Lesson 1.

NO.1 - WEIGHT TRANSER, SHAKING IT ALL ABOUT

Introduction

It was a corner I knew well. Very well.
I came up to the braking point at a round 140mph as normal and brought the speed down for the corner. No problem. The car 15 feet in front of me had done the same and we mirrored each other up to the clipping point. On the exit, however, it was a different story.
It was a damp track and we were all running a fairly soft set-up so that weight transfer wouldn't cause drastic understeer or oversteer. I'd got a slightly better entry into the corner and by the clipping point I was about 6 feet from his bumper.
It was about this time that I noticed his back end dip slightly and as we exited the corner he drifted wider and wider until eventually he put a wheel on the grass and pitched the car backwards onto the grass.

So what went wrong? Well sensing I was close enough to make a move on the exit, he got a bit over excited and booted it to try and get out ahead of me. This shifted all of the cars weight backwards and made the front go light.
Having lost a lot of grip at the front, he tried to force the car to turn by giving it more lock. Sadly, this made things worse and the car's inertia simply forced it forwards and onto the grass.

The Basics.

While most students at racing schools are taught the skills of balancing a car, the reasoning and principals behind it rarely are. Knowing how what you are doing is affecting the car will help you with your driving and also with the set-up.

When you balance a car, you are controlling it's weight transfer using the throttle, steering and brakes. Many manuals with racing sims simply state that braking shifts weight forwards, accelerating shifts it back and cornering to the side.

But why does the weight shift? How can it when everything is strapped down?

How much a car reacts to weight transfer depends on the weight being transferred and the height of the Centre of Gravity. A heavy Nascar saloon with a higher Centre of Gravity reacts a lot more compared to a light Formula 1 car with a lower Centre of Gravity.

Lets explain some fundamental laws.

Most of us remember Newton's laws from school. These laws apply to all large moving objects. In our context of a racing car they are:

1. A car moving in a straight line at a constant speed will keep moving that way until acted on by an external force.

Because of wind resistance and the friction between the tyres on the road, a car rolling in neutral will slow down and stop. This is an external force acting on it.
If there were no external forces, then the car would, in theory, continue to roll for ever. This tendency for the car to want to keep moving is the cars Inertia and is concentrated at the Center of Gravity.

2. When a force is applied to a car, the change in motion is proportional to the force divided by the mass of the car.

F=ma a the equation most used to express this theory. F is the force, m is the mass of the car and a is the acceleration or change in motion. A larger force (such as heavier braking) causes quicker changes in motion. Heavier cars, with greater inertia, react more slowly to these forces. This law explains why Indycars are quicker than Nascars, they have large amounts of power (force) and very little weight (mass).

3. Every force applied to a car by an object is matched by and equal and opposite force from the car on the object.

Under braking, you cause the tyres to push forward against the road and the road pushes back, slowing the car down.

Looking at Dynamics.

Still with me?

The picture below illustrates the forces at working during a 1g braking manoeuvre. 1g means that the force being applied to stop the car is the same as the weight of the car.

In fig. 1, the C of G is the Centre of Gravity. g is the force of gravity pulling the car down. Gravitational forces act on an object through its Centre of Gravity. Don't ask me why. Ask Albert Einstein.

Lf is the force on the front tyre exerted by the road and Lr the force on the rear. These forces are there all the time, even when stationary, stopping the car falling to the centre of the Earth.

Because we accept these forces as natural we tend to ignore them. However, when looking at the dynamics of a racing car, we can't ignore them. They effect the ability of the tyres to stick and differences between the front and rear forces cause understeer and oversteer.

Analysing Weight under Braking

Refer back to fig. 1.

If the car is stationary or coasting (neither accelerating or braking) and weight distribution is 50-50, Lf would be the same as Lr. Lf & Lr always equal g (the cars weight). The car isn't flying so there are no vertical forces. g points down and is counteracted by Lf and Lr which point up. This is explained by Newton's third law.

Now what happens when the car is moving and you brake? Well Lf becomes greater than Lr. As a racer would say, the "rear goes light".

Let's look at the braking forces, Bf & Br in fig.1. They push on the tyres against the cars direction. This, in turn, pushes on the wheels, which pushes the suspension, which pushes on the car, slowing it down.

Note one thing. These forces don't act on the Centre of Gravity where the car's Inertia is. They are slowing the car down by pushing at ground level. However, the Inertia of the car is trying to keep it going at the Centre of Gravity. This causes the top of the car to try and go over the bottom or rotate.

The braking forces causes a rotating movement, or torque, about the Centre of Gravity. Let's examine this a little more.

We all know the trick of pulling the table cloth out from under the glass. This demonstrates this principle well.

In the first picture, the glass isn't moving and, due to Newton's first law, the glass doesn't want to move.

Now, when you pull the table cloth ( f in the second picture ), you are generating a force at the bottom of the glass against the Inertia at the Centre of Gravity. The force pushes the bottom of the glass and the Centre of Gravity, which doesn't want to go with the bottom, stays put. This causes the glass to topple or rotate. Taller objects have a greater tendancy to rotate and this is greater the harder you pull.

Back to figure 1, the braking torque tries to put the car onto its nose and, as it actually doesn't, forces must be counteracting this tendency, by Newton's first law. g can't be doing it as this acts through the Centre of Gravity. The only ones left are good old Lf and Lr which work by making Lf greater than Lr. Basically the ground pushes harder on the front tyres to try and stop the car tipping forward.

Calculating Forces

This section isn't important, but worth a look.

How much does Lf exceed Lr?. The braking torque is proportional to the sum of the braking forces and the height of the Centre of Gravity. The counterbalancing torque resisting the braking torque is proportional to Lf and half of the cars wheelbase (if weight distribution is 50-50) minus Lr times half the wheelbase since Lr is helping the braking force tip the car.

Lf has a lot to do to counteract the braking torque and the force of Lr lifting the back of the car.

Confused? I am and I just typed it!

O.K. Lets say that the Centre of Gravity is 50cm above the ground. We'll also say that the wheelbase of the car is 250 cm. We're pulling a 1g braking manoeuvre so we'll say that the car is 1500kg making the braking force 1500kg as well. Summarised we get:

1500kg X 50 = Lf X 127cm - Lr X 127cm

Lf + Lr = 1500kgs (this is always true).

Using algebra, we get:

Lf = 750 + (1500/5) = 1050kg
Lr = 750 - (1500/5) = 450kg

This translates to adding 300kg of weight to the front and taking 300kg off the back, or shifting 661 lbs in a 3306 lb car.

This is a lot!

If we call the height of the Centre of Gravity h, wheelbase w, weight G, braking force B and static weight distribution d expressed as a fraction of weight in the front, we can use:

Lf = dG + Bh/w

Lr = (1 - d)G - Bh/w

You can also calculate weight transfer during acceleration by treating acceleration as negative braking. Using Newton's second law. If you have acceleration figures in 'gees', multiply them by the weight of the car to get the acceleration force.

If you want to be really clever, you can do the same for cornering forces. Replace the wheelbase with the track of the car. d should be 50% unless you include the drivers weight if offset from the centre line.

Conclusion

O.K. That's weight transfer covered. You should have enough of an understanding to know what happens to a car's weight as you drive. You've also got enough information to bore everybody stupid down the pub.

This may seem like a lot of information to take in, but just concentrate on the principles and important bits. It will be useful later.

Next tutorial, we'll look at tyre grip and adhesion before putting the first few tutorials together to look at set-up.