Hey guys, it's time for another post on my blog. After we discussed the topic "Traction Circle" last time, we will continue with the dynamic axle load distribution and its effects on the driving
behaviour of your car.
The weight of the car is distributed over the four tires and the two axles to which the tires are attached. The two axles are connected to the car body via springs and dampers. Since springs and dampers are not a rigid unit, the car body moves with every driving manoeuvre we perform.
If you are driving on the road just take a look on how your car behaves when you are braking for a red light, when you take off when the lights switch to green on a junction or when you are just turning onto the parking of your favourite supermarket:
- When we brake, the vehicle dives with the front and the rear end lifts up.
- When accelerating, the rear of our vehicle squats down and the front comes up.
- In left turns the vehicle rolls to the right and in right turns the vehicle rolls to the left.
With each of these movements, the weight acting on the respective axles and wheels changes accordingly, here we speak of dynamic axle load distribution. Dynamic because it is a constantly
So, thats it for basis - let's go through the different driving conditions step-by-step.
We arrive at our braking point at the end of the start and finish straight and hit the brakes hard. At this moment the following happens: The brakes starts to decelerate the car and since the weight of the car wants to continue at the original speed for the time being, the car plunges down at its weakest point.
The weakest point in this case is the area of the springs and dampers on the front axle. Weak doesn't mean that something happens here that shouldn't happen - but a spring and a damper are not as rigid as the frame or the body.
As the car is diving at the front, the rear of the car will rise, because the bodywork does not deform. This whole movement causes the weight to shifts its centre of gravity, which is for arguments sake in the middle at constant acceleration, to the front.
The increased weight on the front axle pushes the tyres more into the asphalt. To illustrate it a little more clearly, one could imagine that we could speak of a kind of "downforce". The pressure from the weight pushes the tyre more into the asphalt, increasing the tyre's ability to transfer grip.
Excursion: Traction Circle
If we now imagine the Traction Circle for the front tyres, the radius of the circle would increase during the braking phase, as the 100% limit would move upwards.
For the rear tyres, the exact opposite is true: because they are no longer pressed into the asphalt as strongly as before the braking phase, the ability of the tyre to transfer grip is reduced.
When accelerating, it actually looks exactly the opposite: The rear of our vehicle is pressed down (we say "the rear squats"), as the centre of gravity moves to the rear of the car. Physical it is the same principle as under braking: The weight wants to stay in its current state (stationary) and therefor it moves to the back. As the rear of the vehicle sits down, the front axle naturally lifts accordingly. By "sitting down" the grip increases on the rear axle as the rear tires have more pressure on the asphalt and are therefore able to transfer more grip. The front tyres have less pressure and are therefore not able to build up the same lateral grip as at the beginning of a corner, see Excursion above.
Cornering (constant movement):
When cornering, let say at a constant speed for argument sake, the car rolls to the outside as the weight in the corner always pushes outwards and this increases the pressure on the outer wheels. If you look at Formula 1, for example, you can see that it is usually always the inner wheel that locks up. The reason is mentioned above: The inner wheel is unloaded, it is not pressed as hard on the ground and therefore there is less grip available and the wheel tends to lock up more quickly.
In reality, the weight transfer in a corner is a little more complicated, as a constant speed is not very common. Cars will brake into a corner (decelerating and cornering at the same time) and cars will power out of a corner (accelerating and cornering at the same time), so that the above mentionend examples intermingle.
Next time we will take a closer look on how these weight changes can be used to improve the performance going into a corner - we will talk about "Trailbraking".
Until then, keep it on the black stuff! :-)