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- - Tyres - - Suspension - - Driving - -

INTRODUCTION

There are many aspects to a vehicle which affect the way the vehicle handles. These include the tyres, the vehicles mass, weight distribution, suspension geometry, aerodynamics and the way the vehicle is driven. This section will go through some of these principles to help drivers (whether they be road users, trackday addicts or indeed racers) understand what is going on whilst they are driving the car, and will also give some mathematical background to the physics involved, as well as practical advice. This section gets reasonably technical. The important points are therefore in bold, and are reviewed at the end of each page.

When driven, all vehicles undergo forces laterally (left and right) and longitudinally (forwards and backwards). Now, there are two important things to remember, these are:

  1. These forces are created and controlled by the drivers inputs.
  2. These forces are strongly connected.

For example, a driver has input into the throttle and steering; the throttle control will clearly cause a longitudinal force between the road and the tyre, pushing the car forwards. The steering input will cause a lateral force between tyre and road, causing the car to corner. How it does this will be explained later. However, not only are these basic forces created, but there are other forces created in both directions, such as weight transfer, which affects the vehicles response.

The example above also shows that both the primary forces mentioned act through the tyres; the tyres are generally the only contact between the vehicle and the road, and as such all the force acts through them. The forces laterally and longitudinally are therefore strongly connected, which will be explained later. It seems therefore, that tyres are a logical place to start.

 

Tyres

As mentioned tyres have the job of transferring the forces from the vehicle to the road, in order to change the speed and/or path of the vehicle. In order to do this, clearly there must be a certain amount of friction between the tyre and the road (termed ‘traction’ from this point onwards). In general there are three main factors which determine the amount of traction available. These are:

  1. The coefficient of friction between the tyre and the road surface. This depends on several factors, including road surface, road temperature, tyre compound, and tyre temperature.
  2. The second is the size of the tyre footprint (basically the amount of rubber in contact with the road). The greater this area, the greater the traction available.
  3. Finally, the vertical load acting on the tyres. This can be due to the weight of the vehicle, or aerodynamic forces acting on it to increase the force acting downwards through the tyre.

 

In the vertical direction, tyres act like a spring, meaning the forces generated are related to deflection, whereas in the lateral direction they act like a damper, meaning the forces generated are dependant on velocities.

Because of this, in order to generate force (either laterally or longitudinally), tyres have to slip a small amount (a velocity difference between the tyre and road surface must exist) in order to generate a force. In the lateral direction, this is known as ‘slip angle’ and is measured in degrees, as per the figure below.

  • The red arrow represents the direction of heading
  • The blue arrow represents the direction of travel
  • The yellow angle shows the point at which the force that is created acts
  • The grey region is the contact patch
  • The area highlighted in green is the slip region, whereas the rest of the contact patch is the adhesion region.

 

As your cornering speed and cornering force increase, the tyre points in a different direction to the direction of travel. This is the slip angle, and is the angle between the red and the blue arrows. The diagram shows the deformation of the contact patch during cornering, which is an elastic deformation. It is this deformation that creates a side force just behind the centre of contact. The difference between the centre of contact and the point at which this force acts is the pneumatic trail.

As the slip angle increases, the sliding region takes over from the adhesion region, so the pneumatic trail reduces. It is the pneumatic trail multiplied by the cornering force that gives the aligning moment, which is felt through the steering wheel by the driver. As slip increases, the adhesion region reduces, and this aligning moment reduces, which gives the driver warning of a reduction in grip.

The following is a typical graph of slip angle against traction force, for both a regular road tyre (in green) and a racing tyre (in yellow) on an arbitrary vehicle. Obviously, high performance road tyres will fall somewhere in the middle of the two values.

As mentioned, the above graph is fairly ‘typical’ for the tyres concerned. A few things can be seen. Firstly, the initial part of both graphs shows how the amount of cornering force is pretty much proportional to the amount of slip angle you have. Secondly, the peak traction of both graphs is in the 8 – 10 degrees of slip angle region. (This will reduce on a wet surface) Any more than this (or indeed less than this) and the tyres are beyond there optimum cornering force.

The actual shape of the graph differs slightly dependant on tyre manufacture, compound, temperature, pressure and tyre construction. Some tyres give a lot of warning to the driver when they are about to ‘let go’, and as a result are easier to drive fast, but tend to feel sloppy as this graduated response results in a slow or numb feeling back through the steering wheel to the driver. Conversely, some tyres can give far less warning to the driver, but they do tend to be more instant in their response and so give a more direct feel to the steering. Generally, this is true of higher performance/racing tyres. In general, (as can be seen from the graph) at low slip angles, it is correct to say that the lateral force is proportional to the slip angle.

We can say that:

Lateral Force = Tyre Cornering Stiffness x Slip Angle

When cornering, in order to drive fast and to use the tyres maximum traction, it is important to try to remain in this 8 – 10 degree slip region. Generally, sudden steering inputs (and indeed sudden use of throttle/brake as shall be explained later) cause the tyres to go beyond this region. It is therefore important to be smooth with all driver inputs in order to remain in this maximum traction zone.

In the lateral direction, this slip is measured as a percentage of tyre speed to road speed, and is referred to as ‘percentage slip’. Typical profiles are shown on the graph below, again with the yellow line being the race tyre and the green line being the road tyre.

 

The graph shows that the peak for slip in the lateral direction is around the 10% slip region. This will reduce on a wet surface. As with the cornering case, staying in this region will result in dramatically more traction being available to the driver. Therefore, like the steering case, smoothness is the key; sudden violent inputs into the throttle/brake will cause the slip to rise well above this optimum range, thus resulting in reduced traction.

This percentage is calculated by dividing the difference between the road speed and the wheel speed by the road speed. Clearly a negative percentage will be for the braking case, whereas a positive percentage will be for the accelerating case. However, both profiles are roughly the same.

With both the graphs, it is worth mentioning that the peaks can move up and down with changes to tyre pressure and temperature.

So, this is all well and good, but how does this work in practice? Well, as mentioned above in order to gain the most traction from the cars tyres, it is necessary to drive as ‘smoothly’ as possible, and to feed in the throttle, brake and steering inputs. In terms of the tyres themselves, there are a few things that can be derived from the above information, and a few things that cant!

Firstly, in terms of tyre size, generally the wider the tyre, the higher the peak lateral traction level will be. Likewise, in order to gain more longitudinal traction (for improved acceleration and braking) in theory a larger diameter tyre is preferential in order to lengthen the tyres footprint. However, an increase in diameter will rapidly increase the rotational inertia of the wheel and also the unsprung weight, which may counteract any gains made in traction (especially on bumpy surfaces). More information on unsprung weight is in the suspension section.

Secondly, thinner profile tyres will tend to increase the response of the vehicle to steering inputs. This is because a reduction in section size will increase the stiffness of the tyre, which will increase the lateral force for a given slip angle. Likewise an increase in tyre pressure will have the same effect. Likewise, fitting tyres to a larger width rim will 'stretch' the tyre sidewall over a larger distance, stiffening it up. This will have a similar effect o running lower profiled tyres as detailed above.

Whilst on the subject of pressures, it is worth explaining what effects these have on the tyres. Essentially tyres are designed to work best at a certain operating temperature. Assuming that the correct tyre has been chosen for the vehicle and application in question, the following applies:

Assuming that the tyre pressure is in roughly the right ball park, an overheating tyre is generally a symptom of the tyre pressure being too high. This is because the sidewall is too stiff due to the pressure, and this causes the tread to flex more, causing it to overheat. Likewise an under inflated tyre will not get to temperature since most of the distortion is occurring in the sidewall rather than the tread. Following this theme, on trackdays, road tyres will tend to overheat fairly readily, as the forces they experience causes the tread blocks to flex more than in normal driving conditions, causing them to heat up excessively.

In terms of getting the right ballpark pressure, the ideal is to have a tyre that has equal temperature across its width after hard use. Tyres which are overheating or wearing excessively in the middle demonstrate tyre pressures which are too high, as the tyre bows outwards in the middle (known as 'spiking'). If you don’t have facilities to measure tyre temperature when you come off the track, a rough guide can be the way the tread looks; a slightly grained effect on the rubber surface indicates a correct pressure under hard use. Shiny surfaces near the middle suggest over inflation as the middle is in contact excessively and overheating.

TYRES 2>>>

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