Twin Traction Beam (commonly written "TTB") is a type of independent front suspension used on 4x4 (four wheel drive) trucks. It is descended from the Ford Twin I Beam suspension of the 1950s, and has been found on Ford, Toyota, and Jeep vehicles. In both cases, rather than a solid front axle ("live axle") the axle is broken into two suspension members. In rear wheel drive trucks, the members on early Ford vehicles (and others) literally were in the shape of a tapered I-beam. In four wheel drive trucks, the differential is integrated into one member, while a short axle shaft with U-joints transfers power to the other wheel. Both the two and four-wheel drive versions of this suspension are intended to provide maximum suspension travel with minimum camber or track change.
With a straight axle, camber of both wheels is inextricably linked. When one wheel's camber goes positive or negative a number of degrees, the opposite wheel performs the equal and opposing camber shift. Independent suspension decouples right- and left-wheel camber partially or completely in order to improve handling through an increase of traction. In the rear wheel drive Twin I Beam design, the left axle member is attached to the right frame rail, and vice versa. In Twin Traction Beam designs, both members are generally attached to drop brackets attached to a front bracket. They can utilize leaf springs, or with an additional member run back to the frame, coil-type springs.
TTB's greatest strength, then, is suspension travel as compared to other independent front suspensions. Unfortunately, it does have a number of down sides. Like the Twin I Beam from which it is descended, camber is tied to the position of the suspension. As the suspension compresses, wheel camber becomes more negative. However, mounting the axles to the opposing sides of the vehicle (even just a few inches, as in the 1990s Ford pickups) can reduce or eliminate the problems with body roll and camber as seen in swing axle suspension, e.g. on the Volkswagen Beetle.
Most modern four wheel drive trucks utilize a double wishbone front suspension, with a subframe-mounted differential. In this design, upper and lower suspension arms connect to the frame and a front subframe to which the differential is also attached. This provides reduced unsprung mass as compared to TTB. However, in order to provide the same amount of suspension travel, a double wishbone design will either need to utilize arms of inequal length, deliberately causing camber change, or experience significant wheel thrust as the suspension is compressed, resulting in a change in track which results in odd steering effects.
Another advantage of the TTB suspension is stability. Because the axle is longer, there is simply reduced pivot and even body roll with such a suspension. This produces a vehicle which is considered by some to be superior for high-speed travel off-road. TTB is superior to other independent front suspension types for rock crawling and the like due to increased travel, but still cannot compare to the incredible travel of a live axle. In racing, it is often used with a portal gear axle, which increases travel through the use of a geared relocation of axle output. Portal gears were most commonly used with live axles in the Unimog truck produced by Mercedes but they are available as a bolt-on upgrade for many Dana (and other) axles.
Most of the TTB systems used in the US were produced by Dana; early Jeeps and Rangers have a Dana 28 axle known to be weak, but later examples of both got the Dana 35 which is much more credible. Full-size pickups like the Ford F250 and Jeep Wagoneer carry the Dana 44 until the late 1990s, when the Dana 50 became available. Even then, Ford only put the Dana 50 under the Super Cab and not regular cab F250; F350 has the straight Dana 60. The Dana 44 is considered weak for even half-ton off-road applications, but the Dana 50 is typically considered to be almost the equal of a straight axle.