Does your rod or custom wander from side to side when driving on a straight road? Do you have to constantly fight the steering wheel? Does the frontend feel "light"? Or does your steering not self-center after a turn? You may have some frontend alignment issues, the subject of which could fill an entire book. However, we're going to concentrate on the basic subjects of camber, caster, and toe-in here, as well as bumpsteer, though you should also be aware of topics such as Ackermann angle, scrub radius, and kingpin inclination. The basics of suspension geometry, as well as a diagonally identical wheelbase, spring rates, and wheel offset, must be adhered to if you want your car to handle well.
"But I'm not tackling a ground-up build," you may say; "I'm only putting on new hairpins," or "I'm swapping in the complete front suspension clip from a later-model car." You'd be surprised just how many times jobs like these will cause a change in geometry. Take that last example for instance; perhaps you've replaced the front section of your chassis with a Camaro front clip or similar. The geometry's built into that clip, right, so what can go wrong? Plenty! For starters it depends on the front-to-rear angle of the chassis 'rails. Is it the same as the donor car? Maybe, but probably not. Is your car now on a rake, even if it's only caused by larger rubber out back? If so, that front clip is now leaning forward, and you've likely gone from positive caster to negative, or at best seriously reduced the angle. But what is caster?
CasterCaster is the rearward or forward angle, from vertical, of the kingpin on a beam axle-equipped rod, the angle measured through the upper and lower ball joints on a double wishbone (IFS) suspended car, or the angle measured through the internal damper and lower ball joint on MacPherson Strut- equipped cars. That last one probably isn't relevant to most of "our" types of car, but worth knowing.
Positive caster is what you should be aiming for when setting up a front suspension, where the top of the kingpin leans back toward the rear of the vehicle, or where the upper ball joint is farther to the rear than the lower on an IFS car. Caster angle affects the steering, allowing the wheels to run straight and causing them to self-center after a turn, as well as determining how heavy the steering will feel. Most of us have seen I-beam-equipped rods with negative caster, which can't be much fun to drive. A common cause of negative caster on such rods is the mounting point of the split wishbones. Often, such rods will have a rake and the rear end of the wishbone is simply not mounted low enough because the builder didn't want long brackets hanging off the chassis 'rails. In such instances, geometry really should come before appearance, but hot rodding is often about aesthetics as much as engineering. Form over function.
Of course with a four-bar or hairpin, the kingpin angle can, to a large extent, be adjusted at the batwing on the axle; but this isn't the case with split 'bones, as they're solidly mounted at their forward end. A common solution is pie-cutting and welding the wishbone near the front end, but this isn't something we'd recommend unless you're a very competent welder.
So what's the ideal caster angle? Opinion is split, and every suspension and alignment expert has his own opinion, but general consensus puts it between 1 to 6 degrees. Chassis Engineering recommends 3 to 5 degrees for a beam axle, while Brent VanDervort of Fatman Fabrications told us they build 1 1/2 degrees into IFS systems with a manual steering rack, and 3 to 4 degrees with a power rack, for optimum steering response with each system. Gary Heidt at Heidt's Hot Rod Shop said, "We build 1 degree of caster into our independent front suspensions, but the builder can go to as much as 3 degrees if they so wish." Art Morrison of Art Morrison Enterprises, on the other hand, told us, "It really depends on the application and what the car is going to be used for, and the kind of speeds it will be driven at, but typically we'll build a minimum of 4 degrees into the frontend on one of our chassis."
The greater the caster angle, the higher the stability of the vehicle, but at the expense of ease of steering. Picture the way an old Top Fueler's front wheels "fall over" when turned hard. Stability is paramount over steering with anything up to 15 degrees of caster in this instance, but that's not what you want on a street-driven car. Stay within the 1- and 6-degree parameters-though from the advice above it would seem the angle is dependent on whether you're using a beam axle or IFS-and all should be well, providing you have built in correct camber.
CamberCamber is the measurement, in degrees from vertical, of the inclination of the wheels when viewed from in front of the car. It is known as negative camber if the top of the wheels lean toward the center of the car, while it's positive camber if they lean outward. Excessive camber will cause tire wear, but a little negative camber is conducive to good road-holding. Art Morrison said, "We like our cars to handle, so our Tri-Five frames, for instance, will typically have 8/10-degree of negative camber on the front end, with 1/16-inch toe-in."
With beam or tube axles, the camber is built into the axle and not easily altered if incorrect. It should be very close to vertical, with a half-degree of positive camber. With an independent suspension, such as with twin A-arms, if it's taken from another vehicle, such as our fictitious Camaro clip from earlier, the camber should remain unchanged from the donor vehicle factory specs, unless excessive weight is put upon it and the A-arms are almost at their full extent of travel (not an ideal situation in which to use them anyway). If you have an aftermarket IFS, the camber (and caster to a certain amount) is usually adjustable by moving the inner mounts of the upper A-arms to suit.
Most manufacturers of Mustang II-based IFS systems build in some form of adjustment for just this reason, be it slots, shims, eccentric shafts, or threaded rod ends. Back to Brent VanDervort at Fatman Fabrications: "We prefer the slotted or shimmed adjustment technique, as it preserves the axial alignment of the upper control arm shaft and bushings. An eccentric shaft puts the bushing out of alignment and can cause wear and binding. Threaded rod ends cause additional difficulty since disassembly is required to make any adjustment." Incidentally, '74-78 Mustang II camber should be 1/4-degree negative.
Toe-InToe-in is the angle at which the wheels point when viewed from above, but this is going to be virtually impossible to see unless you drive a fenderless rod or your tires are hanging out of the wheelwells. If they point slightly toward each other, it is referred to as toe-in, with toe-out obviously when they point slightly away from each other. On rear-wheel-drive cars, the forward-most edges of the tires will attempt to point outward under acceleration, so they are set with a minimal amount of toe-in as a counteractive measure.
The amount of toe-in dialed in depends on the diameter of the tire, as manual alignment gauges work by measuring the distance between the front of the tires across the car; and obviously larger tires will appear closer together than low-profile tires when angled slightly toward each other, despite the actual angle being the same. Therefore, around 30 minutes, or a half-degree, should be dialed in when using smaller-diameter tires, and 20 minutes with larger tires. The actual adjustment is made by turning the tie-rod ends or steering rack in or out until the gauges read correctly.
BumpsteerBumpsteer is closely associated with toe-in, and is common on cars that have been lowered, especially in situations where a steering rack has been added in a non-stock location. Imagine an independent front suspension moving up and down as it does every time it traverses a bump. It will move through an arc as the lower ball joint pivots around the lower control arm's inner mounting point on the chassis.
A farther arc is defined by the outer tie-rod end or spherical rod end on the steering arm, pivoting about the ball joint inside the steering rack. Where these arcs differ, the tie rod will either pull the wheel in or push the wheel out when the suspension moves up or down. Hitting a bump causes a change in the steering, hence the term bumpsteer, which can make for some unpleasant handling characteristics. The greater the variation in the two arcs , the more severe the problem, as toe-in changes during suspension travel, causing the car to wander violently with no input from the driver.
Factory suspensions and most aftermarket IFS systems will have all but the most mild bumpsteer engineered out from the outset, though lowering a car can increase the likelihood of its occurrence as the suspension travel arc is increased, or at least will start higher than it was originally designed to. If you're swapping front clips or adding a steering rack where previously there was a box, there are a number of points to observe that will help to eliminate bumpsteer.
First, ensure the lower control arms and steering arms are parallel when the car is at ride height. Horizontal would be preferable but not as important as them being parallel, as they cannot move in the same arc if they are not parallel to start with. Second, the pivot point of the control arm and the ball joint pivot inside the steering rack must be the same distance from the chassis centerline. Again, this ensures similar arcs will be attained.
One area we haven't mentioned is bumpsteer on rods with steering boxes and beam axles. Obviously such cars don't have control arms or steering racks, but the principle is the same. When such a car has a steering box mounted on the chassis with a Pitman arm and drag link, the latter should be parallel to, and the same length as, the axle locator, be it a split wishbone, radius rod, or hairpin. Any discrepancy in these lengths will cause bumpsteer, as the arcs in which the axle and drag link travel are different. If the steering box is mounted farther forward than the rear mount of the axle locator, you can avoid a lot of bumpsteer by ensuring the drag link is located at such an angle that an imaginary line along the drag link continues past it and intersects the axle locator at its pivot point. Of course this applies more to traditional rods, as those with cross steering, such as Vega systems, won't have the geometry that causes bumpsteer in the first place. Cowl-mounted steering boxes, however, should also be mentioned since the geometry can cause horrendous bumpsteer if there is a lot of suspension travel, though of course most rods have a minimum of travel anyway. Again, there's that form-over-function scenario. Of the four alignment issues so far discussed, bumpsteer is the hardest to rectify because it invariably will require some form of fabrication and movement of pivot points.
Kingpin InclinationKingpin inclination angle is the angle measured from vertical to an imaginary line drawn through the center of the upper and lower ball joints on an IFS system. By offsetting the ball joints, a vertical arc is produced as the wheels turn. This angle, coupled with correct scrub radius, has a direct bearing on how easy it is to steer the car and its responsiveness to steering corrections.
Scrub RadiusIf the KPI is correct, the line drawn through the ball joints, or through the kingpin on a straight-axle car, will meet the centerline of the front tire where it meets the road, or just slightly to the outside of the centerline. The distance from the tire center to the kingpin inclination angle contact point on the road is called the scrub radius. Changes to wheel height and offset will affect the scrub radius, and can lead to prematurely worn tires, bearings, and steering components.
Four-Wheel AlignmentSo now that you know how to build correct suspension geometry into your next project, what do you do when it comes to setting the alignment perfectly? For total peace of mind and a rod or custom that will take you safely wherever you choose to drive, a visit to your local alignment shop should be on your schedule. Not only will they adjust and set your frontend alignment, they'll be able to measure and set the alignment on all four wheels. We haven't touched on it here, but the rearend can affect handling too, and not just by being mounted out of square in the chassis.