Without a full understanding of all the factors at work in suspension, wheel and geometry setup, it’s easy to make mistakes that end up making your car feel worse than it did before. It’s not just a case of switching parts willy-nilly.
Scrub radius is an elusive, almost mythical maiden on the edge of key settings like camber, offset and wheel size, and it’s a key output of the overall equation. Essentially it’s defined by the location of the point in space where an imaginary line traced down through the centre of the suspension column crosses the one drawn vertically down through the very centre of the tyre. The suspension is angled, the tyre less so; the two lines will meet somewhere.
If the two lines cross exactly at the tyre’s contact patch with the road, there’s no scrub radius, which I think is what TLC were singing about in their 1999 hit single No Scrubs. If the lines cross below the contact patch, theoretically underground, then it’s positive scrub radius. Above the contact point? That’s negative scrub radius.
It can make a big difference to the way a car steers, accelerates and stops. Different axle loads and drive configurations need different setups, and that’s before you even get to tuning it for the desired handling characteristics. Car makers have a butt tonne of difficult jobs to do, and this is one of them. Change one thing on the suspension and you start a chain reaction that can ultimately nullify one of your key objectives.
At zero scrub radius the general consensus is that it can make a car feel a little unstable at the front end through corners and under hard braking. On the other hand, when stationary the steering has to turn the contact patch where it lies, which takes more effort and wears the tyre more. It’s rare these days for a car to have truly zero scrub radius; a little of something is better.
Push those wheels out with spacers or install fully adjustable coilovers, for example, and the radius can become positive. That forces the tyre to ‘scrub’ the ground as it turns in its arc, adding a different kind of wear and reducing its life. On the up-side, rear-wheel drive cars use positive scrub radius at the rear to help keep the front wheels pointing straight, even when you release the steering wheel. It’s useful for some racing setups and comes as standard with most double-wishbone suspension designs.
Positive scrub radius does no favours for braking if for any reason there’s a different amount of force between sides of the vehicle, say, if the left-hand wheels have less grip and the ABS backs the system off on that side. This kind of setup will see the car want to fight its way in the direction of the wheels with more grip. Extreme positive scrub radius can make the steering very heavy, too, and it was only really viable on old cars with very thin tyres.
Most of us have negative scrub radius on our cars because it tends to go hand in hand with MacPherson strut suspension. It helps driven front wheels find the straight-ahead, which is good for corner exits or if you have a sudden tyre deflation. Another handy side-effect is that if you hit standing water with the wheels on one side of the car, the negative scrub radius works against the car’s natural inclination to yank into the water, mitigating the swerve.
Tuning suspension for negative scrub is the safer way to do it. It generates a slight toe-in force that reduces any tendency to change direction in the way that positive scrub radius can allow. As a result, in cars with diagonally-split braking systems, if one circuit fails then the remaining braking force on one front and one rear wheel will still pull the car up in a straight line. For the road, slightly negative is the way to go. If you’re changing wheels, tyre sizes or suspension alignment, make sure you keep the flighty maiden scrub radius in mind.