The basics of Weight Transfer

Weight transfer is one of the more commonly misunderstood terms in vehicle dynamics. In reality, this topic gets pretty complicated once we factor in more physics, kinematics, considerations for sprung and unsprung masses, geometric and elastic differences, damping, tire characteristics including slip angles, etc. The whole story gets a little too long. Today, we will try to cover the basics and the common misconceptions with weight transfer.

One of the first things to clarify is that weight transfer is not related to what the driver feels, but rather what the tires feel. The opposing forces we feel in the car pushing us in the opposite direction of the turn, throwing us forward as we brake, and pushing us into the seat as we accelerate, are primarily due to inertia and other references or feedback such as roll angle and forward dive. However what eventually connects the car to the road are the tires, or more accurately the 4 tire contact patches.

Most of us would be familiar with the classical friction theory which shows that the amount of friction (or sideways resisting force) is directly proportional to the vertical load as follows:

Where the friction coefficient (mju) is static assuming a no slip condition between both surfaces. What this suggests, is that the larger the vertical load, the larger the amount of sideways force that would be generated. This sounds pretty good, especially since rubber is one of the few materials to have a friction coefficient much greater than 1. Right? Except not. When something sounds too good to be true, there is always a catch. Sure, it's difficult to push a stationary car from the side into a parallel lot for example. The key difference between the stationary 'tire model' block above and a real tire is that real tires rotate.

The mechanisms in which a tire generates grip is quite complex as well, but is generally thought to be the result of 3 main factors: molecular adhesion, deformation and wear/tear. A whole book could be written on tires alone (Hans B. Pacejka), but what we can take from this is that a tire needs a slip angle to generate grip. We'll leave it at there now and get back to weight transfer.

Weight transfer is the transfer of weight from the inside to the outside tires of the car (for example from the left to the right of a car that is turning left), from the rear tires to the front when the car brakes, and from the front to the rear as the car accelerates. The following describes in general (without consideration of suspension geometry and sprung/unsprung mass), a simplified 2 dimensional illustration of the factors that affect weight transfer:

In the car above (without suspension apparently, jacking up the inside wheel as it turns to its left), weight is transferred to the outside tire (Fz), which is responsible for generating the resultant grip or Fy required to keep the car in its intended path. The weight transferred to the outside wheel is a function of the CG height and track of the car in 2 dimensions, which leads to a common misconception that stiffer springs allow you to transfer more weight. The spring rate is not part of this equation, thus does not influence the steady state weight transfer during a corner.

But some of what stiffer springs offer are:
- Reduced time for the load to be transferred, thus increasing the response of the car.
- Reduced body roll, dive and pitch giving the driver more confidence in the car's capabilities.
- Reduced height (usually), which lowers the CG height and reduces weight transfer.
- For custom springs, the ability to adjust the front/rear distribution of roll stiffness

wait, reduce weight transfer?

Another misconception about weight transfer is that it is good, that stiffer springs transfer more weight so we get more grip. We've seen from the above that the Fy (what we are ultimately interested in), is a linear function of Fz, which means the greater the amount of weight transfer, the greater the amount of Fy or sideways grip we get. However, due to the visco-elastic properties of the rubber compound in the tire, the increase in sideways grip is not linearly proportional to the increase in downward force or weight transfer (Fz), a property known as 'Tire Load Sensitivity'. If we revisit the 3 mechanisms that contribute to tire grip, a major contributor would be the deformation of a tire as the rubber tries to 'wrap around' the irregularities in the road (which are vast valleys under a magnifying glass). As the load is increased past a certain limit, the valleys in the road would be pretty much filled up, leaving only 2 mechanisms left for grip.

Source: Milliken & Milliken

We can non-dimensionalize the sideways grip with the vertical load or weight transfer by removing the effect of the weight of the car for different kinds of cars and setup to give the term 'lateral force coefficient'. What is then seen, is a reduction of maximum sideways grip as the amount of vertical load increases. The slip angle required to achieve the maximum is also increased.

If we consider the fact that the inside wheel sees a reduction in vertical load, and hence a reduction in sideways grip, then what Tire Load Sensitivity suggests is that we lose more grip than we gain with weight transfer.


To reduce sideways weight transfer, here are some options:
- Reduce the mass of the vehicle
- Lower the centre of gravity height
- Increase the track of the vehicle (via wheel spacers or wider tires in production cars)

This pretty much sums up the basics of weight transfer. Note that the above was a simplified 2 dimensional explanation without the considerations for suspension kinematics, sprung/unsprung masses and transient effects. Hope it was useful. :)