the effect of the engine braking to manageable levels so the rider does not have to be so precise with letting the clutch out. On most units, the amount of clutch slip and the point at which the slip initiates can be adjusted by changing the rate of and the preload on the clutch springs. Another method is similar to the old racer’s trick to reduce engine braking by increasing idle, and current electronics systems slightly open a ride-by-wire throttle on deceleration. These systems can offer anything from a simple 1-2-3 level of adjustment to fully variable engine braking based on rpm, gear position, and more.
In series that don’t allow elaborate electronics for engine braking control, however, we have to look to the chassis and ways of adjusting rear tire load in order to control how the motorcycle behaves on corner entry as the rear tire slips. In general, with more load the rear end will track straighter entering the corner, with less it will kick out more sideways. One way to address this is to raise or lower the whole bike, which affects how much load transfers under braking; with the motorcycle lowered slightly, for example, less load transfers to the front on corner entry and the rear tire stays more in line.
Another option is to make changes to the rear suspension to adjust the rear tire’s reaction to a given load, rather than trying to adjust the amount of load. As an example, consider a rear shock setup with an amount of preload such that it takes 20 kg of load to even move the suspension; under braking, the rear tire will begin to skip and float across the pavement and go sideways if load goes below that 20 kg, because the suspension is topped out. By adjusting the spring rate or preload, we may be able to change that value so that the suspension begins to move with less or more load, altering when and how much the rear tire skids. Going a step further, we can even get creative with the top-out spring inside the shock to make that adjustment and influence corner entry behaviour without affecting the remainder of the suspension travel.
It all becomes a juggling act, with the slipper clutch, electronics package and suspension setup each playing a part. In an ideal setup, the bike backs into the corner just the right amount without requiring those precise clutch and rear brake inputs, allowing the rider to focus on more important aspects of the corner entry.
– By Andrew Trevitt