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Motorsport Wheel Alignment: Castor

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Castor

04.01

00:00 - Castor is an element of the wheel alignment that's important to understand, however in most cars, it's either not adjustable or tricky to adjust quickly and easily at the racetrack.
00:10 This, coupled with the slightly more difficult task of accurately measuring castor angle for us in the home workshop, tends to limit its usefulness.
00:19 That being said, while we may not be making regular adjustments to castor, it's still an element we may want to alter when initially developing the suspension system so it's important to understand what it is and how it affects our car.
00:32 Castor is only relevant at the front end of the vehicle and is the angle between a vertical line drawn through the centre of the wheel and an imaginary line that the wheel will turn through when the steering is turned.
00:45 In a MacPherson strut suspension system, this is a line drawn through the strut top and the lower ball joint while in a double wishbone suspension system, it's a line drawn through the upper and lower ball joints at the hub or upright.
00:59 When the imaginary line angles upwards and back towards the rear of the car, this is referred to as positive castor and this will have a self centering effect on the steering.
01:10 What I mean by this is that if we're driving a car around a corner and we let go of the wheel, it will naturally tend to straighten by itself.
01:17 This is an effect that you've probably already noticed while driving your car.
01:21 Likewise if you've ever watched in car footage of a driver drifting a car, you'll often notice that once the drift is initiated they release the steering wheel and it will naturally counter steer by itself, usually faster than the driver can manually turn the wheel.
01:37 This is the effect of castor in action.
01:39 This self centering effect is due to the fact that the centre of the tyre contact patch is behind the point where the imaginary steering axis contacts the ground.
01:50 This produces what is referred to as castor trail and when the tyre contact patch is behind the steering axis, this simply forms a lever arm that generates a force called self aligning torque.
02:03 The greater the positive castor is, the larger this lever arm becomes and the higher the self aligning torque is.
02:10 In other words, the more positive castor we have, the more force is required to move or deflect the front wheels from the direction of travel and this improves straight line stability.
02:21 A subtle aspect of castor however is that as we turn the wheel, we'll also see negative camber on the outside wheel in a corner increase too.
02:30 And this is referred to as camber gain.
02:33 This is an advantage because it means that we can run less static camber in a straight line without compromising the tyre contact while cornering.
02:42 Another advantage of positive castor is that as we increase the castor angle, it increases driver feedback or feel through the steering wheel.
02:50 Since we have a higher self aligning torque with more castor, as the car begins to understeer, the deflection force is reduced and the net effect is that the steering will start to feel lighter in the driver's hand.
03:02 Likewise if there's a lot of grip available, the steering will feel heavier.
03:07 This kind of feel is important for the driver to be able to know how much grip is available and what the car is doing.
03:13 On this basis you may be thinking that if some amount of positive castor is good then more must be better but of course it's not that simple.
03:21 First of all, as we add positive castor, we increase steering effort, so this needs to be balanced to suit the driver, particularly if you don't have power steering, as it can make the car very difficult and tiring to drive.
03:35 Secondly, too much castor can increase the effect of torque steer in powerful front wheel drive and four wheel drive vehicles.
03:42 So the optimal castor setting is very dependent on the vehicle and the power level.