Steering Box Upgrades
Not ready to step up to rack-and-pinion, or just want to maintain the originality of your car? There are several good choices out there for a drop-in steering box upgrade. In most cases, an updated quick-ratio box with a modern spool valve can provide as much as 90 percent of the improvement that a rack-and-pinion conversion provides. Plus, installation tends to be a much easier affair, with little or no concern about a loss of turning radius, header clearance, or transmission compatibility.
Early on in the invention of steering systems, most were arranged in a more or less parallel design, meaning that both front wheels turned the same amount in a turn. This works well for low-speed performance, like in the buggies and carts it was originally designed for, but it causes a great deal of problems as speed increases.
Boiled down to it simplest form, the Ackerman concept orients steering angles so that all four wheels rotate around a common point in a turn that correlates with the rear axle. To accomplish that, the front wheel on the inside of a turn rotates more than the outside wheel. Why is that a better design? First, imagine a circle. Now, imagine the outside tire of a car is following the outline of the circle. The inside tire has less distance to cover since it is further inside the circle. In a parallel steering arrangement, the wheels actually end up fighting against one another since the inside wheels want to trace the same line as the outside and end ups scrubbing. In a pure Ackerman arrangement, the inside wheels turn a tighter radius to compensate and create a condition where each tire can cover the correct amount of ground. This arrangement is ideal for most situations, and every car on the road today uses some variation of the Ackerman principle in their steering design.
Things do get quite a bit different in the world of road racing and circle track as pure Ackerman is not necessarily the desired arrangement since many other factors come into play, and energy carried into a turn is far greater. As a matter of fact, some racers use reverse- or anti-Ackermann geometry to compensate for the large difference in slip angle ratio between the inner and outer front tires experienced during high-speed cornering. High-end data acquisitions systems can reveal what adjustments need to be made, but knowing how to correctly read your tires can be every bit as effective.
Caster and camber are the two principles most enthusiasts are familiar with since they are two of three components addressed in a standard front-end alignment. To understand caster, it's important to understand the concept of trail, which is most easily demonstrated by a shopping cart's front wheel. The steering axis is located ahead of the wheel, so when the cart is pushed forward the wheel will follow directly behind the steering axis, making it self-straitening and therefore stable and easy to control. If the steering axis were placed vertically above the wheels, there would be zero caster effect and the wheels would tend to wander. The longer the distance between the steering axis and the wheel, the greater the force, or trail, exerted.
On a car, this same force is applied by titling the steering so that the steering axis falls on a point ahead of the wheels' contact patch with the road. Higher caster angles improve straight-line stability, but they also increase steering effort, so most street cars stay at 3 to 5 degrees of positive caster, while racers have been known to experiment with slightly higher angles to promote camber gain through turns.
While caster is a forward or rearward rotation of the wheel from vertical, camber is the amount the wheel is tilted inward (negative) or outward (positive) from vertical. You've likely seen heavy negative camber on the front wheels of race cars, or on an IRS-equipped car with a heavy load in the rear. There are actually two camber angles that are critical to steering and suspension performance: camber relative to the road and camber relative to the chassis. Ideally, the wheel should always operate at a slightly negative camber relative to the road, but this can be challenging since the forces exerted by suspension travel, body roll, and suspension dive must be taken into consideration.
To counteract roll, suspension is designed to travel in an arc toward the chassis, which helps maintain more of the tire's contact patch on the road in a hard turn. Camber adjustments are independent of this and preload the wheel with more or less static camber. This same motion creates camber gain, which is added to the static number and needs to be taken into consideration for serious track cars. From the factory most cars are dialed in to gain slightly positive camber in a hard turn; this creates understeer, which limits cornering ability and helps keep inexperienced drivers safe. Street cars set up for handling often run -.5 to -2.5 degrees of static camber. While that doesn't sound like much, that extra negative camber makes a dramatic difference in a how a vehicle feels through a corner. It also leads to dramatically shorter tread life and can cause the front end to track grooves in the road, so more is not necessarily better.
Of course, that same negative camber setting causes the inside front tire to lose contact patch in a turn, but for open track, autocross, and aggressive street cars, setting the tires for negative camber is the only viable option. As for circle track where surfaces are usually banked and turning two directions isn't a factor, typically the outside tire will be set for negative camber while the inside would get positive camber to compensate.