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A radius rod (also called a radius arm, torque arm, torque spring, and torsion bar) is a suspension link intended to control 
Other suspension components can be made to act as radius rods under certain conditions. For example, the rear suspension of the original 1966
wheel
A wheel is a rotating component (typically circular in shape) that is intended to turn on an axle Bearing (mechanical), bearing. The wheel is one of the key components of the wheel and axle which is one of the Simple machine, six simple machin ...
motion in the longitudinal (fore-aft) direction. The link is connected (with a rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds.
Types of polyisoprene ...
or solid bushing) on one end to the wheel carrier or axle
An axle or axletree is a central shaft for a rotation, rotating wheel and axle, wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In ...
, on the other to the chassis
A chassis (, ; plural ''chassis'' from French châssis ) is the load-bearing framework of a manufactured object, which structurally supports the object in its construction and function. An example of a chassis is a vehicle frame, the underpart ...
or unibody
A vehicle frame, also historically known as its ''chassis'', is the main supporting structure of a motor vehicle to which all other components are attached, comparable to the skeleton of an organism.
Until the 1930s, virtually every car had ...
of the vehicle. Radius rods are typically mounted ahead of the wheel. In that position they resist dive under braking
A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.
Background
...
forces and wheel hop under acceleration
In mechanics, acceleration is the Rate (mathematics), rate of change of the velocity of an object with respect to time. Acceleration is one of several components of kinematics, the study of motion. Accelerations are Euclidean vector, vector ...
.
Radius rods are also sometimes used in aircraft with fixed (non retractable) undercarriages. Radius rods in aircraft must be regularly inspected because their failure will cause unrestrained swerving of the wheel.
Radius rods are customarily made of stamped steel
Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
or aluminum
Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
for lightness, as they are part of the vehicle's unsprung weight
The unsprung mass (colloquially unsprung weight) of a vehicle is the mass of the suspension, wheels or tracks (as applicable), and other components directly connected to them. This contrasts with the sprung mass (or weight) supported by the ...
. Improvements in composite materials
A composite or composite material (also composition material) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a ...
technology make possible plastic suspension links, although they are still uncommon.
Other suspension components can be made to act as radius rods under certain conditions. For example, the rear suspension of the original 1966 Oldsmobile Toronado
The Oldsmobile Toronado is a personal luxury car manufactured and marketed by the Oldsmobile division of General Motors from 1966 to 1992 over four generations. The Toronado was noted for its transaxle version of GM's Turbo-Hydramatic transmissi ...
used shock absorber
A shock absorber or damper is a mechanical or hydraulics, hydraulic device designed to absorb and Damping ratio, damp shock (mechanics), shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typic ...
s to connect the rear beam axle
A beam axle, rigid axle, or solid axle is a suspension (vehicle), dependent suspension design in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once commonly used at the rear wheels of a vehicle, but hist ...
to the frame; these shocks controlled longitudinal wheel movement, but their damping action prevented the forces of those movements from being felt by the passengers.
On a vehicle with longitudinal leaf spring
A leaf spring is a simple form of spring (device), spring commonly used for suspension (vehicle), suspension in wheeled vehicles. Originally called a ''laminated'' or ''carriage spring'', and sometimes referred to as a semi-elliptical spring, e ...
s, such as a Hotchkiss drive
The Hotchkiss drive is a shaft drive form of Transmission (mechanics), power transmission. It was the dominant means for front-engine, rear-wheel drive layout automobile, cars in the 20th century. The name comes from the French automobile manufact ...
suspension, the springs themselves provide some longitudinal wheel control. One or more torque arms may be added to provide supplementary wheel control, particularly for vehicles with strong torque
In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...
such as drag racers. On some leaf spring-equipped vehicles, such as 1960s Chrysler Corporation
FCA US, LLC, doing business as Stellantis North America and known historically as Chrysler ( ), is one of the " Big Three" automobile manufacturers in the United States, headquartered in Auburn Hills, Michigan. It is the American subsidiary of ...
cars, the springs are mounted so that a lesser portion of the spring's length is forward of the wheels, improving wheel control on acceleration. A side effect of such positioning is that the longer, aft portion of the springs may not be stiff enough to control wheel movement under braking forces, which can lead to pronounced wheel hop in hard stops.
References
{{DEFAULTSORT:Radius Rod Automotive suspension technologies