Torsion bar or Coil Spring?

[ve7brz]

This thread is intended strictly for discussion. Some thirty-five years ago I had an argument with the proud owner of a Crysler produce who claimed the torsion bar front suspension was superior to a coil spring suspension. I argued that a coil spring is simple a torsion bar in disguise. My agument was that the twisting of the wire in the coil was the dominant factor, not the small bending moment produced under deflection. Comments?

 

[GregLocock]

You were right. A coil spring is a torsion spring, when properly designed.

Major reason why torsion springs are used: neat package with some suspension layouts

Major reason why coil springs are used: torsion springs have poor control over ride height in service due to fretting of the sockets in which the spring is held. Cheers

Greg Locock

 

[ve7brz]

Tx Greg

 

[jambel]

The problem of transmitting a torque or rotary motion from one plane to another is frequently encountered in machine design. Typically, circular bars are used for such transmissions chiefly because in these bars a plane section before twisting remains plane after twisting, i.e. there is no warping of the section after loading. You should be interested in determining the maximum load a circular bar is capable of sustaining, the stresses in the bar as well as the deformation or twist of the bar. To do this, you need to establish the relationship between the applied torque, the stresses and the deformation of the bar.
Consider the bar of circular cross-section twisted by couples T at the ends. Because the bar is subjected to torsion only, it is said to be in pure torsion.

Bar Subjected to Torsion
Assuming that the end is fixed, then the torque will cause end the other end to rotate through a small angle f, known as the angle of twist. Thus the longitudinal line on the surface of the bar will rotate through a small angle to position ¢.

Consider a small element abcd of length dx within the elemental segment of the bar, the magnitude of shear strain, g, is given by where r is the radius of the circular bar and df is the angle of rotation of one cross-section with respect to another. df/dx is the rate of change of the angle of twist and representing this by q, we obtain where q is the rate of change of f with respect to length, i.e. angle of twist per unit length.
For the case of pure torsion, the angle of twist per unit length is constant along the length of the bar, hence

(Note) that the above expression is based only on geometric concepts and is therefore valid for any circular bar in pure torsion, irrespective of its material behaviour (elastic or inelastic, linear or non-linear).
For linear elastic materials, shear stresses are proportional to shear strains and the constant of proportionality is the modulus of rigidity, G. Hence ie.

To determine the relationship between the applied torque T and the stresses it produces, I consider equilibrium of the internal forces and the externally applied torque, T. Considering an elemental area dA within an elemental ring of thickness dr situated at radius r from the centre:
If the shear stress in the elemental area is t, then the shear force on this area is tdA. Moment of this force about the axis of the shaft/bar is (tdA)r = Gqr2dA. That is dF = tdA dM = (tdA)r = Gqr2dA
Total resisting torque about the axis of the shaft is the summation taken over the entire x-sectional area, of the moments of all individual elements, i.e. where = polar moment of inertia = Ix + Iy For a circular x-section of radius r
The product GIp is known as torsional rigidity while GIp/L is called the torsional stiffness, defined as the torque required to produce a unit angle of rotation if one end of the bar with respect to the other end. The torsional flexibility is the reciprocal of the torsional stiffness and is defined as the angle of rotation required to produce a unit torque.

 

[mloew]

Jambel,

What is the source for the technical content of your post?

Best regards,

Matthew Ian Loew

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jambel]

40 years plus desigin engineering OJT

 

[contritebarbarian]

Any comments on increasing side intrusion stiffness and/or isolating the passenger compartment from any frontal crash by way of a nice and long torsion bar?

 

[BillyShope]

You might also point out to your Mopar friend that the major contributor to the handling difference between the '56 and '57 Chrysler products was not the change to a torsion bar front suspension, but the change to the rear leaf spring design, resulting in a tendency toward roll oversteer. (Which, as poined out in the Student Workbook for Race Car Vehicle Dynamics, is merely a ruse to fool the driver, as tire loads remain unchanged.)