The Basics: Load Transfer

The Basics: Load Transfer

Oct 21, 2022

In the previous episode the tyres were the discussion topic. As said, one of the most important topics to maximise the tyre performance is the normal load.

The normal load is the vertical force that each tyre receives from the forces acting on the car. Generally speaking, the more load, the better the tyre performance, so greater lateral and longitudinal accelerations will be achieved for the same conditions.

But how this normal force is generated?

In tourism car, most of this normal load is created by the weight of the vehicle whereas in formulas, where the aerodynamics are the main topic, the normal load also depends on the speed and the aerodynamic coefficients, in this case, CL. The downforce of a vehicle can be estimated with the following formula:

Where:

  • ρ is the air density,

  • the CL is the downforce coefficient,

  • the A is the reference aera and the V is the vehicle air speed.

  • Thus, the F1 teams try to achieve the largest CL of the car.

However, how this load is distributed along the vehicle is as important as the amount of load.

In terms of vehicle dynamics, the weight and aerodynamic distribution play a major role in the control and stability of the vehicle.

The weight distribution is defined as the percentage in weight that the front axle receives compared to the total weight. This weight distribution is defined by the distribution of the parts of the vehicle and its weight. By knowing the load from the front and the rear axle, the position of the centre of gravity can be defined in the longitudinal axis of the vehicle.

On the other hand, the aerodynamic distribution has the same definition but it defines the centre of pressure.

Both centres are defined as the points where both loads do not generate a torque.

An important point is worth to mention is that the centre of pressure usually changes with the speed due to the aerodynamics performance and vehicle attitude.

In simple analysis, the distributions may be assumed constant, but, for a detailed performance analysis some sensors may be used to determine the load at each tyre:

  • Strain Gauges

    These are very small printed boards that are able to register the stresses where they are installed on. These are based on different wheatstone bridges with electric resistances that change their values with micro elongations. These are transformed to stresses using a stiffness matrix.

  • Linear potentiometers

    The linear potentiometers are usually mounted in parallel on dampers and springs to determine which is the deformation of the suspension elements. By knowing the suspension stiffness and the motion ratio, the vertical load can be determined.

Once the vertical load is obtained, the static load (weight) can be subtracted to calculate the dynamic aerodynamic load at each axle.

If both options, there must be some data logging or telemetry to obtain the data of both sensors.

Finally, to know the vertical load at each tyre, the load transfer must be considered.

Both load transfer in cornering and acceleration are calculated taking into account the centre of gravity height, track and wheelbase and accelerations.

The wheelbase is defined as the distance between front and rear axles whereas the track is the distance between the centre of the tyres on the same axle.

This load transfer can be intuitively understood as when you are a passenger in a car. When you are in standstill and the traffic light turns green, you notice that your body is pushed backwards. The same happens when taking a turn and you lean towards the outer turn.

With the same analogy, when the vehicle is accelerating, more load is transferred to the rear axle while the load is moved to the front one when braking.

The lateral load transfer follows the same principle: the load is transferred from the inner to the outer tyres. It also exists the case with the combined case with long and lat weight transfer.


So, do you think the load transfer is good or bad?

Generally speaking, the load transfer reduces the lateral force capabilities. This is explained using the tyre concept, so if you have not checked the tyres article, I would recommend you to read it before!

Before going to the main matter, it is important to define the grip coefficient, which is the tyre force (lateral or longitudinal) divided by the vertical load of the tyre.

By analysing the tyre grip for different vertical load, it decreases with the vertical load due to the characteristics of the tyre curves, for instance, Fy vs. slip angle. In other words, although the tyre creates more lateral forces, the tyre efficiency decreases with the vertical load.

An alternative analysis can be by checking the values from the Fy vs. slip angle curves. If the outer wheel increases 200N, the inner decreases 200N. However, the increase in lateral force in the outer should be around 150N while the loss in the inner is about 250N. Thus, a 100N of potential lateral force have been lost.

The overall lateral force of the vehicle is lower than without load transfer.

Although the vehicle stiffness does not modify the total weight transfer, it changes the load transfer distribution based on the axles' stiffness (roll and pitch stiffness). This is why is very important to setup the suspension stiffness to maximise each axle. As a general idea, the softer the axle, the less weight transfer will receive the axle.

In F1 cars, the engineers must find a balance between the vehicle stiffness to find the vehicle stability and balance and the vehicle attitude since it is crucial for the aerodynamics to keep the vehicle far away from pitch, yaw and roll to extract the maximum potential of the aerodynamic package.

The setup topic will be covered in future posts but it is all about finding the balance of the vehicle while extracting the maximum performance.

How can we reduce the load transfer?

  • Reducing the vehicle weight

  • Lowering the centre of gravity

  • Larger track and wheelbase. However, it is important to check the axle stiffness due to the change in the track value because depends on the track squared.

  • NOT: The total load transfer cannot be reduced by softening the vehicle's suspension. The vehicle setup is used to modify the distribution of the load transfer.

  • NOT: By changing the roll centre height, the amount of load transfer will not change. It only changes the elastic and geometric weight transfer. The elastic weight transfer is a key factor to determine the vehicle's attitude.

And that's all for today!

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