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.

[timetravelers]

Thanks for this additional analysis. I took your original advice and tried unhooking one end of the bar to see what happens. (While at Bleakley @ Unadilla, they found the nut at one end of the bar was so stressed, it had jammed the nut and they had to burn it off with a torch.) The difference in ride, negotiating bridge expansion joints was amazing. Instead of a massive clunk, shaking the whole coach, it became a much smoother and a lesser bump. There is more sway in turns, but I happily trade this for the smoother ride.

I removed the whole bar and threw it in the trash while in Maine after traveling north from GA.

The final improvement came last winter, when I finally addressed the shocks, finding one front Koni completely without friction damping. With the new shocks came a another noticeable improvement and now we can drive over interstate bridges with NO FEAR! And if the whole dash could be disassembled, the wood squeaks and rattles originally caused by the sway bar could be eliminated

[cmillsap]

Thanks, David for the information. Tela and I suffered with a lot of bouncing on our initial trip from Florida (where we purchased the bus) to California. The bus rode and drove like an old truck. We knew we had to do something to improve the poor ride and driving characteristics of our new (to us) bus.

As you know, when we got home the first thing I did was to have new Koni shocks installed on my LXI. During the process, we went through a testing procedure trying to find the best combination of ride quality and drivability.

First we drove the bus over a 10 mile portion of I-10 with the old shocks and the anti-sway bar intact. It still drove like an old truck. Then we changed the shocks, checked that the caster was the same on both wheels (which it was). The second drive over the same portion of the road was vastly improved. The bounce over bridge abutments and dips in the roadbed were reduced to an acceptable level. Then we removed the anti-sway bar and drove over the same portion of I-10. The ride and drivability was more improved. The bounce was reduced to almost none and the softness of the ride over imperfections in the roadbed felt great.

Now we could truly feel the comforts of an air ride chassis.

There were no acute turns on I-10 but we made several low speed turns going to and from the interstate. I have to say that I did not experience any over-sway of the bus's body although we didn't travel around any curvy roads at a higher speed.

Given our experience, I just do not think an anti-sway bar is needed on an LXi. Obviously, with the anti-sway bar intact, the side-sway will be less than without it, but not much less. The LXi's center of gravity is pretty low which help limit the side-sway around curves even without the anti-sway bar.

So I guess it all boils down to personal preference but I agree with Morey, I will take the great soft and smooth ride we have now and live with a little more sway.

Chuck

[davidbrady]

(08-27-2013 11:57)timetravelers Wrote:  Thanks for this additional analysis.

(08-27-2013 19:06)cmillsap Wrote:  Thanks, David for the information.

My pleasure guys. I really appreciate the scientific laboratory validation and real world experience that you've both provided. Your experience is identical to mine.

As you guys know, to experience the improvement in ride quality requires a cobblestone like road surface. Slow undulations or smooth payment doesn't illustrate the benefits.

It'd be interesting to experiment with a light weight bar to see if we can find a happy medium between good ride and snappy handling. I may do that some day, but I know the bar would have to be 1 5/8" diameter or less assuming the same geometry as or stock bars (width, bends, and arm length). I'd limit any additional roll stiffness to 300 ft-lb/deg.

A while back Brad Winter gave me the dimensions of his BMC's steer axle anti-sway bar. I still have them and will eventually run the numbers to see if it's a possible fit to our application.

For now, I'm loving the ride and the handling!

[JD33]

David, I do not experience any of the adverse ride qualities that the LXi's seem to? Perhaps the LX has a less robust anti sway bar? Could that be? If you like I can measure the diameter of mine tomorrow and report back.

[davidbrady]

Yes Jim, I'm very interested. So far we know of two bars that were used in the LX and LXi's, a 1.75" diameter and a 2.125" diameter bar. Pictures are always good.

A complete set of dimensions as shown here: would be best since there may be other geometries in use, but that's definitely above and beyond the call of duty. :-)

[JD33]

Hi David, I will try to get the other dimensions when possible, but interesting to note is the the diameter of the bar is 2 1/8". That dimension seemed odd so I even had our shop foreman check it also

[davidbrady]

Thanks Jim,

Since I don't have first hand experience with the LX, I removed the reference to it from the title of this thread. It's really not fair for me to speak for the LX; nonetheless, it would be interesting for you to remove a vertical link and find a bumpy road to drive down - interesting for me anyway! :-)

[JD33]

Hi David
I was not planning on doing that seeing as it rides and handles exactly as it should (IMO)
However if do decide to try it for some reason I will sure let you know

[davidbrady]

(08-29-2013 16:54)JD33 Wrote:  Hi David
I was not planning on doing that seeing as it rides and handles exactly as it should (IMO)
However if do decide to try it for some reason I will sure let you know

I'm glad it works well for you Jim and that you're happy with it.

I'd like to reiterate to the group that the anti-sway bars used on the LX and LXi are not Ridewell components. These are aftermarket bars built by Addco and added by Wanderlodge. Bill Mattocks, principal engineer at Ridewell, told me that the bars aren't their's and that the suspension works just fine without them; furthermore, the Addco bar isn't spring steel, these are just 1020 mild (low carbon) steel bars.