Balance & Stability For Greater Driver Control

To get the best overall driving performance out of a car you immediately have to become involved in physics. This is because as soon as you engage first gear and begin to drive away, there is a whole raft of stuff that begins to happen, each part of which has its own effect upon the behaviour of the vehicle.

When the car is parked and unattended it is as physically stable as it can be in terms of balance. You can sit and watch it for weeks and it will not do anything untoward. It is only when you put a driver in it, and that driver sets the car into motion, does the level of stability become altered.

When we drive a car along a road the balance is affected in different ways, and the biggest affect occurs as a result of the actions of the driver, and due to the manner in which he or she will operate the driving controls. For example, when we accelerate, the rear of the car dips on its suspension, and when we brake, it is the front that is similarly affected.

Many think the reason is that of the weight of the car moving around as if it were full of liquid that surges rearward upon acceleration and forward under braking. But even if that is not so, which is isn’t, it is perhaps sometimes helpful to think of it in this way.

Having you ever been thrown over the handlebars of a bicycle?

Many people, at some time in their life, rode a bicycle, and if they were unfortunate enough they may have fallen off through being thrown over the handlebars, perhaps whilst trying very hard to stop for an emergency. This generally happens when applying the front brake too harshly.

The standard bicycle brake system consists of two rubber blocks mounted on a device called a calliper, which moves the brake blocks toward each other as the brake is applied by the rider. As the two rubber blocks begin to clamp the rim of the wheel moving between them, they produce resistance to the rotation of the wheel. This of course is what causes the bicycle to slow down.

The momentum of the bike
against the effect of the brake

When the brake clamps upon the moving wheel rim it effectively tries to take the brake with it. As the brakes are physically attached to the bicycle frame, and these now being now clamped onto the wheel rim, the wheel rim tries to take the whole bike with it. Therefore, if the forces involved and greater than the weight of the cycle and rider, the rear wheel will lift off the ground.

Conversely, if anyone has performed what is known as a wheelie on a bike they will know that to perform the trick it is necessary to select a low gear, push very hard on the pedals and pull backwards on the handlebars to get the front wheel off the ground.

What is actually going on here is that helped by the weight distribution of the rider over the whole area of the bike, energy is delivered through the drive chain more quickly than the bike is able to accelerate. As that energy has to be dispearsed to somewhere, the drive sprocket ends up climbing along its own drive chain. That is what causes the front wheel to lift.

You may have seen a motorcycle do this, and the rider will achieve it by applying a rate of engine power that is greater than the ability of the motorcycle to increase ground speed. After all, the energy has to go somewhere.

How this applies to the balance of the car

These forces are also acting on your car as you accelerate, but due to the overall mass of the vehicle, relative to the available engine power, it can’t physically lift the front off the road. However, the torque in the drive axle will have enough influence to cause the car to rise at the front and dip at the rear. Only something like a drag strip car will lift its front wheels off the ground under acceleration.

You can see that we have already affected the balance within the car, just by increasing or decreasing speed, and we haven’t even got to our first corner yet. When we do, it will bring on a whole lot more points for discussion.

We can’t drive in straight lines all the time

Whilst the car will be in its most stable state when motionless, it will be at is most well balanced whilst moving if it is travelling in a straight line, with a constant throttle opening. However, we can’t drive around in straight lines all day, as there comes a point when we will have to change direction.

Operating the steering is another factor that causes a change in balance within the car. As we turn the steering wheel we experience an increase in loading to one side of the car, which is equal to a decrease in loading to the opposite side. The reason this happens will be dealt with in a later edition concerned with cornering forces, but for now it is good enough to know that steering to the left or right causes a loading on one side of the car, or the other, with the greater loading occuring upon the side at the outside of the curve.

It is the weight of the car that produces downforce
and is is downforce that produces tyre grip

If we think of the weight of a particular car, and so as to provide a nice round figure with which to work with, to illustrate a point, and we’ll say that car weighs in at 1,000kgs. Imagine that when stationary the car has a weight distribution of 250kgs per wheel. This is the individual amount of ground pressure, or down force, each will deliver.

Down force is what makes our tyres grip the road, and you could say the harder the tyre is pushing down onto the road surface the greater the grip it will yield. By the same token, if the amount of down force is reduced, so is the level of grip offered by the affected tyre. So you can see already, by altering the balance of the car we immediately change the level of grip offered by each individual tyre according to the amount of down force is acting upon it.

As we can’t maintain an even distribution of down force throughout the car, and we can’t remain either stationary or driving in straight lines at a constant speed, the best situation we can hope for is a good compromise.

Whilst it is a compromise, the car will be at its best balance
with the loading kept equal over pairs of wheels and tyres

As already covered, the loading on the rear wheels increases, and proportionately decreases at the front, as a car accelerates. Providing the change in balance results in an equal amount of down force being carried by matched pairs of wheels, that is the best compromise we can hope for.

So, if you accelerate in a straight line the loading on the two rear wheels will be the same as each other, as will the situation be at the front, albeit front and back of the car are now different to each other. The same occurs when braking in a straight line, and both of these can be regarded as good situations.

When it comes to cornering, for the best result we need to apply the same rule, but this time the matched pairs of wheels, down force–wise, will be one at the front and one at the back, but both on the same side of the car. Therefore, in the case of our red 1,000kgs sports car, this might be 150kgs per wheel on the inside of the bend and 350kgs per wheel on the outside, which is good.

When driving through a bend there will be a small increase in the amount of resistance to the motion of the car that occurs between the tyres and the road surface, therefore demanding a greater amount of energy to maintain a current road speed. If not countered this added friction will cause the car to decelerate.

To achieve the best balance point it will be necessary to apply a little more power, just enough so as to keep the car moving at a constant speed to overcome the added drag. This will also maintain the desired distribution of down force in terms of the division between the four wheels. We refer to this as driving on a neutral throttle, which is a situation where the car is under power, but neither decelerating nor is it accelerating.

When driving through a bend in this way the car will be at the greatest level of stability and therefore as far away as possible from the dividing line that marks the point between in control and out of control.

To maintain the ultimate level of stability whilst cornering we cannot brake, we cannot accelerate and we cannot even lift off the power, because to do so would disturb the ideal balance. Likewise, when braking or accelerating on a straight road, by giving the car some steering input at the same time, that would also adversely affect the state of balance.

Preparing The Car For Driving a Bend

You may be wondering, if we are aiming at not altering anything when driving through a bend, how do we brake to get our speed right for the bend? Surely we have to brake, or at least change gear? The answer is that we don’t need to alter speed whilst actually in the bend, because we use something called The System of Car Control to set the car up ready for the bend.

We also use something else, called The Limit Point Analysis, which is a bend assessment technique used to determine what speed we can safely drive the bend, and before we reach it. Details of how this is achieved are covered in later chapters within this series.

So, just to recap. To get the best handling performance from your car, and to deliver the smoothest and most flowing drive, always apply power, brakes and steering in such a manner, and in such a sequence, so as to always maintain the distribution of down force evenly divided between pairs of wheels. Also, the changes in that distribution need to be completed smoothly and progressively, as any sudden change will have a severe destabilising affect.

It is when you significantly upset the balance of the car
it will spin hopelessly out of control

Before we finish, there is one common driving scenario that completely destroys that ideal attitude of a car, and you will most commonly see it happen on motorways and dual carriageways.

Imagine a car driving along lane–3 of a motorway, and due to inattention, the driver has not realised quickly enough the traffic ahead has slowed to an almost stop. Not having time to pull up within the distance available, our driver instantly reacts by applying heavy braking. However, upon seeing a suitable gap in lane–2, he steers towards that lane. This is when the car is liable to spin hopelessly out of control, and here’s why.

When the brakes were first applied, even though that application may have taken place in a rough and hurried manner, at least at that time the down force loading was equal across the front of the car. So by default the two rear tyres matched each other too. However, when the steering input went in, everything then went to pot.

Braking and Steering Thows The Car Off Balance

What immediately occurred when the driver steered left whilst braking heavily, the front right tyre would take on the biggest share of total available down force, whereas the diagonally opposite, the rear left, would have the least share of overall down force.

The down force through the front left would be less than that of the front right, and would be different again to that of the left rear. In fact, all four tyres would be bearing different degrees of down force so that no two would match. The end result is the car will pivot about the most heavily loaded wheel and that usually means getting into a spin.

The moral of the story here is, apply emergency braking in the first instance, and if stopping distance is running out, always release the brakes before changing direction. Once you have settled the car again into a straight line again, you can then re–apply the brakes.

It is not the presence of a difficult incident that will cause a driver to crash, it is what the driver will most commonly do in response to that incident that does the damage.

Julian Smith
Ride Drive Limited

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This page was last updated
Thursday, 27-Jan-2011

Balance & Stability For Greater Driver Control