Anyone ever use balsa core for an ultralight build

[Larry C]

Larry, you say your floor is only 1/8" ply on BOTH sides, with foam. Your foam is only 1" thick? Wow, how solid does it feel when you sit on it? How far apart are your stringers?

Becky,
See my album for how I constructed the floor.

Much of my build is and will be an experiment to see if light weight boat building techniques can carry over to teardrop building. Also, I am using as much material I already have lying around, into my build. Case in point is my floor framing is a recycled picnic table that was built with pressure treated Southern Yellow Pine. about 30 years ago. This is much nicer lumber than what is available today. This is not the lightest wood for sure, but it is quite strong, even at 1" thick.

Andrew once stated that the stringers are only there to stop one (or 2) from falling through the floor, and serve little structural purpose. Its the perimeter walls that give the structure it's strength. The stringers are 16" apart and 12" at each end. The floor is for a 5' X 8' trailer. All lumber and the foam were thickness planed to exactly 1" so the 1/8" skins would be flat.The skins were glued to the foam and the wood with silica thickened epoxy.

I then added 3 oz. style 120 fiberglass to both sides of the floor. The glass dramatically increased the stiffness of this panel. I tested it by suspending it between saw horses at the ends, and then I got on it. It exhibited almost no flex. Remember this is without the benefit of the structural walls. Also, if you look at my chassis, you will see that it will offer very little support for the floor, it must be self supporting.

My inside walls will be constructed using 1/8" cedar strips with thin glass both sides, not glued to plywood, just the strip panel glued to the minimal wall framing and foam core. The framing is mostly around the doors (2) The wall perimeter framing is already super light weight. It was made by laminating Bass Wood strips. The outer skin will be 4MM marine plywood.

After seeing how strong the floor is, I am confident my walls will be strong and light. The walls will be epoxied to the floor. This joint and others will also get an epoxy fillet and tape. Spars are 3/4" X 1-1/4" recycled SYP. Roof will be single layer 1/8" Russian Birch plywood. The entire outside will be glassed.

This is just an experiment on my part. It may all blow apart going down the road, but from past experience, light weight construction can be taken much further than most think. Just look at what is being done with foamies. If you try a similar floor, remember you can always add a second layer of 1/8" ply if your not happy with the results, but if your planing a 4' wide trailer, 1/8" should work fine.

Larry C

[Larry C]

Becky,
I forgot to mention I gave my floor the knee test also. No creaking or cracking at all. IMO/ 1/8" Ply with glass is strong enough.

Larry C

[Corwin C]

The equation for calculating the force from wind pressure is simple:

F = A * P * Cd

where
F = Force
A = Area (sf)
P = wind pressure (Psf) [P = .00256 * V^2, V = wind speed in MPH]
Cd = coefficient of drag [2.0 for flat plate (perpendicular to wind, worst case scenario), 1.2 for long cylinder]

A 4x8 sheet of material in a 40 mph wind will have 262.1 lbf applied to it...
A 4x8 sheet of material in a 60 mph wind will have 589.8 lbf applied to it...
A 4x8 sheet of material in an 80 mph wind will have 1048.6 lbf applied to it...

Just for comparison, that 52'x8' semi trailer laying on the side of the road in a 60mph wind had 7667.7 lbf applied to it (60mph wind doesn't tip them over very often).

As you can see, these forces are not trivial. However, are they big enough to worry about?

Now I'm going to make some assumptions ... 500 lb trailer (ultralight), 80 in wide (tire to tire) (6.666' wide to keep units consistent), evenly loaded, no tire slippage, Cg 2.5' from the ground (center of trailer, which I think is high). Soooooo, there is 500 lbf * 3.333 feet (half width) = 1666.6 ft.lbs of torque keeping it in place and it will take 1666.6 ft.lbs / 2.5 feet = 666.6 lbf to start to tip it over. As you can see, a worst case 60mph wind isn't going to do it, but an 80mph wind could have you in trouble.

Now, before you panic, realize that the speed that the air coming around a semi would be equal to the semi's speed (in reality it's much slower at subsonic speeds and falls off quickly with distance) and it will hit the side of the trailer at an angle, reducing the available force significantly. If you are traveling 60mph and the semi is traveling 60mph the wind will hit the trailer at a 45 degree angle, reducing the force by 0.707 times. In other words, there will only be 417 lbf available (ABSOLUTELY WORST CASE) to tip it over. This is also completely ignoring the air comming around your own vehicle which will reduce that angle even more. It's just not going to happen. Even at 80mph the ABSOLUTE WORST CASE would be 740.9 lbf, but I can't imagine setting alongside an 80mph semi, inches away, for any length of time. First of all, there are very few places where that situation would even be legal. A semi going in the opposite direction wouldn't apply force long enough, and one going the same direction you would have to be AWFUL CLOSE to get that kind of force.

In the real world, the forces on the trailer in a highway environment are very dynamic. The equation may be simple, but in reality it's not. There will be additional aerodynamic forces trying to keep the trailer upright and in line with the TV as well. In most situations, you will have nothing to worry about, but there are probably some unusual situations where we may be treading the line a little. My thoughts are that an aware driver will be able to sense these problems and I would hope take measures to prevent them (let the semi go by). I would be more concerned with storing it outside in a wind storm (with little to nothing in it) or traveling in a severe crosswind than passing a truck on the highway. As a rule of thumb, if you build one of these ultralights and the semi's start pulling over because of the wind, you may want to seek shelter as well for the same reasons.

Does anyone else have any additional thoughts?

[Larry C]

The equation for calculating the force from wind pressure is simple:

F = A * P * Cd

where
F = Force
A = Area (sf)
P = wind pressure (Psf) [P = .00256 * V^2, V = wind speed in MPH]
Cd = coefficient of drag [2.0 for flat plate (perpendicular to wind, worst case scenario), 1.2 for long cylinder]

A 4x8 sheet of material in a 40 mph wind will have 262.1 lbf applied to it...
A 4x8 sheet of material in a 60 mph wind will have 589.8 lbf applied to it...
A 4x8 sheet of material in an 80 mph wind will have 1048.6 lbf applied to it...

Just for comparison, that 52'x8' semi trailer laying on the side of the road in a 60mph wind had 7667.7 lbf applied to it (60mph wind doesn't tip them over very often).

As you can see, these forces are not trivial. However, are they big enough to worry about?

Now I'm going to make some assumptions ... 500 lb trailer (ultralight), 80 in wide (tire to tire) (6.666' wide to keep units consistent), evenly loaded, no tire slippage, Cg 2.5' from the ground (center of trailer, which I think is high). Soooooo, there is 500 lbf * 3.333 feet (half width) = 1666.6 ft.lbs of torque keeping it in place and it will take 1666.6 ft.lbs / 2.5 feet = 666.6 lbf to start to tip it over. As you can see, a worst case 60mph wind isn't going to do it, but an 80mph wind could have you in trouble.

Now, before you panic, realize that the speed that the air coming around a semi would be equal to the semi's speed (in reality it's much slower at subsonic speeds and falls off quickly with distance) and it will hit the side of the trailer at an angle, reducing the available force significantly. If you are traveling 60mph and the semi is traveling 60mph the wind will hit the trailer at a 45 degree angle, reducing the force by 0.707 times. In other words, there will only be 417 lbf available (ABSOLUTELY WORST CASE) to tip it over. This is also completely ignoring the air comming around your own vehicle which will reduce that angle even more. It's just not going to happen. Even at 80mph the ABSOLUTE WORST CASE would be 740.9 lbf, but I can't imagine setting alongside an 80mph semi, inches away, for any length of time. First of all, there are very few places where that situation would even be legal. A semi going in the opposite direction wouldn't apply force long enough, and one going the same direction you would have to be AWFUL CLOSE to get that kind of force.

In the real world, the forces on the trailer in a highway environment are very dynamic. The equation may be simple, but in reality it's not. There will be additional aerodynamic forces trying to keep the trailer upright and in line with the TV as well. In most situations, you will have nothing to worry about, but there are probably some unusual situations where we may be treading the line a little. My thoughts are that an aware driver will be able to sense these problems and I would hope take measures to prevent them (let the semi go by). I would be more concerned with storing it outside in a wind storm (with little to nothing in it) or traveling in a severe crosswind than passing a truck on the highway. As a rule of thumb, if you build one of these ultralights and the semi's start pulling over because of the wind, you may want to seek shelter as well for the same reasons.

Does anyone else have any additional thoughts?

Corwin,
Thanks for clearing that up. My head hurts after reading this. I guess the bottom line is a passing tractor trailer is not going to topple my ultralight, but if that same truck is seeking shelter from the wind, so should I. If that wind continues, and I must also continue, I will just fill the sucker with rocks and carry on.

I wish Roly would chime in. Nobody builds lighter TD's than him, and he has traveled all over the country. It would be interesting to hear from someone that has actually been there and done that with an ultralight.

[StPatron]

Thanks for that info, Corwin.

1. I'll play with those numbers, substituting 75-80 mph for truck speed. That's more realistic (the speed limit), at least where I drive.

2. While avoiding a potential danger situation is always wise, the alternative of pulling over to the side of the highway every time an approaching semi is seen doesn't seem realistic (available shoulder issues aside). There are limits imposed by safety. I'll adjust my speed to avoid a close encounter on a narrow bridge, etc. but I'm not willing to become a potentially dangerous roadblock every time I sense danger. If I ever build a foamy standy (for example) I want the confidence that I can get out on the road with the big boys in safety, not causing a hazardous situation for myself or other drivers.

I suppose I'm not as focused on property damage. I cringed when I read about Miriam's trailer being destroyed by the tornado, but.. as those of us living in tornado alley say, material items can be replaced. Lives can't. I don't want to sign on to this forum and read about an ultralight trailer blow-over, followed by a safety chain drag and an out of control tow vehicle... causing injury or death to one of our members.

Have you seen Mike's foamy standy build? I'd be interested in what your calculations reveal given my narrow, no-shoulder, two-lane highway, 75 mph, oncoming semi-trailer scenario.

[StPatron]

I wish Roly would chime in.

It's always good hearing from Roly, no doubt!

I don't see his builds as potentially dangerous in this scenario. His "sail" area (cabin size) isn't much larger than a postage stamp.

I heard a report this morning of a pick-up with a bed-mounted billboard being toppled over during last night's winds here. Haven't been able to get any more details yet.

[Corwin C]

2. While avoiding a potential danger situation is always wise, the alternative of pulling over to the side of the highway every time an approaching semi is seen doesn't seem realistic (available shoulder issues aside). There are limits imposed by safety. I'll adjust my speed to avoid a close encounter on a narrow bridge, etc. but I'm not willing to become a potentially dangerous roadblock every time I sense danger. If I ever build a foamy standy (for example) I want the confidence that I can get out on the road with the big boys in safety, not causing a hazardous situation for myself or other drivers.

I suppose I'm not as focused on property damage. I cringed when I read about Miriam's trailer being destroyed by the tornado, but.. as those of us living in tornado alley say, material items can be replaced. Lives can't. I don't want to sign on to this forum and read about an ultralight trailer blow-over, followed by a safety chain drag and an out of control tow vehicle... causing injury or death to one of our members.

Have you seen Mike's foamy standy build? I'd be interested in what your calculations reveal given my narrow, no-shoulder, two-lane highway, 75 mph, oncoming semi-trailer scenario.

This is the realm where it gets complicated ... I'm not suggesting that you should pull over when you see a large truck coming at you at freeway speeds. Yes, the forces are there, but for an exceptionally short time. Remember, gravity is a 24/7 force and a truck going the other direction would be fractions of a second. Given the short pulse of force, you will possibly feel it, but as far as tragically disrupting the path of your trailer/TV, it's just not enough. It may be noticeable, even unpleasant, but tragic ... I severely doubt it.

Also, I was assuming a worst case scenario, traveling the same direction, giving the forces time to cause problems. But, to get those forces, the clearance between vehicles would be measured in inches, not feet (airflow at low speeds compared to the speed of sound dissipates very quickly.) Another factor in our favor is that as soon as the trailer starts to tip, the Cd drops (no longer perpendicular) and there is a vertical component to the forces as the air spills upward along the surface of the trailer side. This would actually help the situation (pushing the side of the trailer down) rather than making it worse.

OK, now lets look at Mike's standy at 75mph. How heavy would it have to be to stay stable. Its profile appears to be 10'x6' with the corners all rounded off. So it's profile area is less than 60 sqft. Just looking at the pictures, I'm going to guess closer to 50 sqft (if we measured it, I bet it would be lower). It appears to be 5' wide at the wheel centerline. CG up high, lets say 3' from the ground. Here again I'm looking at worst case scenario. F=A*P*Cd (50*(75^2*0.00256)*2.0=1440 lbf. Again, figuring the torques, the wind applies 1440lbf * 3ft gives us 4320 ft.lb. The weight it would have to be to stay stable is 4320 ft.lb/2.5 ft is 1728 lbs. Also, factor in the angle effect 0.707*1728 = 1221.7 lbs. Even though he's building very light, I question whether he will be able to be under this figure loaded and ready to go. And remember again, this is an extreme. I'm looking forward to Mike finishing up and trying it out in the real world. I'm sure he'll let us know how it turns out.

I won't say that it's impossible, because it is possible to get blown over. It's just unlikely ... unless you build something like this. In the episode, if you haven't seen it, this "motorhome" does tip over, but not on the highway ... it's blown over while parked in the wind. Also notice when the large truck passes him going the same direction, (watch the tree) it sways outward, then is sucked inward ... I suspect that there is some Bernoulli effect going on there as well.

[StPatron]

Thanks for your time and detailed explanation, Corwin. The best factual information and data I've been able to obtain, much appreciated.

Living in an area of the country where wind related vehicle blow-overs are fairly common, I'll admit to having an awareness and healthy respect for the dangers. That has also been reinforced by my highway motorcycling background, I clearly recall bracing for impact with each oncoming semi blast. We are all a product of past experiences.

I've sat on the other side of this table before. During my own build, I was questioned by several good friends about the blow-over danger and my argument for safety and stability was based on the non-existent accident history for similarly designed builds. Where were you then? I could've provided real numbers for them to chew on.

I think we can agree that there are indeed design limitations that impact the stability of an ultra-lightweight towed trailer. And, I hope that if you notice a build that appears to be pushing the safety limits that you'll hop in with your expertise. We could use your advice. Thanks again.

[Corwin C]

The real lesson here is to keep weight as low and even as possible, and smaller profiles will have fewer issues than larger ones. If your trailer is susceptible to crosswinds, carry some extra beverages (as was suggested in the foamy forum) or steaks or dutch ovens, or whatever in the conditions that make it questionable. Ballasting a vehicle to make it easier to handle in adverse conditions is a concept centuries old and it works. It is also something that can be done almost anywhere. Just don't go over the capacity of your trailer or TV.

[StPatron]

Seems we're back to square one....

If ballasting an ultra-lightweight trailer is essential for safe road handling performance, then what's the point of that style build? I thought the objective was to reduce the towing load to accommodate low-powered tow vehicles. Silly me. The objective, instead, appears to be able to transport more cast iron and beer.

[angib]

It sounds like foam, lightweight 1x1 framing, 4mm marine ply and a layer of light fiberglass cloth is the way to go.

When I talked of a fabric inner skin, it is purely 100% cosmetic and has absolutely no structural purpose - it just allows you to have exposed (and again, non-structural) foam on the inside of the trailer walls but the fabric stops the little dings and dents it will get from looking too bad.

Do you think that method is adequate for the floor also, and if so, how close together do the stringers need to be?

The floor is subject to much more load, so needs to be built tougher. Unlike many builders here appear to think, it does not need to be strong enough for a crown of people to stand on - I suspect the most critical load is the 'knee test' where you lean into the trailer with most of your weight on one knee.

I think the floor is a place where a sandwich construction makes sense - say 1/4" ply + 1.5" hard foam + 1/8" ply bonded together. But the floor can also be made single skin about as lightly - say 3/8" plywood with 1x2s (standing vertical, not lying down) at 12-16" centres?