Are you talking about the scrub radius or just tire scrub?
The scrub radius can be adjusted with changing the offset on the wheelaxle.
As you can see in the picture the scrub radius is influenced by the wheeloffset and the location of the pivotpoints on the outer side of the wishbones. This picture is of a Mcpherson strut, for a double wishbone like we use in our modelcars the dotted line is drawn through the outer pivot points/pivot balls.
The distance between the two dotted lines is your scrub radius, so by moving your wheel more to the outside by increasing the amount of shims on the outside of the upright (or changing to +0.xx hexes on your touring car) you will increase the scrub radius.
It's pretty save to say more shims on the outside of the upright increases the amount the scrub radius.
On some cars you can also get different front uprights with a different kingpin angle to change the scrub radius.