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Re: 18mm vs 22mm
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Posted by REM in Seattle [Email] (more from REM in Seattle) on Wed, 14 Jan 2004 10:00:35 Share Post by Email
In Reply to: 18mm vs 22mm, Geoff, Tue, 13 Jan 2004 14:22:54
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The following is the technical description of the NG900/9-3 rear axle assembly from the Saab literature:

Twist-Beam Rear Axle

“The new Saab 900 rear axle is designed to provide dynamic roll oversteer, which Saab engineers believe gives better straight-line stability and more positive turn-in control than roll understeer. This makes the Saab 900 more responsive, and at the same time more damped in its directional movements--it's a characteristic that has been a fundamental part of the Saab handling philosophy for many years.”

“The H-shaped axle side arms are of a heavy gauge metal, and additional welds reinforce the assembly. This design is lighter than its predecessor and provides more longitudinal compliance, to further improve ride comfort. The front ends of the side arms are connected to the body through large-diameter, well-tuned rubber bushings, with direct-acting compact, taper-wound coil springs. Separate gas-pressurized telescopic shock absorbers are attached behind the wheel hubs, while maintenance-free wheel bearings are mounted inside welded hub carriers.”

“For fine-tuning the roll stiffness at the rear, a separate stabilizer bar is mounted inside the open-channel cross tube of the rear axle assembly, with a second stabilizer bar bolted externally behind. Extra plates are welded to the rear axle mounts to improve lateral stiffness.”

So the rear sway bar being replaced is only one of two sway bars that make up the Saab rear axle assembly. The stock rear sway bar on my 1998 900SET was 15mm in diameter. The formula for torsional stress says that, all other dimensions being equal, the force from the bar is proportional to the diameter of the bar raised to the 4th power (D^4). So if the stock 15mm diameter bar is set as the baseline, a 18mm diameter bar will be 2.1 times as stiff, a 20mm diameter will be 3.2 times as stiff, and the 22mm diameter bar is about 4.6 times as stiff as the stock 15mm diameter bar.

However, you also ned to take into account the balance between the front and rear suspension. While the 22mm diameter rear sway bar works well with the base and SE suspension; that will not be the case with other spring rates. Lowering springs are typically stiffer than stock springs, and the change in stiffness may not be the same for the front and rear springs. For example the Viggen has 25% stiffer springs in the rear while the front springs were stiffened only 5% and the front sway bar diameter was reduced 2mm, to 24mm. So fitting a 22mm diameter rear sway bar to the viggen would upset the front/rear handling balance. A 18mm sway bar would be a better choice in that instance.

Rob Mustard (aka REM in Seattle)
1998 Saab 900SET 5-door; Automatic; 65,000 miles
1991 Alfa Romeo Spider 5-sp; 94,000 miles
1984 Saab 900T 3-door; 5-sp; 242,000 miles RIP



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