If you’re ready to learn more about why sidecar rigs do what they do, we’ll dig a little deeper into the dynamics of cornering. What follows is not a prerequisite to the lessons or exercises in Driving A Sidecar Outfit, but explains more about the dynamics of cornering for those who would appreciate additional information. This chapter will be clearer if you have completed all the practice exercises in the book.
The following is most true for conventional motorcycle/sidecar combinations. Outfits with a narrower track and a taller profile are affected more by cornering forces, and require much more physical input from the driver to keep under control. A high performance rig or a racing outfit with a wide track, low profile, steerable sidecar wheel, etc. is less affected by tipover forces. But all sidecar outfits respond to the same physical forces in much the same way.
The simplistic concept of cornering is that when centrifugal force exceeds gravity pulling the sidecar down, the rig tips over. But that concept is incomplete. There are forces other than centrifugal force that we can harness to keep the rig under control, including steering, hanging off, and tire slip.
The most obvious cornering forces are gravity and centrifugal force. Gravity is a constant. That is, the force of gravity pulling the sidecar down is the same regardless of speed. However, centrifugal force increases as a function of speed. So, it might seem that the tipover forces are determined entirely by speed. In other words, if you increase speed around a turn to the point where centrifugal force exceeds gravity, the outfit should tip over. Ergo, the only practical tactic for preventing a tipover is to reduce speed.
However, let’s note that a sidecar outfit can be driven around a curve with the sidecar wheel off the ground, at various speeds, without tipping over or running wide. Apparently, gravity and centrifugal force aren’t the only forces involved. Steering, hanging off, and tire slip also affect balance.
Terms
To prepare for a more detailed explanation of the forces, let’s define them. We use the terms “roll”, “pitch”, and “yaw” to describe spatial movements, terms that are borrowed from aviation. Let’s note that all objects tend to roll, pitch, or yaw around their centers of mass, or what we usually call the center of gravity (CoG).
Gravity is that mysterious force that pulls everything toward the center of the earth. Gravity is a constant that does not change due to the speed of the object. Speeding up or slowing down will not increase or reduce gravity. The pull of gravity is always straight down, not perpendicular to the road surface.
Centrifugal Force is described as “a force that tends to impel a thing or parts of a thing outward from a center of rotation”. Attach a brick to a string and swing it around your head. The force you feel pulling the brick outward is centrifugal force. The force you must apply to the string to keep the brick in a circle is “centripetal force”.
Inertia is not a force, but rather “a property of matter by which an object remains at rest or in uniform motion in the same straight line unless acted upon by some external force.” Inertia is what makes a sidecar rig want to keep moving straight ahead at the same speed. We’ll usually refer to the affects of inertia as “forward energy” (kinetic energy pulling forward).
Traction is the friction or ‘grip” between a tire and the road surface. Traction is dependent upon the rubber compound, tire profile, road surface, surface contaminants, and how much weight is pressing down on the tire. Traction is a variable, and we have some control over how traction is used.
Yaw is a rotation of the rig around a vertical axis. Steering, tire slip, engine thrust, and braking can all cause yaw.
Pitch is a vertical rotation around an axis lying perpendicular to the centerline of the bike. Engine thrust, braking, and hills contribute to pitch.
Roll is a rotation around an axis parallel to the bike centerline. Cornering, steering, and surface camber changes cause roll.
Center of Gravity
Of course, a sidecar outfit isn’t a single object, but a combination of several different objects of mass, including the driver, passenger, motorcycle, and sidecar; each of which has its own CoG. To keep things simple, we usually average out the various masses, as if they had a “combined CoG”. For most rigs, the combined CoG will be somewhere in the vicinity of the driver’s right knee.
Obviously, if the driver or passenger move around, or an additional load is added to the outfit, the combined CoG will be in a somewhat different location. Let’s also note that if the various masses (weights) are spread far apart, the combined CoG could be in the same location, but the rig would be more sluggish to change direction. For the moment, let’s pretend that outfits have reasonably centralized mass, and the dynamic forces and properties act only on the combined CoG.
The reason a rig with a low center of gravity and a wide track will have better tipover resistance is that centrifugal force acts on the CoG closer to the ground. Centrifugal force has less leverage to roll the outfit. The combined CoG might also be farther away from the left tipover line due to the wider track. The result of wide track and low CoG is that centrifugal force will overpower tire traction rather than tip the sidecar up. In other words, the rig will slide sideways before the sidecar flies.
With a taller or narrower outfit, carrying weight in the sidecar—or moving driver weight toward the sidecar—helps keep the combined CoG within the tipover lines. The disadvantage of ballast in the sidecar is that it affects acceleration and braking. The driver hanging off left or right allows a dynamic change in balance without adding extra mass.
A combination with a low CoG will have better tipover resistance than a taller outfit.
Hanging off right moves the combined CoG toward the right, increasing tipover resistance. Carrying ballast in the sidecar will also result in the combined CoG being more within the tipover triangle, but ballast adds mass that makes acceleration and braking more sluggish.
Roll around the CoG
All objects (including sidecar rigs) tend to roll, pitch, or yaw around their centers of gravity (CoG). The inertia of the sidecar rig at highway speed provides a significant resistance to turning. The outfit wants to go straight ahead at the same speed. When we’re trying to get an outfit to go around a corner, the “centrifugal force” (resistance to turning) is merely the rig wanting to get back to a straight line, due to its inertia. Likewise, when the brakes are applied, the resistance to slowing is simply the inertia of the outfit wanting to maintain speed.
Since tire traction is down at ground level, and the combined CoG is much higher, braking causes a pitch change, and steering tends to cause roll. This is why an automobile or sidecar rig leans (sways) toward the outside of a curve.
While inertia resists the outfit changing speed or direction, there is much less inertial resistance to roll. As you can affirm for yourself, a quick swerve toward the sidecar can lift the sidecar wheel off the ground without much change in direction. What makes this possible is that inertia is resisting sideways movement of the rig at its CoG, but not significantly resisting roll. Front tire traction is powerful enough to push the outfit into a left roll—lifting the sidecar.
Note that the outfit doesn’t roll around the left tipover line as much as the tipover line moves under the CoG. The roll movement isn’t exactly around the CoG, since the outfit must also rise or fall vertically due to the resistance of the road surface.
On a conventional motorcycle/sidecar combination, tire traction is typically powerful enough to cause roll--either lifting the sidecar wheel up, or forcing it back to the ground. And with the outfit balanced on two wheels, front wheel traction is sufficient to keep it balanced like a two-wheeled motorcycle.
Steering more toward the right rolls the outfit left, lifting the sidecar. Steering moves the position of the left tipover line more under the CoG.
Steering more toward the left, moves the tipover line left under the CoG, rolling the sidecar down. Of course, once the sidecar tire contacts the ground again, little additional roll in that direction is available.
In the “flying the car” exercise, driving slowly around the small circle, it may seem that speed is what lifts the car, and novice students typically try to lift the sidecar by increasing speed. The novice may be frustrated because as soon as speed is increased to the “tip up” level, the survival tactic is to instinctively steer wider—which brings the sidecar down again.
By contrast, a skilled sidecar driver can fly the car around the circle at a very slow speed, when centrifugal force is almost absent. The point of the “flying the car” exercise is that steering is a primary force in balancing the outfit, and speed is secondary.
The same truth applies to flying the car in a straight line, as you may test for yourself. With the sidecar flying in a straight line, slowing down has no affect on balance. Slowing to a complete stop does not pull the sidecar down.
Likewise, if you fly the sidecar in a right-hand circle, you can increase or reduce speed (within limits) and still maintain balance with steering, until the sidecar tire contacts the ground.
To recap, steering not only controls direction, but also has an effect on roll (lean). Steering slightly more toward the turn causes the rig to roll more toward the outside, lifting the sidecar up. Steering slightly wider (away from the curve) causes a roll toward the curve, pushing the sidecar down. It’s very important to keep sidecar lift controlled in a sharp right turn, because when the outfit rolls left, the combined CoG moves left. Once the combined CoG lifts up high enough to move over the left tipover line, recovery requires a very dramatic left swerve. Slowing down will not help recovery.
Steering controls both direction and roll. With the car flying, steering slightly to the left brings the car down.
It’s important to control roll early, because once the center of gravity moves over the tipover line, it’s very difficult to regain balance.
Tire slip
Another of the forces we can use to control roll is tire slip. In a corner, rubber tires on a vehicle will have some “side slip” as the flexible rubber attempts to force the mass to change direction. The difference between the centerline of the wheel and the direction the wheel is actually moving is called the “slip angle”.
Increasing the driving force (by rolling on more throttle) typically increases rear tire slip angle. The wheel is still pointed “straight ahead”, but the tire is actually slipping a little bit sideways (“drifting”).
We can use tire slip to help manage roll. Increasing rear tire slip in a right turn actually moves the tipover triangle toward the left, helping roll the sidecar down. Increasing the side slip results in a slightly larger radius of turn, similar to steering the front wheel slightly wider. The outfit will yaw slightly toward the right, without increasing the rate of turn.
Drifting the rear tire controls roll by moving the tipover line under the CoG. Drifting the rear tire also creates some yaw, pointing the front end more toward the curve.
Of course, just rolling on the throttle will increase speed, and that would increase centrifugal force. To increase rear tire slip angle without increasing speed, the technique is to lightly apply the front brake while maintaining a leading throttle. Front wheel braking not only helps keep speed from increasing, but also increases the slip angle of the front tire.
Squeezing on some front brake while rolling on more throttle increases the slip angle on both front and rear tires, while maintaining speed. Squeezing on more front brake will increase tire slip and limit the roll.
How much steering force can the front tire generate?
The front tire can generate a significant steering force. Let’s imagine a theoretical sidecar rig with a 580 lb motorcycle, 200 lb sidecar, 180 lb driver, and 150 lb passenger. Assuming half the load of the sidecar is supported by the rear motorcycle wheel, the weight on the front wheel might be 478 lb. That means the front tire is capable of a steering force of 478 lb.
How powerful is the steering force? Based on these numbers, the weight on the front wheel would be 478 lb, which means the front tire could generate a steering force of 478 lb—more than the weight of the sidecar and passenger in this example.
If it isn’t obvious, with the sidecar flying, the weight on the sidecar wheel would be 0, and the weights on the motorcycle wheels would increase to 557 rear and 553 front. That explains why the front wheel has so much power to control roll.
A high performance rig with low CoG and wide track is much more resistant to roll, and therefore it’s harder to fly the sidecar, especially with a passenger on board. It’s much more likely the tires will slide before the sidecar can be rolled up. However, if the sidecar can be lifted, balance is possible even with wide, low profile tires.
The bottom line is that there are forces other than gravity and centrifugal force affecting a sidecar outfit. The “advanced” techniques suggested in the lessons help manage the forces for better control.
