Chassis Alignment Checkup

The first step in diagnosing a handling problem is to obtain accurate weights, which will determine the correct tire inflation pressures. Scale readings should be taken when the motorhome is fully loaded with cargo, fresh water, fuel, and passengers, just as it would be during your travels. Weighing the motorhome on a typical truck scale provides the weight of each axle, which is good, but it’s best to obtain individual wheel weights in order to determine whether major side-to-side variations exist. The Recreation Vehicle & Safety Education Foundation offers motorhome weighing by individual wheel and analysis at each FMCA Family Reunion and at other events. Visit www.rvsafety.com for details and a schedule.

Tire inflation pressures that exceed what is required to carry the measured load will contribute to a rougher ride. Excessive pressures also minimize the contact patch of the tire, which reduces traction because of crowning of the tread. An underinflated tire, on the other hand, cannot safely carry the measured weight and may be subject to damage from overheating. If accurate scale readings have not been obtained, for safety reasons it’s best to favor higher pressures.

A leaf-spring suspension is fairly simple, but air-ride suspension systems on larger diesel-powered motorhomes require special attention. Air bags inflate to the pressure necessary to maintain the corre ct ride height, which is controlled by ride height sensing valves. The valves measure the distance between the suspension and the frame and then add or remove air from each bag in order to maintain that distance. The pressure in the suspension bags varies according to the motorhome weight. When the coach is full of fuel and loaded with cargo, the pressure is higher than when the vehicle is lightly loaded, but the ride height remains constant.

If the ride height is not within specs, the weight may shift between the front and rear axles, resulting in an overworked axle. The ride height also affects the driveshaft angle. A ride height not within specs can result in driveline vibrations that prematurely wear out universal joints. Because ride height can affect per-axle weights, to ensure accurate scale readings, it’s important to have the ride height checked and, if necessary, adjusted before weighing the coach.

Motorhomes with tag axles utilize a proportioning valve to split the load between the drive axle and the tag axle. The optimum split is one-third of the rear weight on the tag axle and two-thirds on the drive axle. This provides maximum traction for both braking and acceleration, and helps to even out the tire wear between both axles.

If the tag-axle air bag pressure is set too high, the tag axle will carry extra weight and will transfer more weight to the front steer axle while removing weight from the drive axle. This reduces braking ability and traction on the drive axle and could potentially overload the front axle. If the tag-axle pressure is lower than intended, more weight rests on the drive axle. This reduces the weight on the front steer axle and the tag axle. If the pressure is too low, braking capacity will be reduced.

Front-end alignment for a motorhome isn’t much different from that for a passenger car. The toe setting relates to the two steer tires and is a measurement of the difference in distance between the front of the tires and the back of the tires. Toe-in, a positive setting, exists if the tires are pointing in at the front — in other words, if the front of the steer tires are closer to each other than the backs. A negative setting, or toe-out, exists if the tires are pointed out at the front. The toe is set by adjusting the length of the tie-rod or rods.

Ideally, the vehicle will have zero toe-in as it travels down the road, but typically the flex in the tie-rod causes the wheels to toe out slightly, so most alignment specifications call for a slight amount of positive toe-in. It’s important to use the correct alignment specs called for by the chassis manufacturer.

Improper toe-in causes tire feathering, with rounded edges on one side of the tread and sharp edges on the other. Excessive toe-out increases wear to the inside of the tire, while excessive toe-in wears the outside of the tire. The toe setting is the most common alignment adjustment on any vehicle.

Camber, the second component of a front-end alignment, measures in degrees the vertical angle of the wheel when looking at the vehicle head-on. A tire that leans outward at the top, away from the center of the vehicle, has positive camber; a tire that leans inward at the top has negative camber. A solid I-beam axle uses kingpins that pivot in bushings at the ends of the axle; the kingpins and bushings are not adjustable. As long as the bushings are not worn and the axle is not bent, the camber setting should not move. Independent front suspensions are a bit more complex; the control arms are adjustable, so the camber can be changed, if need be.

Caster, the final component of a front-end alignment, is the angle of the kingpin or ball joints from vertical. When a wheel is turned via the steering wheel, the wheel pivots along this axis. If this axis is perfectly vertical, it is said to have zero caster. If the top of the wheel’s axis leans forward, it has negative caster, while a wheel that leans rearward has positive caster.

Caster affects the effort required to steer and the ability to track straight without constant correction on the wheel. An example of extreme positive caster is a custom Harley-Davidson motorcycle with extended forks. In this case, the vehicle tracks nice and straight down the highway but requires extra effort to turn and an increased turning radius. And, it puts extra wear on the sides of the tread when turning.

In comparison, an example of extreme negative caster is a grocery shopping cart that has a worn wheel bearing. The cart wobbles and shakes, and it’s difficult to drive in a straight line. A motorhome without enough caster will want to dart and dive and will be difficult to drive. Most chassis alignment specifications require a few degrees of caster to add some stability to the handling.

Having the front end properly aligned is important, but handling issues will remain if the rear axle or axles are not aligned correctly. For a long-wheelbase vehicle, such as a motorhome, a four- or six-wheel alignment is imperative to achieve good handling.

Chances are a motorhome chassis is aligned before it is shipped to the coach builder. But when the motorhome is built, significant weight is added to that chassis, and things can change. If a new motorhome isn’t handling as it should, it may need a complete alignment to verify the rear-axle thrust angle. A rear axle that is perfectly perpendicular to the frame rails has a zero thrust angle. If the axle is offset to an angle other than 90 degrees, it has a thrust angle, which is measured in degrees.

Gasoline-powered motorhomes generally utilize a leaf-spring suspension. If the axle was put in correctly, it should be in line with the frame. If not, the spring hangers or leaf spring-to-axle mounting plates may need to be adjusted.

Diesel-powered coaches with air-ride suspension generally have adjustable rods or shims that can change the thrust angle. Some manufacturers specify a slight thrust angle to help compensate for the crown in the road, while others prefer a zero thrust angle. Be sure to check with your chassis manufacturer for the correct settings.

In the case of tag-axle motorhomes, this goes one step further, because both the drive and tag axles can have thrust angles. A worst-case scenario is a coach with a positive thrust angle on one axle and a negative thrust angle on the other. If that situation is not corrected, the motorhome will wander all over the road.

A new motorhome may drive and handle perfectly right off the lot, but that isn’t always the case. If your motorhome exhibits any of the characteristics we’ve described, follow the above steps to correct them. If you’ve had your coach for a while and it’s not handling like it used to, it may be time for an alignment check. But first, check for worn parts. Worn kingpin bushings, tie-rod ends, ball joints, or other steering components can change settings, and until replacement parts are installed, no amount of adjusting will improve the handling. Once correctly aligned, and with proper tire inflation pressures and ride height, your motorhome should be a joy to drive.