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
One of the basic elements of frontend geometry is caster, which is the tilting of the stee
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.
These illustrations show the equipment that was once commonly used to adjust caster on ear
In most cases toe-in is used to compensate for the cumulative effect of all the tolerances in the steering system. Without a slight amount of toe-in when the car moves forward, any slack in the various steering would let the front of the tires spread apart, resulting in toe-out.
Most straight axle street rods are sensitive to toe adjustment, and this is another one that will be influenced by the type of tires being used. Radials require less toe in, 1/16 to a 1/8-inch is typical, bias ply tires require a little more, 1/8 to 3/16-inch is usually required.
While it's not very scientific, a good indicator of proper toe setting is tire wear. Sometimes the wear can be seen, but another way, albeit dirtier, is to run your hand across the tire's tread. If sharp edges are felt while going from outside in, it's an indication that more toe-in is required. If a sharp edge is felt going from inside out, more toe-out is needed. Of course, if the treads feel about the same from both directions, the toe is just right.
Another set of illustrations showing how camber was once adjusted on Fords. Camber is bein
While there is no adjustment for king pin inclination, it does influence the amount of wheel offset that should be used because of a thing called scrub radius. Theoretically, if a line is extended through the kingpin to the ground, it should intersect the ground at the center of the tire, as that's where it will pivot when turning. Increasing the offset of the wheel and moving the centerline of the tire out means the tire will scrub sideways as it's being turned, which can increase steering effort, the greater the offset the more noticeable the increased effort will be. In extreme cases deep-dish front wheels will increase the tire's leverage on the spindles, causing the steering wheel to jerk around on rough roads.
Toe-out on turns is another factor that is determined by the components used. When a car turns a corner, the inner wheel must turn tighter than the outer. That is accomplished by the position shape of the steering arms. If a line is drawn from the centerline of each king pin to the middle of the rear axle, those lines should pass through the tie rod ends, even if the tie-rod ends are in front of the axle. This arrangement makes the inside tire turn sharper than the outside tire.
A common mistake is to turn early Ford spindles around to get the tie in front of the axle for frame clearance. What this does is create toe-in on turns; the outside tire turns sharper than the inside, and the result is one tire is dragged sideways.
King pin inclination is built into the spindle. It not only establishes the scrub point of
When it comes to tracking changes, some chassis are adjustable, some aren't. Often this is a function of the chassis construction, but changes can be made to cars with triangulated four bars in the rear or a Panhard bar. Tracking can be checked with a straightedge or string and a tape measure, or most alignment shops can do it.
Steering control alignment is another factor that is determined by the arrangement of components and some portions are adjustable, others are not. The most frequent problem with steering linkage causes bumpsteer, which is a result of the drag link traveling in a different arc than the steering arm on the spindle. The diverging/converging angles of the two will cause the car to wander as the suspension works up and down.
A seldom recognized problem with steering linkage is the relationship between the Pitman arm and the drag link. When the steering is centered, the Pitman arm should be 90-degrees to the drag link. If the angle is more or less, the car will respond more quickly one way than the other when the steering wheel is turned off center.
The scrub radius of a solid axle is determined by the king pin inclination and the offset
While understeer and oversteer are typically factors dictated by the design of the chassis and suspension, there are adjustments that can be made for fine tuning (see table below).
Bias Ply Versus Radial TiresWhen it comes to selecting tires for a solid axle street rod appearance is often more of a factor than performance. For some traditionalists, that means bias ply tires are the only way to go.
While bias ply tires are perfectly safe, there are some precautions that should be observed. To properly support the tread, rim width is important, so always follow the manufacturer's recommendations. Never use low air pressure to improve ride quality, as the edges of the tread will wear excessively and handling will be compromised. On the other hand, over-inflation will make the tread bulge increasing wear in the center and reducing the contact patch which will also compromise handling. Interestingly, bias ply tires are generally lighter than radial so they do reduce unsprung weight.
Of course the big difference between bias plys and radials is ride and handling. The flexibility of the radial sidewalls mean that they will ride better while the stiffness of the tread results in less slip angle, which makes them more responsive and have better road feel when cornering.
Sometimes building a street rod requires compromise, and it is true that independent front suspensions do have some advantages when it comes to ride and handling. But that is not to say a properly installed and adjusted solid axle suspension cannot provide good ride and handling as well. It can and it will look like a traditional hot rod doing it.
|To Decrease ||To Decrease ||Adjustment |
|Understeer ||Oversteer || |
|Higher ||Lower ||Front Tire Pressure |
|Lower ||Higher ||Rear Tire Pressure |
|Larger ||Smaller ||Front Tire Section |
|Smaller ||Larger ||Rear Tire Section |
|More Negative ||More Positive ||Front Wheel Camber |
|More Positive ||More Negative ||Rear Wheel Camber |
|Toe-out ||Toe-in ||Front Toe |
|Toe-out ||Toe-in ||Rear Toe |
|More Positive ||More Negative ||Front Caster |
|Soften ||Stiffen ||Front Springs |
|Stiffen ||Soften ||Rear Springs |
|Smaller ||Larger ||Front Anti-sway bar |
|Larger ||Smaller ||Rear Anti-sway bar |
Called toe out on turns, or Ackerman, when a car turns a corner the inside tire has to tur
Ackerman is established by the relationship between the kingpins and the steering arms. Li
Often overlooked is the relationship between the Pitman arm and the drag link. When the wh
The rolling Pete and Jake's chassis uses 5 degrees positive caster, 0- to 1-degree positiv
In some circumstances Ford spindles are reversed to provide steering clearance. This also
In this overhead view of a Pete & Jake's chassis, note the relationship of the steering ar
An interesting tidbit: Aftermarket axles are made in 2 and 2 1/4-inch perch widths. Howeve
In some cases caster angle will be greater than the angle of the front crossmember; as a r