Steering in Bicycles and Motorcycles By J. Fajans

Counter Steering

Motorcycle physics are well understood, and used in design by engineers at all of the major manufacturers (you'd better hope they are as well, or you bike would be unsafe to ride). The salient factors are summarized in the accessible and outstanding book Motorcycle Design & Technology by Gaetano Cocco which was written in collaboration with the engineers at Aprilia.

The physics of steering in motorcycles and bicycles is complex, and there is no doubt that countersteering provides the major control over steering, but shifting the rider's weight, and in the extreme, "hanging off" is an important part of racing competitively (though not really necessary for street riding).

Countersteering alone introduces a delay of about 1 second in getting the bike into a turn, because the steering column turns one way for a second, and then reverse. Hanging off allows the rider to speed up the transition to the proper lean angle, and can on 500cc GP bikes reduce the 1 second lag to 1/2 or 1/4 second.

For most of us riding on the street, fast lean transitions are not an issue, but in racing (as Keith Code notes in A Twist of the Wrist) it is everything. Code's schools are oriented more for the street rider than the competitive racer, and thus he can simplify his explanations and get away with it. But changing the center of gravity of the bike has a nonnegligable effect on lean transition and turn.

The first rider to competively hang off was Jano Sarranin, who brought the technique in from ice racing. Ice racing bikes weight in at about 70 kilograms -- the same as an average rider, and in the range of the weights of 125cc and 250cc GP racers. 500cc and street bikes are about twice as heavy, and thus when the rider throws his weight around, it has much less effect -- but is still useful in getting into the turn quickly. With cruisers, forget it; plus cruiser riders tend to steer rather than lean.

Gyroscopic force is at the root of this motion, but because a motorcycle is essentially two linked casters, with multiple points of input from the rider, the effect of gyroscopic forces are complex. Start by considering the basic gyroscopic forces as shown here (courtesy of Gaetano Cocco , Motorcycle Design & Technology). Now when the rider countersteers, this is converted to a roll then to a yaw motion. By shifting his center of gravity (CG) the motorcyclist can augment this roll acceleration.

The process of making a countersteered right turn can be broken into five somewhat arbitrarily divided steps:

1. You initiate the turn by applying a torque to the handlebars, steering the front wheel to the left.

2. The wheel steers to the left. The rate at which the steer-ing angle increases is set primarily by the moment of inertia I, of the wheel, fork, and handlebars around the steering axis, and by the "trail" (described later.)

3. As the bike is now turning to the left, a centrifugal torque leans both you and the bike frame to the right. Gyroscopic action also leans the bike to the right, but, as I will show later, its effect is negligible.

4. Transmitted by the fork, the increasing lean attempts to lean the front wheel over as well. For the first time, gyroscopic action becomes important, as the wheel responds to this "leaning" torque by attempting to steer to the right, thus counteracting the steering torque. The steering angle stops increasing.

5. The leaning torque overcomes the steering torque and the wheel steering angle decreases. Note that the lean continues to increase because the bike is still turning left.

6. As the bike has now acquired substantial leaning velocity, the lean increase cannot end instantly. Driven by the still increasing lean, the wheel steering angle passes smoothly through zero and then points right. The centrifugal torques reverse direction, eventually halting the lean increase and balancing the gravitational torques. As no more leaning torque is applied to the wheel, the steering angle stabilizes, and the bike executes the desired right turn.

Alternately, the required lean can be generated by throwing your hips in the direction counter to the turn (what you have to do to hang off). Throwing your hips is how a bike is steered no-hands. The sign of the effect is subtle, but a half-hour session with a bicycle (don't try this with your motorcycle) in an empty parking lot should convince you that while riding no-handed, you steer the bike by leaning your shoulders in the direction of the desired turn.

Since angular momentum is conserved by a sudden shift of your shoulders, your hips move the opposite way, thereby leaning the bike the opposite way as well. With the bike now leaning, the bike’s "trail" becomes important. As the steering axis is not vertical, the point of contact of the wheel with the road "trails" the intersection of the steering axis with the road. The trail makes the bike self-steer: when the bike leans to the left, the front wheel steers left; when the bike leans to the right, the front wheel steers right. This effect is easily demonstrated by standing beside a bicycle and leaning it from side to side. (The trail is the single most important geometric parameter which enters into the handling of a motorcycle.)

The premise of the no-BS bike is that you either body steer, or you countersteer. It eliminates any input at all from the handlebars. But except on a bicycle, it is virtually impossible to shift the center of gravity around sufficiently with your body weight alone to actually no-hands steer. Code's no-BS bike is a Kwak600 like the rest of his school bikes and weighs in at about double the weight of a 70 KG rider. Even with a small "trail" it just way too heavy to body steer.

Where shifting the center of gravity is important is as an augment to countersteering. It helps lean the bike faster. But the majority of the leaning torque will come from countersteering.

CG shift in combination with body steer leans the bike faster than with countersteering alone. This is why it is competitively important. especially with light bikes like supercross and ice racing.

Here is a possible explanation as to why Keith Code's no-BS bike is so difficult to ride. The appropriate way to shift the CG is to weight the pegs (this is your point of contact with the motorcycle chassis rather than the fake handlebars on Code's bike), line up your outside leg (i.e., the left leg on a right turn) with the tank (i.e., spine aligned with the bike) and throw the right knee straight out, and move your bottom off the seat to the right. This provides a smooth and definitive shift of the CG.

On Codes no-BS bike, it has been suggested that riders are lazily resting their rears on the seat, and pushing the handlebars one way, while shifting their body the other (i.e., they push the handlebars left and their head right for a right-hand turn). Because of this, there is little change in CG. And there is no effective input to the handlebars (since they are rigidly attached to the frame), while the mind is expecting a countersteering input. Thus a combination of almost no-shift of CG, along with misleading signals to the riders brain about the countersteering input make it difficult to ride. The no-BS bike is actually not properly set up for the demonstration. It locks the rider to the body of the bike through a false replacement of the front caster (the steering fork). If you use your hands as on the no-BS bike, you will tend not to move any of your body below the waist; above the waist, you'll throw your hands out one way, and your head out the other, pretty much negating any lateral weight shift. But good riding dictates that the body be locked to the bike through the rear caster -- i.e. by putting your weight on the pegs. The appropriate way to demonstrate body steering is not with a set of false handlebars, but by providing stabilizers (e.g., like long training wheels) on either side of the bike, and eliminating the handlebars all together. It would be interesting on the no-BS bike, to get someone who is used to doing stunts on dirt or sport bikes ... get them to stand off way to one side on just the inside peg and holding the inside handlebar

Body steering alone is slow, simply because you can't lean the bike very far and fast with body steering alone. The force input from changing CG just is not that great.

But combine force input from the bars, generating gyro roll torque, and add to that some more torque from body shift, and you move the mass of the bike (which has its own inertia which increases with mass). The more torque input, the faster this mass accelerates.

Newton's second law of motion: force = mass * acceleration

The acceleration of "roll" acceleration, and in this case determines how quickly you lean.

you should keep your spine aligned with the bike (this certainly feels a lot better to me, and is a confidence builder). I use the knee as a feeler as well to determine lean angle. Throwing your body weight around is a bit complex, since for every motion there is an equal and opposite counter motion ... as well as the fact that all of the parts of your body are attached to each other, with relatively little give.

It has been suggested, in fact, that we learn over time to both shift the CG with the body, and at the same time provide the appropriate torque input to the handlebars, as a part of shifting our weight. There are a lot of variables, and perhaps this is why it takes so long to learn to ride well (and why different riders have different styles).

Interestingly, gyroscopic action's effect on bike steering is generally indirect. A bike is two gyros liked through the steering axis. Applying torque to the front gyroscope produces a pitch (lean) at right angles. The bike continues to lean until a new equilibrium position is found. That position is determined by centrifugal force acting in one direction, and the yaw of the bike (induced by torque input to both the front and rear gyros by the pitch (lean) torque input) around the radius of curvature of the turn.

The engine in racing (where RPMs are high) provides most of the direct gyroscopic effect, which is the reason that Yamaha and Suzuki have produced their GP bikes with two counterrotating crankshafts.

 

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