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-   -   Sway bars and straight-line stability (https://www.irv2.com/forums/f115/sway-bars-and-straight-line-stability-123996.html)

Vanwill 05-16-2012 05:31 PM

Sway bars and straight-line stability
 
Since there have been several posts about Class A coaches whose owners think that they do not “handle” or “track” properly when traveling down smooth, relatively straight highways, I decided I would start another, rather than add any further conflict in an existing one. The reason I am doing this is that I have a personal experience with a coach that handled terribly, and seemed almost disconnected between the steering wheel and the road. Note that my experience pertains to a Class C motorhome, NOT a Class A, but as a lifelong mechanic I think the two experiences are closely related.
The scenario: A 1995 Fleetwood Jamboree Rallye 31’ on a P32 Chevy chassis with 454 engine. The coach wandered horribly and scared the crap out of me when driving it home from FL to NC. I did many things to correct the problem—tie rod ends, idler arm supports, drag link replacement, and steering box adjustment. Each new repair seemed to improve the situation, but CLEARLY did not make a dramatic improvement. I had all but given up and decided that all vehicles this large and top-heavy would feel “sloppy” on the road and would leave one weary after a few hours of “working the wheel”. There was one problem that I had purposely not corrected. The coach had a front anti-sway bar. One of the two forward mounts was completely MISSING its bushing. Since conventional wisdom says that sway bars have no effect on straight-line performance, I had simply chosen to focus on other areas that might improve the wandering. After all, sway bars (or more correctly anti-sway bars) only come into play when the coach is navigating a curve—a situation which would cause the coach to lean. Only when I had pretty much given up on the whole situation did I replace the bushings in both the front anti-sway front mounts. I just did it as “normal” maintenance—fixing something that needed fixing, but wasn’t really pressing. Eureka!! Even a short test drive near home that was meant to test an added tranny cooler, NOT the sway-bar bushing replacement, showed an unmistakable, DRAMATIC change in tracking. It was nearly rock-solid. It could be steered with one hand. No constant wheel-working. The wheel, for the first time, actually seemed directly connected to the road it was traveling. The change was so dramatic, and it was so easy to remove the replaced bushing, that I drove back into the motor home shed and quickly removed the recently replaced bushing, leaving one side connected just as before, but removing the LH bushing entirely. In less than twenty minutes, I was out for another test drive. It was back to the same sloppy, meandering coach it was before. Drove back into the shed and replaced the bushing. Ten minutes and I was back on the road. Once again, it was almost all I had hoped for.
So what conclusions can I draw from this experiment? That in this ONE PARTICULAR CASE, on this ONE PARTICULAR COACH, an anti-sway bar made a remarkable difference in straight-line performance, when theoretically it should have made no difference at all. And the experiment was done so correctly, that there could be NO question about the results. Admittedly, this test was on a Class C that weighed 15K pounds, not 28K like some typical Class A’s.
As a life-long mechanic, machinist, toolmaker, tinkerer and graduate mechanical engineer, I have come to a conclusion. My conclusion is open to question; my RESULTS are not. My CONCLUSION is that perhaps something as large, heavy, aerodynamically lacking, softly-suspended, and whose center of gravity is as high as a motor home, is more sensitive to the action of an anti-sway bar than one might reasonably expect in other more “normal” applications of anti-sway bars. At the same time, I realize there are posters who have added anti-sway bars in an attempt to cure “wandering” problems with their coaches and been very disappointed. Draw your own conclusions, just as I did. If any of you who think (rightfully) that anti-sway bars should have no effect on straight-line handling and that I am trying to start a flame-war based on my experiences, be assured I am NOT. Take this information for what it is worth—a person accurately reporting his personal experiences.

Mike Canter 05-16-2012 06:29 PM

Given the way a sway bar works then removing one bushing would preload one side of the front suspension and cause erratic behavior. I don't think that is a logical test to the fact that a sway bar comes into play on a straight and level road or not. Given your engineering background removing the whole sway bar and test driving it and then putting it back with two good bushings would have been a more logical test.

deSanford 05-16-2012 07:32 PM

From a "layman's" point of view having put sway-bars on two motor homes I can tell you that it is one of the BESTS improvements that I've done. deSanford....I NEVER leave home without them.

norwestie 05-16-2012 09:13 PM

MY experience in regard to Class A DPs wandering: a new 2009 Tiffin Phaeton which does not have independent front suspension vs. my current 2000 Monaco Dynasty with IFS. With the Phaeton, I was constantly correcting the wheel to track straight on the road. With the 12 year old Monaco, no problems keeping centered in the lane. Is it the independent front suspension or something else? Dunno, but I love how the Monaco drives.

Mike Canter 05-16-2012 09:29 PM

My 2004 Monaco with a straight beam front axle does not wonder at all. Interesting I did not know Monaco used IFS in the past. So whys the stop using it?

Mr_D 05-16-2012 09:43 PM

Lots of variables in steering problems. Even how you load the rig can make a big difference.
Our DSDP has a 14,600# IFS and no sway bar. I don't have any problems with steering wander. But then neither did our 2000 DSDP with beam axle and no sway bar.

Dunner 05-17-2012 02:54 AM

Quote:

Originally Posted by Mike Canter (Post 1179066)
Given the way a sway bar works then removing one bushing would preload one side of the front suspension and cause erratic behavior.

How much "preload" do you think the rubber bushings that hold the sway bar to the axle are going to put on that one side? With the link bushing missing or the link removed, the sway bar is just going to move up and down with only slight resistance as MH moves up and down or side to side.




Mike Canter 05-17-2012 05:39 AM

Dunner, those sway bar bushings are very critical in holding the bar to the frame so that there is equal and opposite forces applied from one side to another if the frame rocks on the suspension. The sway bar HAS to be in a neutral position with no forces applied This is why they wear and also way the make replacement hard poly bushings for sports and racing suspensions. If you remove one bushing on one side then there is no longer a neutral position where equal forces are applied to both sides of the suspension so therefore one side is going to have more force in the neutral position (preload) than the other side. Also when the frame now tilts on the suspension it is going to be all wrong. Don't know exactly how much force because that depends on the thickness of the bar being used. I can assure you that it would be critcal. On the rear anti-roll bar (racing sway bar) in our race car we load the car up with fuel and put the driver in it and then adjust the linkage joining the bar to the rear axle to the loose or neutral position. If we don't check that and do it on a regular basis the car will turn when under full power and acceleration.

George Schweikle 05-17-2012 06:08 AM

Vanwill,

Having a rubber bushing removed from one sway bar mount means that the bar is not effective until it moves in the bracket opening and contacts something solid. This may not occur until the coach has experienced considerable lean, and by replacing the missing bushing, you have reduced the lean.

When the coach leans, you may be experiencing "bump steer". This is a condition where the steering geometry gives a steering input as the independent suspension moves through it's travel. If the coach leans, one side "steers" more than the other, and you have to correct to stay in a straight line. This is a condition that race car owners take considerable care to eliminate. I know the earlier GM class A P-30 chassis had this problem, and somebody actually produced a bump steer kit for these but I didn't know about it I sole my 1883 Winnebago Chieftain. This thing had a severe bump steer condition that was exaggerated whenever there was a side wind. Also, as you said about softly sprung RV chassis, it may not take much lean to experience the bump steer effect. I know that my Winnie took lots of steering just to keep in a straight line, even with little or no wind, and it had front and rear sway bars. The bump steer condition was just that bad.

We bought a new Safari Trek in 1995, on the P-30 chassis, but the steering geometry had been significantly improved. I could be in a pack of 18 wheelers, buffeted by their air, and steer with one hand on the wheel. This was literally a different world than with our 1983 chassis. Having said that, we had a 1976 Midas Mini, class C, on a GM chopped van chassis, and I don't remember any steering instability. This was 23 ft. long, and your 31 ft length may exaggerate the steering condition. Or, maybe the class C chassis was changed from 1976 to your version.

At any rate, you solved your problem, but I wanted to offer what might be an explanation of the root cause.

ahicks 05-17-2012 06:10 AM

I think the OP's point is that the sway bar has a very definite effect on handling when going in a straight line - contrary to some popular opinions. This would agree with what I was saying regarding the roof line being in a near constant side to side motion most of the time, even when the chassis is tracking straight down the road/within it's lane. I think all of us would expect screwed up handling with one bushing missing. Preload or not, there's no way the bar is supported well enough to force it to twist allowing it to help resist sway?

Mike Canter 05-17-2012 06:31 AM

Ahicks, like I said you cannot say that on that sway bar configuration because it was not in a normal configuration. You can compare it by having it installed correctly with good bushing against totally taken out but cannot compare it with just one bushing to two good bushings. It is not a valid comparison and any findings are false.

ahicks 05-17-2012 06:37 AM

Mike, you can split hairs if you like, but with that bushing missing, that sway bar's effect on handling was effectively negated in all but the more extreme conditions. His point is valid from where I'm sitting.

Dogpatch 05-17-2012 06:57 AM

Quote:

Originally Posted by ahicks (Post 1179514)
Mike, you can split hairs if you like, but with that bushing missing, that sway bar's effect on handling was effectively negated in all but the more extreme conditions. His point is valid from where I'm sitting.

I agree. One persons experience corrected his issue. No denying it.

Thanks Vanwill! Perhaps your experience will prompt others to at least investigate this avenue and have the same results.

Dunner 05-17-2012 09:22 AM

Let me simply this for sake understanding preload.

"If" a one piece sway bar were mounted with ball or roller bearings on the axle, you would have virtually zero preload. The bar could move up or down at will. If you connect one end link to this sway bar, it would still not have any preload and take minimal movement of the coach to move it up and down, and have no preload. If you connect the other equal length end link to the other end of the sway bar, it would still have zero preload. If the coach moved up and down, there would be no preload. It is only when the coach leans from side to side or hits a bump or hole that the sway bar is loaded.

If you put un-equal end links on the sway bar, it would then have equal but opposite preload to both sides, unless the sway bar us held stationary at some point where it mounts to the frame.

This is all assuming a one piece anti-sway bar.





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