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Definition of Monocoque chassis

Monocoque means "one shell" or "single skin". Almost nothing is a true monocoque -there's always some load condition that can't be sustained by a single shell. It's become a marketing term.

If you take a pure shell and attach something to it, say an axle or an engine and transmission, you need attachment points. This creates a point load that cannot easily be supported by a pure shell without reinforcement. As a result, frames, stringers, bulkheads, and other structural components are required. A jetliner's airframe makes extensive use of these frame reinforcements to resist bending and to serve as attachment points for tail sections, wing cantilevers, and landing gear.

About the closest you'll find to a true monocoque in bus construction is the old GM High Liners, Silversides, 4106's, etc. They were constructed using aircraft manufacturing techniques. The aluminum skin lent torsional and bending strength to the structure, but close inspection reveals the bulk heads, framework, and sub-frames required to distribute suspension and engine point loads. Because of this, the bus structure is more properly termed a semi-monocoque. The presence of frame rails also rules out the possibility of a true monocoque construction. Blue Bird Wanderlodges, for instance, use c-channel frame rails for suspension and engine attachment points, but the bus body itself is a monocoque. Combined they comprise a semi-monocoque. By simply bolting the c-channel frame rails to the bus body the torsional stiffness of the frames rails is increased by a factor of 10.

Prevost and modern day MCI's use a spaceframe. The skin is largely along for the ride, although the skin does provide some strength and the fasteners provide damping. So even they might be termed a semi-monocoque. I'm not exactly sure of the Foretravel method of construction. Perhaps someone can make a case for it being a true monocoque.

David Brady
'02 Blue Bird, Wanderlodge, LXi

David Brady
Asheville, NC
2004 Prevost H3 Vantare
2002 Wanderlodge LXi (sold)

Re: Definition of Monocoque chassis

Reply #1 –
David,

Your description is accurate of ALL "monocoque" coaches including Foretravels-- they are more accurately semi-monocoque.

Of course, the older pre-slide out coaches were "more monocoque" as they did not have to have frame rails in the center section of the coach (at least on the Foretravel Unihomes).  As soon as holes were cut in the side to accommodate slides, frame rails had to be added to support that area.

Brett

Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Definition of Monocoque chassis

Reply #2 –
That's sounds good Brett. In the aircraft industry one would call the holes in the body left by the slides, 'cutouts'. Cutouts require framing to restore strength. And, as you pointed out, slides would indicate another notch moving from pure monocoque to semi-monocoque.

David Brady
'02 Blue Bird, Wanderlodge LXi
David Brady
Asheville, NC
2004 Prevost H3 Vantare
2002 Wanderlodge LXi (sold)

Re: Definition of Monocoque chassis

Reply #3 –
Howdy David and Brett, 
Thanks for this; A great description and explanation...I'll be bookmarking this one for future reference..
Dave Abel
Dave and Nancy Abel
'00 U320 36' WTFE  Build #5669
'10 Jeep Wrangler Unlimited Rubicon
Livingston, TX  SKP's Fulltiming

Re: Definition of Monocoque chassis

Reply #4 –
This is one of the best understandable definations that I have found.  It was written by Bob Gummersall, a writer for RVersOnline.org.  It is no longer on their web page:

"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."
1994 U225
build #4514

Re: Definition of Monocoque chassis

Reply #5 –
Pat,

It's a beautifully worded piece, but it's fundamentally incorrect. The author confuses 'space frame' with 'monocoque'. He describes a box of stressed steel or fiberglass skin construction , where the skin is in tension, as 'frame rail'. He's got it backwards.

A pure monocoque is a pure shell, an eggshell. All the stresses are born by the outer skin.

A frame made up of diagonals like a truss bridge is a space frame, this is the current crop of long distance highway coaches: Prevost, MCI, Newell,

A semi monocoque is a shell with frame elements designed to distrubute point loads into the shell. It describes the continuum between the pure monocoque and the pure space frame, and this is why it's an almost meaningless term. It's too general since it describes virtually every motorhome or bus on the road.

David Brady
'02 Blue Bird, Wanderlodge, LXi
David Brady
Asheville, NC
2004 Prevost H3 Vantare
2002 Wanderlodge LXi (sold)

Re: Definition of Monocoque chassis

Reply #6 –
Seems like this fellow just wants to show the rest of us how great he is.  Wonder why he is here instead of on a Wanderlodge site ?
Gary B

Re: Definition of Monocoque chassis

Reply #7 –
Gary, The term "Rabel Rousing" comes to mind or maybe its a case of realizing how unwonderful the BBB (Blue Bird Bus) is and attempting to feel good some how in a goofy manner ?
My opinion FWIW

Re: Definition of Monocoque chassis

Reply #8 –
Gentlemen, let's keep up our standards of respect on the forum, with the way we treat and speak about each other.

Tone is very difficult to convey in an email, and I welcome the divergence of opinion and sharing of information. The fact that a Blue Bird owner is on this forum is because he is considering the brand at some point in the future.
Tim Fiedler    2025 LTV Unity MBL on Order
2000 Chevy Tracker 2 Door Convertible 4WD Now lifted 4.5"
1985 Citroen 2CV6 Charleston
Murphy Rebel on wheels with 175HP Titan
Cessna P337
1980 48' Westport MY (!/4 Share)

Re: Definition of Monocoque chassis

Reply #9 –
Yes Tim...fully agree.  We welcome everyone interested in exchanging thoughts and ideas pertinent to Foretravels on this forum.  We can always use these  :help: to add to our inflections in writen mode.  We do have a sense of humor from time to time.  :snap:
 
 ;)
Peter & Beth Martin
No Forrest? What have you done?
MC# 15890 until Dec 2016; FMCA #F329677
Cincinnati, OH

Re: Definition of Monocoque chassis

Reply #10 –
Quote from: Tim Fiedler –
Gentlemen, let's keep up our standards of respect on the forum, with the way we treat and speak about each other.

Tone is very difficult to convey in an email, and I welcome the divergence of opinion and sharing of information. The fact that a Blue Bird owner is on this forum is because he is considering the brand at some point in the future.

Thank you, Tim.  This needed to be said.
George Stoltz.  Retired from full-time living in a great Foretravel and now are back to living in a traditional sticks and bricks in Florida.

Re: Definition of Monocoque chassis

Reply #11 –
Thanks Tim, Peter, and George,

Tim you're exactly right. The reason I check in on this forum is to learn about Foretravels since it is a couch I'd consider owning someday. There are lots of things I like about Foretravel. First, Foretravel is a great American company. I like their values and their commitment to to the customer and their commitment to quality. Second, I like the motorhome itself. It's relatively lightweight, get's great fuel mileage, has a good chassis, and it has a great ride and road manners. In 2004 I bought my BB from Steve Mitchell at Parliament Coach and have often looked at and driven the Foretravels that Steve sells.

This particular thread is interesting to me cause bus chassis design and construction has long been an interest. I think it's notable that I did not start this thread. I would never take it upon myself to start a new thread on your Foretravel forum entitled: "Definition of Monocoque Chassis". For a non-Foretravel owner who doesn't post that often, that would be incredibly bold. What happened is that there was a discussion about monocoque chassis on a different thread, I posted a reply, and one of the Foretravel moderators took my reply and put it in a new thread. It's a good thread with interesting commentary, but I want folks to know that I didn't create it. If I had, some might say that I'm showing off, which really isn't the case.

As far as comparing Wanderlodges with anything else that's out there. I guess I've been in this rodeo long enough to know that all motorhomes and buses have their pluses as well as their minuses. Each has their strengths and weaknesses. In essence to some degree, the grass is brown everywhere! :) The fact that Wanderlodge is factory-less is a real disadvantage. Sure, we have the Blue Bird School Bus company, which really is a huge benefit, but with Wanderlodge gone, there is a lot that is suddenly missing. A manufacturer that's still in business is a huge benefit. Treat your factory well... :)

I invite you all to come over to WOG (Wanderlodge Owners Group). We encourage owners of any brand to participate, as I'm sure is also true with Foreforums.

David Brady
'02 Blue Bird, Wanderlodge, LXi
NC
David Brady
Asheville, NC
2004 Prevost H3 Vantare
2002 Wanderlodge LXi (sold)

Re: Definition of Monocoque chassis

Reply #12 –
Quote from: travelite –

This particular thread is interesting to me cause bus chassis design and construction has long been an interest. I think it's notable that I did not start this thread. I would never take it upon myself to start a new thread on your Foretravel forum entitled: "Definition of Monocoque Chassis". For a non-Foretravel owner who doesn't post that often, that would be incredibly bold. What happened is that there was a discussion about monocoque chassis on a different thread, I posted a reply, and one of the Foretravel moderators took my reply and put it in a new thread. It's a good thread with interesting commentary, but I want folks to know that I didn't create it. If I had, some might say that I'm showing off, which really isn't the case.

Correct.  At the request of other forum members who contacted me offline, I split David's post off from another discussion in FT Tech Talk since it was more of a broad RV technical nature than specific to the coach being asked about in that thread.  We try to keep the top board under "Foretravel Motorhome Forums" - Discussions, Tech Talk, Renovations, and Classifieds - as FT-focused as possible and have the broader discussions in Fire Ring.  I apologize if it caused any confusion.

Michelle
Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320

Re: Definition of Monocoque chassis

Reply #13 –
Hi Dave,
  Thanks for any contributions you have. I have been a member of the WOG many years. Always have been treated well. I used your group as I was researching which coach I would purchase. These forums are the best places to learn about a coach. Keep hanging around.
Raymond Jordan
1997 Foretravel
Maryland
Raymond & Babette Jordan
1997 U 320
1998 Ford Explorer XLT

Re: Definition of Monocoque chassis

Reply #14 –
Dave B, I too appreciate both your expertise and ability to make a complicated issue more understandable. I hope you will appreciate that sometimes we misinterpret the reason someone shares their thoughts. I'm sure that some of my posts on subjects on which I have lots of experience have been interpreted as showy by some but they have also been appreciated a great deal by others. My posts are always presented with humility and a desire to share information useful to others. That has been my read on your posts as well.
The selected media item is not currently available.Kent Speers
Locust Grove, OK
1993 U300 SSE 40' (Restored at FOT 2009) Build 4323
720 watts Solar
6V92TA DDEC Silver Engine
2014 Subaru Outback

Re: Definition of Monocoque chassis

Reply #15 –
Thanks to all who have attempted to explain the subject in plain English so those of us who aren't engineers can understand at least some of it.

I'm one of those who is on several brand-specific forums, because we're still learning (and there is A LOT for us to learn). We haven't even decided on the class of RV we want for FT use, much less the brand. The more we read, the shorter the list of possible manufacturers becomes. There have been some additions to the list, as we learn more about a particular brand. Foretravel is one of those additions. I'm finding that those coaches that are on our list all have many points in common, and I guess I shouldn't be surprised at that. We want to buy our third coach first; that is, we don't want to be trading coaches two or three times in the space of a year or two before we get it right.

Part of the reason the Foretravel is on our list is the knowledge base available on this forum. DW is becoming increasingly interested in them, to the point that I'm spending quite a bit of time on this  site. Thanks again to all who so generously contribute their hard-learned knowledge so that those of us who follow may benefit.