Many rear-drive cars and trucks use a live axle at the rear -- that is, the rear axle incorporates the differential and halfshafts into a single rigid unit that moves up and down with the rear wheels. Live axles are cheap and rugged, and they ensure that the camber of both rear wheels remains constant as the wheels move through their suspension travel. (The main drawback of live axles is that they are heavy, and that mass is part of the vehicle's unsprung weight, which is detrimental to ride and handling.)

A live axle must be located -- that is, its movements in all planes must be limited -- and it requires some means of transmitting the torque generated by the wheels to the body of the vehicle.

There are two basic methods for transmitting drive torque:

• Closed driveshaft (torque tube): The first option is to enclose the driveshaft in a torque tube, connected rigidly to the axle housing and linked to the transmission via a single universal joint (which allows it to move relative to the transmission without affecting the shaft's rotation). Drive forces from the wheels are transmitted through the rigid torque tube to the transmission mount. Torque tubes were fairly common until the early 1960s (they were used by, among others, American Motors, Buick, and Chevrolet), but they fell out of favor because the torque tube adds significantly to the unsprung weight.
• Open driveshaft: The second option is to leave the driveshaft open and use a universal joint on each end, allowing the axle to move relative to the driveshaft. Since the linkage between the axle and the driveshaft is now flexible, the driveshaft cannot transmit drive torque. Instead, the axle requires one or more trailing arms (also known as radius rods) to carry drive forces to the body.

Regardless of whether the driveshaft is open or enclosed, the rear axle's motions must be limited so that the rear wheels remain firmly on the ground as much as possible. Any time the vehicle accelerates, decelerates, or changes direction, forces are exerted on the axle assembly that attempt to move or twist it in different directions. Acceleration causes the axle to squat and axle windup (twisting the entire axle assembly, as well as the wheels); if the windup is severe, it can cause axle tramp, where the entire axle hops up and down. The same forces can also steer the rear axle, changing the direction of the rear wheels. On deceleration, the rear axle tries to rise as weight shifts forward (called brake dive, because it causes the nose to dip as the tail rises), sometimes causing the rear wheels to break traction with the ground and hop. Meanwhile, in turns or on uneven pavement, lateral forces attempt to displace the axle assembly laterally. For the axle to do its job properly, it must be connected to the body with locating members -- control arms or links -- that exert leverage to resist these forces.


Simplified diagram of a five-link rear suspension. Lower links (yellow) transmit drive torque to the body; upper links (orange) stabilize the axle; a Panhard rod (green) locates the axle laterally. Many GM and Ford cars of the sixties and seventies used a four-link version of this system, omitting the Panhard rod and angling the upper arms toward the centerline of the body, so that they would provide lateral location. (Diagram © 2007 Tennen-Gas; used under a Creative Commons Attribution ShareAlike 3.0 license)

Automakers have developed a number of common methods of locating a live axle. One approach, popular at GM for many years, uses four trailing arms, two above the axle, two below it, angled inward so that they resist lateral motion of the axle. Another approach, used by Buick in the 1960s, uses two lower control arms and a single upper arm, mounted next to the differential, with a lateral track bar (a Panhard rod) or parallelogram linkage (Watts linkage) to limit lateral motion. Both the three-link and four-link layouts are reasonably effective, but the control arms and track bars make the rear suspension more complex, and thus more expensive to build.

A simpler third option is Hotchkiss drive. In a Hotchkiss layout, the axle is suspended by a pair of longitudinally mounted semi-elliptical leaf springs, which serve to locate the axle, as well as supporting the weight of the body. The front portion of each spring functions like a trailing arm, transmitting drive torque to the body and resisting squat and axle tramp. The rear portion of the spring acts as a leading arm, resisting wheel hop under braking. The stiffness of the springs also serves to resist lateral motions. By making the springs perform multiple duties, Hotchkiss drive is very simple, and thus very cheap. Since it has few parts, it's also very sturdy, which is useful for heavy-duty vehicles like trucks.


Diagram of a Hotchkiss drive layout. The leaf springs (yellow) act as both springs and control arms, locating the axle and transmitting drive torque to the body. (Diagram © 2007 Tennen-Gas; used under a Creative Commons Attribution ShareAlike 3.0 license)

The drawback of Hotchkiss drive is that while the springs can perform all these various functions, they don't necessarily do them well. The flexibility of a leaf spring limits its usefulness as a control link; if the spring isn't very rigid, it will move in response to the various forces on the axle, rather than resisting them. Making the springs stiffer makes them more effective in controlling axle movement, but it also makes the ride firmer, sometimes uncomfortably so. The more powerful the engine, the greater the problem. Torquey engines like the big-block V8s of the muscle car era can exert so much force on the axle that the only ways to adequately limit axle movement are to (a) make the springs brutally stiff or (b) add auxiliary control arms (popularly known as "traction bars") to help control the axle, which is anathema to the whole rationale for using Hotchkiss drive in the first place.


A U.S. Army Jeep shows off its Hotchkiss-drive suspension. (2006 U.S. government public domain photo; source)

One stopgap method, which Chrysler used for many years, is to change the position of the axle on the springs. The spring rate of a leaf spring is proportional to its length. If you move the axle forward, toward the leading ends of the springs, the front section of the spring will be shorter, and thus stiffer, allowing it to better control axle tramp without making the ride harsher. The drawback is that the opposite is also true. The rear portion of the springs (the section aft of the axle assembly) will be softer, and thus less able to resist wheel hop on deceleration. Powerful Chrysler cars of the sixties had good axle control on acceleration, but were prone to violent wheel hop on hard braking, sometimes with harrowing results.

Another stopgap, employed at various points by Chevrolet and Ford, among others, is to "stagger" the rear shock absorbers, mounting one ahead of the axle, the other behind it. In this way, the shock absorbers are made to perform double duty, resisting axle tramp. Staggered shocks can be reasonably effective for street cars, but may not be adequate for really high-powered applications like drag racing.

Whence the name "Hotchkiss drive?" The layout was popularized by the French firm of Hotchkiss et Cie as early as 1905, devised by engineer Georges Terasse. Few things in the automotive world are ever really new, however, and similar layouts had previously been used by other automakers, including Cleveland's Peerless Motor Company. Nevertheless, the name has stuck, even if it's not entirely accurate.

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