ALL THE TRACTION YOU NEED
FROM THE ENGINE TO THE WHEELS
Even with the most powerful engine and finest mechanics, we cannot move without it. Let’s take a look at traction and one of the most evolved designs currently available for 4x4 vehicles
IT ALL TAKES PLACE inside that black oval, the area of contact between the tyre and the road, where engine torque is discharged to create a reaction with the ground. In effect, the third principle of dynamics – every action has an equal and opposite reaction – tells us that that if a car moves forwards the ground moves backwards!
Obviously this is theoretical: the mass of our planet is incomparably superior to that of any vehicle so it does not “notice” the force exerted by an object, no matter how big or powerful it is.
This extreme example apart, the concept to be understood is this: vehicle traction is based on grip, in other words, the ability of a tyre (or train wheels or caterpillar tracks) to transmit parallel forces to ground with limited slipping or – better still – with no slipping at all. The term “parallel” is very important because it tells us that perpendicular forces (typically, weight and aerodynamic load) have little effect because they could just as easily be supported by blocks of wood.
In reality, a tyre must also absorb a certain amount of surface irregularity and from this point of view it can be said that it helps the work of the suspension, but this is not its main task.
Saying grip is easy
Grip depends on a variety of factors, the most important of which are the material composition of the contact surfaces (in our case, the tyres and the road) and the force that presses the tyres against the ground. Operating temperature is also very important.
Tyre work load is very critical because it varies continually depending on vehicle speed, which can cause considerable changes in vehicle stance: you will have noticed how a vehicle tends to “crouch down” when braking and then rise up again on acceleration. To begin to understand the reason for this behaviour we must remember that braking and acceleration forces are applied at the point of contact between the tyre and the ground. We might imagine that vehicle inertia is applied to the barycentre, but it is a much higher point. When braking, torque is generated (with distanced points of force application) and it makes the car “roll” forward: nose up, tail down. An example might help. What would happen if something grabbed our ankle where we were running? We would fall forwards. But if something pulled our feet forwards when we were standing still, we would fall backwards: a vehicle does not “fall” but the load is transferred to the back and causes the rear suspension to “seat”.
This is why sports car stance is low (to limit barycentre height) and stiff, which limits load transfer – and, as a consequence, pitching – when accelerating or braking.
But our untiring tyres must not only transmit engine and braking torque, they must also generate the lateral forces needed to allow the vehicle to corner. Suppose we turn the steering wheel when driving in a straight line: the front wheels would no longer be parallel to the direction of travel but would be angled by “x” degrees. This incidence generates a lateral force that causes the vehicle to leave its rectilinear trajectory and creates – again – torque that causes the vehicle to lean to the side (roll) with a mechanism that is similar to pitching. In this case, two forces are also applied at distinct points: the inertia of the barycentre and the lateral force in the area of contact between the tyre and the ground.
These horizontal, longitudinal and transversal forces continually change in accordance with an infinite number of combinations, for example when cornering at a constant, moderate speed (with little aerodynamic resistance and, as a consequence, low levels of engine torque), the forces will be mainly lateral. But braking is sufficient to introduce longitudinal forces: can the tyre also transmit these stresses without slipping?
To analyze the overall limits of tyre grip, a special graph is used, the ellipse of grip force. To construct it we use a Cartesian plane in which axis y – vertical – represents longitudinal forces Fl (acceleration and braking, with negative values for the latter) and axis x (horizontal) showing transversal forces Ft created by the change in direction. The ellipse is given by the points that represent the maximum Fl and Ft the tyre is able to support and maintain sufficient grip. If there were only longitudinal force, it would be represented by an arrow along the vertical axis, but if there were only lateral force, the graph would show a horizontal arrow. Each of these infinite combinations of the two forces is represented by a point on the plane and if this point falls inside the ellipse, the tyre will guarantee the required grip. Going back to what I said previously, we can consider that this ellipse becomes bigger with the increase in load, because the forces that can be transmitted increase with the increase in load.
Divide and collaborate
It seems to be clear that it is not possible to expect great acceleration from a wheel that is being steered and vice-versa. A possible solution would be to divide the traction forces between the tyres so that each one can work more easily in our famous ellipse.
This strategy was conceived and then adopted many decades ago, but in recent times different solutions have flourished thanks to the progress made in electronics.
Four-wheel drive can now be used on different vehicles, one per axle, and not only with the traditional solution of two transmission shafts and various differentials. Widespread examples of this design are Peugeot’s Hybrid 4, vehicles that add to classic all-front mechanics a rear axle driven by an electric motor.
But classic applications have also massively exploited the versatility and flexibility given by using mechanical components controlled by electronic systems.
This is the case, for example, of the AWD platform presented by Jaguar for its XF and XJ Model Year 2013. The system continually varies torque between the two axles thanks to an advanced Transfer Case Control Module (TCCM). TCCM controls the level of grip and driver input by suitably dividing front and rear torque to best safeguard the feeling of rear wheel drive that Jaguars have always had.