Anti Sway Bars 101 (LXi and LX)

[davidbrady]

We've had many discussions on this forum and on other forums about the anti-sway bars used on LXi wanderlodges.

There's a few things that bear mentioning.

1) just like air springs, the sway bar acts thru the tires. This means that we need to look at forces at the wheel, or wheel rates.

Our beloved Ridewell stick axle steer suspensions have a motion ratio of about 1/2; that is, for a 1 inch motion at the wheel the air springs are compressed 1/2 an inch. Likewise, for a 1 inch motion at the wheel, the sway bar is deflected 1/2 of an inch.

2) there's a characteristic of suspensions called "roll stiffness".

If you lean on the top corner of the bus, up by the goat rails, the bus will roll over slightly on it's lengthwise axis. The degree to which the bus tries to remain upright (vertical) is roll stiffness and it's measured in torque per degree of rotation, or ft-lb/deg.

3) the bus chassis itself has a resistance to twist called "torsional stiffness" which is also measured in torque per degree, or ft-lb/deg.

A while back when I was doing my Finite Element Analysis on Woody's slideout conversion I discovered that our Wanderodges have a torsional stiffness of around 12000 to 15000 ft-lb/deg.

4) as the anti-sway bar does it's job it twists the bus chassis.

The job of the sway bar is to keep the steer axle parallel to the bus chassis; that is, it resists with it's spring force any rotation of the bus body relative to the steer axle. To do this the sway bar applies a countering torque to the bus body. This means that any sway bar deflection will result in bus body deflection in the twisting sense (think wringing out a bath wash cloth).

The question then is how much bus body twist should be allowable for a given anti-sway bar twist? If both the bus body's and the sway bar's torsional stiffness is 15000 ft/lb per deg, then for every degree rotation of the bar you'd have one degree of twist in the bus chassis. This isn't good. This kind of bus body twisting will wreak havoc with suspension geometries, ride quality, handling, and wear and tear on the bus body and interior components. A reasonable maximum for anti-sway bar torsional stiffness as a percentage of bus body torsional stiffness is 10%. This means that if the sway bar twists 1 degree the forces transmitted into the bus body result in a twist of only 1/10 of a degree.

If our bus body has a torsional stiffness of 15000 ft/lb per degree, then we need to limit our suspension's torsional stiffness (acting thru the wheels) to 1/10th of that or 1500 ft/lb per degree.

So what's wrong with the sway bar used in the LXi, well it has a torsional stiffness at the wheels of around 1200 ft-lb per degree. Couple this with the 1200 ft-lb/deg of torsional stiffness from the air springs brings us to a total torsional spring rate of 2400 ft-lb/deg or roughly 16% of the bus body's torsional resistance.

In short, the anti-sway bar adds excessive roll resistance such that the bus body is being twisted like wringing out a towel.