[MrE]

Derek,

I tried finding a link to an article that changed the way I look at motor homes forever but unfortunately it is now a dead link. I did however find reference to it in a 2006 post on this site.

12-03-2006, 04:22 PM

#2

fulltiming

You might take a look at this article written by Bob Gummersall, the Chief Technical Officer for RVersOnline.org.

The Chassis under 99% of the Class A Motorhomes manufactured today is a basic Frame Rail. One Hundred % of over the road passenger buses are built on monocoque or semi-monocoque chassis. Full monocoque chassis surround the complete vehicle with frame members. Semi-monocoque chassis use frame members on the lower half of the vehicle, and that provides a very strong base for the rest of the coach. I wonder why that is? A monocoque construction technique is like that of a girder type bridge with support elements diagonally placed between vertical and horizontal elements. Like bus chassis a motorhome semi-monocoque chassis use less weight and gain more strength. Like Greyhound type buses and all new automobiles, this technique provides more rigidity while providing huge inside storage and living space. Frame rails are used in most trucks from pickup to 18-wheelers and the cab is always separate from the payload body. That is because, no matter how big and strong the frame rail is, there is significant torque turning, or twisting, from the front to the rear of the vehicle. In order to limit the damage from this twisting process, truck chassis manufacturers heat treat or temper the rails after key holes are drilled to accommodate components to be attached. Drilling new holes or welding any new components to this hardened frame rail, voids the warranty because it is therefore weakened. Special fasteners, called huck bolts, are normally used to attach truck components to the frame rail because normal bolts no matter how tight they are installed, will eventually loosen.

Motorhome manufacturers use the front and rear caps, the side walls, roof and floor to stiffen the box against this always present torque or twisting. They use special glues and fasteners to attach large sheets of plywood and fiberglass to a simple steel or aluminum frame for all six sides of this box to make it stay together. If perfectly done, the box sides will stiffen the whole vehicle. If not perfectly done, fiberglass will be delaminated, rear overhangs will droop, front and rear caps will crack, many unfixable rattles will develop, and the structural integrity in case of an accident will be weakened. I have seen roll over accidents where all six sides of the frame rail chassis came apart. I have seen roll over accidents of monocoque or semi-monocoque chassis that have simply been righted and driven away. I have not seen any roll over accidents with frame rail chassis where all six sides stayed together. I have not seen a single roll over accident with a monocoque or semi-monocoque chassis where the six sides did not stay together.

If you ride in a 20 year old passenger bus or semi-monocoque motorhome you will find that it is still tight and almost rattle free. It is rare if you find a 20 year old frame rail chassis that that tight. There is really no comparison between the chassis types concerning passenger safety. The monocoque or semi-monocoque wins every time.

So why don't more motorhome manufacturers use a semi-monocoque chassis? The reason is primarily cost. Spartan, Freightliner, Ford, and Union Bay (used to be Chevrolet) supply frame rail chassis to volume motorhome manufacturers. Some makers like Winnebago, cut a frame rail in two, and build a center section that is semi-monocoque design to strengthen the vehicle and gain large storage compartments. All other makes of monocoque or semi-monocoque coaches, manufacturer custom chassis to meet their own specific requirements. Newell, Vogue, Monaco, Foretravel, and Country Coach are the major coaches makers that use custom designed semi-monocoque chassis. These companies have a chassis division that supplies them with proprietary products.


I have bolded several significant sentences.