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Author Topic: Structural engineering anyone??  (Read 3210 times)
paulcjhastings
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« Reply #15 on: January 27, 2008, 06:31:29 AM »

I am real sure that the last comment will insure NO One will help out!
I am doing slide outs on a MCI EL3. I gathered all the info and drawings I could and then hired a local structure guy ( also gave him Daves book). For a fee -- NOT FREE - he did a set of drawings for the structural mods needed. There are a lot of mods not directly around the hole in the side. Many mods to the the underside of the bus and other parts of the bus. We used 40 LBS of wire in the wire welder making the mods!
Be safe - not sorry. There is more to it than meets the eye.
JimH


Did I say I wanted free help? Nyet, I asked for an engineer to consult on the job. As far as the joke, every engineer that I know thinks its funny and applies to some guys in their field of study, one can also change it to read lawyer, accountant, county employee, etc. Perhaps you did not read the line before the joke.


I reread Galeys book yesterday, I'm going to check my figures using his equations and see what happens.
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Paul Hastings
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« Reply #16 on: January 27, 2008, 09:30:08 AM »

Hi Paul and others.  Since I am follicly challenged AND and engineer, I hear all of the bald jokes and engineer jokes.  I had not hear this one.  I read it to my wife and we both had a good laugh!!! Grin

I have been trying to think how I would address this if I were doing a slideout.  Engineering analysis has come a long way since I was in college (slide rules then) and the technology is so much better today.  Software for FEA analysis is so powerful.  However, <soap box mode on> many engineers (most young engineers) do not have a lot of practical sense and often believe everything the computer tells them.  I supervised a bunch of young engineers and it took a huge amount of effort to get them to step out from behind the computer for a minute and think about what it was telling them.  I got involved in a fun project with a bunch of the new engineers where I became the "grunt" doing the fabrication and welding.  They would ask what I thought of their design and I would respond that I was only the welder (inside, I knew they had made some terrible engineering decisions).  After the design failed, I would then explain what there problem was.  That was great fun!  <end soapbox>

Back to how I would approach the project.  I truly believe that a rigorous analysis could predict that a slideout will not work (especially if the analysis looked at the impact of the reinforced structural area had on the overall frame structure (change in structure flexibility, torsional rigidity, and possible creation of high stress areas).  However, we know that it has been done many times.  I think I would do enough analysis to make sure I did not compromise the strength of the bus and also do enough analysis to make sure that no other major design issues were created (mostly flexibility issues) and then get on with it.  I think it was Bob Belter that responded that you can add a lot of extra weight to an over-designed slide.  I really respect Bob's engineering knowledge, but I can't really see where more than 100-200 pounds would be added to an over-designed structure.  Even if I am off by a factor of two, that is not much weight in the scheme of things.  I would not try to engineer the design for the lowest weight, as the margin for error would be very small.

I often wonder how many folks with slides develop problems after lots of miles on the road.  I have talked to several folks who have had to have major rework done on factory installed slides on Prevost based motorhomes.  One person had to have his top dollar Prevost based coach in the shop several weeks while the factory rebuilt the very low mile coach slide.  I think this was a Prevost factory slide.  As I recall, it was a structural issue.

It would be interesting to start of poll/thread to see what sort of history bus folks have had with their slides.

Some folks will point out that almost every new RV has slides.  However, recall that almost all of these units have frames and not comparable to most of our buses that are monocoque or multi-tube steel tube (like the old bird cage racing frames -  think Maserati for you old racing fans:  http://en.wikipedia.org/wiki/Maserati_Tipo_61) construction.

Sorry, kind of rambling this morning, but I hope a couple of these random thoughts might pertain.

Jim
« Last Edit: January 27, 2008, 09:56:59 AM by rv_safetyman » Logged

Jim Shepherd
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Ray D
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« Reply #17 on: January 27, 2008, 10:47:21 AM »

BusGuy,

I would be interested in what mods you made to the underside and other places, I put two slides in and everything is still opened up.  Might be something you know that I need. 

Thanks,
Ray
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NJT5047
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« Reply #18 on: January 27, 2008, 08:28:38 PM »


Regarding Jim's statement about problems with  RV slides...they are huge problem causing devices. 
Cables screwed, hydraulic lines and cylinders damaged, slides binding, slides won't retract, slide seals leaking, won't unlatch to extend, won't lock in place when retracted...and the list goes on. 
However, RV buyers want slides for obvious reasons.  Factor that with demographics indicating most RV owners won't use their RVs more than a few weeks a year...if that much, and it paints a bleak picture.   Still, RVs, even those with real frames, are not well built items.  Typically very poorly built and under-engineered. 
As you state--bus owners with slideouts would be where to look for useful data.
Since Prevost is the dominant slide/bus builder, you're gonna have a problem getting data on a busnut board.  Prevost slide out coach owners are not typical 'busnuts.' 
I doubt Prevost will discuss this issue?  They may not have many problems and if anyone does it right, I'd guess that Prevost has the program.   Any good pix of a partially completed Prevost?  Good place to start an engineering design! 
FWIW, Va Coach builds large slides in MCIs for sure, and probably other bus brands too.  There's some pix on Sawyer's Bus Sales that link to Va Coach showing the construction of a long slideout in an MCI 102A2.   The bus was here in Carolina del Norte.  I have met the owner of that coach several times, but he's been MIA for a couple of years now.  I believe he has a very nicely done shell sitting around.  The bus is distinctive.  Maybe he's posting on this board under another name? 
The amount of room a slide ads is incredible.  The problems associated with them may be incredible too.  My ADHD would worry the $%+ out of me wondering if the slides would retract... if wasn't enough,  watching the slide bounce in the hole in the wall while driving down the highway would be exciting. 
Someone made a good point regarding leveling...if you gotta have slides, use a 3 point leveling system so that the bus cannot be torqued side-to-side.     
JR

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JR Lynch , Charlotte, NC
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luvrbus
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« Reply #19 on: January 28, 2008, 06:59:10 AM »

Paul i don't know if he can help you are not but BJ the owner of Gus a MCI bus did a unbelievable job on engineering for slides on his bus.i don't have his phone number but he is a good friend of the owners of Custom RV Interiors in Vancover,WA 1-800-955-3957  hope this helps
« Last Edit: January 28, 2008, 08:25:25 AM by luvrbus » Logged
kyle4501
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« Reply #20 on: January 28, 2008, 07:46:02 AM »

Yes, slides can be done successfully, but most aren't.

As an engineer, my opinion is the reason you aren't getting an eager list of engineers to sign up for this project is because it is a major task. You say you'll pay, but there is much more to it than that.

What you may not realize;
1 - The very small number of engineers actually qualified or with the experience to do this sort of design.
2 - The time required to do it right. (even $25/hour adds up fast, & I usually wouldn't trust an engineer that'd work that cheap. If he was any good, why wouldn't he be collecting over 5x that doing consulting work?)
3 - The very real liability issues involved (You can thank the lawyers for that).

I get a kick out of those ignorant enough to actually believe you don't need an engineer to do something of this magnitude with a reasonable expectation of success. (The engineering doesn't have to be a result of a formal degree.)

BTW, I've worked with lots of engineers that should be shot & burned. So, just because you find an 'engineer' doesn't mean he knows any more than you.

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Rick Brown
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« Reply #21 on: January 28, 2008, 09:29:20 AM »

If you don't mind curling up with a "Statics and Strength of Materials" college text book each evening for a couple of months and math doesn't scare you then you might do it yourself.

One of the first questions that came to mind when I raised the roof and replaced the steel windows section in my 4905 was how strong is what I will take out related to what I will put back in (aluminum sheet and rv windows).  You can make an estimate of the strength of the section you must cut out if you understand (or are willing to guess) what its strength is contributing to: i.e. is it holding  the roof up or does it prevent the coach from folding up.  If you can get that sorted out then the rest of the job is pretty much just more of the same to make sure you put back in is at least that much plus the extra load of the slide-out.

I, personally, would be leary of any engineer who would do this job for less than you could buy a coach with a slide-out for.  If you work on your coach for pleasure then buy the book and start with the numbers;  I've found it to be pretty satisfying.

Disclosure:  I've had a college course in statics & strength, but that was a long time ago.

-RickBrown in Reno, NV
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kyle4501
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« Reply #22 on: January 28, 2008, 09:52:56 AM »

Statics & strength are the easy part almost anyone can do.

It's the dynamic loading, fatigue issues, & stress concentration considerations that seperate success from failure.

Raising a roof is a very different structural issue than cutting a hole in the side.

Roof raise = larger moment of inertia.
Hole in side = smaller moment of inertia.

Rick is spot on in his comment about cost of engineering this vs. buying a unit with a factory slide.

I'm not trying to be negative, just trying to provide more information to allow one to make the best decision for their needs.
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niles500
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« Reply #23 on: January 28, 2008, 10:35:10 AM »

Kyle, Jim and others are certainly right about the dynamic loads in caged frame construction. Most people think in 3 dimensions, but the load factor at any given structural connection point are almost infinite in the number of load planes that must be considered - this is why vehicle manufacturers have milions of $$$$ invested in software - can you armchair it? Certainly others have with success, but I have seen a few carcasses out there that were attempted slide retrofits and appeared to have been abandoned after running into complications? FWIW
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« Reply #24 on: January 28, 2008, 10:52:40 AM »

I had an RV door put in my bus to replace the original entrance door.  I went to four places with the bus to get price estimates.  1st guy wouldn't give a solid price, but talked about $3500.  2nd place wanted $2100 to do the work.  3rd place was a large bus body shop in Northfield, MN, but they refused to do it for liability reasons.  4th place wanted $900 to do the work and got the job.

Of the four places I went to, only one was bigger than a one or two man operation and they were concerned about liability.  The others apparently didn't care about liability.

The moral of the story is you might find someone desperate who will do the job, but I would bet most would be concerned about liability.
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Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
Rick Brown
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« Reply #25 on: January 28, 2008, 03:33:20 PM »

It occurred to me that nobody asked what was to be slid out.  If you are talking about something the size of a sofa or bed, something alcove like that could be fit through a hole 5 feet or so high, wide enough to lie down in, and a foot off the floor, then I'll bet some of the advice given here would have been different.  I think the way to keep the engineering manageable is creativity and pragmatism in selecting what you want to push outside the bus.
-RickBrown in Reno
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