We have many experts on the forum better equipped than I to offer insight based on knowledge and experience..The intent of this thread is to invite discussion on the issues that affect the stability of our motorhomes. Please feel free to challenge or add to my comments...
What makes a coach stable?
I do not think that its any one thing.. I believe there are 6 area's that conspire to deliver a white knuckle or tranquil experience... Of the five the wheel base is not something that can be changed.. I believe that while there is a general truism that the higher the ratio the better, however most of the commentary is anecdotal... The consensus is that a ratio of .54 and above is the magic number, however there are many with lower ratio's that report that their coach is very mannerly.
Tire are the least mysterious. Good condition and properly inflated and rated for the weight of your coach and balanced is about all that can be done.
I believe that the key to optimizing the stability of your coach lies in having it well balanced in terms of weight distribution...
Weight and balance
Equal weight on each corner of both axles will yield the best results.
I think that a brief discussion of center of gravity is in order.
Ideally having the Center of gravity as close to mid coach as possible. There are three centers of gravity, length wise, height., and side to side. There is little if anything that can be done about the height, and calculating it can not be done with out weighing both axles then jacking up one axle taking some measurements including the amount of weight shifted to the axle not being raised..Center of Gravity Height = [ Level Wheelbase x Raised Wheelbase x Added Weight on Scale / Distance Raised ] x Overall Weight ..
The side to side Center of Gravity Location (off-center to heavy side) = Track / 2 - [ Weight on Light Side / Overall Weight ] x Track Would require knowing what the drivers side weighs versus the passengers side and then the overall weight.. The simplest way to get your CG in the center of the coach is to insure your left to right weights are equal on each axle
The length wise is a simple calculation; Center of Gravity Location (behind front wheels) = Rear Wheel Weights / Overall Weight x Wheelbase.
Keeping the center of gravity as low as possible and as near the center is where you are wanting to be...
Ideally your coach will have some sort of steering stabilization which offer resistance to movement and return to center pressure to achieve neutral steering designed to keep the front wheels from castering from external forces like road surface irregularity or wind by resisting the external forces.
Ride height where applicable properly adjusted, shocks and air bags where applicable in good repair and rated for the weight of the coach. The suspension components need to be matched to the weight of the coach to control the travel so that the suspension does not bottom out or porpoise.
The issue here is rear stabilization to prevent tail wagging which will affect steering. This is most often brought on by an excessive tail overhang which will use the rear wheel as a fulcrum. Rear sway bars or stabilizers and momentum control such as airbags or heavier shock absorbers can inhibit the tail wagging affect of excessive overhang
Properly inflated for the weight of your coach, Balanced...
As mentioned the general consensus is a wheel base ratio to overall length is that the higher the number the better. However if the coach is well balanced this negates some of the need for a higher ratio
Although it may seem complicated, alignment is simply making sure the wheels are operating parallel with one another, and the tires meet the road at the correct angle. So that the motorhome will not pull or wonder.
Three basic wheel angles determine whether a vehicle is properly aligned. All three of these angles need to be properly set for alignment to be correct.
Camber is the inward or outward tilt of a wheel compared to a vertical line. If the camber is out of adjustment, it will cause tire wear on one side of the tire's tread.
Caster is the degree that the car's steering axis is tilted forward or backward from the vertical as viewed from the side of the car. If the caster is out of adjustment, it can cause problems in straight-line tracking. Caster has little affect on tire wear.
Toe refers to the directions in which two wheels point relative to each other. Incorrect toe will cause rapid tire wear to both tires equally.
The thing to keep in mind is that there is no one size fits all silver bullet. Each chassis configuration present its own unique profile in terms of what will help or make a difference.
Well put. The only thing I'd add is that even two identical coaches, on the same chassis, that rolled off the assembly line one right after the other may have different issues. Each MH is, as you say, not subject to a one size fits all silver bullet.
Test drive it. Test drive another. Then test drive each again. Put a lot of thought into any aftermarket "improvements". Gather feedback from those that have used that same improvement.
One manufacturer used to have problems with porpoising. Seems to have been resolved by upgrading shocks. From what I've read, the chassis manufacturer now installs them at the point of assembly.
Don't go nuts. Use common sense. Be methodical in your approach.
2004 Coachmen Aurora on a FORD V-10 Chassis
2004 Jeep Liberty Don't take your organs to Heaven. Heaven knows we need them here!
Good post. I think that wheelbase, while important, must take into account whether or not there is a tag axle. This brings up a question.....how does one calculate wheelbase when the chassis has a tag axle.
Also, windage is an important factor to consider. A "boxcar" designed MH is not helping to avoid sway.
I have steer safe stabilizers on my front wheels and I highly recommend them. Also have Centramatic automatic front wheel balancers which makes stabiity better on the road in the event of tire wear.
Proper weight ratio per axle helps also, my ex- Travel Supreme w/spartan seemed to track very nicely. When the fuel tank was full it had a nicer, heavier feel on the front axle, as the fuel was consumed it seemed lighter on it's front but also rode nicely either way- wind never was a factor. I agree each wheel should be equally loaded with weight, IFS to me seems to handle better than a straight axle. This may be a theory but if force and momentum energy builds up on the chassis dynamicly the IFS seems to prevent the synergy of the two from discharging into the steering wheel. One more critical measurement would be centerline to centerline of each side of the coach independantly- 1/2" can make a world of handling difference.
Johnny T, I must have missed something here. If we did achieve a truly centered CG in the longitudinal (front to back) axis would not the tires and wheels on front be carrying twice as much load per each as the duals (and sometimes even tandems) on the back. Was that ideal only a theoretical and unachievable ideal or can we really do it?
Johnny T, good post. I would add one thing to your list and that is how the stability of a rig changes (or doesn't) when towing.It's hard to asess that dynamic before you own thw rig. However, they all don't have the same stability impact when towing. Althoug I am very pleased with how my rig handles without a toad I have noticed that it seems to improve when towing. This may be anicdotal but we do live in a world of perceptions.
JohnnyT, I agree great post but my thoughts are under the KISS principle for ME, (Keep It Simple Stupid.) #1 GOOD driver, #2 confident driver, #3 weight & balance in check... That said, if you have number one and number two, it should take care of number three. And I might add, all other related items. JMHO
Interesting topic. However I would suggest that you have missed maybe one of the most important factors in determining "stability" of a coach.
Driver impression. This is a subjective area but ultimately the driver decides if he/she is "satisfied" with the stability of their coach. What one finds acceptable another very well might not. Unfortunately the drivers previous life experiences and their expectations for the stability of their coach will make this opinion very subjective.
As they gain time behind the wheel that opinion will change. It could go either way, toward the good or bad.
Now bring into play your question.
Are we looking to quantify ways to recognize a "stable" coach ??
In your list I would expand the alignment issue. You have not included (and most never think about it) the relationship between front axle and drive axle. They need to be aligned to each other. Ever see a vehicle going down the road "crooked" to one side or the other ??
Add in a "tag" axle and the tag needs to be aligned to the drive and the drive to the steering. Anything less and you get a coach that can be "pushing" or "dragging" itself down the road. An unruly coach and in addition poor tire wear. Yet the steering is "aligned" (proper castor,camber, and toe).
Have fun, this will be an interesting thread.
Never argue with an idiot, they take you down to their level and beat you with more experience....
Since owning my Bluebird Wanderlodge I have found the stability to be incredible. I attribute the following to it's success:
Extremely Heavy duty chassis
Eight air bag system
Superb placement of Generator, LP Tank, Fuel, Fresh water, sewage tanks, and stowage compartments located in basement.
Much heavier coach for it's size...37' equals 37400 pounds.
Excellent placement of kitchen and bathroom.
Also did an excellent job in keeping all the fixed load as low as possible within the entire coach.
These coaches do not get blown around by passing traffic, and handle much better then the majority of coaches on the road.
Former Bluebird Wanderlodge Owner/Advocate....but still love them!