Building a Tube Buggy: Safety and Strength Questions

[hydro joe]

HI, I have builded a tube buggy out of square 1"1/2 x.085 and was told that it was unsafe so I'm looking into it. Was my design unsafe?
My question is round stronger then square if so how much?
Question 2 dose welding weaken the steel?
Question 3 if round is stronger how much stronger is it wend its bent?

 

[mgb_phys]

Round tube is typically about 20% lighter for the same overall diameter than square tube
Square tube is stiffer but on paper for the same weight of material round tube would always win.

BUT square tube is a lot easier to cut and weld, especially for complex angle joints - so for something like that it would be my first choice.

The safety risk is probably you driving into a tree at 60mph or off a cliff rather than the chassis breaking!
edit - some cross bracing might be good

 

[hydro joe]

Thanks, you wood not believe the bashing i got. You wood have thought i builded it out of paper.

 

[hydro joe]

Lets say you have 2' run of dom how much wight would it take and the same with cromo and mild steel

 

[FredGarvin]

I would be much more concerned about things like weld quality than round vs. square.

With typical box tubing and round tubing, the material is typically structural steel with relatively low properties (not saying you can't get some with elevated properties). In the case with structural shapes, unless the tube is pre heat treated, you are working with a steel in the annealed state. That is what the localized area around a weld will be. So in this case you are probably not deteriorating the material properties. That being said, with different materials, different things can happen during the welding process depending on material and process used, inert gas, etc... It is a very broad field that requires a fair amount of knowledge.

If you are interested in some absolutely fantastic reading that is a bit old school but incredibly informative material on welding (at an extremely low price!) I would recommend you look at purchasing:

https://ssl.lincolnelectric.com/lincoln/apdirect/item.asp?prodnum=DWS

https://ssl.lincolnelectric.com/lincoln/apdirect/item.asp?prodnum=TS

I have a few of Blodgett's books and they are the best I have seen on welding. I can't believe how inexpensive they are for the amount of information they provide.

BTW...nice bit of work there. Congrats!

 

[hydro joe]

Thanks, I will buy them.

 

[nvn]

hydro joe: You gave us the dimensions of a square tube cross section. But what round tube cross section do you want this compared to? You would need to state the specific round tube cross-sectional dimensions (outside diameter and wall thickness) for the comparison.

And it would also be good if you state the material specification for the specific square tube and round tube you want compared.

 

[Mech_Engineer]

The problem is not that you made it out of square tube, but that the tube you used is relatively thin walled mild steel and won't take any appreciable impact. Additionally, you're severely lacking in triangulation, and it's likely your frame will readily band/crumple under any kind of rollover condition.

I would recommend going over to a site like www.pirate4x4.com/forum/ for advice on your situation. It's probably salvageable with some strategic triangulation, but right now it's not all that great. The Pirate guys will tell you like it is, so don't expect hugs and kisses over there.

 

[hydro joe]

I been on sites that love to beat you down and its hard to get info.
I'm looking for real answers not what people think. The rock crawling guys do not have any rule book that states what we should use.
I've fond a tube buggy that's is accepted by rock crawlers. The f toy, the base modal is made of HREW hot roll electric weld 1.75x.120 wall tubing.


As for as rolling it I've dune that.

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And in this roll the cage moved 2" to the right on the top. Down pic
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I thank you guys for your input.

 

[hydro joe]

hydro joe: You gave us the dimensions of a square tube cross section. But what round tube cross section do you want this compared to? You would need to state the specific round tube cross-sectional dimensions (outside diameter and wall thickness) for the comparison.

And it would also be good if you state the material specification for the specific square tube and round tube you want compared.

The tube is hot roll electric weld 1.5x.095 wall both round and square and a span about 24"

 

[Mech_Engineer]

Well hell if you've already rolled it and the cage held up, I'd call that a successful test. You might consider adding some triangulation spars for some added stiffness in the event of a multiple rollover, but it seems to me you're already basically there. I attached a picture of what I mean.

Off-roading is a fundamentally risky hobby (I'm an off-roader too). Have fun driving the vehicle and don't worry about what people are saying. There is no fundamental problem with using the square tube you've got, and it seems to have already proven itself to be a strong safety cage. It doesn't have to meet any specs because it isn't a race vehicle, it's just a backwoods trail rig.

 

[mgb_phys]

I don't think anybody was trying to beat you down.
I would want to add some diagonal cross bracing behind the seats just to stop the roof crushing (imagine a box sliding sideways), you could also put some diagonals on the nearly square back side 'windows'
Some diagonals on the lower panels under the 'door' between the side bar and the frame would stiffen the lower frame a lot.

I like it - wish I had one ps. were the rollovers a deliberate test!

 

[hydro joe]

Well hell if you've already rolled it and the cage held up, I'd call that a successful test. You might consider adding some triangulation spars for some added stiffness in the event of a multiple rollover, but it seems to me you're already basically there. I attached a picture of what I mean.

Off-roading is a fundamentally risky hobby (I'm an off-roader too). Have fun driving the vehicle and don't worry about what people are saying. There is no fundamental problem with using the square tube you've got, and it seems to have already proven itself to be a strong safety cage. It doesn't have to meet any specs because it isn't a race vehicle, it's just a backwoods trail rig.

Well i like what you have suggested.

 

[hydro joe]

I don't think anybody was trying to beat you down.
I would want to add some diagonal cross bracing behind the seats just to stop the roof crushing (imagine a box sliding sideways), you could also put some diagonals on the nearly square back side 'windows'
Some diagonals on the lower panels under the 'door' between the side bar and the frame would stiffen the lower frame a lot.

I like it - wish I had one ps. were the rollovers a deliberate test!

I felt that behind the seats needed more as well so i installed 1"3/4x1/4 wall hopping that would do.
Heres what they like to say about my square buggy. <link deleted>
Rolling over was not deliberate but i new it was getting close.

 

[nvn]

hydro joe: In post 1, you mentioned a 0.085 inch wall thickness. In post 9, you mentioned 1.75 x 0.120 inch wall thickness; although you did not say square or round. In post 10, you mentioned 0.095 inch wall thickness. Is the wall thickness in post 1 a typographic mistake? Or is the 0.095 in post 10 a typographic mistake? Just wanted to check with you to make sure which sizes are correct for the comparison you want.

 

[Mech_Engineer]

Well in the pictures in the other forum you can see that the cage bent sideways when you rolled it on its side. You need to add something like ax X-brace behind the seats, like the attached picture.

Seriously though, what the hell is going on with your suspension? It's just weird...

 

[mgb_phys]

Seriously though, what the hell is going on with your suspension? It's just weird...

Looks like an extreme version of the independant chassis on a Unimog

 

[hydro joe]

It has a one link that's made of 2"5/16 trailer balls and 12" hydraulic powered rams at all corners that helps weight shift. The red part right under the springs is a swing arm. under normal driving condition the axle rests against the bottom of the swing arm bringing into play. The axle has the ability to drop away from the swing arm. It is only located by the 1 link and track bar.




Here a video of the roll.http://www.youtube.com/watch?v=5oL-BM5_9VE"

 

[Mech_Engineer]

You would have avoided a lot of problems if you just aired down in that rollover video you posted. Anyway, I can't lie- the more I see of your rig the less I like it. However it's yours not mine.

Also, get some 4 or 5-point harnesses in that thing, and stop hanging out the side! You're lucky you didn't get crushed in that rollover.

 

[nvn]

hydro joe: For a 38.1 x 2.413 mm (1.5 x 0.095 inch) round tube, versus a 38.1 x 2.413 mm square tube, the moment of inertia of the square tube is 1.70 times the moment of inertia of the round tube. Therefore, in an orthogonal direction, the square tube elastic bending strength is 1.70 times the elastic bending strength of the above round tube. In the 45 deg direction, the square tube elastic bending strength is 1.20 times the elastic bending strength of the above round tube. I have not investigated plastic bending strength yet.

 

[hydro joe]

You would have avoided a lot of problems if you just aired down in that rollover video you posted. Anyway, I can't lie- the more I see of your rig the less I like it. However it's yours not mine.

Also, get some 4 or 5-point harnesses in that thing, and stop hanging out the side! You're lucky you didn't get crushed in that rollover.

Air down was not an option the carb will cut out,but i put in holy pro injection. As far as a 4 or 5-point harnesses it takes away for site.
What part or parts do you don't like if i can ask.

 

[hydro joe]

hydro joe: For a 38.1 x 2.413 mm (1.5 x 0.095 inch) round tube, versus a 38.1 x 2.413 mm square tube, the moment of inertia of the square tube is 1.70 times the moment of inertia of the round tube. Therefore, in an orthogonal direction, the square tube elastic bending strength is 1.70 times the elastic bending strength of the above round tube. In the 45 deg direction, the square tube elastic bending strength is 1.20 times the elastic bending strength of the above round tube. I have not investigated plastic bending strength yet.


The buggys weight is 3400 LB if that helps.
THANKS for helping.

 

[Mech_Engineer]

Air down was not an option the carb will cut out,but i put in holy pro injection.

AIR DOWN YOUR TIRES. You should be running like 10-15psi max in those monsters, it will give you better traction and a smoother ride off-road.

As far as a 4 or 5-point harnesses it takes away for site.

If you're likely to roll over, you should have a 4-point harness MINIMUM. If you feel it takes away from your sight, it because you're dependent on hanging out the side of your vehicle to drive, which is dangerous and essentially useless. All you can see when hanging out the side of your vehicle is your front left tire, it doesn't help you drive your vehicle. Get 4 point harnesses, this really is just as important as making sure your cage is strong.

What part or parts do you don't like if i can ask.

I don't like that your cage bends sideways in a rollover like that one (this means it is not strong enough), I don't like that you don't have proper seat restraints, and I don't like your suspension setup- it unloads too easily and while it flexes a lot it also performs poorly in off-camber situations like seen in your rollover video.

 

[Mech_Engineer]

You posts would be a lot easier to read if you used the "QUOTE" button at the bottom of a previous post when replying to it.

 

[hydro joe]

You posts would be a lot easier to read if you used the "QUOTE" button at the bottom of a previous post when replying to it.

I am dyslexic and I'm figuring it out sorry.

 

[hydro joe]

Thank for you honest opinion, the buggy is a work in progress.

 

[nvn]

hydro joe: For a 38.1 x 2.413 mm round tube, versus a 38.1 x 2.413 mm square tube, the square tube plastic bending strength, in an orthogonal direction, is 1.50 times the plastic bending strength of the above round tube. In the 45 deg direction, the square tube plastic bending strength is 1.4142 times the plastic bending strength of the above round tube.

Regarding question 2 in post 1, welding generally weakens steel. Even though the weld metal is often stronger, the base metal generally becomes weaker in the heat-affected zone.

 

[hydro joe]

Thanks