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Monocoque vs. frame rail chassis
#1

Back in the 90's there was an excellent article on motorhome chassis written by Bob Gummersall, the Chief Technical Officer for RVersOnline.org that totally changed how I viewed motorhomes. There are still many articles written by Bob on their website, but the chassis article has been removed. My assumption is that RV manufacturers applied pressure to have the article removed as it pointed out how unsafe frame rail chassis motorhomes are.

Recently a user shared pictures on the FaceBook group "Class A Diesel Pusher Motorhomes RV Coaches" showing the aftermath of a blown steer tire.

       

There was a post by another user asking which was better, a Spartan on Freightliner chassis. I posted the following response which was removed with a warning.

   

This is a copy of Bob's article preserved by @fulltiming on Luxury Coach Lifestyle. Michael Day's post

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.

Jon & Chris Everton
1986 40' Dog House #86
450 hp ISM 5 spd ZF Ecomat 2
2004 Range Rover L322 Toad
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#2

Good article Jon thanks for posting, I continually gripe about working on our coach but every time we dig into the substructure part the coach or the components that support the systems I tip my hat to Newell for building a solid platform for these big rigs. I am not a guru on motor coaches but in auto and aviation I know a good deal in the fabrication department and consider the coach we bought built like an old Douglas DC3. Like every airplane these must have continual maintenance and if it goes without then problems occur, but if I was to have a roll over this would be the one I would do it in, not a head on for sure but a roll over I think if strapped in you should survive.

1999 45' with tag axle, #504 "Magnolia"
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#3

I have always been impressed by the construction Newell uses. never ceases to amaze me when I am hanging out under the bus in the pit just in awe of all the heavy gauge steel used.

--Simon
1993 8v92TA #312
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