What is a Torsion Bar?
In this brief article, we will discuss the Torsion Bar, its features, advantages, and applications.
As per the Britannica definition-
“..torsion bar, rod or bar that resists twisting and has a strong tendency to return to its original position when twisted. In automobiles a torsion bar is a long spring-steel element with one end held rigidly to the frame and the other end twisted by a lever connected to the axle. It thus provides a spring action for the vehicle..”
The Torsion Bar Suspension uses a Torsion Bar instead of a helical Coil Spring as the main Load-bearing member in the suspension. The Torsion Bar suspension could very well be called a ‘Variation’ of the Double Wishbone suspension since the basic setup is the same.
Like a Steel Coil Spring or leaf Spring, the Torsion bar is yet another suspension element that absorbs energy of road bumps coming through the tires and converts this into elastic deformation.
In the case of coil springs, the elastic deformation happens in the form of compression and expansion, whereas in the case of the torsion bar, the elastic deformation happens in the form of a twist in the torsion bar.
What are the purposes of a torsion bar?
The 2 main purposes served by a torsion bar in the vehicle suspension are:
- A main spring element
- A Roll Resistance element or Anti Roll Bar
Torsion Bar as the main Suspension Spring
The Torsion bar is basically a long Rod that twists and resists the vertical suspension motion, giving the suspension its vertical spring rate. In most cases, the attachment ends of torsion bars are splined. One end of the torsion bar is fixed onto the Chassis, while the other end ends at a lever, also known as a ‘Torsion Key’, which bolts onto the Suspension Lower Control Arm.
The lower control arm’s chassis attachment point can be considered as a hinge, which is both in line with and fixed to the Torsion bar at its axis. So whenever the wheel moves vertically up and down, the lower control arm rotates about its hinge, twisting the torsion bar in the process.
The stiffness of the suspension is achieved by converting the Torsion bar’s resistance to twist motion, into a vertical resistance at the wheel by using the lower control arm as a ‘Lever’.
Unlike a Coil Spring type of suspensions, which has a fixed installed, the position of the torsion bar key has to be adjusted in order to vary the car’s corner ride height. There usually is a threaded arrangement to alter the rotational spline position of the torsion bar in relation to the chassis.
The ride height of the vehicle can be either raised or lowered by rotational adjustment of the torsion bar’s “Zero-twist” rotational position. The spring rate of torsion bar is a direct derivative of the bar’s overall diameter, length, and its basic material composition.
Torsion Bar as an Anti Roll Bar
While negotiating a turn, the sprung mass of the vehicle’s body produces a lateral force effectively at the center of gravity (CG), in direct proportion to lateral acceleration. In a majority of passenger vehicles, the CG lies outside of the roll axis (The roll axis is a line that joins the front and rear roll centers). As a result, the lateral force creates a moment acting about the roll axis that tends to roll the body.
Body Roll is resisted by the suspension roll stiffness, which is a function of the vehicle’s spring stiffness and the stiffness of the anti-roll bars, if any.
The “Understeer” or “Oversteer” behavior of the vehicle can be engineered by changing the proportion of the total roll stiffness that comes from the front and rear suspensions. Increasing the proportion of roll stiffness at the front increases the proportion of the total load transfer that the front axle reacts to. This results in an understeer effect. Increasing the proportion of roll stiffness at the rear axle has the opposite effect and decreases understeer and increases oversteer.
Advantages of the Torsion Bar as the main Suspension Spring
Ease in Ride Height adjustment
- The Torsion bar suspension’s biggest advantage is in the ease of Ride Height adjustment. The torsion bar’s spline can be rotationally adjusted at the chassis end without dismantling the wishbone mountings or knuckle ball-joints.
- Off-road vehicles that have a front double-wishbone suspension, often need to be raised in ride height for better articulation. There are limitations in the extent to which ride height can be raised because of potential interference between the Upper control arm and the Coil Spring in extreme rebound. This problem is solved by eliminating the Coil Spring in the case of the Torsion Bar suspension.
Advantages of a Torsion Bar for Roll Resistance
- The Torsion Bar used as an Anti-roll bar allows body roll reduction without increasing the suspension spring stiffness. This implies that body control can be improved without much of a compromise on ride quality.
- A Torsion Bar, used in the form of an anti-roll bar, can also be used to tune the handling balance of a car, by increasing or decreasing the stiffnesses of the front and rear anti-roll bars and proportioning them to get the desired vehicle dynamic response.
What are the disadvantages of Torsion Bar suspension?
- The Torsion bar spline mechanism and lower control arm hinge need to be greased or lubricated regularly. The Coil Spring, in contrast, does not require any maintenance as such.
- The Torsion bar occupies a considerable length along the chassis. It is more suited to a body-over-chassis type of vehicle like an SUV. The torsion bar would not be suited to small wheelbase monocoque cars.
What Cars Use the Torsion Bar as a Suspension
The origins of the Torsion Bar Design approach can be traced back to Ferdinand Porsche’s patent in 1931 for Torsion Bar suspension, that was meant for Racing Prototype cars mainly. But it was not until 1948 that Porsche implemented its own invention on a series production sports car, the Porsche 365.
The first passenger car implementation of this concept was in the Citroën Traction Avant (1934), which had an independent front torsion bar suspension. Later on, Armored combat vehicles like the T-72, Leopard 1, Leopard 2, M26 Pershing, M18 Hellcat, and the M1 Abrams from World War II. The Torsion Bar was commonly used as the preferred choice of suspension set up on these World War II vehicles.
In the post-World War era, a notable example was Tatra’s 1948 T600 Tatraplan which incorporated a rear torsion bar suspension.
Though most of the vehicles today mostly have a strut suspension or air suspension, there were many modern-era high selling cars that had a Torsion Bar type of front suspension. Some of the examples were:
- Volkswagen Beetle upto 1966 model year, after which the suspension changed to Coil Spring
- Early Porsche Racing prototype cars
- GM Light-duty 4-wheel drive trucks from 1988- 2007 model years. Eg. GMC Sierra, Chevy Silverado
- Ford Ranger 4WD variant from 1998-2012 (Mazda B-Series based) model years
Apart from these, many American-built trucks and SUVs from Chrysler, Mitsubishi, Mazda, Nissan, Isuzu, and Toyota used a Torsion Bar setup in the front suspension.
In this brief article, we have discussed the Torsion Bar, its features, advantages, and applications.