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.