Motorhome Suspensions and Handling
By: Bob Gummersall
[Ed Note 2011: While many improvements have been made to suspension systems since this article was first published, a careful reading of this will provide a basic understanding of the different types of suspension systems used in today's motorhome offerings.]
Three basic types of suspensions are used in Class A motorhomes. They are leaf/coil steel-springs, air bags, and torsion bars. Within each category there are various methods of positioning components that effect the weight capacity and handling characteristics. This FAQ will describe the basic differences between these suspensions. Before we start, it is interesting to note that no modern over-the-road bus chassis use spring suspensions. Most school buses use leaf spring suspensions. Most light and medium duty trucks use spring suspensions. After you read this article, you will understand why this is true.
LEAF/COIL SPRING suspensions are the least expensive and the hardest riding. Handling on the highway is significantly affected by crosswinds or the wind forces created when an 18 wheeler blows by the motorhome. The close mechanical coupling between the axles and the frame of the motorhome transmits noise, vibration, and side forces. Except for small rubber or plastic bushings, there is a steel to steel connection. The bumps in the road are transmitted directly into the chassis of the coach and neither sound nor vibration is dampened. To see the worst case in the ride characteristics of this type of suspension, find a way to take a ride in the rear seats of a school bus. Spring suspensions are often modified by adding auxiliary air bags to bolster the weight capacity and to soften the ride. After-market steering stabilizer and sway bars are frequently added to increase highway directional stability and reduce tipping. Spring suspensions have significant leaning when turning corners and very hard riding over bumpy roads. Leaf springs eventually sag and require added leafs to correct the ride height, especially from side to side. Because most roads are crowned to let rain water run off, driving many miles on a slant causes the passenger side to eventually sag. The older the motorhome the greater the chance for the necessity for this repair.
AIR BAG suspensions can be four air bags or eight airbags and significantly soften the ride characteristics. Air bag suspensions use automatic pressure regulators on side-to-side and front-to-rear to dynamically adjust the air pressure in each air bag to compensate for the tipping forces the coach is experiencing. Thus while turning a corner, the outside air bags are automatically inflated to compensate for the leaning of the coach. The vehicle thus goes through corners and through high side winds with far less tipping motion. Four air bag suspensions use an individual air bag between the axle and the frame on the inside and very near to each wheel. Solid axles are called "live axles" and are one piece and ridged. Both Spartan and Freightliner now offer front independent suspensions systems that use an air bag inside an A-frame supporting each front wheel. This independence allows each front tire to follow the terrain that it faces and thus results in a smoother and more stable ride than a live axle suspension. Independent front suspensions have been used in automobiles since the 1940's but have only recently found their way into motorhomes and some passenger buses. Eight air bag suspensions have an axle carriage that allows two airbags to be placed above and to the front and rear of each wheel. The result is much greater side-to-side support and less tipping than a four-bag system. Since the ride height is automatically and dynamically adjusted, there is never any sagging. Airbags can develop leaks or blow out, but they are relatively inexpensive to replace. Airbag suspensions are much more compliant to side-wind conditions and the resulting sway. The softness can sometimes cause porpoise(ing) like action while driving on undulating roadways. This is normally dampened properly by shock absorbers. Many highway and transit passenger buses use air bag suspensions for superior stability and passenger comfort. When parked, it is possible to dump the air out of the airbags and lower the coach for easier egress. Some more sophisticated systems can also raise and lower the ride height while traveling down the road. The driver can lower the whole coach while traveling on a flat interstate or raise it up to negotiate a steep driveway. The air in the bags significantly reduces the noise and vibration transmitted from road surfaces.
TORSION BAR suspensions use a steel bar in a sleeve connected to a lever arm to provide the up and down action. Newer torsion bars are encased in hard rubber inside the sleeve to further buffer the road noise and vibration between the axle and the chassis. Eagle passenger buses made this suspension famous and many entertainers will only use Eagle buses for touring because of their ride characteristics. Torsion bar suspensions are less effected by porpoise(ing) and have excellent resistance to tipping. Foretravel and Safari are the major motorhome builders that use or have used this suspension system. Ride-height is adjusted by loosening the lever arm and twisting the bar with special tools and then tightening the arm. Adjustment is limited and when the bar ages, replacement is relatively expensive. Torsion bar suspensions are used on some travel providing lower ride levels and thus a lower center of gravity. Torsion bars are used in some automobiles and light duty trucks.
I have driven buses and motorhomes with all types of suspensions and personally favor the eight air bag system for its excellent side to side stability, lower noise and vibration coupling, comfort and automatic ride height adjustment. The independent front four air bag system is also very good. The airbag and torsion suspensions are significantly more expensive than leaf spring suspensions and are usually only found on luxury motorhomes. As I have stated in other FAQ's, I feel that a used luxury motorhome with semi-monocoque chassis and air/torsion suspension is a better buy than a new leaf/coil spring chassis motorhome.