The case for fitting an air assist suspension kit for a motorhome is compelling in terms of safety, occupant comfort and even in reduction of wear of the steel suspension parts. This article begins with an outline of the fundamentals of vehicle suspension and a brief discussion of conventional arrangements. It then proceeds to describe air assisted suspension, to explore the benefits of this type of suspension in some detail and finally to outline the numerous solutions on offer from Marcle Leisure.
The Purpose of Vehicle Suspension
In order to fully understand the benefits of air assisted suspension, let us first of all go ‘back to basics’ and discuss fundamentals. A vehicle suspension essentially has two functions…
To keep the wheels in contact with the road surface as much as possible
To insulate the occupants of the vehicle against noise, vibration and general harshness arising from unevenness of the surface over which the vehicle is being driven.
The suspension is the major contributor to…
the ride quality of the vehicle: are the occupants well insulated against vibration and harshness arising from unevenness of the road surface (‘good’ ride quality) or not (‘bad’ ride quality)? Furthermore, when the vehicle is driven in a straight line over an uneven surface, does it remain relatively level (‘good’ ride quality) or does it pitch and rebound significantly (‘bad’ ride quality)?
the handling of the vehicle: does the vehicle remain level and true when negotiating corners or bends (‘good’ handling), or does it tend to drift sideways or roll (‘bad’ handling)?
Both ride quality and handling are affected by the amount of payload within the vehicle. The vehicle occupants may experience significant differences in the behaviour of the vehicle between the unladen and fully laden states, in particular with regard to ride quality.
Conventional Suspension Arrangements
A conventional vehicle suspension comprises of an assembly of mainly steel parts: springs, dampers (or ‘shock absorbers’) and linkages.
Motorhomes and light commercial vehicles commonly use bow-shaped ‘leaf springs’, as shown by the diagram. The springs continually store and release energy whilst they are deformed from their original shape as the vehicle is driven. The dampers absorb and release energy from the springs, helping them to recover their original shape significantly more quickly than they would without assistance. In the absence of dampers, the vehicle would tend to bounce during a period in which the springs continue to oscillate before relaxing back to their original shape.
The stiffness of the springs (‘spring rate’) and the degree of motion absorption provided by the dampers (‘damper rate’) combine to determine the degree of firmness or compliance of the suspension. A softer or more compliant suspension is often desirable in terms of ride quality, whilst a firmer suspension is normally preferable in terms of handling. Suspension designers therefore have what is often termed a ‘ride-handling compromise’ to contend with when determining spring and damper rates for a vehicle.
Air Assisted Suspension Arrangements
Air assisted suspension arrangements incorporate two air springs or ‘bellows’ between the chassis and the axle at the rear of the vehicle – one close to each of the two ends. The air springs are essentially air-filled cylinders having a rubber wall and sealed by either…
…a rigid ‘plate’ at each end, made of metal or plastic
…a rigid ‘plate’ made of metal or plastic at the top end, and a rigid ‘piston’ (again made of metal or plastic) at the bottom end.
Air springs for motorhomes are normally sealed with a plate at each end. The walls often have a convoluted shape, hence the term ‘bellows’.
The straight-sided type of spring sealed to a piston at the bottom is commonly known as a ‘rolling lobe’ air spring. This is because it has a ‘lobe’ at the bottom that skirts the piston and ‘rolls’ up and down the piston wall as the spring is compressed and elongated in use.
This diagram shows an air bellow fitted between the chassis and the axle. Here the bellow mounting brackets are attached by making use of existing holes and fixtures, which is typical of an air assisted suspension arrangement.
When fitting on an air assisted suspension system there is very little or no need for modification to the existing suspension parts. Most commonly, the only alteration required is to remove bump stops (or ‘jounce bumpers’) from the chassis and/or the axle in cases where the air spring mounting brackets are to be fitted in their place. Bump stops take the form of solid blocks of rubber that are fitted to a conventional suspension in order to prevent metal-to-metal contact that would otherwise arise if the suspension was fully compressed. Such a condition could arise with the vehicle either at rest (overloading) or in motion (negotiating an extremely uneven surface or ‘potholes’).
It is very important to note that air springs operate alongside the standard-fit leaf springs and dampers – they ‘assist’ in suspending the vehicle.
Advantages of Air Assisted Suspension
Having explored the fundamentals of vehicle suspension, discussed conventional suspension arrangements and introduced air assisted suspension, let us now consider the many benefits of fitting air assisted suspension to motorhomes.
Despite the fact that they are not inherently rigid, they contribute sufficient stiffness to the suspension that handling is not compromised – in fact most users report a significant improvement. At the same time, the air column within the spring helps to provide for a very comfortable, compliant ride. Here are the main benefits in some more detail…
- Vehicle Levelling:
Simply by varying the air pressure in the springs, the vehicle can be levelled both front-to-rear and side-to-side. Keeping the vehicle level optimises stability, ensures correct headlamp beam distribution and reduces tyre wear arising from uneven distribution of weight.
- Straight Line Stability:
Straight line stability is greatly increased at higher speeds, and when subjected to buffeting from strong cross-winds or large overtaking vehicles.
- Ride Comfort:
As previously mentioned, air springs help significantly in the absorption of shock loads from uneven road surfaces and therefore general ride quality is much improved. Furthermore, the amount of variation in ride quality between the unladen and fully laden vehicle fitted with air suspension is significantly lower than is the case for the same vehicle conventionally suspended.
- Reduced Body Roll:
Body roll is the tendency of a vehicle to lean to one side when the vehicle is cornering, or negotiating bends or roundabouts. The roll condition arises from the forces that the vehicle experiences under such conditions. Air suspension has proved to significantly reduce roll.
- Fatigue Reduction and Wear Compensation:
Suspension fatigue is reduced, so helping to prevent steel leaf springs from sagging under repeated or constant loading.
Any sagging already present can be compensated-for simply by varying the air pressure within the springs to suit. This is a particular benefit for motorhomes, which by their nature are always fully laden - especially so if fitted with a rack and carrying bicycles, for example.
- Temporary Raising of the Rear of the Vehicle:
For systems fitted with an air compressor, the driver is able to raise the rear of the vehicle temporarily in order to reduce the risk of damage to the underside that may arise from ‘bottoming-out’ when traversing protrusions in the road surface (e.g. speed bumps), or when boarding or alighting from ferry boats
Air assisted suspension systems are uncomplicated in their design and so are relatively simple to fit although, mainly for safety reasons, it is recommended that fitting is carried out by a qualified vehicle mechanic. Once fitting is complete, it is also generally recommended that the metal parts of the system are coated with a protective substance such as body wax. Very little maintenance is required other than to keep the air springs clean and to maintain pressure within them. Much like a tyre, the springs will lose some air over a period of time.
The Marcle Leisure Air Assisted Suspension Range
Marcle Leisure offers an extremely comprehensive range of air suspension solutions covering the majority of popular motorhomes and vans in common use today. The majority of these solutions are for motorhomes and light commercial vehicles having a conventional suspension arrangement with leaf springs, and are supplied by Dunlop Systems and Components.
Several options for inflation of the air springs are available. These range from a simple console having two Schrader-type valves for connection of an external air supply; to a complete on-board inflation solution having an individual pressure gauge for each air spring, and an associated air compressor and reservoir.
|Simple Console for External Inflator Line
Marcle Leisure also offers solutions for other types of suspension…
Al-Ko Chassis Solutions
The increasingly popular Alois-Kober or Al-Ko galvanised steel type of chassis is provided-for within the Marcle Leisure range.
The diagram here shows this type chassis fitted with a rolling lobe type air spring for suspension assistance, a system again supplied by Dunlop Systems and Components.
For cars, motorhomes and light commercial vehicles having a helical-coil-sprung suspension, a solution supplied by the Firestone Corporation and known as ‘Drive-Rite’ or ‘Coil-Rite’ is offered.
Coil-Rite comprises of two air springs which are fitted within the existing coil springs. The resulting arrangement provides for all of the benefits of air assist suspension described within this article.
The addition of an air assisted suspension system to a motorhome or light commercial vehicle provides a host of benefits…
- A level vehicle stance is maintained
- Straight line stability is much improved
- Ride comfort is significantly enhanced
- Body roll on cornering or when negotiating bends or roundabouts is much reduced
- Suspension fatigue is lessoned and sagging can be compensated-for
- The rear of the vehicle can be raised on a temporary basis, for example just before boarding or alighting from ferry boats
Marcle Leisure offers an extremely comprehensive range of air assisted suspension solutions, not only for vehicles having the most commonly used leaf spring type of suspension but also for those having an Al-Ko chassis and even those with a coil sprung suspension arrangement. Explore the Marcle Leisure website to discover the full range of solutions available.