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Topic: Chassis construction photos from Don and welding on your FT (Read 8946 times) previous topic - next topic

Chassis construction photos from Don and welding on your FT

Today I was cutting and fitting some of the new square tubing and new angle iron sill. Took a few pics to document the process and thought I would share because if you are like me, you will find it interesting to see what our coaches are like under the skin. Seeing the frame work this way makes it clear why some frame members are doubled up and most are not. My belief is that they used (at least in these years) the same basic framework across models and even perhaps, lengths as well. In the lower left of the pic that shows the entire width of the utility compartment, you can see the opening where the hook-ups pass through. To the right of that just to the left of the divder that seperates (and supports the weight of) the waste tanks and the FW tank, is another potential hook-up opening where the piece of angle iron that spans that gap would be cut out in a coach with a different layout. In our 99' U270, the grey and black tank are on the left side of the compartment divider and the fresh on the right. I have seen pictures of 320's and 295's where the waste and fresh tanks are reversed and the divider is moved toward the front of the coach accordingly. This way they can build the subframe according to one plan and adjust it to fit the floor plan of whichever model is on the build list. So, in other words, I don't believe that the strange seeming doubled up frame members are for particular structural purposes other than to facilitate the building process and have sub assemblies ready to use over a range of models and floor plans. Just my thoughts, I welcome any observations you might want to share...
Cheers!
Don

Re: Chassis construction photos from Don and welding on your FT

Reply #1 –
Don...

Are you welding those new pieces back into the frame? And, if so, are there any special precautions needed when doing that? I have some frame members around the generator (which is right under the driver's seat position where it gets spray from the front left tire) that I am thinking about replacing and wondering about the welding process.

Craig

Re: Chassis construction photos from Don and welding on your FT

Reply #2 –
Hi Craig,
Yes we (my son and I) are welding in the new metal, and yes, according to Foretravel, there is a whole host of items to turn off and disconnect. Such as the batteries, the Allison ECU, the ABS brains, etc. I believe it would be okay if you clamp the ground quite close to where you are working (my understanding of what happens at the mothership), and you aren't using an old school AC Arc welder (so my son believs), or of course, a gas welder. However, I am going to do the reccomended disconnects before starting the major work.. We did tack weld some nuts to the broken Roloks that were to ate up to use the double nut method, but so far no indication that there was any deterimental effect from that, and the ground clamp was right there where we were working. His welder uses DC at 24 volts...
Don
Quote from: wa_desert_rat –
Don...

Are you welding those new pieces back into the frame? And, if so, are there any special precautions needed when doing that? I have some frame members around the generator (which is right under the driver's seat position where it gets spray from the front left tire) that I am thinking about replacing and wondering about the welding process.

Craig

Re: Chassis construction photos from Don and welding on your FT

Reply #3 –
Don... thanks.... I had thought there were some precautions. I don't have ABS (I'm pretty sure) or an ECU on a U225 with the 4-speed Allison. Maybe someone can set me straight on that. In 1993 there was not a lot of computer  operated systems on the coaches.

Craig

Re: Chassis construction photos from Don and welding on your FT

Reply #4 –
Quote from: acousticart –
Hi Craig,
Yes we (my son and I) are welding in the new metal, and yes, according to Foretravel, there is a whole host of items to turn off and disconnect. Such as the batteries, the Allison ECU, the ABS brains, etc. I believe it would be okay if you clamp the ground quite close to where you are working (my understanding of what happens at the mothership), and you aren't using an old school AC Arc welder (so my son believs), or of course, a gas welder. However, I am going to do the reccomended disconnects before starting the major work.. We did tack weld some nuts to the broken Roloks that were to ate up to use the double nut method, but so far no indication that there was any deterimental effect from that, and the ground clamp was right there where we were working. His welder uses DC at 24 volts...
Don

Don,
Here is the Foretravel Welding Checklist
Welding Checklist

Re: Chassis construction photos from Don and welding on your FT

Reply #5 –
Don,
Thank you for posting photos and the progress.  Please continue to do so as the project moves along.  It looks like you and your son are doing very nice work.

Re: Chassis construction photos from Don and welding on your FT

Reply #6 –
Let me second John's thanks on your postings, Don. The descriptions and the photos are fascinating.
jor

Re: Chassis construction photos from Don and welding on your FT

Reply #7 –
Don, you have been so thorough in this process I am guessing that you talked to Foretravel about removing the fiberglass clear across the bottom of the chassis. I would be concerned that removal might compromise the monocoque structure of the Foretravel chassis. Am I correct that you discussed this with the factory and found that it was OK to remove all of the glass. Do you have to do anything special when re-glassing to insure the new fiberglass will withstand the stress on chassis at this critical point?

Re: Chassis construction photos from Don and welding on your FT

Reply #8 –
Kent,  regarding your comment on the fibreglass skin below floor frame on Don's coach, do  you think the thin skin of it is a part of the strength in the frame? Seems to me to be too thin to help in any way, other than a weather stop!! I am not an engineer so may be way off.
John

Re: Chassis construction photos from Don and welding on your FT

Reply #9 –
Kent,
To answer your question, no I didn't talk to Foretravel about removing the skin. I did it simply because it was necessary to access the rusted subframe. The bottom skin, as well as the skin on the floor of the storage/utility bays, is a sheet of FRP (fiberglass reinforced plastic) less than .090" thick, and as John surmised, is a weather cover with perhaps a tiny bit of "R" value thrown in. I will be replacing the bottom skin with a new sheet of  FRP. The wall structure on the sides of the coach, on the other hand, surely does provide some of the strength as they have actual fiberglass bonded to luan. Even there though, I am sure that it is the welded steel that does most of the work. The metal which I will be welding in along with some extra gussets will make our particular U270 much more structurally sound and integral than it has been in a number of years. I will document the process as I go for the community's entertainment...
Regards, Don

Quote from: Kent Speers –
Don, you have been so thorough in this process I am guessing that you talked to Foretravel about removing the fiberglass clear across the bottom of the chassis. I would be concerned that removal might compromise the monocoque structure of the Foretravel chassis. Am I correct that you discussed this with the factory and found that it was OK to remove all of the glass. Do you have to do anything special when re-glassing to insure the new fiberglass will withstand the stress on chassis at this critical point?

Re: Chassis construction photos from Don and welding on your FT

Reply #10 –
Don... are the beams you are welding back into place the same as those you have removed? What is the wall thickness? I'm probably going to use a flux welder unless I run into lots more of a problem than I anticipate; then I'll invest in a better welding unit. (Or maybe keep an eye on craigslist... some serious bargains on there lately.)

You aren't just doing a repair... more of a structural restoration. I'm glad I stumbled in to this forum in time to be here while you document it. I really appreciate the postings!!

Right now I figure I can stabilize the rust on the steel in the generator compartment by removing the scale and then spraying it down with a stabilization spray (rust converter) then repaint with a durable paint. If I discover any seriously debilitated structure my first choice would have been to sister it but I'm thinking that what you're doing is probably a much better solution.

Thanks... and keep 'em coming. :)

Craig

Re: Chassis construction photos from Don and welding on your FT

Reply #11 –
Quote from: acousticart –
Kent,
To answer your question, no I didn't talk to Foretravel about removing the skin. I did it simply because it was necessary to access the rusted subframe. The bottom skin, as well as the skin on the floor of the storage/utility bays, is a sheet of FRP (fiberglass reinforced plastic) less than .090" thick, and as John surmised, is a weather cover with perhaps a tiny bit of "R" value thrown in. I will be replacing the bottom skin with a new sheet of  FRP. The wall structure on the sides of the coach, on the other hand, surely do provide some of the strength as they have actual fiberglass bonded to luan. Even there though, I am sure that it is the welded steel that does most of the work. The metal which I will be welding in along with some extra gussets will make our particular U270 much more structurally sound and rigid than it has been in a number of years. I will document the process as I go for the community's entertainment...
Regards, Don


Don,
Here are a couple of photos of what foretravel did to reconstruct mine for comparison.

Re: Chassis construction photos from Don and welding on your FT

Reply #12 –
I am not an engineer but I spent many years working with Polymers and Polymer Structural Members. We used to make hundreds of structural polymer members by bonding fiberglass under pressure to both rigid urethane insulation board, plywood and even phenolic board and honeycomb. Some of these polymer sandwiches were used in the RV industry.

I was under the assumption that the outer skin of the Unicoach was part of the structural monocoque frame. I have never opened up a bay floor or the underskin like you did to see what the makeup was. I always thought the bottom of the coach would be a sandwich of a fiberglass outer skin, approx 100 mils, bonded to rigid urethane insulation board and a steel structural skeleton, bonded to either another layer of fiberglass or plywood with an upper skin of fiberglass. The 1" box metal frame would be embedded in the middle of the sandwich. If the fiberglass skin was not bonded to either the plywood or rigid insulation board then the fiberglass skin is probably inconsequential and not part of the monocoque structure. If, however the plywood or insulation board was bonded to the skin, you may want to talk to the engineers at FOT.

I have a very high regard for the expert knowledge and experience of many of our forum members so I am probably wrong about my assumption, but just in case the bottom of your 1999 U270 is part of the structural integrity of the chassis, I thought I should explain my thoughts further. Making an error in this regard could be catastrophic.

Re: Chassis construction photos from Don and welding on your FT

Reply #13 –
Quote from: Kent Speers –

I have a very high regard for the expert knowledge and experience of many of our forum members so I am probably wrong about my assumption, but just in case the bottom of your 1999 U270 is part of the structural integrity of the chassis, I thought I should explain my thoughts further. Making an error in this regard could be catastrophic.

I don't think you're wrong on this, Kent. I think it would depend on how (or whether) the FRP "skin" is bonded to the structure. Even though the skin is extremely thin it still would offer resistance to compression (as a result of the movement of the structure in torsion) and could contribute significantly to the strength of the member. Certainly worth talking to the people who had a hand in the design.

Craig

Re: Chassis construction photos from Don and welding on your FT

Reply #14 –
Kent, my fg panel looks like it is held to base by a row of Rolocks (or such) across the width every few feet and is not glued to underside. Would that constitute some rigidity for frame? I feel that if this does not then the meer fact of all those bolts actually depletes the strength of each of these cross tubes by having so many holes in them. Surely another way to hold the FG sheet up would be better/
as I said I am not an Eng'r but figure if you drill a whole bunch of holes in the underside of a beam or box tube some strength is lost. Maybe the Mexican sun has got to me too much!!!

Re: Chassis construction photos from Don and welding on your FT

Reply #15 –
John, I really don't know the answer to that question. Your assumption sounds logical. I just find it hard to believe that the fiberglass skin is not part of the monocoque structure. The 1" steel grid would not appear to have enough strength to hold the whole thing together. I think it is supplemental rather than the foundation of the frame structure.

Re: Chassis construction photos from Don and welding on your FT

Reply #16 –
Quote from: John Haygarth –
if you drill a whole bunch of holes in the underside of a beam or box tube some strength is lost. Maybe the Mexican sun has got to me too much!!!

I can see how you might think this but it's not necessarily true. In some cases you can put holes in the web of a structure and have very little loss of strength (at least in the direction you want). Consider a truss versus a girder, for instance. A girder, with a solid web, is a common structural component. But if you make much of that web empty space it becomes a truss and is almost as strong and a LOT lighter. Turn the truss sideways and much of that strength is lost, of course... so you have to understand the direction of the force(s) you want to withstand.

In the case of the thin fiberglass covering on the bottom of the FT unicoaches (and unihomes) you can easily create a model to see how this can aid the resistance to torsion of the structure. Set up some popsicle sticks on edge and glue them together in a flat box shape (like the steel in the bottom of the coach). Once it's dry pick it up and try to torque (twist) it in various directions.

Now glue some paper to the top and bottom of the structure and try to torque that. I bet it will a lot stiffer.

This would be because the twisting of the box structure is converted into tension and compression on the paper... and this is exactly what the paper will resist the best. Of course, if the paper weren't well glued to the box structure then it would add nothing to the strength of hte structure and would just slip around.

Bolting the fiberglass to the steel frame of the coach may not be the best way to secure the material... but it's probably the best practical way. And while the holes for the bolts probably do weaken the beams a little, they don't weaken it enough for a failure.... if they did then all those 1987 Unicoaches would be failing all over the place and we'd hear about it.

Craig

Re: Chassis construction photos from Don and welding on your FT

Reply #17 –
Kent,
I appreciate your concern, but in any case, the structure of our coach was already compromised by the damaged metal and there is only one way to get at it. I would have been happy to let Foretravel do the work to repair and replace the rusted framework, but in our case, it was not an option. Next best scenario, figure out what to do and how to do it myself. I was all set just to do the standard bulkhead fix, but I saw signs I couldn't ignore. Upon peeling back the skin on the floor of the compartment, my worst fears were revealed and you can bet I was devasted. Our dream RV had become a nightmare.

Had I just done the standard fix, I am quite sure that it would have held for awhile, but the stresses and contnuing rusting would have eventually (probably sooner rather than later) broke the welds around the rust thinned metal and left the rear 1/8" box tubing firmly bolted to the angle iron but seperated from the forward going longitudinal frame members leaving us with an even worst case scenario. I am not an engineer by trade, nor do I play one on TV, but I do trust my sense of how things work in the world enough to be confident in a good outcome. Hopefully the documentation of the process will provide convincing evidence that my fix will provide a permanent soloution to my problem. True, the FRP was bonded to the styrofoam, the four small areas of plywood. However, the wood, for the most part, had turned to mush, and the rust on the metal had all but released the fiberglass skin, and the bond to sthe styrofoam was negligible. That said, I intend to bond the new skin in place, but I don't believe that it contributes significantly to the structural integrity other than to protect the metal from moisture and corrosive elements. All the metal will also be coated with POR-15 after the welding is done with the overlaping parts painted beforehand with high zinc weld-through primer prior to welding.

The plywood, which by staying wet and holding moisture next to the steel frame, is being replace with 3/16" sheet aluminum in the areas where fixtures like the water pump and the like were screwed to (the screws likely contributing to letting water get to the wood). The aluminum will be mechanically fastent to steel gussets similar to what you see in the photos posted by Barry, which by the way, were not a feature in the original construction of the subframe on our coach.

Thanks for the replies, thoughts, concerns, and observations, keep em' coming!
Don

Re: Chassis construction photos from Don and welding on your FT

Reply #18 –
Kent, the structure is 1 1/2" box tubing, for the most part 16ga or thinner, but 1/8" thick at the bulkhead joint. There is also 1 1/2"X1 1/2"X1/8" longitudinal angle iron on the sides.under the alumin
Quote from: Kent Speers –
John, I really don't know the answer to that question. Your assumption sounds logical. I just find it hard to believe that the fiberglass skin is not part of the monocoque structure. The 1" steel grid would not appear to have enough strength to hold the whole thing together. I think it is supplemental rather than the foundation of the frame structure.

Re: Chassis construction photos from Don and welding on your FT

Reply #19 –
John, I believe the primary purpose of those Roloks is to tie the subframe to the vertical compartment walls, which are in turn bolted to the floor above, thus supplying support and cutting down the span of the box tubing of the subframe.
Don
Quote from: John Haygarth –
Kent, my fg panel looks like it is held to base by a row of Rolocks (or such) across the width every few feet and is not glued to underside. Would that constitute some rigidity for frame? I feel that if this does not then the meer fact of all those bolts actually depletes the strength of each of these cross tubes by having so many holes in them. Surely another way to hold the FG sheet up would be better/
as I said I am not an Eng'r but figure if you drill a whole bunch of holes in the underside of a beam or box tube some strength is lost. Maybe the Mexican sun has got to me too much!!!