Skip to main content ↑ ↓
Topic: Chassis construction photos from Don and welding on your FT (Read 8946 times) previous topic - next topic

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

Reply #20 –
Quote
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.

Aluminum connected to steel. Have you given any thought to galvanic corrosion?

And which one, steel or aluminum, would be corroded?

best, paul

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

Reply #21 –
Quote from: Paul Smith –
Aluminum connected to steel. Have you given any thought to galvanic corrosion?

And which one, steel or aluminum, would be corroded?

Excellent point! I think the corrosion could be mitigated by using some sort of insulation. Fasteners could still be a problem, however. Teflon washers, maybe?

Craig

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

Reply #22 –
A quick call to James or Rance at Xtreme would provide the answers...

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

Reply #23 –
I have thought of that Paul... not being enough of a chemist or materials scientist to say with certainty, but I figure that have both materials coated with POR-15 should help with that, but mostly prevent future moisture coming in contact which could provide the electrolyte need for that process to take place. In any case, I feel fairly certain that removing the wet wood sponge from the equation will be beneficial in the long run. I have considerd using some composite material like Starboard in place of the wood, but the 3/16" aluminum will actually let me add about an inch and a quarter of rigid foam insulation below it.

Also, the original configuration had a thin strip of aluminum inserted between the bulkhead joint that wrapped around the edge of the fiberglass reinforced plactic botoom skin. With the addition of road salt and water, perhaps that was a contributing factor to the failure of the Roloks... Interestingly enough, the structure of the top of our coaches is also aluminum, while the sides are steel. My guess is that the persistant presence of moisture to act as an electrolyte would be required to create the ion exchange required to create a galvanic reaction.
Don
Quote from: Paul Smith –
Aluminum connected to steel. Have you given any thought to galvanic corrosion?

And which one, steel or aluminum, would be corroded?

best, paul

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

Reply #24 –
Don, my only point was that if the area being repaired is an integral part of the monocoque structure, I would check with engineering at Foretravel to see if you need to tie the glass, insulation, metal framing and bay flooring glass together. If you use the POR 15 on the steel structure, you may not be able to get an adequate bond between the metal, the skin, and the foam. You also need to know the structural rating or density of the foam and the specs of the resin used to attach all of the components. You will need to match the fiberglass specs, flexural strength and compressive strength of the resin to that of the original resins used and total thickness of the skin to insure uniformity of the system.

I'm not trying to be an alarmist and I hope I am wrong and things are very simple for you but if I am correct about the importance of that section of the chassis and you don't get it right, you could have an unrepairable catastrophic chassis failure. I'm just suggesting that you ask the questions before you start putting everything back together. I would start with Mark Harvey and FOT. If it is part of the chassis integrity, it won't be much harder to do it the right way. You will just need to make sure you get the correct components. You have already done most of the hard work.

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

Reply #25 –
Maybe a thin coating of the paste used in electrical connection of alum and copper wires would provide protection after all the prep work and painting is done in final installation. I also use pure copper sulphate on many connections on the coach-elec/frame/bolts etc and have found so far this works well. Anyway, very interesting reading and thought process by all, and I am sure FOT are reading and stealing some ideas from you all.I personally think Don is right in that keep any moisture away from joints and you have it beat (as best as humanly possible)
John

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

Reply #26 –
Quote
Excellent point! I think the corrosion could be mitigated by using some sort of insulation. Fasteners could still be a problem, however. Teflon washers, maybe?

Teflon could mitigate 99% of the potential problem. But that leaves the most critical part vulnerable: the shank of the bolt. Us anal engineers might try some silicon in the donut made because the hole is a bit larger than the diameter of the bolt....

best, paul

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

Reply #27 –
Quote from: Kent Speers –
Don, my only point was that if the area being repaired is an integral part of the monocoque structure, I would check with engineering at Foretravel to see if you need to tie the glass, insulation, metal framing and bay flooring glass together. If you use the POR 15 on the steel structure, you may not be able to get an adequate bond between the metal, the skin, and the foam.

I think that the Roloks were there for the express purpose of tying these parts together; why else use them? If the design engineer didn't envision strain on this joint then they would have simply used screws. So if they worked before then they (or something similar) would still work. I do agree that there does need to be a good mechanical bonding between all those bits but I think that fasteners would  do the job. In fact, at least from a corrosion standpoint, the POR 15 will help.

However, as far as Don's theory about keeping moisture out of the area... I don't think it's possible to keep moisture out given the environment (wet roads, salt, humidity, sand and gravel, etc.). I think I'd try to mitigate galvanic corrosion as much as possible by insulating the dissimilar metals from each other. I would make sure that the aluminum bits and the steel bits never touch. Whatever you do, do not "bond" aluminum and steel together!

Teflon washers and silicon would be an easy and cheap solution.

But I also agree that a phone call to FOT or MOT would be a good idea.

Craig

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

Reply #28 –
Craig,

I really like your popsickle stick and paper model idea. I once did a finite element analysis (FEA) of a space-frame touring bus (Prevost like). It used frame elements with a similar section to what Don has found in his Foretravel. The structure was plenty strong but wasn't very stiff. Then I added the fiberglass outer skin and the stiffness was dramatically improved. As you know, the skin adds stiffness and the fasteners used to adhere it add damping. The net effect was a much higher resonant frequency. What you say about girders versus trusses where the strength can be maintained in the direction needed while greatly lightening the structure also rings (so to speak:)). If it's stiff and light, then it resonates at a higher frequency and will be that much more insulated from road inputs. The net effect is better ride and handling and metal fatigue resistance. An alternative to the fiberglass skin is to add diagonals to adjacent corners; i.e., turn all those rectangles into triangles. I'm with everyone on the dissimilar metals idea too, and on the idea that you can't keep water out. This is why if it were me I'd add some diagonal 1.5" members, or some sheetmetal skin (14 gauge), or some gussetting as in Barry's coach, and I'd leave ample room for water to escape. Don, you're doing excellent work. Looks really good. Please forgive me for my eng'r ways. I simply can't resist commenting on a project of this nature. Excellent work and thanks for the photos!

David Brady
'02 Blue Bird, Wanderlodge LXi
NC

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

Reply #29 –
Quote
I once did a finite element analysis (FEA) of a space-frame touring bus (Prevost like).

"Finite element analysis" sure brings back memories of my 4 years at UCB. In fact, we just returned from the funeral of Jerry Goudreau who headed a group at LLNL developing FEA code that was used for nonlinear FEA of car crashes - thereby saving auto manufacturers big $$$ (it reduced the number of actual test needed - the code was actually developed for weapons purposes). Professors Bob Taylor and Karl Pister and others were there.

best, paul

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

Reply #30 –
From all this engineer talk maybe FT needs to let contracts to some Real professionals that have real world experience. :)
Gary B

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

Reply #31 –
Don... travelite's suggestion of improving stiffness with corner bracing sounds like it might be a great idea. It would add very little weight and cost almost nothing (probably just use scraps from what you've cut for the frame members). Even if you just installed two in opposite corners of each rectangle.

Craig

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

Reply #32 –
Quote from: wa_desert_rat –
Don... travelite's suggestion of improving stiffness with corner bracing sounds like it might be a great idea. It would add very little weight and cost almost nothing (probably just use scraps from what you've cut for the frame members). Even if you just installed two in opposite corners of each rectangle.

Craig
Good point and one that Kenworth failed at.  Fishtails are very important in adding strength to to anything your welding  on.  My old Kenworth had a cracked frame because the Kenworth engineers forgot that stuff.  But some good old southern engineering and welding fixed things up.

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

Reply #33 –
Quote
Don... travelite's suggestion of improving stiffness with corner bracing sounds like it might be a great idea. It would add very little weight and cost almost nothing (probably just use scraps from what you've cut for the frame members). Even if you just installed two in opposite corners of each rectangle.

If I understand the proposed construction, the aluminum plates would do a great job of increasing lateral shear stiffness parallel to and below the plane of the FT's frame, if attached with enough bolts - say every 6 inches. This also means the length of the aluminum plates should be a least a long in the direction perpendicular to the length of the FT as their length in the long direction of the FT.

This assumes the lower tubular framing is well-connected to the FTs frame above. Such as vertical steel-framed shear panels up to the frame.

best, paul

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

Reply #34 –
I think Don is doing the right thing by replacing the rusty metal and doing it properly.  I think the fiberglass skins no matter how thin add strength to the structure if properly bonded.  If one steps back from the coach and looks at the cargo compartments with the doors open, they are a lot like a cardboard box without any ends, and a box like that folds up flat very easily.  The coach does have a rigid metal structure at the ends near the wheels that prevents that from happening.  The basement floor is fastened in place at the front and rear with horizontal bolts, and it is held up in the middle with vertical bolts into the bulkheads.  I have never checked, but the top of the middle bulkheads are probably bolted to the main cabin floor from above. All the rectangular tanks once in place will add some resistance to the cargo floor moving fore and aft.  There is also one fore and aft vertical bulkhead in my coach that forms the back of the battery compartment that should add rigidity to these compartments.  One should be able to bond the skins to the metal frame by using epoxy.  Epoxy bonds to metal much better than polyester resin will and also will bond to the polyester resin skin.  On the top skin one could just place bags of sand to apply pressure until it cures.  The bottom skin presents more of a problem and should be done first, but an old or cheap air mattress or two should be able to lift the skin into place and apply enough pressure until it cures.  One could paint the tubing and sand to bare metal for the epoxy, or sand blast and coat all the metal with epoxy, maybe a bit of overkill though.  Definitely put the foam back in place as it insulates the tanks.

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

Reply #35 –
I should add something.  The foam should also be bonded to the skins.  Do a test on a scrap piece to make sure the foam is not dissolved by the uncured epoxy.  I don't think epoxy will do that, but uncured polyester resin will dissolve some foams. 

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

Reply #36 –
Quote from: Jerry Whiteaker –
I should add something.  The foam should also be bonded to the skins.  Do a test on a scrap piece to make sure the foam is not dissolved by the uncured epoxy.  I don't think epoxy will do that, but uncured polyester resin will dissolve some foams. 

Jerry, you are correct, epoxy should be used and any 100% solids (no solvent) epoxy should not melt the urethane foam. If the skin is polyester, then go ahead and glass with polyester resin but try to use one with similar physical properties. I hope that Foretravel used epoxy resin but maybe not.

Rather than using the POR 15 for rust proofing, a good application of epoxy resin will be more effective and allow for a better bond to the skin and the foam. Remember, epoxy is the coating of choice for coated rebar in bridge construction.

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

Reply #37 –
Craig et al.,
Regarding the question of galvanic corrision: I do think I will be able to keep moisture out of this area, but the talk of galvanic corrosion got me thinking about some things, so I did some research... just in case. It appears that the  chances for corrosive damage to the steel is almost nil in this configuration. First of all, it is the aluminum that would be the annode in this scenario and the corrosion would be mostly limited to the point of contact. Secondly, since the Aluminum is right next to mild steel in the Galvanic series, there isn't much difference in voltage potential, meaning the galvanic action would be very slow, but in any case would be protective of the steel. Also, the aluminum plate which I plan to use is 3/16" thick and any corrosion will be a long way down the road, even if moisture was persistently present (but I am determined to mkae sure that doesn't happen!).

Finally, if it is likely to be a problem, why would Foretravel use a piece of aluminum trim "inside" the bulkhead joint? You might say so that it would be the sacrificial anode and protect the steel exposed to road salts and the like, and that does make sense to me... but that doesn't explain the substantial 1/8" thick extruded anodized aluminum bottom sill trim which runs in direct contact with the steel frame along both bottom edges of the coach, electrically connected by many screws, as well as all of the bay door trim that holds the struts and weatherstriping, all screwed into the steel framework. Could be that Foretravel missed that one, but I don't think so. I have been too busy working the project to post more, but I will in the near future. In the mean time, I appreciate all the comments- which I take in the spirit in which they have been given. That is to say, out of concern for the wellbeing of one of our beloved Foretravel coaches :D
Don

Quote from: wa_desert_rat –
I think that the Roloks were there for the express purpose of tying these parts together; why else use them? If the design engineer didn't envision strain on this joint then they would have simply used screws. So if they worked before then they (or something similar) would still work. I do agree that there does need to be a good mechanical bonding between all those bits but I think that fasteners would  do the job. In fact, at least from a corrosion standpoint, the POR 15 will help.

However, as far as Don's theory about keeping moisture out of the area... I don't think it's possible to keep moisture out given the environment (wet roads, salt, humidity, sand and gravel, etc.). I think I'd try to mitigate galvanic corrosion as much as possible by insulating the dissimilar metals from each other. I would make sure that the aluminum bits and the steel bits never touch. Whatever you do, do not "bond" aluminum and steel together!

Teflon washers and silicon would be an easy and cheap solution.

But I also agree that a phone call to FOT or MOT would be a good idea.

Craig

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

Reply #38 –
Don, good job and interesting topic. Hope you are taking lots of pics for a later-show and tell!!
Have you got any of the bulkhead area you had to fix that we can see now?
Glad I do not have any of these issues as mine is tight and dry.
John H

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

Reply #39 –
Quote from: acousticart –
Craig et al.,
Regarding the question of galvanic corrision: I do think I will be able to keep moisture out of this area, but the talk of galvanic corrosion got me thinking about some things, so I did some research... just in case. It appears that the  chances for corrosive damage to the steel is almost nil in this configuration.

I thought aluminum and steel were farther apart but hadn't look at the chart in years. At any rate, the aluminum plate is a much better idea than wood.

Mostly we were all just talking through potential problems and, like engineers everywhere, thinking about the worst case scenario and how to avoid it. You have a good handle on what you're doing so don't mind us. :D

Craig