Summary

00:00 - Explain scrub radius and why it's important to have scrub radius set so that the car toes in on braking and why it's important with ABS cars.
00:08 So for a start, what Michael's asking about there, the definition of scrub radius is the distance from your steering axis, so let's say, let's start with this example here.
00:17 So it's the distance from the steering axis which in the case of the MacPherson strut is going to be defined from our top pivot point where it pivots on the car through our lower control arm, so it's going to have some angle like that typically.
00:30 And the scrub radius is the distance from the centre of the tyre at the contact patch to where that point, that steering axis intersects the ground.
00:37 So there are a whole lot of different theories out there and a whole lot of different reasons you would run different scrub radius in different cases.
00:42 So one of the things that often happens with scrub radius is people go ahead and put a really big offset wheel on their car or maybe they've got spacers, wheel spacers on their car and what that ends up doing is it ends up spacing, increasing your scrub radius, so the scrub radius was the distance from the centre of that tyre where it's at the contact patch to where that steering axis intersects the ground so if you're going to put a big offset wheel or a wheel spacer in, you're increasing that scrub radius.
01:06 Now the important thing to understand about that is that as you're increasing that scrub radius, you're effectively increasing the leverage arm that you've got around the steering axis so if any longitudinal forces, so braking and acceleration, that's going to mean that that wheels are going to be trying to take, basically steer for you or put a steering force out of your hands.
01:27 So the reason that's important is because when you've got a different force on one from the other, so obviously if you've got an equal force on both wheels and you've got the same offset on both of them, you're going to have an equal and opposite force applied to the steering rack, so that's in a perfectly theoretical example, that's not going to start pulling the wheel out of your hands but in reality you never have the same forces on one side or the other.
01:50 So the whole time you're increasing that scrub radius, you're going to be increasing the amount of steering force coming back through the steering wheel, little bit of an unstable type force coming through the steering wheel.
01:59 Now what I think Michael's talking about or getting at there is that depending on how much compliance you have in your suspension and depending on the position of your steering rack as well, so for a lot of cars, particularly things like front wheel drive cars, will always have that steering rack at the rear behind the front axle.
02:15 One of the problems with that is that any compliance in the system can tend, well will tend to toe out the front wheels which can make any stability problems even worse.
02:25 So guys, that was a, sorry that was a bit of a long winded explanation for that but that's why scrub radius is important and why just putting bigger and bigger offset wheels on your car or bigger and bigger spacers in behind the wheels is going to be a negative thing, typically, for controlling stability and certainly when you've got compliance which is just the amount, all of the suspension bushings and everything are flexing as you go.
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