History and Detail on Solid-Axle Front Suspension

This issue marks the final 50th Anniversary tribute to ROD & CUSTOM magazine, and it is interesting to note that while hot rodding has gone through several phases and trends over the past five decades, many of the rods on the road today are equipped with suspension setups based on designs first employed by Henry Ford nearly a century ago. While independent suspension was the rage for several years throughout the '80s and '90s, a return to traditional style has created a tidal wave of interest in period-correct solid-axle front suspension setups, as well. What we have noticed, however, is that the growing nostalgia movement has attracted many new faces to the rodding scene, and many of these people are totally unfamiliar with the dropped-axle suspension design, which is a must-have for any traditional hot rod but otherwise totally obsolete on any production car built after the Eisenhower administration. Since building, maintaining, and even driving an old car necessitates having a general idea of how it works, we thought it might be a good idea to take a look at the basic principles of how this suspension system functions and perhaps debunk some long held myths in the process.

THE BASICS
From the first Model T until the end of the '40s, Ford Motor Company relied on a solid I-beam front axle as the backbone of its front suspension designs. Since both roads and tire technology were fairly primitive in the '30s, Henry's engineers built passenger cars to ride high with lots of suspension travel, which came in handy when navigating rough terrain at whatever pace the 60hp engines of the day could sustain. The basic principles behind this design centers around an iron solid axle supported by what is known as a buggy spring, which looks like an inverted "U," runs lengthwise across the axle, and is bolted to the frame with shackles. The spring effect is dampened by friction shocks, which are basically alternating pieces of steel and high-friction pads sandwiched together and bolted between the axle and frame. Tube shocks were not introduced until the '40s, although most hot rods use them today. The axle is supported by what is known as a wishbone, which was essentially a piece of steel shaped like its namesake that bolts to both ends of the axle up front and comes to a point in the middle of the car with a reticulating ball bolted to the transmission mount (see picture number 1). This piece triangulates the suspension system, and the ball allows the wheels to travel up and down independently as the car navigates potholes and other road hazards. Finally, steering involves a box that bolted to the frame near the firewall. The steering column feeds into the top of the box and an arm (called the pitman arm) sticks out the side of the car and moves back and forth, pushing or pulling a rod that is bolted to the front left spindle, thereby turning that wheel left and right. Since both front wheels are tied together, this allows the car to turn. Henry Ford's system worked extremely well for its intended purpose, which was allowing small cars with skinny tires and 60hp engines to navigate rough country roads. As time wore on and hot rodders discovered the joys of big engines and lowered suspension, however, things began to change.

EARLY HOT RODS
Prior to World War II modifying old Fords for speed was a very limited engagement, mostly restricted to a few racers and speed-crazed hobbyists. These early rodders hopped up their flathead engines and stripped unnecessary parts off of their cars to save weight, but they left their suspension setups relatively unchanged. Only after the war, when Detroit started to crank out bigger and brawnier V-8 mills, did frontends begin to change. Suddenly flatheads were no longer top dog, and with the removal of the stock engine and transmission and the addition of a Cadillac or Lincoln engine, the reticulating ball wishbone setup was no longer applicable. As a result, early hot rodders cut the ball out of the system and bolted the ends of the wishbone to the framerails, devising what is now known as a split wishbone. About the same time people started to realize that the lower you got the frontend of your car, the better it would look and handle, and "dropping" the axle was the best way to accomplish this goal.

"Original Ford axles had roughly 1 inch of drop in them," explains Magnum Axle Company co-owner Fred James. "If a guy wanted to drop his I-beam axle, he would have to go to Mor-Drop in the South Bay area or Dago's down in San Diego, where they would heat and stretch the ends of the axle. Some of them looked pretty good, but some turned out like an hour glass, which could cause a frontend shimmy or other problems. It became apparent pretty quickly that something new was needed, which is where tube axles came into play."

In the late '40s and early '50s Bell Auto (the same company that makes Bell helmets today) started offering the first dropped tube axles. They would cast ends and weld them to a tubular centersection, creating a new part that looked good and could drop the front end of car as much as 5 inches. Unfortunately, Bell Auto stopped manufacturing their axles by the end of the decade, and soon hot rodders were once again relegated to digging up good used I-beams and having them dropped by shops that specialized in such operations.

"Back in 1972 when I was working at Blair's Speed Shop, there wasn't really a good new dropped axle you could buy," recalls So-Cal Speed Shop President and famed rod builder Pete Chapouris. "You could buy a good used axle at a swap meet and send it to Mor-Drop, but with all due respect to them it might take two or three months to get it back because they were overwhelmed; it became a real issue. Then in 1975 or so Jim Ewing started Super Bell and came out with a nice tube axle, which eliminated the need to go to wrecking yards completely. I could order ten at a time and they would show up on my doorstep. It was a revolution."

By the time the late '70s rolled around, both tube and I-beam dropped axles could be purchased new right out of a catalog, which lit several creative fires and influenced quite a few new developments in street rod suspension technology. Rodders could suddenly choose what kind of axle they preferred and what sort of radius rod setup they wanted to support it.

TUBE VS. I-BEAM
While tube and I-beam axles both accomplish the same basic task, each has its own positive and negative traits that need to be taken into consideration. Tube axles look more aerodynamic, are built out of smooth steel tubing (making them easier to chrome or paint), and are extremely rigid. I-beam axles are either cast or forged, so they have a porous surface that can be more difficult to chrome without leaving nickel shadows in the channel that runs down the center of the axle. I-beams are also slightly flexible and can twist as a car goes over uneven terrain. "The only reason to run a tube over an I-beam or visa versa is personal preference," says James. "Since the nostalgia wave has kicked in, the ratio has really increased in favor of the I-beam, so we are actually developing a 5-inch dropped I-beam that will offer the best of both worlds: the drop of a tube with the traditional look everyone wants."

What is interesting to note here is that for many years the only radius rods available were either the split wishbones or a more refined version called hairpins, both of which bolted to a single mounting point on either side of the frame. This worked great with the dropped I-beam axles that were available at the time, because the radius rods allowed the axle to move up and down and the flexibility of the I-beam allowed the axle to twist slightly when necessary to accommodate strange torsion loads going up driveways or over speed bumps. However, when the tube axle became popular, some argued that its rigidity and inability to flex could exert extreme pressures on the single mounting point of the radius rod, which could cause it to tear right out of the frame. As a solution to this problem, Pete & Jake's came up with the four-bar radius rod system, which had two mounting points on the axle and two mounting points on the frame, with aircraft-style rod ends on all four corners, allowing plenty of movement at all angles. The system worked great, and yet to this day many rodders are still mystified as to whether a tube axle can properly function safely with a standard split-wishbone or hairpin setup.

James takes issue with the steadfast rule that four-bar systems must be employed with tube axles, explaining that there isn't a great amount of travel built into the frontend of a hot rod in the first place. "If a car's total front suspension travel is 6 inches, meaning that the wheels can move 3 inches up and 3 inches down, then the most an I-beam axle can twist is 6 inches in both directions," James says. "Now if one wheel is loaded and one wheel is unloaded as a car with a four-bar goes though a driveway diagonally, the radial twist in the four bar during that period is greater than the radial twist in an I-beam axle, because there is more freedom of motion allowed by the four-bar system. If anything was going to break from work hardening or twisting, it would be the four-bar setup since it is built lighter than a heavy-duty tube axle, but that has never been a problem in the many years that the four-bar has been on the market."

According to Chapouris, who helped design the four-bar setup in the first place, one must consider more than just the axle type when choosing a radius rod, as the steering setup comes into play as well.

STEERING
Since original Ford steering boxes usually wore out long before the rest of the car, rodders in the '40s and '50s resorted to using '40 Ford boxes, and then later F-100 units as replacements, which were basically more heavy-duty versions of the system already in place. With a dropped axle, split wishbones, and a stock or F-100 steering box in place, the rod that runs from the pitman arm to the spindle (called a drag link) was forced to operate at an unusual angle due to the geometry change. What would happen is that, as the frontend went up and down, it pulled the steering rod back and forth along with the rest of the suspension, causing the wheels to turn on their own, which is called bumpsteer. Essentially, chassis flex could make the wheels turn without any driver input on the steering wheel. This problem became even worse in the late '60s, when Mustang steering boxes began to replace F-100 units. Due to the way they were designed, the Mustang boxes needed to be mounted with the pitman arm pointing up rather than down like a stock box, so the drag link became very short. The shortened drag link further accentuated the bumpsteer problem.

A four-bar system allowed rodders to move the Mustang box farther back, stretching the drag link and eliminating quite a bit of bumpsteer. This occurs basically because the longer the drag link, the less it will be affected by the suspension travel as the car moves down the road. As an example of this phenomenon, lay a pencil next to a straightedge and measure its length. Then move one end of the pencil up about an inch, leaving the other end touching the straightedge so it is now at a diagonal angle. Notice that the overall length covered by the pencil got shorter. This is exactly what the drag link of a car does as the front suspension goes over a bump, pulling on the spindle and turning the wheels. If the pencil (or drag link) was twice as long, the overall length would change half as much when moved, thereby reducing the bumpsteer problem by 50 percent as well (see illustration 12). Because of this discovery, the combination of a four-bar and a Mustang box totally revolutionized solid-axle front suspension in the late '60s and is still a fantastic setup to this very day.

The last big leap in traditional front suspension steering technology came along in the '70s, with the advent of a small economy car by Ford's rival General Motors. The Chevy Vega utilized a steering box with a pitman arm that swept left to right, as opposed to fore and aft. With one of these boxes mounted far forward on the inside of the left framerail, the drag link can run laterally across the chassis where it connects to the steering arm on the right spindle. This system, called cross-steering, allowed the use of a long drag link that was unaffected by bumpsteer, so the car would not only drive better and be safer, the owner could also use whatever type of radius rod he wanted without worrying about the steering setup (see illustration 3 and 13). Cross-steering is nothing new, as Ford utilized it on the Model T and the '35-48 models, but for the first time rodders were able to take advantage of this system with a heavy-duty box that they could buy brand new.

"The Vega box is still the best deal for a traditional dropped-axle frontend," James says. "There are a couple of people who make them now under license from Saginaw, including Mullins and Flaming River." Chapouris agrees, explaining that cross-steering allows builders to take full advantage of all the different parts and possible setups available through the aftermarket, since it not only makes the car safer, it takes bumpsteer out of the equation.

FINAL SETUP AND ALIGNMENT
The way Fords were originally set up with a drag link running fore and aft didn't force the frontend to one side or the other when you turned the wheels. With cross-steering, however, when you turn the steering wheel, the drag link's pushing against the inside of the right spindle not only turns both wheels, it also has a tendency to put a load on the shackles that hold the spring to the axle, thereby trying to literally shift the chassis laterally over the axle. The way to eliminate this problem is by using a Panhard bar, which ties the axle to the frame with a pivot point on either end (see illustration 13 for further details).

Once you have chosen an axle, spring, radius rod design, steering setup, and Panhard bar, the only thing left is to align everything so the car will travel down the road straight with a minimum of effort. The first thing to adjust is the camber, which is essentially the amount the tire leans in or out at the top. The only way to adjust this with a solid axle is by bending it, but don't fret, the process is painless. Stock axles were made with 1.5 degrees of positive camber, meaning the top of the wheel sticks out 1.5 degrees farther than the bottom. This is the perfect setup for a bias-ply tire and is the standard even today for all new axles. With radial tires neutral camber (zero degrees, or straight up and down) is optimal, so the axle will have to be pulled down a bit in the center. Any good commercial alignment shop can accomplish this, as new big rig trucks still utilize solid-axle front suspensions. In most cases this can even be done without harming the chrome plating or paint, as long as the shop in question is careful and uses insulating items between the clamps and the axle.

Next up on the alignment checklist is caster, which is the amount the axle is leaned back. Stockers had between 3 and 4 degrees of caster, but once you lower a car and slap on some big meats, that number usually comes down to a degree or so. The farther an axle is leaned back, the more stable the car will become, but the turning radius increases as well. About 6 to 8 degrees of caster is appropriate on a car with modern tires driving on modern roads.

Finally the toe needs to be set, which is a measurement of whether the tires are pointed straight down the road or angle in towards each other or out towards the curbs. Hot rods should run about 1/8 inch of toe-in, meaning the fronts of the tires should point just slightly in toward each other. This will make the car run tack straight down the road.

Special thanks to Pete Chapouris at So-Cal Speed Shop, Fred James at Magnum Axle Co., and Jerry Slover at Pete & Jake's Hot Rod Parts for their help with this story.