The Science Of Straight Axles - Traditional Tech

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.

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.

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.

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.