A radius rod (also called a radius arm, torque arm, torque spring, and torsion bar) is a suspension link intended to control wheel 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. Thailand, Malaysia, an ...

or solid bushing) on one end to the wheel carrier or axle, on the other to the chassis or unibody of the vehicle. 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 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. Backgroun ...

forces and wheel hop under

acceleration In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by t ...

. Radius rods are customarily made of stamped steel or

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

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 th ...

. Improvements in

composite materials A composite material (also called a composition material or shortened to composite, which is the common name) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or ...

technology make possible plastic suspension links, although they are still uncommon. Lotus 49C Motor links

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 used shock absorbers to connect the rear

beam axle A beam axle, rigid axle or solid axle is a 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 historically they have als ...

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 commonly used for the suspension in wheeled vehicles. Originally called a ''laminated'' or ''carriage spring'', and sometimes referred to as a semi-elliptical spring, elliptical spring, or cart spring, i ...

s, such as a Hotchkiss drive 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 equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...

such as drag racers. On some leaf spring-equipped vehicles, such as 1960s Chrysler Corporation 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