Kingpin InclinationKingpin inclination angle is the angle measured from vertical to an imaginary line drawn through the center of the upper and lower ball joints on an IFS system. By offsetting the ball joints, a vertical arc is produced as the wheels turn. This angle, coupled with correct scrub radius, has a direct bearing on how easy it is to steer the car and its responsiveness to steering corrections.

Scrub RadiusIf the KPI is correct, the line drawn through the ball joints, or through the kingpin on a straight-axle car, will meet the centerline of the front tire where it meets the road, or just slightly to the outside of the centerline. The distance from the tire center to the kingpin inclination angle contact point on the road is called the scrub radius. Changes to wheel height and offset will affect the scrub radius, and can lead to prematurely worn tires, bearings, and steering components.

Four-Wheel AlignmentSo now that you know how to build correct suspension geometry into your next project, what do you do when it comes to setting the alignment perfectly? For total peace of mind and a rod or custom that will take you safely wherever you choose to drive, a visit to your local alignment shop should be on your schedule. Not only will they adjust and set your frontend alignment, they'll be able to measure and set the alignment on all four wheels. We haven't touched on it here, but the rearend can affect handling too, and not just by being mounted out of square in the chassis.

Art Morrison Ent.
5301 8th St. East
WA  98424
Kiwi Steve's Hot Rod Shop
Chassis Engineering
P.O. Box 70
West Branch
IA  52358
Magnum Axle
Classic Performance Products
175 East Freedom Avenue
CA  92801
Fatman Fabrications
Rodparts Warehouse
Heidt's Hot Rod Shop
111 Kerry Ln.
IL  60084
Speedway Motors
P.O. Box 81906
NE  68501
Jim Weimer Rod Garage Unisteer Performance Products
JPL Street Rods Walton Fabrication
1933 W. 11th St., Ste. H Dept. CRM
CA  91786