Firm foundations?

If you're looking at coachbuilt motorhomes for sale, you should compare the different models’ chassis foundations before buying, explains John Wickersham

By John Wickersham
Published: July 31, 2015

Practical Motorhome goes behind the scenes at Al-Ko - Get the low-down on Al-Ko chassis with Practical Motorhome's technical expert John Wickersham - Copyright: Practical Motorhome Productions

Get expert advice on chassis from John Wickersham – since the moulded body on his self-built model was designed to match the cab of a Fiat Ducato with a light commercial chassis, that decided the structure needed to support its GRP shell

This 1999 Fiat Ducato Maxi was ordered, and a chassis specialist added full air suspension and extension members between the front and rear wheels

Long rear overhangs often lead to overladen rear axles so the usual idea of bolting on these extending members was discounted

Since commercial chassis might not always suit motorhome constructions, this purpose-made structure from Fiat was exhibited in 2007 – its low chassis, wider rear track and cab cut-out helped a lot

When Bailey Caravans prepared to build motorhomes around 2008, it decided to create low-line coachbuilts and this led to the use of Fiat’s Peugeot-badged cabs with galvanised Al-Ko AMC chassis

At the Al-Ko Kober factory, various types of base vehicle are fitted with lightweight motorhome chassis and torsion-bar rear suspension instead of Fiat’s commercial-type of leaf springs

Whereas ‘camper motorhomes’ use vans that have a ‘floor pan’ and not a traditional chassis, a few coachbuilt models from Dethleffs employ what are called platform cabs from firms like Fiat

Auto-Sleepers (the Mezan) and certain models from Bürstner, Chausson and Hobby are built on these low-level floor panels, which include wheel covers; platform cabs often suit low-line motorhomes

With A-class coachbuilts, there’s no point purchasing a base vehicle complete with a cab when the motorhome builder will be creating an all-encompassing body enclosure

As a rule, A-class models are constructed using what is called a ‘chassis cowl’ base vehicle – this has a bonnet-free power unit with fascia instruments and many have an Al-Ko chassis added

In the Al-Ko Kober factory car park there are usually rows of back-to-back cabs that have been ordered by motorhome manufacturers whose projects require an Al-Ko lightweight chassis

Depending on a firm’s model dimensions, an Al-Ko chassis is assembled in the factory and all the units are bolted together; recessed seatings for bolts achieve an accurate register

Whereas one cab might be coupled to a single-axle chassis, another might need a tag-axle configuration – this type of chassis is assembled to suit the client

As a rule, commercial types of chassis coupled to a cab are quite high from the ground to maximise clearance for industrial users – for this motorhome project, an Al-Ko chassis can be much lower

Some types of motorhome are designed with a double floor so that storage lockers, cable runs, pipework, heating appliances and so on are mounted under the living area; this Al-Ko chassis permits that

Seat and safety belt assemblies for interior travel seating (protected here in polythene sheet) can be bolted directly through a ply floor-panel and into the chassis below to create full anchorage

As revealed in Bailey crash testing experiments, floor supporting outriggers bolted to chassis were essential, however, some makes don’t have them at all

A completed chassis is subjected to numerous tests before it is deemed ready for the later conversion work – adding weights before testing brakes on this rolling road is only one of the many checks

The Al-Ko Kober weight plate is affixed to the base vehicle and maximum axle loads are determined by the type of chassis and running gear fitted – the MTPLM for this vehicle is 3850kg

Providing the body fitted by a motorhome builder doesn’t extend rearwards beyond the chassis members, installation of items like an Al-Ko towbar or bike rack are simple bolt-on operations

Watching this Hymer going through checks by a rust treatment specialist was enough to confirm that its owners need to keep a constant watch on the condition of the chassis and related items

Under-floor inspections often reveal that the deterioration of an otherwise serviceable motorhome needs urgent attention – in reality, it’s better to get work done before these problems arise

Keeping a commercial chassis painted undoubtedly helps for a few years, but to do it properly needs good access – and even then, the rust will soon re-appear, especially if you travel all year round

In the end, it has transpired that thorough preparation of chassis members followed by careful treatment using good quality rust-protection products was needed to extend the life of my vehicle

Because a vehicle’s chassis is hidden below the floor, many potential owners don’t appreciate that it determines features such as the height of the habitation area, the scope for underfloor storage in the vehicle and the security of extra ‘travel seats’, as well as the ease of adding a towbar and taking care of maintenance matters.

Of course, a chassis is also linked to other underfloor items, referred to as ‘running gear’. However, we will leave the merits of different kinds of suspension, braking assemblies, wheels and tyres for another occasion.

Building and using a pair of coachbuilt motorhomes since the late 1980s has certainly taught me a number of lessons. Attending automotive crash-testing sessions has also revealed some of the weaknesses within professionally built models. The same can be said of conducting live-in tests using manufacturers’ loaned motorhomes; the time spent within them often reveals unexpected features.

It is fair to say that there are far more intriguing items of motorhome equipment to grab the interest of prospective purchasers, but a prosaic feature such as a chassis can have an impact on several ownership issues. It might also come as a surprise that there are several different types of chassis.

The five variations shown in our gallery above are well-engineered products, which are strong and reliable, as well as offering contrasting strengths – and weaknesses. Moreover, each has its particular merits and suitability for different types of motorhomes.

If we sought to find one key difference, it would be the fact that vehicle manufacturers’ chassis are usually made to meet commercial and industrial needs. The Al-Ko AMC lightweight galvanised chassis, meanwhile, are especially (though not exclusively) designed to fulfil the leisure-styles of motorhome users. When the Al-Ko AMC chassis was introduced in the early 1980s, a motorhome manufacturer would purchase a light commercial vehicle complete with a multi-purpose, heavy, rugged chassis with leaf springs. This would be sent to Al-Ko Kober to have the chassis cut away at a point to the rear of the cab. The steel sections were then scrapped and an Al-Ko chassis would be coupled-up to the cab in its place.

This is undoubtedly a wasteful process and Fiat recognised that the use of its products by motorhome builders could be improved. That’s when it started supplying cabs in pairs with no main chassis at all. The company also designed its own motorhome-specific chassis and cabs with pre-fitted swivelling seats. These units included items like camping tyres, which are designed to meet a typical motorhome’s pattern of use.

And please note: although some Al-Ko AMC chassis have been installed on rear-wheel-drive base vehicles, most are designed for front-wheel-driven models.

Chassis types: a commercial steel, painted chassis

My 16 years using a self-constructed motorhome based on a Fiat light commercial chassis have been revealing. For instance, component specifications for all light commercial vehicles are intended to suit the relatively short life and high mileages expected of most working vans. In contrast, motorhome owners typically cover lower, irregular mileages and expect their vehicles to provide enjoyable holidays for around 20 or more years.

With that in mind, my brand new chassis was meticulously injected internally with Waxoyl rust retardant before the body was fitted. However, its external white paintwork showed rust patches quite quickly and was given its first re-spray after seven years of ownership. A further re-spray was required after its 10th birthday and that was done with the help of a garage hoist.

In spite of this attention, the chassis, towbar and parts of the cab soon needed serious anti-rust treatments. After comparing several specialist service providers, a two-day concentrated refurbishment with two operators working simultaneously was carried out by Rustbusters in Poole, Dorset.

Chassis types: Al-Ko AMC systems

The fact that an Al-Ko chassis has a galvanised finish and needs no maintenance is a great asset. However, after driving on treated roads in winter, you’d be advised to hose-off excessive salt from the surface. Equally, if you hit an obstruction on the road and expose the chassis’ bare metal, it’s important to get the damaged area treated with a galvanising repair product.

Also, be aware that these chassis soon lose their smart shiny appearance, and that is normal. If the subsequent grey looks less attractive, don’t worry; it actually indicates that the treatment is following an intended ‘hardening’ process.

Other points to note include the following:

Computer-designed chassis members are light but still achieve the rigidity that they need
Motorhome designers’ floor heights can be achieved more easily when a chassis is specifically ordered
An Al-Ko torsion bar suspension system is fitted to suit a new motorhome’s projected use
Since these chassis are bolted together rather than welded, there are instances when a damaged section can sometimes be replaced
Al-Ko accessories, such as towbars, bike racks and belted travel seats, are made to bolt on to the chassis with the minimum of difficulty
Different types of chassis can be supplied, for example double-floor-unit types as shown in our images above.

Maintaining a painted commercial chassis

When it came to my own motorhome, the condition of its chassis was deteriorating quite rapidly after 12 years. Urgent maintenance involved welding a small steel plate on the floor below the driver’s door where holes had appeared near the sill. Fortunately, refurbishment preparation underneath, followed by the application of Waxoyl rust inhibitors, were done in time before more serious failings materialised below. It is only now that I feel that this 15-year-old vehicle will probably be in safe running order when it reaches its 20th birthday.