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Motorsport Wheel Alignment: King Pin Inclination & Scrub Radius

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King Pin Inclination & Scrub Radius


00:00 - We've just learned about castor angle and seen how that can affect the steering effort, straight line stability and driver feedback.
00:07 However as I mentioned, we can't consider castor in isolation.
00:11 We also need to consider the kingpin angle and scrub radius as all three of these elements work together.
00:18 As with castor, kingpin angle is an aspect of the production suspension geometry of the car and is not likely to be adjustable.
00:26 It's only when we get to professional level motorsport that we may have the option of adjusting the kingpin angle.
00:32 However for the sake of having a complete understanding of the terminology, it's still worth considering.
00:38 The kingpin angle or kingpin inclination as it's also known as, is best understood by viewing the suspension from head on.
00:46 What we want to do is draw a line through the upper and lower ball joints of our front suspension and compare the angle of this line to vertical.
00:54 If we're dealing with a MacPherson strut suspension setup then this line projects from the centre of the strut top, through the lower ball joint.
01:01 You may have already worked out that this is the same axis we used to define the castor angle only this time we're viewing the car from the front rather than the side.
01:10 It's easiest to understand the effect of the kingpin angle if we plot the movement of the axle as we turn the steering wheel.
01:18 What we find is that as we turn the steering wheel through its travel from lock to lock, the axle actually moves in an arc.
01:26 If we plot the movement of this arc we find that the apex of the arc is the point where the steering wheel is pointed straight ahead.
01:33 Either side of straight ahead, the arc drops which means that when we turn the wheel, we're actually trying to lift the front of the car.
01:41 This makes the steering always want to stay centred and the required steering effort increases the further we turn the wheel from straight ahead.
01:50 Similar to castor, this increases straight line stability and steering feel.
01:54 Of course just like castor, we can have too much kingpin angle which makes it very difficult to turn the steering wheel.
02:01 However another important consideration is that as we increase the kingpin inclination, the wheel will tend to move into positive camber as we turn the steering, hence reducing the camber gain we achieved with positive castor.
02:15 For this reason, we'll find that in most instances the suspension design in factory cars will include more castor angle than kingpin angle, ensuring that the net result is camber gain as we turn the wheel.
02:28 Another term that's linked to kingpin inclination is the scrub radius.
02:32 And to understand this we'll come back to our diagram of the wheel.
02:36 Scrub radius is the distance between the centre of the tyre contact patch and the point where the steering axis intercepts the ground.
02:44 The scrub radius can be affected by the kingpin inclination as well as the width and offset of the wheels fitted to the car.
02:51 A little scrub radius is a good thing as it helps with steering feel and allows the driver to get feedback on the front end grip.
02:58 Where scrub radius becomes more important is if you're fitting wheels with an aggressive offset which moves the centreline of the tyre contact patch further away from the car centreline.
03:08 This naturally has the affect of increasing the scrub radius and this can be detrimental, particularly in powerful front wheel drive and four wheel drive cars as it increases the effect of torque steer.
03:19 While you're unlikely to have the ability to adjust the kingpin inclination, it's important to understand how you're affecting the scrub radius if you do decide to bolt on wider wheels with more offset and how this may affect the handling of your car.

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