Angular deflection of a torsion bar is pretty easy to visualize. A thicker bar or stronger steel will produce less deflection for the same load. It may surprise some to know that a coil spring acts pretty much like a torsion bar wrapped into a coil. Torsion bars offer easier fabrication, easier adjustment of ride height and some advantages in packaging. Otherwise, there’s not much difference.
The comments about corrosion problems reminded me that a torsion bar in my ’61 Valiant broke as I was backing out of a parking place—a couple of years after purchasing it new. The black tar-like protective coating on the bar had been chipped—probably from kicked-up road trash. Omaha salt did a job on the exposed steel and I got a beautiful classic fatigue failure. That brought the reality to my courses in strength of materials and statics. With the Valiant, I was able to remove the broken ends and motor on in to the dealer for a free replacement. It’s possible that torsion bars may be more exposed to damage than coil springs that are somewhat hidden. I don’t recall coil springs being coated except by overspray from rustproofing the bottom of the car.
Long torsion bar life depends on its metallurgy, original surface finish, coating and protection from road trash damage. Therefore, it would be a good idea for most of our ’57 to date 300’s (I think the 300M’s had struts) to inspect the torsion bars’ coating for continuity. Holes in coating are called “holidays” in pipelinese and should be cleaned and the underlying steel carefully inspected for corrosion pits or physical damage like a nick. These can greatly concentrate stress at that point and lead to premature and unannounced failure. Tolerance for this kind of damage is zero.
Brentwood, CA (Saw 103 F here a little earlier today but a warming trend is predicted)
OK - so Torsional stiffness Torque/Rotation = KG/L where K = Pi/2 x (Radius)4 ---- (Thats radius to 4th power or squared twice - cant seem to make superscripts work) and G is a constant for steel and Rotation is in radians.
But all you want to know is difference in stiffness, so take G as a constant and just look at difference in R4th/L
And since L is constant here at 40 in, difference in stiffness is simply (.505)4 / (.495)4 or 1.08329 - translated 8.3% difference in stiffness between 300 Sport (or Newport Sedan / Conv, etc.) at 0.99 and H at 1.01
H vs New Yorker 12.8% stiffer
H vs Newport wagon 17.5% stiffer
And by the way, to convert to stiffness you need distance from center of rod to centerline of tire - if you want absolutes, need to go thru more calcs incl dealing with G Shear Modulus and radians.
But unless you are playing with different wheel offsets (going to aftermarket wheels - moving out increases moment arm and increases torsion which effectively "softens" car spring rate - moving in effectively "stiffens" but you get interference) or changing hub dimensions (such as may be if changing to disc brakes, etc).
PS - If you are also playing with Anti-sway bars on front only, it would not be unusual to soften spring rate if increasing sway bar stiffness on that end - otherwise you would need to play with back of car to keep handling balance (oversteer / understeer).
For example, if you add a rear sway bar to a car that did not originally have one, and you do nothing else (no change to rear spring rate or better front bar increase in stiffness to balance handling), be prepared for rear end to come around fast in a hard turn - potentially severe oversteer condition. (Personal experience, lesson learned)
Edward Mills Antique Tractors 1930-1960 Antique Cars 1960-1985
On 6/20/2016 1:45 PM, 'Rich Barber' c300@xxxxxxx [Chrysler300] wrote:
Posted by: "Rich Barber" <c300@xxxxxxx>
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