Caster and SAI Revisited

[johnc]

I have said repeatedly that caster increase the load on the inside rear wheel. I've said that based on testing I've done on a set of scales. I was wrong. Here's an article from mark Ortiz that explains Caster and SAI. I called and talked with him about it for about an hour last week and I agree with what he states in the article. I don't know how I ended up seeing an increase of load on the inside rear wheel in the test I did, but it looks like the test was wrong.

 

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 it’s 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 – 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 can’t 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 don’t 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 drivers’ 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 car’s 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. People’s willingness to live with such effects in the pursuit of more speed will vary.