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Over The Network

A motorhome handling primer

by Gary Brinck

Vehicle handling is a complex issue and is often perceived differently by different drivers. Each motorhome chassis manufacturer makes multiple versions of the same chassis, with different wheelbases and different GVWRs, so one cannot simply compare Freightliner to Spartan or Workhorse to Ford. Furthermore, most of the chassis manufacturers cannot control the weight distribution after the various coach manufacturers add the motorhome body [and sometimes even enhanced suspension components], nor after the owner loads his gear aboard. This results in an almost infinite variety of motorhome handling characteristics, even within the same chassis type or coach model. And even when the coach manufacturer is also the chassis designer, the same two or three chassis still have to do duty under a variety of motorhome models and thus are still subject to a lot of variation in loading and weight distribution.

Weight distribution has a major effect on the vehicle center of gravity [VCG] and VCG in turn is a major factor in handling on a large, heavy vehicle. Motorhome handling improves dramatically as the VCG moves down [lower] and toward the center of the axles, both side to side and fore and aft. Another major and related factor is the length of wheelbase relative to the overall length of the coach. A longer wheelbase generally places the VCG more between the axles rather than toward either end. A rule of thumb is that the wheelbase should be more than 50% of the overall length, with 53% or more deemed to be "good". Because there are so many factors in good handling, it cannot be categorically stated that a wheelbase ratio under 53% will result in poor handling, but all experts seem to agree that "more is better".

Coaches with a long rear overhang and/or heavily loaded in the rear (e.g. generators, air conditioners, water tanks or rear engine) will have a tendency to teeter-totter around the rear axle in the vertical direction, resulting in a fore and aft plunging movement called "pitching". To minimize this condition, the front axle should be loaded in the same proportion to its load rating (GAWR) as the rear. If the rear is near max load (common in motorhomes) but the front is only 50-60% of its max rated capacity, pitching is a given. Shock absorbers with a heavier (stiffer) motion resistance can sometimes alleviate the symptoms, but the only real fix is to redistribute weight. However, stiffer shocks will generally degrade the overall ride because more wheel bounce is transmitted to the frame. A chassis with a tag axle is much more resistent to pitching than a single rear axle because the second axle resists the teeter-totter effect.

A movement similar to pitching is called "porpoising", named for the high arching maneuver of a dolphin surfacing in the ocean. In porpoising, the front end of the motorhome leaps into the air when going over a dip or short rise in the road. This may throw the front seat passengers towards the roof and be quite alarming. If it fails to steady out immediately, this is a sign of worn and the front shocks should be replaced immediately (and perhaps the rears as well). However, some rigs will leap once and steady right out. In this case the shocks are doing their job OK but the passengers are still unhappy. A stiffer shock absorber can be used to better control the rapid rise of the front suspension, but the overall ride will be a bit harsher as a result.

A high positioned VCG and maximum weight will result in more vehicle lean or "roll" during cornering and a perceived loss of control is emergency maneuvers. To avoid this, heavy objects should be stored as low as possible and weight evenly distributed from side to side. Sometimes "anti-sway bars" are added to help reduce roll by stiffening the resistence to the roll movement. This doesn't stop the rolling, but it slows down the action and may reduce the feeling of uncontrolled movement. Modern motorhome chassis usually have anti-sway bars designed in, but in some cases it may be possible to add another or substitute a stiffer one.

Weighing your motorhome is the preliminary step in diagnosing nearly all handling problems. Besides, you must have accurate weights to correctly inflate the tires, another factor in proper handling as well as for tire safety. Weigh the motorhome's front and rear axles separately and if possible also get the weights on individual wheels. If you can get the weight for each axle, front and rear, and for one side, you can calculate the weights on each wheel closely enough for most purposes.

Another major factor in motorhome handling is the large surface area of the sides. Strong side winds can actually shift the motorhome on its suspension to the point where the body is skewed at an angle to the chassis as it move down the road, called "crabbing". Speed should be reduced whenever this happens, since the force acting on the sides is proportional to the square of the air speed and thus even a small difference in speed can make a huge difference in the amount of force applied. Vehicles with a larger rear overhang will typically be more affected by side winds because they will tend to pivot horizontally around the rear axle (the teeter-totter effect again). When caused by gusty winds or passing vehicles, this horizontal motion is called "yawing" - or more humorously, "the tail wagging the dog". The rear end moves sideways in the direction of the wind and pushes the front end the opposite way, requiring a steering correction. The correction alters the direction of the wind force and that requires another, sometimes opposite, steering correction. A small curve in the road or wind direction change can be enough to change the wind force, also requiring a steering correction. A day of driving in variable side winds can leave the driver exhausted, due both to actual work at the steering wheel and the stress of coping with frequent steering changes.

Heavily traveled highways, especially interstates, will develop ruts or tracks which are often nearly invisible. If your vehicle's wheel track (the distance between the front wheels) is different than the space between the road ruts, or if your driving habits have you trying to drive to one side or the other of the worn track, the front wheels will continully be trying to ride up or down the sloped side of the worn track and getting pushed from side to side. This "side play" usually results in many and frequent small steering corrections, sometimes called "fidgeting". To see if rut tracking is causing excessive steering correction, try driving in the left hand lane of a 4-lane highway, which usually has less wear. Leaf spring suspensions are more susceptible to side play than good quality air suspensions, probably because air suspensions have no inherent resistance to sideways movement and thus must have anti-sideplay mechanisms designed in. Crowned roads (high center) and banked roads (tilted sideways, usually at curves) will also have an effect similar to rut tracking as the steering wheels try to run down the grade to the lower side. A device called a "panhard rod" can often provide substantial relief to wheel tracking problems. A panhard rod locks the leaf spring stack in place laterally, preventing axle side play. [Note: the Davis Tru-trac Bar and the Henderson Super Steer Bar are brands of panhard rods designed for motorhome chassis.]

Sloppy fitting mechanical parts in the steering mechanism can also cause fidget steering because a sloppy fit allows the two front wheels to wander even when the steering wheel is held straight. Sloppy fit is a result of wear or sometimes just excessive tolerance in manufacturing. Each part is manufactured to a specification which allows a certain amount of variation, plus or minus, in the size. If too many of the steering parts are at the extreme of their size tolerance, the accumulated tolerance of multiple parts can result in steering which feels loose and requires constant correction. If you are having steering problems, make sure your front suspension is aligned as close as possible to its "dead center" specifications. If the problem persists, have a skilled mechanic check the tolerances on the steering system components.

A device known as a steering damper is sometimes used to help unwanted control steering movement. The devices known by the brand names of Steer Safe and Safe-T-Plus are RV steering dampers and many chassis have similar devices right from the factory. A steering damper is essentially a shock absorber and spring combination that works laterally on the steering system to reduce unwanted movement of the steering wheel. It does not stop movement of the wheels themselves if that movement still occurs even when the steering wheel is held straight, e.g yaw or fidget problems. Steering dampers are most effective at reducing sudden sideways movements caused by the front wheels striking potholes or curbs. Some drivers find they reduce steering fatigue, but fatigue should not occur at all if the underlying handling problem can be identified and corrected.

Because of the variety of motorhome handling problems and unfamiliarity with the terminology, most drivers don't know how to describe their handling problem or what sort of fix to ask for. "Doesn't handle worth a darn" is the usual complaint and "Make it better" the requested action. The average RV shop isn't any more knowledgeable and generally recommends a one-size-fits-all solution, which varies according to the shop or even which service advisor on duty. One will always say "stiffer shocks" while another always recommends a steering damper, with the result that motorhome owners usually end up buying add-on parts until they finally hit on the one that addresses their particular problem. This is also the reason why one owner will swear that a particular handling device is wonderful and another a worthless (but expensive) piece of scrap iron, while another owner will claim the opposite. Each of them had a different problem, on a different rig, caused by different circumstances. Armed with the knowledge of handling problems that I have provided here, hopefully motorhome owners can more quickly - and less expensively - zero on the right solution for their problem.