This chapter discusses front suspensions with straight axles utilizing leaf springs. These suspensions have fixed caster and camber, but toe can be adjusted just as with the strut or A-arm front suspensions. This style suspension was very popular in the early days of drag racing, particularly in the Gasser classes, due to its weight-saving advantage. Today, even though it’s had major improvements over the years, this style of front suspension is hardly ever used except for nostalgia or exhibition drag race cars. There are even clubs that exist just for the preservation of the vintage-style straight-axle Gasser and altered-wheelbase cars.
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Typically, today thought of as “old school,” leaf-sprung and straight-axle front suspensions have experienced a resurgence in the past few years as a result of the growth of the nostalgia drag race craze. In the 1960s, before the auto industry made the wholesale change to independent front suspensions, nearly all cars and trucks used the same style of steering and suspensions. Cast-steel-beam axles and leaf springs were common, easily serviced, and cheap. So, it was only natural that the hot rodders of the day used these pieces in their cars.
It goes without saying (as with anything mechanical) that all components need to be in good condition and repair status, as well as lubricated and maintained. Routine inspection for wear and damage, especially in the case of a car that wheel stands or pulls the front wheels off the ground upon launch, is strongly suggested.
Front axles were typically all of the same basic design, the main difference being the physical size and, correspondingly, the weight. Truck axles were available, and for the abuse absorbed they were used in many of the dirt-track jalopy racers of the time. Drag racers were always looking for a weight advantage and sought out the lighter passenger car axles from Ford, Chevrolet, and Willys. These axles, with simple leaf springs, made a cheap and effective front suspension and steering package.
In order to assist in weight transfer, many racers of the day customized their spring packages by reducing the number of leaves in the pack, then had the remaining leaves arched or curved to an almost extreme amount. It’s a pretty good bet these suspension systems did little for ride quality. They did accomplish what they wanted, which was to move the nose of the car upward, thus transferring weight to the rear tires and hoping the racing slicks gained traction. While it was only marginally effective, it was an improvement, so it became a trend. The nose-up stance became representative of the time.
The next step of the progression was to reduce the weight of the front end. Due to the weight of an original-equipment-style beam axle, some forward thinkers decided a round tube could work as a substitute. Chassis builders of the day were soon making heavy-wall tubing, with bosses welded to the ends that accepted original-equipment-style spindles. Typically, the Ford spindle was chosen for weight considerations. The tube axle was born, and then the straight axle. Chassis/axle builders were turning these things out as quickly and cheaply as possible. A straight piece of tubing with the ends welded on was an easy out the door product. Put the correct length of tube in a jig, secure the ends, use a stick welder—and presto, a race car axle was born. With the added benefit of the straight piece of tubing giving a bit more lift to the front end of the car, there could be a reduction in the weight of the spring package by reducing the number of leaves needed to get the stance desired. Less weight means you go faster.
It was believed that a higher front end allowed for quicker weight transfer, thus giving more traction and allowing the car to get going faster. This isn’t all true, of course, but it was believed then.
The front-end alignment was of secondary importance in this system. The primary concern for the racer was high-speed stability. The camber angle was predetermined and pretty well fixed when the ends were welded to the tube.
Camber relates to the wheels being perpendicular, or straight up and down. If the wheel leans in at the top, toward the frame, it has negative camber. If the wheel leans outward at the top, away from the frame, it has positive camber. For maximum straight-line acceleration, the camber angle should be near zero so that the entire tread (the tire) is flat on the road. Positive or negative camber has a scrubbing effect due to the tire being at an angle, and increases rolling resistance. Near zero camber helps the car accelerate more quickly, as do narrow tires, which have less rolling resistance. The only camber adjustment method available on straight axles is to physically bend the axle. Heavy truck and trailer shops are equipped to do this by securing the axle to a fixed point while applying pressure with a hydraulic ram.
Caster deals with the spindle location governed by the upper and lower ball joints. Caster (unlike camber and toe, which are viewed from the front or rear of the car) is viewed from the side of the car. Caster may be set with each side independent from the other. For street driving, caster usually has negative 1/2 degree of caster on the left side to compensate for the crown built into most roads for water runoff. The side of the car with the least amount of caster pulls toward that side. On drag race cars running down a level track, the caster should be the same on both sides. Greater caster angles improve straight-line stability, but they also cause an increase in steering effort. In a typical street car, the normal range of positive caster is 3 to 5 degrees, but lower settings can be used on heavier vehicles to keep the steering effort reasonable.
Viewed from the side, draw an imaginary line from the top mounting point for the spindle (ball joint) to the lower mounting point for the spindle (ball joint). Caster is the number of degrees (positive or negative) front to back from this vertical line. If the top ball joint is more rearward than the bottom ball joint, the caster is positive. If the ball joint is tilted forward, the caster is negative. Positive caster tends to straighten the wheel when the vehicle is traveling forward, and thus is used to enhance straight-line stability. That is why after turning, your steering wheel slowly returns to center without any correction from you. It’s not usually as quick returning to a straight forward direction as we want so we give it some help by turning the steering wheel back to the straight-ahead position.
With straight axles, positive caster (king pin inclination) is the ticket for stability. A typical Gasser setting starts at 7 degrees and is fine-tuned to as much as 10 degrees. Adjustments are done by placing a wedge between the spring and the axle/spring mounting pad, just like setting pinion angle in a rear-wheel-drive car with leaf springs in the back.
These settings are typical of full-bodied cars. Look at the front end of a front-engine dragster with a straight axle. The positive caster for one of these cars is so extreme (start ing at 25 degrees) that in many cases, when the wheels are at their full left or right extension, they cannot be returned to straight without an assist from a crewmember physically grabbing the tire and straightening it to neutral (straight-forward facing).
The final aspect of the alignment dilemma is toe, which refers to the wheels being parallel with each other, forward to backward (tracking). Toe has a great impact on stability at speed. While you’d think that setting toe to 0 would be perfect and offer the least resistance, that is not the case. Alignment shops set up most muscle-era drag cars with 1/8-inch toe-in so the front edges of the tires are 1/8-inch closer to each other than the rear edges of the tires. If the rear edges of the tires are closer to each other, instead of the front edges, it is called toe-out. The reason for 1/8-inch toe-in is that at 60 to 80 mph (a somewhat arbitrary number), the wind rushing at the front of the car tries to push the front of the tires apart. Setting the toe-in at 1/8-inch is like preloading the rear suspension. It offsets the wind resistance so as the car goes faster and faster, the wheels deflect to 0 toe-in and are straight (parallel) with each other.
Toe settings effect tire wear as well. For minimum tire wear and power loss, the wheels should point straight ahead (0 toe-in) as the car is moving in a straight line at highway speed. Too much toe-in causes the tires’ outside edges to wear, while too much toe-out causes the tires’ inside edges to wear. As not all cars attain the same speeds on the quarter-mile tracks, or even on eighth-mile tracks, the settings for each car may need to be different.
Chuck Lipka of the Geezer Gassers says his car utilizes toe-out. With the front end already up in the air at the starting line (typical Gasser setting), then as the front end rises farther on acceleration, the tires/wheels want to return to a straight-ahead or neutral setting. As the car continues to accelerate and the weight continues to transfer rearward with even more air rushing under the car, the tire contact patch is reduced and lessens the scuff factor. Remember, these cars begin the run with the front end up in the air, not like today’s muscle cars with the front end aerodynamically down for less air resistance and drag.
Straight-axle suspensions have two distinctly different styles of steering. Both styles accomplish the same thing, but with different approaches.
Cars using a straight axle from the 1930s and 1940s typically used a steering box and Pitman arm working forward-to-backward (or push-and-pull). This in turn acted on a relay rod that connected the Pitman arm to a steering arm, or knuckle, that moved the left spindle assembly to the left or right. The left spindle was then connected to the right spindle with an adjustable tie rod that transferred the same steering inputs to each wheel so they worked together. The steering box is typically mounted to the left frame rail, and the connection to the steering arm runs parallel to the frame rail (front to rear).
The other system, and the better choice in the eyes of many, is the cross-steer arrangement. In this style, the steering box mounts to the left frame rail. The Pitman arm connects to the right spindle/steering arm with a relay rod. The right spindle is then connected to the left spindle with a tie rod. This system all but eliminates the bump steer dilemma, especially when the alignment specs of positive caster and toe-in are in sync.
The challenge with cross-steer setups is keeping the relay rod and the tie rod parallel to each other as much as possible, as well as parallel to the ground. When the steering is moved through its full range left to right, the relay rod and the tie rod should remain on the same plane. Placement of the steering box is the key to attaining this specification. If the steering box is too far forward, the relay rod and tie rod can come in contact and in extreme cases they cross.
In a worst-case scenario, the steering momentarily binds or locks up. Then as you apply more steering-wheel pressure to overcome the lockup, the relay rod and the tie rod overcome the bind and the steering wheel is jerked out of your hands, stopping at a full-turn lock. Definitely not a good situation.
These steering systems worked in their day with low speeds, narrow tires, marginally adequate road surfaces, and light overall vehicle weights (especially in the front). The main concern and complaint with this type of system is a problem known as bump steer. The systems are susceptible to dramatic and sometimes unexpected, possibly violent jerking of the steering wheel when the wheel/tire strikes a pothole or a bump in the pavement surface. With the steering box and Pitman arm moving front to rear, striking a pothole or bump transfers in full force directly to the steering wheel. It gets worse with any degree of wear in the front end (such as worn tie rod ends, kingpins, steering box, etc.).
For drag racing, the narrower the front tire, the better. However, don’t get into a situation where safety is an issue because your tires are not rated in a high enough capacity for your car’s weight. Too narrow of a tire on too narrow of a rim in an extreme handling situation (other than straight forward) could cause the tire to collapse, unseat, or come off the rim.
Mickey Thompson makes a narrow front tire in several different sizes that can be mounted on 4-inch-wide rims. They are available in eight-ply to hold up on heavy drag cars. These front tires are Department of Transportation (DOT) approved and therefore may be used on the street for those with dual-purpose street/strip cars. Mickey Thompson and Moroso also make a non-DOT-approved front tire that is much lighter in weight (13 pounds).
Since I occasionally drive my car to a cruise-in, I prefer the DOT-approved tires. Both the approved and non-approved tires have a crowned tread, so that under acceleration while the car is experiencing weight transfer (pitch rotation) the amount of tread on the pavement becomes less and offers even less rolling resistance. These tires are bias-ply designs and are not recommended by the manufacturer to be used with radial tires in the rear.
When choosing any non-original- equipment tires, it’s important to match the front tires as close as possible to the rear tires. Changing the stance of the car to where the front of the car is lower than the rear of the car greatly hinders weight transfer (pitch rotation), as it makes the front of the car heavier and much harder to lift for maximum weight transfer. Smaller-diameter front tires can also be compensated for through spring selection. Smaller-diameter front tires can create better reaction times, but slower 60-foot times. The opposite is true in larger-diameter front tires—they give you slower reaction times with quicker 60-foot times.
With this chapter’s information you now understand the dynamics of using a straight-axle front end. In the Gasser days they were easily available and lightweight. With today’s technology, race cars with straight-axle front ends are outdated and almost non existent, except for nostalgia racing.
Written by Dick Miller and Posted with Permission of CarTechBooks