Improvements in Handling

Motorhomes are never going to handle like a Ferrari Testa Rossa. Still, considering how much size and weight is being thrown around, they aren't too bad. Some chassis can be just fine right out of the box, especially the diesel pushers. However, some chassis can benefit from a bit of assistance. Sometimes a bit of tweaking here and there can make a huge difference. The secret is knowing what is needed. Many RVers spend needless amounts of money installing handling assists that really don't address the problem they have. Hopefully this FAQ might help in that area.

Basic Geometery:

Before we can determine what is needed, we first need to determine what kind of forces are at work in an RV.

An RV has a suspension system. The main purpose of the suspension system is to support the weight of the vehicle. If the springs are worn and sagging, the vehicle won't be at the proper ride height. This will affect the steering geometry as well as a number of other things. If the springs are weak, they also won't be able to properly adjust for bumps and dips in the road nor will it be able to handling leaning, nose diving under braking, or rear squatting under acceleration. Therefore, the first step is to make sure that the springs are adequate to handle the load.

On a new coach this should not be an issue. As long as the coach is not overloaded it should be at the proper ride height. If it isn't, then there is something wrong. Step one is to confirm your weights by taking your RV to a scale and recording the readings on each axle. If at all possible get separate 4 wheel readings, especially if you have a lean or sag to one side. If you are overweight, reduce that weight to conform to the GVW and axle ratings for your coach. If you have weight ratings that are within the designed load specs and the coach still is not at the proper ride height, then you have a problem. Seeing it's a new coach it's probably time to take it to the dealer. If it's a DP with air-ride suspension it may be as simple as adjusting the height sensing valves to give you the proper ride height. If it's a gas chassis with leaf springs, you'll have to find out why these brand new springs aren't doing their job.

On a used coach things change a bit. If you have leaf springs that are sagging, chances are they are wore out. Rather than add helper springs to it, consider replacing the leaf springs with new ones. Helper springs will keep you from sagging but they are more of a second stage thing and won't help as much in leaning or nose-diving. Once you have established that your coach's springs are capable of supporting the load it's safe to go on and analyze what it happening to your coach.

Yaw, Pitch, and Roll:

I know, it sound like we're talking about airplanes here, but the in-motion directions of an airplane aren't all that different than an RV. Either way, there's three dimensions of movement possible.

Pitch is the attitude of the nose. In a plane we'd be climbing or diving (negative pitch) unless the pitch was at zero, in which case we'd be flying straight and level. An RV is no different. We should be riding down the highway straight and level. If every time we hit a bump the nose pitches up or dips down, then it's a pitch issue.

Roll in an airplane is when you drop a wing down. Now when you look out the side window you are seeing either ground or sky. Keep it going and you get a barrel roll. Try not to do that in your RV though. It's really hard on the rooftop air conditioners. On an RV roll is when the RV leans to one side. When you drive something that tall and that heavy you are bound to get a certain amount of lean, especially if the road surface is not level, or you are riding on a crown. Because you are sitting up nice and high, roll will be the first thing you'll notice from the driver's seat. However, if the roll turns into a repetitive sway, you do have problems and these can be addressed.

YAW is when you use the rudder on a plane or boat to turn left or right. Again, when driving straight down the highway you don't want any yaw or you'll be fighting the wheel the whole way. This too is correctable and generally the most common area that needs correction in motorhomes.

Pitch Issues:

Okay, so your springs are just fine, you're riding down the highway nice and level. Then you hit some rough pavement and the front end of the motorhome bounces up and down excessively. Why?

To start with, any action is accompanied by an equal and opposite reaction. When you hit the bump, the coach moves up into the air. After you roll over the bump the coach comes back down. Except, it isn't that simple. Once the coach is thrown up into the air it likes to keep on going. Eventually the momentum subsides as the springs stretch out and it comes crashing back down to earth. Again, it wants to keep on going and compresses the springs real tight before they push the coach back up. Naturally this continues for a while until enough energy is scrubbed off to return to a level ride. In order to keep this from continuing the coach needs a way to dampen this action. That's where shock absorbers (or dampeners in the UK) come in. The shock absorber is designed to scrub off that energy by passing fluid back and forth between two chambers as the shock absorber extends and collapses. There is a real science to designing these things. If the shock is too loose you be riding in a bouncing RV. If the shock is too stiff you'll be in a lumber wagon. Shocks get worn and need replacing. Also, not all OEM shocks are spec'd as great as they could be. Here's a shocker for you - sometimes the manufacturer's are more concerned about how inexpensive they can build it. Big surprise, huh? In this case you will need to get some better shocks to dampen this action. We won't go into which ones to get here because there's always new shocks coming out and that information changes, For that info I'd ask around in the forum section from those who have done that. Remember, if your springs are fine and the shocks are fine, you shouldn't have any excessive pitching.

Roll Issues:

Next, you're driving down a narrow blacktop road with a whole lot of crown. You find yourself leaning to one side. Unfortunately, this is normal. Unless you have an air-ride suspension with a form of automatic 4 wheel height adjusting control, you are not going to be level. Either find a flatter road or drive down the middle of this one.

However, if you are going down a nice interstate highway and every time a truck passes you or every time a gust of wind kicks up you find the coach leaning back and forth you do have excessive roll. Also, if you turn a corner and the leans over like the Titanic you also have a roll issue. First thing is to go back to the shocks. Are they in good shape? They also control Roll as well as pitch but remember - they do not "limit" roll nor pitch, they only dampen it. If you have lots of quick little leans repetitively you may have a shock issue. Better shocks may dampen that action and make it less noticeable. But, if you want to limit how much roll your vehicle has, you need to look elsewhere, namely the anti-roll bar.

Anti-roll bars are sometimes referred to as sway bars but that can be confusing so we'll refrain from doing that. An anti-roll bar is a huge round steel bar, shaped like a horseshoe. The center of the horseshoe is attached to the vehicle frame with pivot points. The tips of the horseshoe point forward and each tip attaches to one of the front wheels. As you go bouncing down the road this anti-roll bar will simply rotate in the frame mounted pivot points. It won't affect a thing. However, when you enter a turn the body want to lean to the other side because of centrifugal force. When this happens the inside tire will tend to lift up on it's anti-roll bar tip while the outside tire will try to push down on it's anti-roll bar tip. Needless to say, the horseshoe doesn't like being twisted and wants to return to being flat again so it tends to restrict this movement. These are steel and they can lose their temper after a while and no longer be as effective as they once were. Also, some coaches have more weight up top and the OEM anti-roll bar really isn't adequate for the task. In that case, installing a stiffer aftermarket anti-rolle bar will make a big difference. Generally this means goes to a bar with a larger diameter. To a point, you do want some flex in the coach. If you were to install a super-stiff anti-roll bar and enter a curve there would be no lean whatsoever. But, you have a tall vehicle, not a go-kart, and chances are that the inside tire would lift and lose traction in the curve. Plus if you went too fast it would flip the coach over much sooner than if the anti-roll bar allowed it some give. Just like shocks, too much can be as bad as too little. The key is that anti-roll bars only affect roll and not yaw or pitch.

Yaw Issues:

This is the big one. The number one complaint of motorhome drivers is that it just doesn't go where they want it to go. Rather than going in a straight line it wanders all over. This is very common in gasoline powered chassis due to their design parameters, which favor cost effectiveness rather than performance.

Before we go too far, I guess it's important to mention tires here. Tires are covered in the Tires and Wheels FAQ in much more detail, but it's important to bring up the fact that tires that are not in good shape, tires that are not properly inflated, and tires that are not of the proper specs for that motorhome will mess up handling. Be sure that your tires are correct before messing with the suspension.

What keeps your vehicle going nice and straight? A number of things, actually. The first thing is to determine that you don't have anything worn out on your RV. If the shackle bushings or control arm bushings are worn, you won't be able to keep the wheels going in the same direction and they'll walk all over the place. This requires jacking the weight off and prying around on them to see if there is any movement. On a solid front axle you need to take the weight off of the front axle by jacking the front end up (you can use the levelers). Then grab the top and bottom of the tire and see if you can wiggle it in and out. If it's tight, there should be no movement.

Also, the steering mechanism needs to be tight. While you have the front end raised, wiggle the tires from front to rear to see if there is any slop in the tie rod ends or drag links. A common wear point is the bell crank bushing. This is where the steering box link connects to the drag link, which operating the steering arm on the left front wheel. There are better grade aftermarket bell cranks available that can tighten up this action and give you greater service. A popular bell crank is available on the Henderson's Lineup website.

Assuming that everything is tight and in order, then it's time to see what the symptom is and if we can rectify it with an upgrade or add-on of some sort.

The most common complaint is that every time a large truck passes the RV driver has to steer right when the truck's bow wake hits the tail of the RV. Then, when the truck passes the RV the RV driver has to steer left to keep the nose back in line as the truck's suction vortex passes. The reason for this is simple - RVs have a very long rear overhang. Think of a teeter totter laying on it's side. The rear axle is the pivot point. Whenever pressure (wind or bow wakes) is applied to the left-rear corner of the RV, it ends to push it away. The front of the RV then goes the opposite direction as it pivots on the rear axle. This is more prevalent is gas chassis RVs because they are lighter and the leaf springs have more lateral looseness due to the sideways flex inherent to leaf springs. This actually tends to push the coach body to the right as the rear axles remain in position. If you could exaggerate it you would see the coach body riding in the ditch with the axles sticking out the left side and the leaf springs flexed at an angle. Diesel pushers are less susceptible to this because they are heavier and the air-ride suspension has lateral locating links that keep the coach body centered over the axle at all times.

It's important to understand that the issue does not lie at the front of the coach. Even though you are making constant steering corrections the root problem is at the rear of the coach. It's a matter of the tail wagging the dog, and a little dog with a big tail at that. Therefore, by trying different front end add-ons you won't be as effective at mitigating the problem. Instead, correct it back at the rear axle, where the problem begins. The easiest way to do that on a gas chassis is by installing a rear track bar to keep the axle laterally located underneath the center of the coach. The number one choice of this is the Super Steer Rear Trac Bar. We used these on race cars in the old days and they were called Panhard rods. One of these will give you lateral axle control just like a diesel pusher. Note that a rear trac bar will not affect up and down movement, nor will it affect body roll or lean. However, when the "18 wheeler push" tries to lay one on you, the body won't pivot on the rear axle and you'll find that the amount of steering correction is drastically reduced.

Another yaw related issue is rut wander. This has nothing to do with the back end of the coach. Rut wander is when the front wheels tend to follow surface irregularities in the road. Instead of you controlling the way the wheels attack the road, the road is controlling the wheels. This is generally caused by looseness or flex in the front end geometry. It may not be worn out but it may just be something in the way it's designed. Certain coaches will have more of this tendency than others, depending upon how the weight is distributed. There are various front end tightening devices available, such as the Davis Tru-Track. These can minimize rut wander and other steering artifacts.

Summary:

There are many variations in handling characteristics. Handling is affected by so many different parameters that there are no hard and fast rules that can equally apply to all coaches. The important thing is apply these steps in the proper manner to prevent chasing problems by throwing unnecessary dollars at them. A good alignment specialist such as Henderson's Line-up can give you expert advise on what is truly needed. If you are doing your own work be sure that everything is in good functional shape. There's no sense adding some mods to your chassis if all that's really wrong is that some parts are worn out. Replace them first, then verify that your tires are up to the task. Finally, if you need chassis mods be sure to address shocks first, then the rear of the coach, before attempting any front end mods. Chances are that once the rear end is working properly you may not need anything done to the front end.

Submitted by Mark Quasius - 2/10/06
 

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