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Race Driving Fundamentals: Traction Circle

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Traction Circle


00:00 - Before working on our driving technique, it's important for any driver looking to progress to have a good understanding of the forces that are applied to a vehicle on the track, as well as the behaviours that a car will display when those forces are applied.
00:14 We're going to investigate these within this set of modules so let's start with what's arguably the single most important concept worth understanding when it comes to vehicle dynamics, the traction circle.
00:26 No matter how good your tyres are, they all have limits and every tyre can generate a specific amount of cornering, braking and accelerating force but not all at the same time.
00:37 Meaning, that if there's more than one force being applied to the tyre at one time, the available grip needs to be shared.
00:43 For example if we're hard on the brakes and generating maximum braking force right on the limit of the tyre's grip, we have nothing left to be able to turn the car.
00:53 In this scenario, in order to put lateral cornering load onto the tyre and make the car turn a corner, we'd need to reduce our braking forces.
01:02 This all might sound a bit messy but there's a very simple and effective way to visualise this balancing act, called the traction circle or GG diagram.
01:12 This is an incredibly useful tool for understanding how much of the car's total grip potential the driver is exploiting at any one time.
01:20 The traction circle is particularly helpful for pointing out deficiencies in the way we're using the combined load of the tyres when we're braking and turning or accelerating and turning at the same time as opposed to either braking and accelerating purely in a straight line.
01:37 This is important because when a tyre is loaded in a combined state during corner turn in for example, it can't produce as much braking force as it could when the car was braking in a straight line.
01:48 The traction circle helps us visualise how we're exploiting both the lateral and longitudinal capabilities of the tyre.
01:56 The traction circle is also known as a GG diagram because it's essentially a plot of the lateral vs longitudinal acceleration or G forces.
02:06 The vertical axis shows us the longitudinal G force, meaning accelerating and braking forces and the horizontal axis is our lateral G force or in other words our turning G force.
02:19 We can then plot thousands of points on this graph which represent the instantaneous combination of lateral and longitudinal G force the car is experiencing during a lap.
02:29 Analysing the result will very quickly make it obviously what lateral and longitudinal forces the tyres can generate are.
02:36 While the maximum attainable G forces in each direction are useful, the real value of the GG diagram comes from our ability to analyse how well the driver is riding the edge of the traction circle as they transition from braking to cornering and finally to accelerating.
02:52 While the traction circle is referred to as a circle, it's actually more of an egg shape, particularly since most cars can generate much higher G force under braking than they can under acceleration.
03:03 If we add in this egg shaped line that combines our maximum lateral and longitudinal forces, we can see what we should be able to achieve when we start combining braking and turning or turning and accelerating.
03:16 Essentially we should be able to achieve a combined lateral and longitudinal G force that neatly rides along the rim of the traction circle as we progress through a corner.
03:26 Let's break down a typical corner to gain a better understanding of how this relates to the traction circle.
03:33 Initially the driver is approaching the corner in a straight line at full power and after reaching the braking marker, the brake is applied and maximum braking force is achieved.
03:43 As the driver reaches the turn in point and wants to begin applying steering lock, some brake pressure must be reduced in order to provide the required grip to begin turning the car.
03:54 We're now beginning to rid along the rim of the traction circle and as the car is turned more, the brake is reduced evenly to free up the required lateral grip needed.
04:05 Once the car is at the apex of the corner, we'll see maximum lateral G force and zero longitudinal G force.
04:12 Finally as the car exits the corner, the driver will begin releasing the steering lock which again frees up some of the tyre's grip that can then be used to accelerate out of the corner.
04:23 The more we unwind the steering, the more longitudinal grip is available and therefore more throttle can be applied so in effect we're not at full throttle until we're close to having the steering wheel straight again.
04:35 Simply put you're adding throttle at approximately the same rate that you're releasing the steering.
04:40 Not exploiting the available traction is a common mistake that costs novice drivers time and we can easily visualise this by using the GG diagram.
04:49 If we look at a typical plot from a novice driver, we can see that while the driver is often able to generate similar maximum G forces to a pro driver, it's the transition where they're almost always giving away potential lap time.
05:03 Instead of neatly tracking around the rim of the traction circle, the novice driver will usually follow a more direct path for a maximum braking to maximum cornering force which means that the full potential of the tyres is not being utilised.
05:16 Learning to ride the rim of the traction circle is not something that you'll pick up overnight and it does take time and experience to master.
05:24 If you're trying to find these traction limits, pick your places when starting out, somewhere safe so that you can afford to have a spin or run wide with no consequences.
05:34 This limit is a constantly moving target of course with weather as well as tyre and track conditions changing the limits from day to day or even session to session.
05:44 So the key takeaways I want you to remember from this module is that a tyre can generate a specific maximum lateral or longitudinal force but it can't do both at the same time.
05:55 If we're generating maximum braking force and want to begin turning the car into a corner, this will require a reduction in braking force to free up the required grip to generate a lateral force.
06:06 Likewise, as we transition from the apex, where we're generating maximum lateral force, and we want to begin accelerating, this will require us to free up some grip by reducing our steering lock.
06:18 The GG diagram is the easiest way to visualise how well you're exploiting the available grip from your tyres as you move through the cornering sequence and where you can potentially improve.

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