Lil Bigfoot Ridged Shell Top -- stronger? - Fiberglass RV


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Old 08-09-2016, 11:49 AM   #1
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Lil Bigfoot Ridged Shell Top -- stronger?

I was going to post this in Toykiter's Reno thread, but it does seem like a new topic.

Toykiter,
Looking at the photo of your ceiling reminds me of a question...

These LB's have that ridge running along the top of the shell, which I think serves to make the roof stronger. Our LB does not have any kind of support between the kitchen counter and the upper cabinets, and the closet on the other side has been removed. I don't see any signs of sag... So does the roof shape on these shells provide a stronger product than the round, smooth egg shells?

(Or is the LB supposed to have those supports in place?)

(Another question might be, if the shell top IS stronger, could it support an AC unit?)

Questions, always questions!
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Old 08-09-2016, 01:07 PM   #2
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Read section 3.1 on improving stiffness of fiberglass panels.
http://www.westsystem.com/ss/assets/...20-%202015.pdf

An arc shape is stronger and can support more weight across its width than a flat shape. The Romans figured that out pretty early on as it allowed them to build wider widths without the use of beams.

With a nearly flat roof you need to incorporate beams that transfer the loads on the roof back to the walls and on down to the floor. In a fiberglass shell the beam (stringer, stiffener, etc) can be inside or outside. Putting it on the outside creates more drag, putting it on the inside reduces head room. Either way you have a compromise.

As to the ridge on the roof, it will have been sized (engineered) to stiffen the roof itself but not necessarily would it be strong enough to support an AC. However if you see in the brochure for the trailer that a roof top AC was a standard option to choose from that is an indication that it might support an AC. But before doing so you should research and see if people who had the AC option got a lot of issues from them.

Hence the use of egg shapes for trailers with arched roofs which don't need beams and maximize a smooth slip stream for economical travel and more interior head room. But the shape of the egg reduces head space at the sides of the walls and they are more difficult to insulate and create the interior walls and fittings. So there is a compromise with them as well.

As to the ridge on the roof, it will have been sized (engineered) to stiffen the roof itself but not necessarily would it be strong enough to support an AC. However if you see in the brochure for the trailer that a roof top AC was a standard option to choose from that is an indication that it might support an AC. But before doing so you should research and see if people who had the AC option got a lot of issues from them.

The Sunraders fiberglass motorhomes used a SIP, structural insulated panel for its roof. It allowed a shallow curve, only a couple of inches away from being flat, and it was a self supporting roof. But after just a couple of years they found out that the AC units on the roof were too much weight for the SIP to support so they had to install two steel rectangular beams in the interior across the width to support the roof fore and aft of the AC unit. I have an early 1980 shorty Sunrader that was built before they started using those beams. Fortunately no one has ever installed an AC on the roof so I don't have a sag in my roof but if I put an AC up there sure enough it would sag after awhile since I don't have the beams in it. I could add a lot more fiberglass layers to the roof but that would increase the weight and that would compromise structure and my engine. My walls were not engineered in thickness to support a heavier roof. You can't change the roof thickness without changing the wall thickness so beams were the practical, low cost, low in weight, fix the company came up with.
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Old 08-09-2016, 03:47 PM   #3
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Are inside closet, cabinet posts needed to support shell?

KC, thanks for your thoughts on this!

I'm still hoping to hear from other LB owners about whether this trailer needs the counter/cabinet support or if the top shell is strong enough on its own. I have a copy of the original brochure, but it does not show that side of the trailer.

The brochure does not list AC as an option, so I'm guessing that the shell is not strong enough to support one.
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Old 08-10-2016, 12:41 PM   #4
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Talking Bigfoot A/C

About your Bigfoot roof a/c: I have a 17.5 Bigfoot that I bought new in 1993, and I assume your L/B is the same as mine structurally. Just came back from a month long trip to Calgary,Alberta over the Coq. and Rockies and then down to Yellowstone and Mt. Rushmore. We went over the border to the US via Big Chief mountain crossing and the roads were bad enough to loose a tire pump off the bikes on the back and almost lost 3 bikes that were strapped on, but, never thought about a/c problems structurally. I have had a 13,500 btu a/c unit since new and last year replaced it for a new 13,500 Dometic unit. I regularly go up on the roof to wax it and inspect the a/c and the rest of the roof. I never have had any problems with any leaks or cracks from weight on the roof. If it will hold my 200 lbs. it will hold an a/c unit. If your worried contact Bigfoot, I believe they are still in Armstrong,B.C...Cheers....Durb.
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Old 08-11-2016, 08:24 AM   #5
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Quote:
Originally Posted by durb747 View Post
About your Bigfoot roof a/c: I have a 17.5 Bigfoot that I bought new in 1993, and I assume your L/B is the same as mine structurally. Just came back from a month long trip to Calgary,Alberta over the Coq. and Rockies and then down to Yellowstone and Mt. Rushmore. We went over the border to the US via Big Chief mountain crossing and the roads were bad enough to loose a tire pump off the bikes on the back and almost lost 3 bikes that were strapped on, but, never thought about a/c problems structurally. I have had a 13,500 btu a/c unit since new and last year replaced it for a new 13,500 Dometic unit. I regularly go up on the roof to wax it and inspect the a/c and the rest of the roof. I never have had any problems with any leaks or cracks from weight on the roof. If it will hold my 200 lbs. it will hold an a/c unit. If your worried contact Bigfoot, I believe they are still in Armstrong,B.C...Cheers....Durb.
Durb, I'm thinking now that the 13.5 LB is not structurally sound for an AC. In looking at their old brochure, AC was an option on the 17's, but NOT on the 13.5. If they were strong enough for AC, surely it would have been an option.

Plus, I did contact Bigfoot when we were going through our axle replacement trauma, but since my 1989 was built before the company closed and reopened, the new management had little advice to offer about my model. Sad face again.
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Old 08-11-2016, 11:55 PM   #6
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Quote:
Originally Posted by Ellpea in CA View Post
Durb, I'm thinking now that the 13.5 LB is not structurally sound for an AC. In looking at their old brochure, AC was an option on the 17's, but NOT on the 13.5. If they were strong enough for AC, surely it would have been an option.

Plus, I did contact Bigfoot when we were going through our axle replacement trauma, but since my 1989 was built before the company closed and reopened, the new management had little advice to offer about my model. Sad face again.
Don't give up so easy, do some analytical thinking and testing, it is good for your brain and stretches your problem solving abilities

Just go up and push on the roof in the area where you would install an AC and see how much flex there is. If it does not move very much it might be OK with a reinforcing curb ring for it to sit on. But if it flexes a lot it would not be OK for bearing the weight of an AC without some major modification.

If it passes the flex test stack up the equivalent weight of an AC unit using something such as 12 x 12 concrete stepping stones and/or stuff on top of a single stepping stone or on a plywood square the size of an AC opening and see what happens to the roof line in that area. You can tell a lot just by simulating the weight load of an AC across a closely similar size span. If you see any kind of sag then forget about putting an AC up there unless you reinforce the roof. But do the testing before you try to solve the problems of reinforcing a roof.
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Old 08-12-2016, 02:17 AM   #7
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L/B- A/C

What I was going to say re: Little Bigfoot a/c is: to remove the vent in the bathroom and take a look to see the structure of the roof. Some excellent ideas already, so do a few of those and if you are still leery about the roof top a/c put in a Fantastic Fan in place of the vent. They move a lot of air and cool things down just by leaving a front side window open and the fan on. In the past I can't remember seeing a L/B with A/C. Good Luck...Durb.
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Old 08-13-2016, 10:02 PM   #8
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Quote:
Originally Posted by durb747 View Post
What I was going to say re: Little Bigfoot a/c is: to remove the vent in the bathroom and take a look to see the structure of the roof. Some excellent ideas already, so do a few of those and if you are still leery about the roof top a/c put in a Fantastic Fan in place of the vent. They move a lot of air and cool things down just by leaving a front side window open and the fan on. In the past I can't remember seeing a L/B with A/C. Good Luck...Durb.
Thanks Durb! The LB doesn't HAVE a bathroom, but we do have a vent over the rear dinette. A Fantastic Fan is the next option, along with (possibly) one of those portable AC units we've read about in other threads.
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