So if you have a king pin the result is different ?
No. Not unless one thrives on semantics.
The correct terminology should be used in all accounts, for any explaintion. Its the generic or grassroot terms that tend to confuse the masses. "Scrub radius" is a function of the steering axis and the wheel position and is defined by the SAE as "kingpin offset", which is the distance between the two points created by the inter- section of the wheel centerline (centerline of tire contact patch) and the steering axis with the ground plane. The steering axis is determined by the "kingpin index angle", which is the plane projected through the upper and lower ball joints and the wheel shaft joint. A zero scrub radius exists when the wheel centerline and the "kingpin index" intersect exactly at ground level. Positive offset occurs when the wheel centerline is located laterally inboard from the steering axis at the ground plane, while negative offset occurs when the steering plane intersects the wheel centerline above the ground plane.
The two major factors that affect the scrub radius are wheel height and wheel offset. Taller wheels than stock will push the scrub radius farther out from the center line which is acceptable to a point. Shorter wheels will pull the scrub radius in towards the frame. This is not desirable in most moderate speed applications as it makes the steering heavier.
Choosing a negative or positive scrub radius for a vehicle's suspension has emerged as a great philosophical debate in the last couple of years. For instanance, in a strut suspension, achieving a negative scrub radius is at least a viable option. However, on IFS suspensions (eg light trucks with 4WD and short long-arm SLA suspensions) packaging the wheel end components and achieving a negative scrub radius has been difficult or impossible. Demand for off highway capability, working payload capacity, and the required brake component size needed, makes achieving even a small positive scrub radius difficult or requires compromising brakes, steering linkages, or other suspension components. In the case of this thread... its not so much of an issue.
Opinions about whether negative scrub is what the average driver wants are mixed. However, there is a trend to provide improved vehicle-handling dynamics, which has increased the focus on reducing scrub radius to a small positive number. The steering axis inclination does contribute to the forces and moments in the steering system, so, on high down-force cars without power steering reducing the inclination will lighten the load at the steering wheel. This parameter also provides a self-centering effect in the steering which is considered to aid in controllability or stability. It also effects the roll center migration of a fixed point dual I beam.
On another note, dealing with the original subject matter, Sumner's Purplesage web site (
http://purplesagetradingpost.com/sumner/bvillecar/bonneville-Index.html ) has an interesting write up on the change they made to the KPI of his suspension and for another matter... an interesting insight into weight and pressure (center of gravity / center of pressure).
I'll second the Carroll Smith books. It should be mandated (required reading) that everyone read the Carroll Smith trio, Tune to Win, Engineered to Win and his book on fasteners. Good information, good satire and great explainations without going "brain child" on the reader.
