Wheel and Tyre Fitment: Scrub Radius

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Scrub Radius

06.28

00:00	It's fair to say that the scrub radius and its considerations are going to be the most technical and maybe most difficult to grasp information in this course.
00:10	Unfortunately though, the truth is that this can't be ignored as it has a significant effect on the handling of the vehicle.
00:17	To add to this, caster angle, kingpin inclination and scrub radius all need to be considered together and are only relevant to the front axle where the steering takes place.
00:29	Caster is best viewed from side on and is the angle between the vertical line drawn through the centre of the wheel and an imaginary line or axis that the wheel will turn through when the steering is turned.
00:41	In a McPherson strut suspension system, this is a line drawn through the strut top and the lower ball joint.
00:48	While in a double wishbone suspension system, it's a line drawn through the upper and lower ball joints at the hub or the upright.
00:56	Since this is the case, it's not something that's changed by fitting different wheels.
01:00	And while it's not something that's always adjustable on factory vehicles, it is commonly adjusted in motorsport.
01:06	We will always have positive caster, which is where the axis angles upwards and backwards as this provides a self centring effect for the steering and aids driver feel for what's happening at the contact patch, the net result being stability and control.
01:22	The distance between the centre of the contact patch and the point where the axis intersects the road is the caster trail, but we'll come back to the effect of this soon.
01:32	The kingpin inclination or kingpin angle as it's also known as, is best understood by viewing the suspension from head on.
01:41	What we want to do is draw a line through the upper and lower ball joints of our front double wishbone suspension and compare the angle of this line to vertical.
01:51	If we're dealing with a McPherson strut suspension setup, then this line projects from the centre of the strut top through the lower ball joint.
02:00	This is very similar to the line that we use for the caster angle, only viewing it from the front rather than the side.
02:07	Again this isn't influenced by our wheels and in most cases won't be adjustable other than on the likes of high level race cars.
02:15	This brings us to the scrub radius, the key topic of our discussion in this module and something that is also referred to as kingpin offset.
02:24	This is the lateral distance between the kingpin axis and the vertical centreline of the wheel at road level.
02:32	The scrub radius is to kingpin inclination as the caster trail is to caster angle.
02:37	It's easy to remember this by understanding that the contact patch will rotate about the intersection of the kingpin axis and the road.
02:46	So, the centre of the contact patch will scrub around this point.
02:51	If the centreline of the wheel is inboard of the centre of rotation, this is called negative scrub radius.
02:57	If the centreline is outboard, this would be positive scrub radius.
03:01	And if the intersection points on the ground coincide, this would be zero scrub radius.
03:07	The kingpin angle will affect this, but of course the centreline of the wheel is directly controlled by the wheel offset so all other things being equal, a lower offset will move the scrub radius more positive and a higher offset will move it more negative.
03:24	As a result of both the caster trail and angle of the axis, as well as the scrub radius and kingpin angle, what we find is that as we turn the steering wheel through its travel from lock to lock, the wheel actually moves up and down in an arc.
03:40	The apex of this arc is the point where the steering wheel is pointed straight ahead.
03:45	And on either side, the arc drops, which means that when we turn the wheel, we're actually trying to lift or jack the front of the car.
03:53	This makes the steering always want to stay centred and the required steering effort increases the further we turn the wheel from straight ahead.
04:02	This increases straight line stability and steering feel.
04:05	But if we have too much caster trail or scrub radius, it can also be difficult to turn the steering wheel at all.
04:12	The caster angle and kingpin inclination also contribute to changes in the lateral load transfer distribution and camber gain during steering.
04:22	But these aren't specific to the wheel in this case, so we won't overcomplicate this module.
04:27	Again, I'd recommend checking out the HPA suspension tuning and optimisation course where these topics are explained in more detail if you're interested.
04:37	However, what's important to understand is that an excessive scrub radius, positive or negative, also means that any out of balance forces acting on the front wheels are transmitted to the steering system.
04:50	So, if one front wheel hits an obstruction, or road conditions are uneven, which will always be the case to some extent, the driver would have to resist this at the steering wheel.
05:01	Put simply, this means that the car will feel more darty and nervous.
05:06	From the other direction, with the force coming from the drivetrain, it will also increase the effect of torque steer in front wheel or all wheel drive vehicles.
05:15	For all of these reasons it's common in motorsport to aim to keep the scrub radius below 40mm or so, either positive or negative.
05:23	Finally, since the scrub radius is the distance from the centre of rotation, increasing it will mean that when steered, the wheels will move further forward and backwards in the wheel well and therefore will need more room for clearance.
05:36	There's been a lot of information covered in this module, so let's recap the main points to remember.
05:43	The offset of the wheel directly changes the scrub radius, where a lower offset wheel means more positive scrub radius and vice versa.
05:52	Kingpin inclination and scrub radius contribute to the lifting effect of the front of the car with steering.
05:59	Caster angle and trail also have an effect on this, but this isn't related to the wheel dimensions.
06:06	The result is a self aligning torque among other things that can be good for steering feel, stability and handling.
06:13	However, too much scrub radius, either positive or negative, can make the steering excessively heavy and handling unpredictable and also increase the effects of torque steer.