Independent VS Single Suspension


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If the roads we towed on were perfectly flat, then our RVs wouldn’t need suspension.  But since they vary from fresh bitumen to potholed horror stretches, our vans need to be able to cope in all conditions.

Without suspension, those bumps would cause the after-shock to go right through the frame of our van. The wheels would lose contact with the road, then skid back again, making towing not just difficult but downright dangerous. Suspension, by keeping the wheels and tyres of our van in contact with the ground, is designed to counteract these forces of gravity, propulsion and inertia when we accelerate or brake heavily.

THREE THINGS SUSPENSION SHOULD DO

Contact.  Keep the tyres of the RV in contact with the road surface.

Handling. Keep the RV from tipping or rolling into a corner as it absorbs the centrifugal forces.

Comfort.  Keep the contents of the RV isolated from the bumps on the road.

MATG_215_TT_1NEWTON’S LAW OF MOTION

Without being too technical, Newton’s Law of motion suggests that all forces have both magnitude and direction.  Simply put, a deviation in the road surface causes the wheel to move up and down perpendicular to the road surface. The magnitude naturally depends on whether the wheel is hitting a large or small bump. Either way the wheel will experience a vertical acceleration as it passes over the deviation.

Suspension absorbs the deviation and reduces the rate of vertical acceleration.  Understanding this concept gives you a better appreciation why you need suspension.

SO MANY CHOICES

Just as there are different styles of caravans in the market, there are also different suspensions and setups. Not all suspension systems are the same, nor do they work or cost the same to manufacture. Some are low-maintenance while others need regular inspections and servicing.  Some are designed for the black top while others are designed for off-road dirt tracks.

To help allay the confusion let’s take a closer look at the more popular types to see how they work.

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THE BASICS

Joe from G&S Chassis says:

The choice of suspension system for a caravan is determined by a number of factors such as its weight, its payload, its layout, and the type of terrain for its intended use. From here, it’s important that the design is engineered and calculated accordingly.

WHAT YOU SHOULD KNOW 

BEFORE YOU BUY

Joe from G&S Chassis says:

It’s important for a buyer to be clear about the intended use of an RV. It’s no good getting a standard slipper leaf spring set up when you intend to take your caravan regularly on to rough dirt tracks.  You’ll need a design suspension suitable for those conditions.

Let’s look at some of the options on offer.

Leaf Springs and Axle setups

This is the simplest set up of the lot. Leaf springs come as single leaf or in packs and are clamped together. The longer the spring, the greater the flex. The greater the number of leaves, the more load it can carry.

Slipper Springs

This is the least expensive type of spring as it is fixed only at the one end. At the opposite end, the spring is flattened out slightly.  This results in a sliding contact with the chassis frame. Offered on many smaller RVs, this works fine provided suitable ratings of springs and axles are selected.

Eye to Eye Spring

Using a form of bush at each end, the eye-to-eye spring can flex more freely and is quieter and smoother in operation. To tune a leaf spring suspension, it’s necessary for engineers to determine the length of the spring, its thickness and the width of each leaf then calculate the number of leaves. Leaf spring manufacturers provide rating charts to guide buyers in the right selection.

Torque – Arm Suspension

Consists of an outer square tube into which a splined shaft surrounded by a rubber/urethane compound is introduced under high pressure.  The stub is welded offset to the splined shaft which provides the action for the tube to twist against the rubber compound under load.  They work well provided they’re not overloaded, but replacing of the rubber can be done only by the manufacturer due to the unique process.

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TANDEM VS. SINGLE SUSPENSION

There are many advantages that Tandem has over Single:

  • Tandem systems wheels cover twice the area on the ground, therefore halving the weight density.
  • Less weight pressure reduces the likely risk of punctures.
  • They are generally considered safer when it comes to a major tyre failure.
  • Tandem systems will track behind a tow vehicle with less tendency for swaying.
  • Load carry capacity is greater.

Tandem Rocker Suspensions 

These are also known as load sharing (not independent) and are designed to distribute the weight across all four wheels. When compared to fully independent, they are a lot cheaper and are the most common tandem suspension setup.  They do require regular maintenance due to bush wear.

Tandem Rocker – Roller Suspensions

This is similar to Rocker suspension although there is a big loop at the end of each spring where a roller runs. The loops allows for small axle movement, while bigger axle movement rocks on the main pivot arm. This type has a longer service life and is less prone to spring breakages.

Independent Suspensions

These types of suspensions are popular and are built specifically for off road use.  The design uses a pivoted leading edge fitted up with a stub axle wheel bearing assembly.  The load is taken by a trailing arm arrangement in conjunction with other components such as coils or leaf springs, and some now use an airbag.  Each wheel is able to move independently of any other wheel.

Simplicity Suspension

This is a load sharing independent design for robust conditions.  It has a swing arm for each wheel and relies on a heavy duty leaf spring arrangement which is load sharing across a centre pivoting fulcrum.

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Independent Coil Suspensions

This type has the ability to keep the tyres in contact with the ground whilst at the same time reducing the overall weight in the suspension set up. Coil suspensions are less prone to breakages when compared to leaf.  However, they do need to be assisted by shock absorbers to prevent excessive bouncing. These suspension set ups do provide a smoother ride over corrugated roads and they require closer monitoring and servicing.

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Air Suspension

Air Suspension uses a tough expandable rubber bag loaded with compressed air. The big advantage is the variable air pressure setting that can be applied depending on the load being carried. This type of setup requires an air compressor and fixed pressure piping that is well protected, and is at the more expensive end of the scale.

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OTHER THINGS TO CONSIDER

We have only briefly touched on the popular types of suspensions currently on the market.  There are a variety of other designs to suit special-purpose applications.

It is important that the suspension system is designed to suit the van for its intended purpose. The selection of wheels, tyres and braking systems all play a vital role as does the configuration of the caravan and how the weight is distributed throughout the design.  Chassis suspension designers go to great lengths when testing to pinpoint the best configurations to provide trouble-free towing, working within ADR guidelines and rules.

MATG_215_TT_4Be careful when you start to modify van layouts.  Make sure the chassis designers approve of these changes. Achieving a well-balanced RV when fully loaded, including the full tare loads on board, requires complex engineering calculations. The result should be a caravan that will tow faultlessly.

Good quality, well-designed suspension suited to the RV’s intended purpose will ensure that its wheels remain in constant contact with the road. This will result in a smooth and comfortable towing experience for you and the contents of your RV.  Here’s to trouble-free and safe travels!