Re: Coupe GT Anti-sway bar for rear

[James Marriott <marriott@micron.net>]

Paul R Anderson wrote:
> 
> Many questions concerning anti-sway bar diameters can be addressed
> by the following statements.
> 
> An anti-sway bar's torsional stiffness (resistance to twist) is directly
> proportional to its cross sectional area.

Almost. It's proportional to the square of the area, or to the diameter
                                 ^^^^^^

raised to the fourth power.

The deflection in degrees is given by:

deflection (twist, deg) = 584 * TORQUE (inch-pounds) * LENGTH (as Paul
said, the twisting part of the beam, inches) / DIAMETER^4
(DIAMETER*DIAMETER*DIAMETER*DIAMETER, inches) / G (torsional modulus,
for all steels 11,500,000 pounds/square inch)

To determine the TORQUE, you need a load, and the load's distance from
the rotational axis of the bar (assuming the link bolt is at a right
angle to the bar's lever arm). Multiply these two together to get the
torque. eg, 50 lb (load) * 7 in (distance) = 350 in-lb. DO NOT double
this number just because you're also twisting the other end of the bar.

> The bar's actual working
> stiffness is also dependent on the length of the lever arm (the
> perpendicular distance from the end of the bar to the torsional axis
> of the bar.  If the arms are twice as long, it will cut the working
> stiffness of the bar in half.  Also to be taken into account is the
> material the bar is made of and whether it is tubular or solid.
> On FWD cars with torsion (T-beam) rear axles such as the
> VW Rabbit/Scirocco/Quantum, and, I believe, the 4000, you already
> have, in essence, an anti-sway bar.  It is T-shaped instead of
> round.  The rear bar adds to the effect of the rear axle beam.

The 4000 ('86, anyway) has an inverted "U" shaped channel, with a round
bar in the bottom of the U (closest to the sky when installed). Chrysler
has copied this design on about 50B minivans, though Audi sure didn't
invent it . . .

One method of stiffening the front (dunno why on a typical FWD) is to
shorten or lengthen one of the AR-bar link bolts. This pre-loads the
bar, kinda like a Charlie washer for the pressurized weenies (hoping to
become a tubular meat-like product myself in the spring ;-) 

> . Addco makes a 1" bar for the front, and the 3/4" rear
> >bar,
> >which seems kind of arse-backwards to me for a fwd car. Perhaps the
> >small diameter of the rear bar is intended to allow for the axle to
> >flex
> >as necessary. Actually, I never knew the rear axle had any movement
> >other than up/down!
> 
> Even though Addco bars are relatively cheap, they seem to work
> well in pairs.  I'd probably go for it.  Try J.C. Whitney for best price.
> That's the only place I've ever bought an Addco from.
> 
> Paul Anderson, Cheyenne, WY  Private email:AndersonPaul@juno.com

hth,
James
NOT A SUSPENSION EXPERT, just a mech eng
'87 4kq, has awful body roll but tracks good enough to PAX 1st
'86 4ks, needs rollers fitted to door handles
'64 Falcon Sprint conv, V-8 & 4 speed, who needs to turn???
Boise, ID, USA