You’ve finally hitched up the new trailer, loaded family and gear aboard and breathed a sigh of relief. But as you pull out of the driveway on your shakedown trip, the undercarriage scrapes loudly as it digs into the pavement. Going down the road, the suspension wallows precariously, headlights point skyward, and the vehicle bottoms out on every dip.
This is nature’s way of telling you the vehicle’s suspension is not up to task.
Our toys – like some of us – have gotten larger and heavier than ever before. Most stock suspensions favor ride comfort over maximum hauling capability and as a result, many vehicles tend to ride low and have poor handling when fully loaded.
Safety and Comfort
Heavy loads combined with weak suspension often leads to difficulty steering and controlling the vehicle, increased tire and shock wear and other problems. Not only can this be uncomfortable, it can be damaging and unsafe.
Before investing in suspension modifications, you should weigh your fully loaded tow vehicle and compare the numbers with the manufacturer’s ratings, especially if it is riding low when loaded. All vehicles are engineered for a peak capacity called the gross vehicle weight rating (gvwr). The gvwr is how much weight a vehicle is designed to carry and includes the net weight of the vehicle, plus weight of passengers, fuel, cargo, accessories and hitch or pin weight applied by a trailer. A vehicle’s gross axle weight rating (gawr), determined by the manufacturer, is
the maximum allowable weight that can be placed on an individual axle.
Using a commercial truck scale, drive the loaded tow vehicle with trailer (or camper) onto the platform, first with the front axle, and take a reading. Compare it to the front axle gawr. Then pull forward until both front and rear axles are on the scale. Take a reading and subtract the
front axle weight from it. This is your rear axle weight; compare it to the rear gawr. If you use a commercial scale with a multi-segment platform you may be able to get all of these readings at once because the scale records the weights for each segment. The information then will be on the printed data sheet you receive from the weight-master.
Finally, pull the whole truck and trailer onto the platform and take a reading. Compare this to the gross combination weight rating (gcwr), which is determined by the manufacturer to be the maximum weight of a loaded tow vehicle and its attached loaded trailer. These ratings may be found in the driver’s doorjamb, glove box, owner’s manual or from the manufacturer.
Tires have maximum weight ratings marked on the sidewalls. These are based on maximum inflation pressure, and there are separate ratings for single- and dual-tire applications. A vehicle should never be loaded beyond the manufacturer’s listed maximum gawr, gvwr, gcwr or tire ratings. If it is overloaded, you may have to carry less water or cargo, upgrade the tow vehicle, get a lighter trailer or camper – or go on a diet!
The suspension system includes springs or torsion bars and shock absorbers, as well as sway bars and other components that connect the vehicle to its wheels. Understanding what these major components do is important if you intend to get the most from them.
Springs (or torsion bars) support the vehicle’s weight and compress and extend to allow the suspension to flex and react to impacts such as bumps. Keeping the vehicle at (or reasonably near) stock ride height means you maintain the designed travel before the suspension stop hits the frame and bottoms out, which translates into a smoother ride. When the ride height of a vehicle is changed from stock, the angles of the U-joints and drive-shaft(s) also change, which leads to problems of wear and vibration. Therefore, your goal should be leveling the vehicle – both side-to-side and front-to-rear.
The “spring rate,” which is measured in pounds per inch of compression, determines how firm the ride is and how much it will sink when weight is added. Springs with a higher rate will not compress as much from a given weight as “lighter” springs. It is important to note that heavier or “helper” springs don’t increase load-carrying capacity and you should never exceed the vehicle’s recommended ratings.
To determine how much ride height changes with load, measure the vehicle height. Using a tape measure, record the distance from the ground to the middle of each wheel well, directly above the axle center-lines. Do this before and after loading and hitching and compare the changes. Weight-distributing hitches should be used with heavy travel trailers, but cannot be used with fifth-wheels, so stiffer springs or air bags may be needed to level the truck.
Coils, wound from heavy steel wire, are the simplest and most common type of springs. The coils vary in length and number of spirals. Replacement coils are available with variable rates, thicker metal for heavier loads and longer lengths. Variable-rate coil springs provide a changeable rate of resistance to help reduce sway and sag.
Leaf springs are typically used on the rear (and sometimes front) of heavier trucks, as well as on the front of many lighter four-wheel drives with solid axles. Leaf springs typically use several layers of metal leaves bound together, so they operate as a unit.
Two-stage leaf packs offer a smoother ride on the primary stage when the vehicle is unloaded and additional support when the secondary stage is employed. If the vehicle rides slightly too low, aftermarket add-on leaves are available that can be bolted onto the existing spring. Heavier replacement springs are also widely available from spring shops.
Variations in leaf width, number and thickness tailor the spring to the load.
Torsion bars – which are often used with independent front suspensions – are straight round bars connected to the vehicle’s frame at one end and the suspension’s lower control arm at the other. They use the twisting property of steel along the bar’s length as a spring force. Torsion bars can be adjusted within a certain range to raise or lower the ride height, and some aftermarket products are available for vehicles equipped with them.
Air Bags/Air Springs
Many of today’s trucks and SUVs are built with softer suspensions to provide a smooth, comfortable ride, which makes accessories such as air springs (also known as air bags) suitable products to help control heavy loads. Air springs have become a popular way to keep vehicles level with varying loads, such as trailers and slide-in campers, without having to install stiff metal springs. Air allows adjustment by varying inflation, and can improve both ride quality and stability.
Air springs install between the frame and the axle to support the load. They’re available for both coil and leaf-spring suspensions and can
often be mounted inside existing coil springs. Kits are offered in varying capacities and tailored to specific vehicles, making installation considerably easier. Accessories are also available to inflate and deflate the bags with a push of a button from the interior of your vehicle.
There’s also a new product called SumoSprings, from SuperSprings Company that looks and performs like an airbag without being inflated with air. It’s one more option in the auxiliary-support aftermarket product line.
Shock absorbers control spring oscillation, or vehicle bouncing. When the shock extends or retracts, an internal piston moves through hydraulic fluid, which is forced through specifically sized holes to resist movement. A shock’s resistance to being pushed in is called compression damping and resistance to being extended is called rebound damping.
Shocks are available with adjustable valving that can tailor operation, for extra control when hauling and softer ride when solo, and even remote controls to adjust them from the dash. Heavy vehicles and those driven on rough roads may benefit from multiple or heavy-duty aftermarket shock absorbers because they resist fade, which occurs when the shock’s oil overheats. Standard factory shocks are often smaller in diameter and contain
less oil than heavy-duty aftermarket units, so they can’t handle heat as effectively. If your rig wallows it’s time for an upgrade.
Many people mistakenly think that shock absorbers support weight and prevent sagging. This is only true when the shocks have air bladders (air shocks) or have coil springs wrapped around them. Adding air shocks or coil-over shocks can help a sagging condition, but it can also overload the shock mounts if they aren’t up to the task; check this before purchasing items.
Suspension arms that pivot as the wheels move up and down usually run on rubber, polyurethane or nylon bushings. As they wear, vibration starts to creep in and the suspension has extra movement from the worn bushings. As a result, handling and tire wear suffer. Polyurethane bushings are generally stiffer and more durable than the rubber bushings and are popular for replacement.
Here’s another oft-misunderstood item. Anti-sway bars – or sway bars for short – use a steel bar connected to the outer portion of the suspension with links that have rubber bushings. “Sway” bars don’t do much for swaying and don’t stiffen the ride going straight, but do help resist leaning in corners. As such, a “sway bar” would be more accurately known as an “anti-roll bar” or “roll bar” for short. Such bars come in different diameters – the thicker they are, the more effective they are. Adding stiffer sway bars is especially helpful with tall slide-in campers, which have high centers of gravity.
Steering Stabilizers, Etc.
A steering stabilizer is essentially a shock absorber for the steering system, and some four-wheel drive vehicles come from the factory with them. Steering stabilizers dampen jolts and wheel shimmying from rough roads or potholes and help stop the steering wheel from shaking back and forth. Aftermarket kits are recommended if you have oversize tires, or if your vehicle shimmies after hitting bumps, crossing railroad tracks or in other
similar situations. There are also track bars and steering-box stabilizers that can reduce wandering and improve steering control.
Qualified technicians should service steering and suspension components. RV shops, truck-spring shops and four-wheel drive centers are good resources for parts and installation. Always inspect the vehicle after modifications and gradually increase speeds only after determining that the vehicle drives safely.
Sources for more information
Air Lift (air springs),(800) 248-0892, www.airliftcompany.com.
BD Diesel Performance (Dodge trac bars, steering-box stabilizers), (800)
Bilstein (shocks), (858) 386-5900, www.bilstein.com.
Edelbrock (shocks), (310) 781-2222, www.edelbrock.com.
Firestone (Ride-Rite air springs), (800) 888-0650, www.ride-rite.com.
Hellwig (sway bars, air springs, compressors), (800) HELLWIG, www.hellwigproducts.com.
Koni (shocks), (859) 586-4100, www.koni-na.com.
Monroe (shocks, air shocks), (734) 384-7809, www.monroe.com.
Rancho Suspension (shocks, springs, steering stabilizers), (734) 384-7804,
SuperSprings, (800) 898-0705, www.supersprings.com.