Technology: Driving dynamics safety

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Technology: driving dynamics / safety

Technology: driving dynamics / safety
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The discussion at the motorcycle round tables is as old as the motorcycle itself. Which is better: a high or a low focus?

Werner Koch

06/04/2009

Those who equate the low center of gravity with easy handling rush forward immediately. Even large motorcycle manufacturers like BMW argued in this direction, as with the K models. At first glance, their theory makes sense. Assuming you’re standing next to the motorcycle, grabbing the handlebars and swiveling the machine quickly from one side to the other, a low center of gravity offers the least resistance and the machine feels light and agile. Logical, because the low center of gravity, i.e. the center of mass of all vehicle components, covers a shorter distance when tilting around the longitudinal axis (an imaginary line from the front to the rear tire contact point) than with a high center of gravity. According to the simple laws of levers, in addition to the machine, people have to support a greater proportion of weight force with a high center of gravity than with a low one. The driver finds this very pleasant when stationary and when maneuvering, because it feels light and stable.

But what happens when the machine moves? Then the scenario just played out no longer applies. The reason: The motorcycle is not tilted by a second system connected via the handlebars and the floor (here the driver standing next to the machine), but by the driver sitting on the machine. And now it has to work against the stabilizing gyroscopic forces of the wheels and let them rotate in an inclined position instead of in a vertical position. How does it work? This is done initially by the steering impulse introduced in the opposite direction. In other words, before a left turn, the handlebars are turned around one to two degrees to the right, whereupon the motorcycle and driver tilts to the left in an inclined position. Entire textbooks could be written to explain this process physically precisely. In simple terms, the following applies: We turn the whole system mentioned at the beginning on its head and replace the person pulling the handlebars with a cornering force on the front wheel. This occurs as soon as the motorcycle takes a curve. And it actually has it easier to bring the center of gravity out of the vertical, the higher it is.

Another phenomenon is called precession and can be tried out beautifully on a dismantled bicycle rim: Simply hold a rotating wheel on the axle and “steer” it to the right, a strong force immediately acts and tilts the rotating rim to the left. This force increases tilting in an inclined position. The whole thing is counteracted by the inertia of the machine and of course the gyroscopic forces of the rotating rear wheel and the engine components such as the crankshaft, clutch, etc. A never-ending story, from which even renowned specialist book authors like Jurgen Stoffregen with his classic ?? motorcycle technology ?? stay out of it as much as possible. We just look at the center of gravity. Let’s take a stack of ten crates of beer ?? when trying, please leave out bottles? and if the bottom wobbles, it wobbles faster than a stack with only two boxes, or?

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When the center of gravity is too high

The machine will then fall too far into an inclined position. The center of gravity, so to speak, has to be slowed down again. You can feel that, for example, with a fully fueled travel enduro. Sensitive motorcyclists notice how, almost unconsciously, they counter-steer slightly in an inclined position in order not to drive even more diagonally. If the center of gravity is too high, this can knock the line with an annoying wobble. In the MOTORRAD top test course with its slalom course, only those machines are really fast that are balanced in terms of the height of the center of gravity. So far we have ignored the tire width when looking at it. But depending on the height of the center of gravity, it also has an influence. Machines with wide tires in particular require more lean angle than is mathematically necessary due to the tire contact area being shifted to the inside of the curve at the same speed and the same curve radius. Because theoretically, with a lateral force of one G, i.e. the dead weight of the motorcycle and rider, only 45 degrees of inclination are required (theoretically required inclination with tire width zero).

With the actually required incline of the machine, the angle that leads through the wheel contact point and center of gravity is added. The higher the center of gravity, the lower the additional incline required. Or the other way around: On the top test course, you can drive faster because the maximum possible incline is always used there. An important factor in racing. With different heights of the center of gravity (usually the whole machine, sometimes just the engine, is raised or lowered), the motorcycle is adapted to the respective route and curve progression. A higher center of gravity tends to be preferred on narrow stretches with long curves (less lean angle required, more grip at corner exit), while it is lowered on fast stretches (less wheelie tendency at full load acceleration, better braking stability, more stable driving behavior). No light without shadow. The high center of gravity can also be the trigger for all kinds of chassis unrest. At high speeds, a slight unwanted steering movement, for example due to air turbulence, is enough to cause the high center of gravity to sway and the motorcycle to swing. Bumps on a slope can also cause instability.

In practice, such problems arise when, for example, the rear of the vehicle is raised extremely by lifting kits, because the sensitive high center of gravity also has an extremely nervous steering geometry (steep steering head, short caster). However, there are limits to the height of the center of gravity not only in terms of driving stability, but also when accelerating and braking. The reason: The long lever arm of the high center of gravity increases the dynamic axle load distribution, which can quickly lead to a wheelie when accelerating and to the rear wheel lifting off with the risk of rollover when braking. Which is why the choice of the height of the center of gravity must be a compromise based on the purpose and development goal.

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