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Professional Motorsport Data Analysis: Lateral & Combined Tyre Forces

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Lateral & Combined Tyre Forces

06.29

00:00 - Cornering performance is obviously a pretty big subject in the world of motorsport data analysis.
00:06 But before we jump into the nitty gritty of it all, let's first touch on some of the fundamentals of lateral tyre grip in much the same way we did when we discussed the longitudinal tyre forces in the braking and acceleration sections.
00:19 All the same parameters that influence longitudinal tyre forces also have an impact on lateral forces.
00:26 The main parameters being vertical load, slip angle, slip ratio, camber, inflation pressure, temperatures and vehicle speed.
00:35 As the slip ratio being the dominant parameter the driver has control over with longitudinal tyre forces, slip angle is analogous for lateral tyre forces.
00:45 A tyre cannot generate any lateral force without first having a slip angle.
00:51 The definition of slip angle is the angle that the tyre makes relative to the direction of travel.
00:56 During cornering, each of the tyres has a unique slip angle.
01:00 Looking at this diagram, which is a bird's eye view of a car in a skid pad, the car is in a pure lateral cornering state.
01:07 Taking a corner of constant radius at a constant speed.
01:12 The path the vehicle is travelling at at any point is tangent to the cornering radius as shown by the large arrow.
01:19 Zooming into the front right tyre, we can see the direction of travel of the tyre.
01:24 Note that the tyre isn't pointing in the same direction as the direction of travel.
01:28 It's steered further in towards the corner.
01:31 The angle between its direction of travel and the direction the tyre is pointing is the definition of slip angle.
01:38 If we add in a line to indicate the orientation of the chassis, the angle between the direction the tyre is pointing and the direction the chassis is pointing is the steering angle.
01:48 It's important to understand that the steering angle is what we're normally logging, not the tyre slip angle.
01:55 It's not possible to measure the slip angle of each tyre without some specialist equipment.
01:59 At least one slip angle sensor is required.
02:03 These sensors tend to be expensive and are typically only used in closed testing environments.
02:08 In many series their use is explicitly banned for cost control reasons.
02:14 Looking at the same diagram as before, the lateral force supplied by the tyre is indicated by this arrow.
02:19 Because the tyre is deformed and is a self aligning tool which makes the tyre want to return to its undeformed state of having zero slip angle, it's the combination of the lateral force and self aligning torque of the tyre that results in the steering feedback force the driver feels through the steering column.
02:38 The relationship between how the forces and torque on the tyre interact with the suspension kinematics, informs how much and at which time the driver gets torque feedback from the tyres during cornering.
02:49 The typical form of a slip angle vs lateral force plot has some similarities to the slip ratio plot we saw before.
02:56 This is a typical plot which in this case is showing for constant values of vertical load, camber, inflation pressure and a slip ratio of zero.
03:07 We can see that the lateral force initially builds up linearly in response to increased slip angle before the rate of increase begins to drop off.
03:15 The force peaks and then the force reduces with increased slip angle.
03:20 As highlighted here, these are the linear transitional and frictional ranges of the slip angle plot.
03:28 One of the implications of this plot is that in the linear section, lateral force builds up quickly in response to initial steering input.
03:35 The slope of the linear range is referred to as the cornering stiffness of the tyre.
03:40 The steeper this line is, the higher the cornering stiffness and the quicker the initial response of the tyre will be to steering input.
03:48 The shape of the transitional and frictional ranges determine how easy it is to drive at the peak of the tyre and how much steering torque feedback the driver will get to help indicate whether they are close to or past the peak lateral force of the tyre.
04:04 In the vast majority of the corners during a lap, the tyres are not operating in the peak slip angle.
04:10 The only time some may be operating close to the peak slip angle is when the car is in mid corner.
04:16 Most of the time over a lap is spent building up and reducing lateral force, rather than being sustained at maximum steady state lateral acceleration.
04:24 During these times of building up and reducing lateral forces, the tyres are always subject to longitudinal forces at the same time.
04:32 Either acceleration or braking.
04:36 We refer to this as combined tyre loading.
04:38 The effects of combined loading are very different for every tyre.
04:42 But what's always true is that compared to the forces a tyre is capable of in pure lateral load, any combined load will always reduce the maximum lateral force available.
04:52 One way to visualise this is by looking at a friction ellipse for a given tyre.
04:57 Which can be used to visualise not only the maximum limits of longitudinal and lateral force but also its combined force characteristics.
05:06 Longitudinal forces are shown in the vertical direction, drive at the top and braking at the bottom.
05:13 And lateral on the horizontal as seen here.
05:15 This point here is the point of maximum drive force, this is the maximum braking force and these are the maximum pure lateral forces the tyre can produce.
05:26 The areas in between represent how the tyre behaves in a combined state.
05:31 Comparing a few different tyres here, tyre A produces significantly more longitudinal force, but at the expense of lateral force.
05:39 Tyre B produces more lateral forces but is not as strong longitudinally.
05:45 And tyre C has less peak lateral and longitudinal but has the strongest combined loading characteristics.
05:53 Like we discussed earlier, depending on the construction of the tyre, the combined braking and acceleration forces may not be symmetric which is what's being shown here in this example.
06:03 We can see that the peak straight braking and drive forces are similar but the tyre's response to combined loading and braking is much broader than in drive.