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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: firstname.lastname@example.org. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
EFFECTS OF CASTER AND SCRUB RADIUS
I continue to enjoy your newsletter and would really like to see some of your insights into the effects and limitations of high caster angle, preferably in conjunction with both large and small scrub radii. It seems to me double-digit caster angles would provide favourable camber-into-the-corner of both front tires during steer and that significant scrub radius would add a jacking effect that would usefully load the inside front tire, but perhaps it's not quite that simple...
The effects described do indeed happen, along with a few other things. More caster does make the front tires lean in the direction of steer. And this combined with more scrub radius (or front-view steering offset) does add load to the inside front and outside rear tires, while correspondingly unloading the outside front and inside rear. This is sometimes called de-wedging, and as a rule it adds oversteer, or reduces understeer. But itís not simply a case of more tire inclination and de-wedging with steer being better without limit.
First of all, it is possible to have too much tire inclination. Tire inclination improves lateral grip up to a point, but beyond that optimum, the tire runs too much on its inside edge, and grip starts to be lost.
Lots of caster and a big scrub radius will result in the car de-wedging a lot with steer, and also rolling outward as the inside front is lifted and the outside front is let down. The roll somewhat reduces the tire inclination gain, and will also tilt the rear tires out of the turn some, with independent rear suspension.
The amount of de-wedging also depends on the wheel rates in warp throughout the suspension system, and the torsional stiffness of the sprung structure.
We will not in all cases want the wheel loads to change a lot with steer. On ovals, it will generally be unnecessary. On really high speed ovals, it will tend to make the car twitchy and make it harder for the driver to be smooth. More moderate caster and scrub radius are preferable for that
application. Applications where we want to go the other way include autocross, street circuits, tight road courses, chicanes, twisty hill climbs, and so on Ė situations where we need the car to respond fast to abrupt maneuvers, and stability and smoothness are not paramount.
Where the vehicle has a spool or locked axle, and runs on a road course, it is particularly common to employ lots of caster jacking. When the turns go both ways, we canít use tire stagger to prevent locked-axle push, and unloading the inside rear becomes crucial to making the car turn, especially on the tighter radii.
Examples of racing vehicles using unusual amounts of caster jacking to overcome locked axle push would include go-karts and Australian Super Cars, and perhaps even Legends cars to a smaller degree. We donít have much freedom on spindle or upright design in a Legends car, but we do have some caster adjustment. On karts, it is common to vary the scrub radius using spindle shims as a way of tuning the vehicle. Sometimes, parts of the frame are made removable as a way of adjusting torsional stiffness as well.
Scrub radius and caster definitely affect steering feel. It is important to remember that a race car is a tool for a human being, and driversí preferences in steering feel differ. It is also important to note that if we are designing our own spindles or uprights, there are additional parameters to consider, which interrelate with scrub radius and caster. This means that there is more than one way to vary caster, and more than one way to vary scrub radius.
If we are adjusting an existing car, or if we are committed to a single upright design, adding caster adds trail. This increases steering handwheel torque per unit of lateral force at the front tires. This translates to higher control effort, especially in hard cornering, and also increases the carís tendency to follow lateral road slope. When we have more freedom on our upright design, we can employ some pin lead to decrease trail, and have any combination of caster and trail that we want.
Even when we have less trail due to pin lead, increasing caster still increases caster jacking.
If we have design freedom, we can vary scrub radius by varying front view steering axis inclination (SAI), or by placing the ball joints closer to the wheel centerplane or further inboard. These two approaches have different effects on steering feel.
If we had zero SAI and positive caster, and some scrub radius, the front end of the car would not only roll oppositely to steer (with corresponding de-wedging of the car), it would also drop with steer. This would cause the steering to try to go away from center at low speeds, and at a standstill. If, conversely, we had no caster and some SAI, the front of the car would be lifted with steer, and the steering would seek center with respect to vehicle centerline.
When we have some SAI and some caster, what happens at a standstill or at parking speed is that near center, the steering seeks center, but at some amount of steer, it tries to go away from center.
SAI also affects camber change with steer. It causes both front wheels to go toward positive camber with steer. This means that when we have a combination of SAI and caster, the outside wheel will gain inclination with steer, but at a decreasing rate, and at some point will start to lose inclination, while the inside wheel will gain inclination at an increasing rate. This may not be too bad, particularly if the front wheels have some static negative camber.
Finally, large amounts of caster and scrub radius can in some cases produce really disagreeable behavior in the steering, in the form of various types of oscillation. Any runout in the tires or pulsation in the brakes will be felt more in the steering. I have had clients running large caster settings on stock cars report steering shimmy at lower speeds. Peopleís willingness to live with such effects in the pursuit of more speed will vary.
CAMBER GAIN RECOMMENDATION
I have built 2 7s type cars and am busy with an exoskeletal car. This car is out and out for track racing.
Do I go for the unequal double wishbone design (top wishbone 2/3 of the bottom)? What should the camber gain be that I design into my uprights?
I do not know how much the car is going to roll.
Does one strive to get the camber gain front and rear the same? (independent rear suspension)
Unequal double wishbone is good. Top wishbone 2/3 of bottom is a reasonable rule-of-thumb recommendation for uprights of typical dimensions. You donít need to worry too much about straying from the 2:3 ratio if packaging or structural requirements dictate.
ďCamber gainĒ typically refers to the rate of camber change per unit of suspension displacement, as measured statically in the shop, moving the wheel or upright up and down with a jack and measuring camber change as the displacement changes.
My standard recommendation for camber gain is 0.6 to 0.9 degrees per inch, which would be roughly 0.02 to 0.03 degrees per millimeter. With typical track widths, this gives camber recovery rates between a bit less than 50% and a bit better than 25%. That is, ignoring roll due to tire deflection, and ignoring other compliances, the wheels lean a bit more than half as much as the body in cornering, but less than ĺ as much, and they donít experience any really huge camber changes due to acceleration, braking, and bumps.
We also need to avoid excessive jacking. That is, we want the roll centers fairly low: modest geometric anti-roll. There is no hard and fast relationship between camber gain and geometric anti-roll when we have total freedom to move all points, but in the majority of cases, where we face
packaging constraints and have committed to some point locations, more camber gain will produce more lateral anti, or higher roll centers.
My general recommendation is that at static ride height, the force line Ė the line from the contact patch center to the front view instant center Ė should slope upward toward the center of the car, at around four degrees. In no case should the slope be more than eight degrees (roll center around 4Ē above ground). This means that the coordinates of the front view instant center, using the contact patch center as a local origin, as in Mitchell software, should have z (vertical) and y (transverse) coordinates with a ratio around .07 z/y, or in no case more than .14.
You will want to make sure you stay within these limits at the highest ride height you design for. Generally, race cars have their ride height tuned to the circuit. With a smoother surface, we can run the car lower, and we should. Generally, that lower ride height brings lower roll centers. Thatís okay.
Itís not bad if the geometric anti-roll is very small, or even negative (roll center low, or below ground), but donít let it get too high. When there is little geometric anti-roll (roll center near the ground), the lateral location of the force line intersection will move all over the place with suspension movement. Some people will tell you that the force line intersection is the roll center, and/or that you can model car behavior by taking moments about that point, and that minimizing lateral (or vertical) migration of that point is vital to good car behavior. It isnít true. Donít worry about it. Cars work fine with little geometric anti-roll. Just donít have a lot, with independent suspension. That does create problems.