In 1965, Ford introduced a novel and interesting independent front suspension for trucks known as the Twin I-Beam. Here’s how it works.
When Ford introduced its Twin I-Beam front suspension for light trucks in 1965,.the idea wasn’t new, of course. Swing-axle front ends had been around for a good long while. For example, in 1933 the British engineer Leslie Ballamy (1903-1991) created an inexpensive IFS conversion for the Austin 7 and Ford light cars by splitting the beam axle in half and adding pivots at the center (above). While the arrangement had some imperfections, including adverse camber change in roll, it was light, cheap, simple,and durable. And on a number of British sports cars from Allard to Lotus 11, it got the job done.
Ford’s Twin I-Beam front end for trucks was a swing axle with a difference. Here, the axles were offset and overlapping, improving the geometry. Each beam, a heavy chrome-moly steel forging just like a conventional straight axle, was approxmately 36 inches long, reducing the camber change at the wheel to a manageable level as the suspension travels through its range of motion.
Also: Instead of a pair of leaf springs to provide longitudinal location and take the steering and braking loads, as with a conventional truck axle, there were a pair of radius arms (also heavy steel forgings). Relieved of the braking and steering loads, the coil springs could be of a lighter rate, producing a softer, more comfortable ride and more responsive handling. Twin I-Beam was introduced on F-100 through F-350 models in 1965, while the heavier trucks continued to use conventional beam axles.
A simple straight axle and leaf springs had worked just fine on light trucks for many years. Why would a pickup need independent front suspension? The same reason passenger cars adopted it a few decades earlier: improved ride and handling. (See our features on IFS history here and here.) By the early ’60s, many owners were using their pickups for daily transportation, and passenger-car comfort could be a powerful selling feature. “You’ve never ridden in any pickup as comfortable—yet as rugged—as the ’65 Ford,” the ad writers declared.
The setup was continually refined through the years, with ball joints eventually replacing the traditional kingpin steering knuckles. In 1980, Ford adapted the concept to four-wheel drive with a setup called Twin-Traction Beam or TTB (above) with a pair of steel housings replacing the forged I-Beams and the drive hardware supplied by Dana.
Long story short: With the Twin I-Beam Ford sought to combine the ruggedness of a beam axle with the superior dynamics of independent suspension. (For a deeper dive into the engineering, there’s a 1965 SAE paper, 650153.) While its unfamiliar features have thrown mechanics some curves through the years, the system has proven out on millions of vehicles over zillions of miles. Ford’s Super Duty F-250 and F-350 trucks continue to use a version of Twin I-Beam front suspension to this day.
1968 Ford F-100
What a great article, thanks. My family had lots of Ford trucks with twin I-beam but I never gave it much thought. How severe is the camber change, I wonder.
The camber change is significant, about one degree per inch of bump or rebound. It’s not a problem in ride or handling, but if the ride height is altered or degraded, tire wear is the result. Fix the ride height and the static camber spec is usually restored.
These things were the bane of alignment shops everywhere!
If I am not mistaken, the length of each beam was more like 63 “ than your article’s 36.
That’s the full track width.
The length of the beam is effectively determined by the distance from the steering knuckle to the opposite frame rail. The beam mount must be at least as rigid as the beam itself. The authors of the SAE papery dryly noted that the mount was more of a challenge than the beam.
They are strong……but heavy. Probably the reason Ford got away from them in 97 with the redesigned F150. They needed weight reduction for EPA mileage, the double A arm has to weigh half as much as the twin I beam setup.
Although the 97 and later F150s did go to an upper and lower control arm design, as did the Ford Ranger , the larger Super Duty Ford pickups retained the Twin I Beam on 2 WD trucks. My 99 F350 has Twin I Beam, and works just fine.
Beg to differ on the camber gain or loss of one degree per inch. Keeping mind that the swing radius of each arm being 36″ that one-inch deflection at the 36 inches would amount to 1/36th of the arch coupled with the 7 kingpin inclination that still would not be that much. One third of a degree should be closer, don’t you think?