Need Schooling - Class A Frames

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Need Schooling - Class A Frames

October 21, 2013, 09:34:41 pm

I continue to read about the strength and weaknesses of various types of Class A frames. Could someone point me to a place that explains what seems to be an important matter? For example, I've read that FT have a superior frame as compared to others, but I don't understand the "why" of it. I'd be grateful. And thanks in advance.

Re: Need Schooling - Class A Frames

Reply #1 – October 21, 2013, 09:37:47 pm

Foretravel, like commercial buses have "unibody" construction, not a frame. More expensive to build, but much stronger.

Virtually all cars today are also unibody construction rather than "on frame".

The entire structure (much stronger structure) is use to make the vehicle structurally sound. Superior to two frame rails in the basement with a light-built house on top.

Re: Need Schooling - Class A Frames

Reply #2 – October 21, 2013, 10:25:39 pm

My coach certainly appears to be body on frame as opposed to unibody. Running gear and suspension attached to frame, body attached. Are newer FT's being built monocoque? Or am I overlooking something?

Re: Need Schooling - Class A Frames

Reply #3 – October 21, 2013, 10:29:50 pm

No, Chuck, you have no frame. Just a big metal box with suspension and running gear attached to it.

Re: Need Schooling - Class A Frames

Reply #4 – October 21, 2013, 10:35:17 pm

Chuck,

Look under the center of your coach-- that large expanse of white Fiberglass. Above all that is unibody/monocoque construction. Open a basement-- no frame rails. The structure of the floor of the basement, floor of the motorhome, walls and roof ARE the structure.

Yes, there are structural members that carry the heavy components-- front suspension, rear suspension and drivetrain. These join the center structure at the bulkheads.

Brett

Re: Need Schooling - Class A Frames

Reply #5 – October 21, 2013, 10:43:03 pm

I'm looking at the approx 3"X6" tubing that the suspension, engine and transmission is bolted to and calling that a frame. MonoQ to me would typically have a body pan with pressed reinforcing shapes in it that would also accommodate the drivetrain and suspension. The body is attached to this tubing network with airbags. So, these....lets call em subframes for lack of a proper term, are free to flex independently of each other because they are not rigidly connected. Am I thinking about this right?

Re: Need Schooling - Class A Frames

Reply #6 – October 21, 2013, 10:45:15 pm

Out of curiosity, when did Foretravel come up with the wonderful monocoque chassis? I know my 1990 U280 was. Did it start in 1988?

Re: Need Schooling - Class A Frames

Reply #7 – October 21, 2013, 10:47:07 pm

Chuck,

Those front and rear sub-frames are pretty well held together by the center (unibody) of the coach. Unless you have bad bulkheads, there really should not be much "independent" flexing.

Brett

Re: Need Schooling - Class A Frames

Reply #8 – October 21, 2013, 10:48:59 pm

Quote from: Brad & Christine Slaughter – October 21, 2013, 10:45:15 pm

Out of curiosity, when did Foretravel come up with the wonderful monocoque chassis? I know my 1990 U280 was. Did it start in 1988?

Yes, it started with the U280 and U300. Then when the U225 and U240 joined the lineup, they were also monocoque construction.

The last "framed" coach Foretravel built was the ORED (Oshkosh Rear Engine Diesel)-- about 1993 as I recall.

Re: Need Schooling - Class A Frames

Reply #9 – October 21, 2013, 10:57:21 pm

Ah so. I am truly red faced for not seeing this before...and I'm all about looking at structure day to day. I think, really, based on what you are saying, the body is not riding on air bags, rather the driveline and suspension is loaded and modulated by them, the bulkheads constrain fore and aft movement. At any rate, it sure works well and I truly appreciate the schoolin'. Unibody it is.

Re: Need Schooling - Class A Frames

Reply #10 – October 22, 2013, 09:44:52 am

<DISCLAIMER>Here is my understanding of the Foretravel semi-monocoque design of the 1990s. I expect experts to clarify or correct improper representations. Any reliance on my presentation ... blah, blah, blah

</DISCLAIMER>

Chuck, my coach is structurally the same as yours. The wheels and tires attached to axles, which in turn are attached to an "H frame." Air springs, shocks, and some stabilizing bars hold the "body" in place over the "H frames."

A picture of a 2007 frame is here: "Google Image Result for,r:75,s:300,i:229&iact=rc&page=22&tbnh=167&tbnw=302&ndsp=19&tx=180&ty=80"

and here: http://www.cleggind.com/drawings_pdfs_pics/Foretravel_Commercial_Flyer_2007.pdf

My understanding is that the floor was reinforced with those two rails when Foretravel started installing slides. The rails do reinforce the system, but are not the primary carriers of the load as they are in traditional twin rail frames.

Re: Need Schooling - Class A Frames

Reply #11 – October 22, 2013, 09:54:39 am

Correct, things changed when slides were introduced. That is one of the reasons Foretravel was late in the market introducing slides-- they required a redesign of the structural elements of the coach.

Said another way, if the walls are a structural component of the vehicle, cutting a large hole in it required redesign. Same for taking a modern unibody car and making it into a convertible-- it requires structural redesign.

Brett

Re: Need Schooling - Class A Frames

Reply #12 – October 22, 2013, 11:07:59 am

Think of your coach having "sprung weight" and "unsprung weight." Everything the airbag touches at the top is sprung weight and everything the airbag touches at the bottom, the rectangular tubing, I beam axle (front), tires, brakes are unsprung weight. The front and rear sub-frames are bolted to the main body structure that has a monocoque design. The result is a semi-monocoque structure. My old 4107 GM Buffalo Bus did not have the sub-frames and was considered full monocoque. Because of the front and rear sub-frames, the vehicle will be a little heavier and a little less stiff compared to full monocoque. This may be the reason for some of the shifting windshield problems. It would have been difficult and more expensive for a small company like Foretravel to design and build a full monocoque vehicle. For me, this translates into a much more careful and slower transition from paved roads to shoulders, rough roads, etc. than I did in my old bus.

Trucks and RVs generally use the "I" beam method of front end construction opposed to independent suspension because of cost (#1 plus a lot of companies don't get it right), complexity and ultimately increased reliability as the coach ages. The fully independent suspension in the front will result in much lower unsprung weight and better suspension control over bumps and on rough roads. One of the big reasons trucks, cars and RVs went to aluminum wheels as they are much lighter than steel wheels (increased payload is another reason) and lower the unsprung weight. The newer RVs with independent suspension will handle and ride a bit better but front end alignment will be harder to do, more frequently needed and because of all the parts, bushings, etc. the maintenance costs can be much higher as everything ages.

Without a major redesign of the monocoque center section, slide outs WILL allow more flexing of the structure, increased weight will load the suspension more compromising the handling, decreasing the fuel mileage and degrading the comfort level slightly on rougher highways. The "convertible" example above hit it on the head.

Pierce

Re: Need Schooling - Class A Frames

Reply #13 – October 22, 2013, 11:35:01 am

Pierce, Brett, J.D., Amos

Once again, I am amazed at the depth of knowledge on this group. Thanks for the great explanation, I do believe I got it now. I had looked at the truss like structure of the coach walls on cold, dewey days and wondered why the extensive triangulation was necessary, I now know why. I can see the role the bulkheads play, the reason for our lavishly equipped coaches relatively light weight compared to others. This has to be costly to manufacture, compared to purchasing a rigged frame/chassis/suspension/driveline and rolling it into the shop to slap a body on.

Chuck

Re: Need Schooling - Class A Frames

Reply #14 – October 22, 2013, 01:49:45 pm

Yes, per Greg Amy's at GV, chassis cost main reason that FT isn't building a lower priced "entry level" coach. And if FT used a Freightliner or other chassis, they would compete with the likes of Tiffin which has material cost efficiencies and automation efficiencies driven by their greater unit volume. So they are sticking up market in the coach world and trying to break into non RV markets (disaster response units) with their chassis and custom work.

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Need Schooling - Class A Frames

October 21, 2013, 09:34:41 pm

Re: Need Schooling - Class A Frames

Reply #1 – October 21, 2013, 09:37:47 pm

Re: Need Schooling - Class A Frames

Reply #2 – October 21, 2013, 10:25:39 pm

Re: Need Schooling - Class A Frames

Reply #3 – October 21, 2013, 10:29:50 pm

Re: Need Schooling - Class A Frames

Reply #4 – October 21, 2013, 10:35:17 pm

Re: Need Schooling - Class A Frames

Reply #5 – October 21, 2013, 10:43:03 pm

Re: Need Schooling - Class A Frames

Reply #6 – October 21, 2013, 10:45:15 pm

Re: Need Schooling - Class A Frames

Reply #7 – October 21, 2013, 10:47:07 pm

Re: Need Schooling - Class A Frames

Reply #8 – October 21, 2013, 10:48:59 pm

Re: Need Schooling - Class A Frames

Reply #9 – October 21, 2013, 10:57:21 pm

Re: Need Schooling - Class A Frames

Reply #10 – October 22, 2013, 09:44:52 am

Re: Need Schooling - Class A Frames

Reply #11 – October 22, 2013, 09:54:39 am

Re: Need Schooling - Class A Frames

Reply #12 – October 22, 2013, 11:07:59 am

Re: Need Schooling - Class A Frames

Reply #13 – October 22, 2013, 11:35:01 am

Re: Need Schooling - Class A Frames

Reply #14 – October 22, 2013, 01:49:45 pm

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