The Science Of Straight Axles - Traditional Tech

One of the great things about street rodding is the way you build your car is open to interpretation; still, the best cars seem to follow a theme. As an example, most builders believe that from an appearance standpoint a solid axle is the only proper choice to slip under a traditional car. Unfortunately, there also seems to be a belief that a certain amount of hardship goes along with the decision. It's like lousy road manners are part of the solid axle package, but the truth is that doesn't have to be the case. Straight axles may have been standard equipment on Conestoga wagons, but just because the basic design has been around for quite a while doesn't mean the theory behind it isn't sound.

Like any street rod front suspension system, proper installation and alignment is extremely important, so let's start with the basic terms used.

Caster is the tilt of the king pins towards the front or rear of the car. Forward tilt is negative caster, rearward is positive. Most cars with solid front axles use positive caster because it prevents road wander and helps keep the wheels pointed straight ahead (drag race and Bonneville cars often use extreme caster for high-speed stability).

Camber is the tilting of the wheels from vertical. Positive camber means the top of the wheel leans out, with negative camber the top of the wheel leans in. Camber's purpose is to bring the center of the tire tread in contact with the road at the point of load.

Toe is the distance between the front and rear of the tires. Proper toe adjustment prevents excessive tire wear and prevents wandering on straightaways.

King Pin Inclination is the tilt of the king pins inline with the axle.

Toe on Turns, also called Ackerman, is the relationship between the front wheels during turns.

Tracking means that the rear wheels of the car should be parallel to the fronts

Steering Control Alignment is the proper adjustment of the connecting linkage between the steering gear, drag link, and the steering arm on the spindle(s).

Understeer/Oversteer refers to how the car handles while cornering. Understeer in a turn means the front tires are slipping and the car isn't going the direction the wheels are turned-it feels like the car wants to keep going straight. Oversteer means the rear end is coming around like a sprint car in a corner, the car is turning sharper than the front wheels are pointed.

Alignment adjustmentsAlthough most early cars with straight axles used positive caster as with most alignment specifications there was a range of adjustment and the amount varied among the manufacturers. Ford specified 4 1/2 to 9 degrees, Chevy called for 2 1/2 to 3 1/2, and Mopars used 1 to 3 degrees.

As there was no method of adjustment for Fords, the standard procedure when these cars were new was to twist the axle to change caster; however, that procedure isn't recommended for contemporary aftermarket axles. In the first place, the equipment to do the job is no longer common and besides, most street rods have caster adjustment of some sort in the radius rods. Cars with parallel leaf springs up front can have the caster adjusted by the use of tapered shims between the main leaf and axle.

One of the most common mistakes when setting caster is making the adjustment while the chassis is not at ride height front and rear. Caster is the relationship of the axle to the ground, not the chassis. That means if it's set at 5 degrees positive with the frame sitting level on a jig, then the car is put on the ground with a 3 degree nose-down rake, the caster will be 2 degrees positive-and you'll be wondering why the car wanders all over the road.

As with caster there will be a range of factory specifications for camber. For examples, early Fords called for 1/4 to 1 degree positive, Chevys 1/2 to 1 1/2 degrees positive, and Mopars 1/4 to 3/4 degree positive. For the most part those figures are still valid, particularly if bias ply tires are used. Keep in mind most contemporary radial tires generate the most cornering force with a trace of negative camber, around a 1/2-degree, but if blasting around corners isn't your top priority, 1/2-degree negative to 1-degree positive (about the maximum for most aftermarket axles) will work fine.

If necessary, changing the camber of a solid axle isn't a sophisticated function; it simply requires bending, something that is still done regularly on trucks and can be done to a stock or dropped OEM axle. However, with aftermarket axles (cast or forged) the best thing is to check with the manufacturer for their recommendation if camber is outside of their specs (usually 1/2-degree positive, plus or minus 1/2-degree).

The last adjustment when aligning a front suspension is toe. The reason for this is adjusting caster and/or camber can change toe, but adjusting toe will not alter the other two. Toe adjustments are made with the tie-rod ends, but before any changes are made the wheel bearings should be properly adjusted and the steering gear centered in its travel.