The Torsion Bar Suspension Diagram

In this brief article, we will discuss the Torsion Bar suspension, its layout diagram, advantages, and disadvantages.

What is a Torsion Bar Suspension?

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. 

What are the Parts involved in a Torsion bar suspension? How does the Torsion Bar Suspension Work?

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. 

We will look at the Torsion Bar suspension system taking the Nissan D22 platform Torsion bar suspension as the example. The layout diagram of the Nissan Torsion Bar suspension would have the following parts:

  • Lower Control Arm
  • Support for the Adjusting Arm
  • Torsion Bar
  • Adjusting Arm
  • Bushing for the Adjusting Arm

The chassis mounting location would mostly be on the Transmission mount crossmember. There is usually a hexagonal hole that matches the Bolt head of the Torsion bar. 

The other end of the Torsion Bar ends at a lever, which bolts onto a Splined Slot in 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 suspension, 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. 

The Chassis Anchoring point is actually to a Lever called a “Anchor Arm”. The Anchor Arm lever can be moved up or down by tightening or loosening an adjustment bolt.

The spring rate of torsion bar is a direct derivative of the bar’s overall diameter, length, and its basic material composition.

Advantages of the Torsion Bar over Coil Spring Suspension

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.

Packaging Advantage

  • 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.

What vehicles use a Torsion Bar?

Some well-known and high selling cars that had a Torsion Bar type of front suspension are:

  • 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

What are the disadvantages of Torsion Bar suspension?

Bump Stop Durability

  • The Torsion Bars’ stiffness is linear and cannot be made progressive unlike coil springs which can be modified to behave progressively.
  • The result of this phenomenon is that the torsion bar suspension, would the bump stop more often and hit harder as compared to a coil spring suspension. 
  • Hence, the torsion bar suspension bump stops would need to be designed heavier and more durable.

Maintenance

  • 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.

Packaging

  • 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.

Conclusion

In this brief article, we have discussed the Torsion Bar suspension, its layout diagram, advantages, and disadvantages.

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