Suspension design on any vehicle is a compromise. As an example, a person who is looking for mastery around curves is more inclined to buy a Porsche than a Cadillac. By the same token, one who is interested in hauling 5,000 pounds of dirt would be mistaken to select an AMC hornet for the task. The point to remember here is that each is built with a different priority in mind. Yet all of these vehicles are intended to carry people and cargo over roads in all kinds of conditions. We will look at some simple theories and dynamics of handling, especially weight distribution, and how each affects and modifies response. These precepts are universal, and apply just as much to the family errand runner as to a 34-foot Class A motorhome.

A chassis is simply the frame to which the body is attached. The engine, transmission and all occupants ride on, or within, the chassis. Besides supporting the static loads imposed on it, the chassis must also withstand the dynamic (moving) forces and stresses placed on it. This would include acceleration and braking loads, in addition to the stresses created when cornering. Supporting the chassis underneath are springs, which flex or give as the wheels move up and down over bumps. This insulates the chassis from many road irregularities.

Why do vehicles need springs in the first place? Let's take a look at the alternative. We are motoring down a billiard-table smooth road with no suspension and solid tires until we spot that small pebble in the road. Hitting the pebble would pitch the vehicle up on that side, and it would come down with a crash. Steering control would be difficult to maintain, not to mention the effect on kidneys and molars. Since the springs are effectively between the wheels and the chassis, the wheels and tires and some friction of the weight of the suspension is referred to as the unsprung weight. The weight supported by the springs is called the sprung weight. The ratio between the sprung and unsprung weight has a tremendous effect on how smooth the vehicle rides, and how well it handles on rough surfaces.

The main concept here is that there are two forces acting against each other. When one wheel hits a bump, it is driven upwards against the force of the spring and the weight of the chassis. The more the chassis weighs, or the less the wheel assembly weighs, the less disturbing or upsetting effect it will have on the chassis. It is simply the unsprung weight reacting against the greater inertia of the chassis, or sprung weight. There is little, but significant, room for improvement in this area, when it comes to motorhomes.

The up and down motion of the wheels is not unlimited. Most people are familiar with the feeling of bottoming, where the suspension has run out of travel and has hit the bump stop. This is a large rubber block that cushions the end of the travel, preventing the otherwise sickening sound of the tire crashing into the wheel well, or metal crashing together, along with a string of bent and broken suspension parts dropping onto the road.

In the early days, it was common practice to put a wheel at each end of a beam axle, with long flat leaf springs between the beam and coach. Each end of the leaf spring was connected to the coach, with the center of the spring clamped to the beam. When the wheels hit a bump, the beam would move upward, causing the leaf springs to flex. This flexing absorbs some of the energy, instead of transmitting it to the chassis.

This same design is still used today on the rear of many vehicles. Up front, however, things are a little more complicated. The problem is that the front wheels steer. Because the wheels are tied together on a single beam, when one wheel hits a bump it affects the other wheel. This causes the steering to feel less precise, especially on bumpy surfaces.

The solution that some manufacturers have turned to is independent suspension. This allows each wheel to move up and down individually, without bothering its counterpart on the other side. Arms in the shape of an "A" (also called an A-frame) connect the wheels to the chassis. The arms move up and down through their travel while keeping the wheels perpendicular to the road. Coil springs usually are used with this type of suspension, one end of the springs on an arm and the other solidly against the chassis. The unequal length A-frame front suspension and the leaf spring rear suspension have been used with great success for many years.

Motorhomes put some staggering loads on their chassis. Let's look at some of them. A motorhome spring first must support the weight of the vehicle at rest. Unfortunately, this is the easiest of its functions. The major force underlying most suspension design and tuning is based on a phenomenon called weight transfer. This is simply an inertia force, the same one that pushes you forward when braking hard. It is an extension of the principle that "a body in motion tends to remain in motion." In this case, while the coach is trying to stop, your body has its own forward momentum to deal with.

During violent or sudden vehicle maneuvers, the tremendous momentum of a motorhome must be taken into account. A motorhome represents a tremendous amount of inertia going down the highway, and trying to change or modify its speed or course is not easy. To give an example of how much weight may be transferred during a severe stop, use this formula:

Weight Transfer = IF * CG / W.

Inertial force (IF) is measured in G's, Center of Gravity (CG) is the height of the vehicle center of gravity in inches, and Wheelbase (W) is the distance from center of front wheels to center of rear wheels in inches.

For example, assuming a 30-inch high cg of a 13,500-pound motorhome with a 178-inch wheelbase, during a .5g stop (not quite an emergency panic stop), 1,137 pounds are taken off the load of the rear springs and transferred to the front! The formula, with sample figures, looks like this:

Weight Transfer = (.5)13,500 * 30 / 178

Weight Transfer = 1,137 lbs.

This accounts for the nose dive that vehicles experience under such conditions. It is also the reason that front brakes wear out faster than the rears. As seen from the formula, factors affecting the severity of the weight transfer depend on the severity of the stop, the wheelbase of the vehicle, and the center of gravity. The center of gravity is the perfect balance point within the motorhome. If you could take a hook and place it at the center of gravity, the vehicle would hang from that point without moving or tilting. The important point to remember here is, the higher the center of gravity, the greater the weight, transfer, and the harder it is for the suspension to keep things on an even keel. It is therefore easy to see that loads placed in a motorhome should be as close to the ground as possible, and that only very light items should be stored up high.

This same weight transfer also takes place when going around a corner. Instead of fore and aft, there is a lateral shift in loading. The same basic formula can be used to calculate it. The most recognizable and important effect from it is body roll (lean). Among all the factors dealt with by a driver, body roll is probably the most disconcerting and unpleasant. Severe body roll also produces unwanted changes in suspension geometry, which decreases the total grip available to the tries.

Tires have a lot to do with how a vehicle corners. As the vehicle enters the turn, the tire tread distorts sideways slightly under the load. This distortion makes the vehicle follow a slightly wider path that where the wheel is actually pointing. The difference between the two paths is called the slip angle. Both front and rear wheels have angles, even though the ones in back do not steer.

Why are slip angles important? They determine the cornering characteristics of the vehicle. When the front tires' slip angle is greater than that of the rear tires, the vehicle under steers. All this means is the vehicle's front end tends to run wide in a corner. If he goes to fast into a corner, the driver must either slow down or turn the steering wheel even tighter to maintain the intended path through the turn. This is how all stock domestic passenger cars are set, allowing safe steering characteristics for the average driver to handle in an emergency situation. Too much under steering creates a "plowing" condition, making the motorhome very reluctant to change course.

Slip angles are affected by many things. The most important for motorhomes are the tires and body roll. An under inflated tire, in addition to being unsafe from a loading standpoint, will run at larger slip angles. This will make the RV feel "mushy" and slow to respond. Slip angles is also affected by body roll, but in a way that is unexpected.

A vehicle's front suspension tries to keep the wheel and tire assembly at right angles to the road surface. As it nears the limits of its travel, the steering geometry can no longer do that and begins to tilt the tire. This can pull part of the tread away from the road surface. The less rubber on the road, the larger the slip angle. If you reduce body-roll levels, the tires stay in better contact with the road, and under steer is reduced, which makes the vehicle much more responsive to steering inputs.

Many Class A motorhomes are built on a Chevrolet chassis. This chassis is referred to as the P-30, a derivative of the forward control Step-Van. This chassis has been around for quite some time, and it shares the same basic suspension design as the C-series pickup trucks and the G-series vans. Because of this, the same principles apply to all three, even though equivalent parts aren't interchangeable.

The P-30 has an independent front suspension, using unequal length A-frame arms and coil springs. Assisting in the task of keeping things under control are air bags inside the front coils and a front 1-1/8" diameter stabilizer bar. The rear uses a solid axle and leaf springs, and includes a rear 1-3/4" diameter stabilizer bar. These bars attempt to keep body roll to an acceptable level by being forced to twist when the wheels fail to move as a unit, such as when the vehicle leans in a corner. A stabilizer bar will have little or no effect on ride quality, since as long as the wheels move the same, the bar does not twist, but just goes long for the ride, so to speak.

The other component and, without a doubt, the one that needs the most improvement over original equipment, is the shock absorber. Contrary to popular belief, conventional shocks do not support the load. If they were removed, they would have no effect on ride height. Springs, if left to their own volition, would oscillate up and down many times after striking a bump. A shock absorber's main function is to dampen this spring motion to an acceptable level. In the case of the P-30 motorhome chassis, the stock Delco shocks are barely acceptable when new, and woefully inadequate after several thousand miles of wear.

Many owners of P-30 based motorhomes have upgraded the standard shocks with something more capable of handling the loads. Shocks fall into two broad categories: conventional and gas-filled. Superior shocks are available in both categories. Generally called telescoping shocks, they are mounted with one end connected to the frame and the other to the suspension arm. As the frame moves in relation to the wheel, the motion is hindered by the shock, which must telescope in and out. Both systems use a piston that is forced through fluid to dampen or absorb this motion. The piston has a series of small holes, or orifices, which the fluid must pass through for the piston to move. By combining theses and tailoring the orifices, specific characteristics and damping action can be obtained.

In addition to firmer valving, high-performance shocks offer greater control and longer life than standard shocks. Because a shock absorbs energy as heat, high-performance shocks use increased fluid capacities and larger pistons. This adds up to better control.

Most motorhomes suffer from sluggish response to steering inputs by the driver. Increasing the response of the coach is a function of many factors, including weight distribution, center of gravity, shock control, and body roll during cornering. The owner interested in good handling from a coach should start by exploring the factors over which he has direct control. First, of course, should be loading the motorhome in a systematic manner, with all heavy items placed as low in the coach as practical. Water tanks that are located aft of the rear axle should have minimum water levels when driving. Top quality radial tries should be fitted to every wheel, if wheels are approved for use of radials. In addition, inflation pressures should be checked religiously, not only from a handling standpoint, but from an overloading factor as well.

Those willing to spend money for additional parts have a couple of options available to them. For most, the addition of better shocks should be first. Also check the market for stabilizer bars, and other devices such as integrated-coil springs/shock stabilizer systems.