The Mark Ortiz Automotive


July 2016

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Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions.  Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to:  Readers are invited to subscribe to this newsletter by e-mail.  Just e-mail me and request to be added to the list.





Per your request for topics, if this one has been discussed, I've missed it: The interaction and effects of spindle offset vs. caster on steering effort, steering angle-induced camber gain and weight transfer.


An auxiliary topic would be discussing the trend to and effects of attaching the front spring mount to the upright rather than the lower control arm (LCA), in a manner that allows it to be offset fore/aft  from the king pin axis thus variably loading/unloading the spring and damper as the wheel is steered.


I have addressed at least the first part of this question before, but not for a while.  At the risk of belaboring points many readers already know, Iíll start at the beginning.


When we steer a carís front wheels, each one steers about an axis, not surprisingly called the steering axis.  In a beam axle front end, the steering axis is the centerline of the kingpin.  In an independent suspension with an upper and lower ball joint, the steering axis is a line passing through the two ball joint centers of rotation.  Some older independent suspensions have kingpins like a beam axle.  In MacPherson strut suspension, the steering axis is a line through the center of rotation of the upper strut mount and the ball joint.


It is possible to have sort of virtual ball joint by substituting two links for an upper or lower control arm.  When these links lie in a common plane, they form a linkage that has an instant center where their centerlines intersect.  This instant center will migrate as the wheel steers, and it will generally lie closer to the wheel centerplane and further from the car centerplane than a ball joint can.


The steering axis usually does not pass exactly through the tire contact patch center, and it usually is not entirely vertical.  It generally intersects the ground plane somewhat ahead of the contact patch center, and somewhat inboard.  It generally is inclined rearward at the top and inboard at the top.


The side-view inclination angle of the steering axis is called caster.  It is conventionally positive when the steering axis is inclined rearward at the top.


The front-view inclination angle of the steering axis is called steering axis inclination (SAI) or sometimes kingpin inclination (KPI).  It is positive when the steering axis tilts inboard at the top.


The side-view distance between the contact patch center and the point where the steering axis meets the ground is called trail.  It is positive when the contact patch center is behind the point where the steering axis meets the ground.


The front-view distance between the contact patch center and the point where the steering axis meets the ground is called steering offset per ISO terminology, or scrub radius per older SAE terminology.  ISO uses ďscrub radiusĒ to mean the radial distance between the two points, which makes more sense.  Per either SAE or ISO, the quantity is positive when the contact patch center is outboard.


The wheel axis or spindle pin centerline can also lie ahead of or behind the steering axis.  I call this pin lead or pin trail.  This is my own invented terminology.  Neither the ISO nor the SAE had a term for this, so I concocted my own.  ďSpindle offsetĒ is another term that is not an ISO or SAE standard term.  Iím not sure what the questioner means by it myself, but I guess it would logically be the front view distance from wheel centerplane to steering axis at pin height.


It will be apparent that the contact patch centerís instantaneous motion path is along a circle concentric with, and perpendicular to, the steering axis.  That circle then lies at a compound angle to the ground plane, since the steering axis is at a compound angle to ground vertical.  When the wheel steers, the contact patch moves up and down with respect to the car.  This causes that corner of the car to rise or fall, and causes the wheel to load or unload.


The effects are similar to what weíd get if we raised or lowered a given corner by adjusting the springs.  The corner we raise gains load, and so does the diagonally opposite one.  The other two lose load.  The diagonal totals and percentages change.  The front, rear, left, and right totals and percentages donít change.


To understand the effects of these different variables on tire loads and forces in the steering, letís look at some hypothetical cases, where most of the parameters have a value of zero but one does not.  (Actually we will find that when we try to change just one parameter away from zero, others change too.)  For simplicity, letís imagine that the tire has no width or compliance, and letís look at how things are changing instantaneously in the condition weíre considering, because once things move we will generally no longer have only one non-zero parameter.


First, letís consider the case where everythingís zero.  The steering axis is straight up in both front and side view, and smack in the middle of the tire.  In this case, when the wheel steers, there is no



jacking and no camber change.  Lateral force at the contact patch creates no moment about the steering axis.  Bumps create little or no moment about the steering axis.  The steering feels like the

steering of a motor boat: no feel; no self-centering.  Actually, such a geometry will have a little self-centering due to tire self-aligning torque, but the steering will be very numb.  Wheel loads will not change with steer.


Now letís add just some trail, but no caster: geometry like a furniture caster.  This means we will also have some pin trail.  There will still be no jacking or camber change when we steer.  If the car is sitting still, the front will move laterally a little when we steer.  Tire drag will tend to create a centering force.  Lateral ground plane force will create a moment about the steering axis.  We will be able to feel cornering force through the steering.  Also, the car will tend to follow a lateral slope of the road surface and weíll have to apply force at the steering wheel to counter that when running straight.


Next, letís consider a case where thereís positive caster but no trail: the steering axis passes through the contact patch center but tilts rearward in side view.  This implies pin lead.  Now there is camber change with steer.  The wheels tilt in the direction we steer.  However, since there is no trail or steering offset, cornering force and bumps do not create a moment about the steering axis, and there is still no jacking or wheel load change.


Next, letís combine trail and caster.  Now we have a steering axis that tilts, at least in side view, and also passes in front of the contact patch center.  Now we get some ride height change with steer.  The car drops when the wheel steers either direction.  Both front corners drop identically when we steer either direction.  This creates a gravitational de-centering force in the steering.  There will be a centering force due to tire drag, since we have trail.  There will be no change in tire load when we steer, because both front corners jack down together.


Note that we donít get any jacking with steer unless the steering axis is both tilted and offset at the ground plane.  Also note that we donít get wheel load changes if both front corners jack identically.


Now letís try some non-zero front-view parameters, with no caster or trail.  Suppose we have some positive steering offset but no SAI.  The steering axis is inboard of the contact patch center, and vertical.  Now we have bumps and braking forces creating a moment about the steering axis, but not lateral forces.  There is still no jacking and no load transfer with steer.


Next, SAI but no steering offset.  Now, the camber goes toward positive on both wheels when we steer.  The inside wheel leans into the turn and the outside one leans out of the turn.  There is still no jacking, and no wheel load change.


Now, both SAI and steering offset.  Now we get some jacking.  As before, the steering axis has to be both tilted and offset at the ground plane to produce jacking.  Now, instead of both front corners




dropping identically as we steer, they both lift identically.  Again, we get no load change with steer.  We do get a gravitational force in the steering.  This time itís a centering force.  The steering seeks

center with respect to the car centerline, but there is no drag centering or transmission of lateral force through the steering.


Finally, letís combine SAI, steering offset, caster, and trail.  Now we get some load transfer.  The ride height change due to caster and due to SAI are additive on the inside wheel and subtractive on the outside wheel.  The inside front corner jacks up more than the outside front corner.  The inside front and outside rear gain load and the other two corners unload.  The front end as a whole rises, at least for small steer angles, so there is a gravitational centering force.  Quite often this effect will reverse at large steer angles and we will get a gravitational de-centering force.


To get maximum load change when we steer, we want a lot of steering offset and a lot of caster.  To increase gravitational self-centering, we need SAI and steering offset.  To get more cornering force felt through the steering, we increase trail.  To get the wheels to lean in the direction we steer, we increase caster.


Now, what about anchoring a coilover, a pushrod or pullrod, or a drop link for a leaf spring directly to the upright?  I think this is a very promising idea, because it allows load transfer and jacking with steer to be controlled independently of camber change with steer and transmission of contact patch forces through the steering.  Things to watch out for when attempting such a layout include the possibility of running joints out of travel and having interferences as things move.