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  #1  
Old April 20th, 2021, 05:30 PM
benjamintx@gmail.com benjamintx@gmail.com is offline
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Default Roll Bar

I'm working on a roll bar design. I have two daughters that I am excited to drive to the local pool/soccer games in my truck and there is just something about having nothing over their heads that is spooking the dad in me.

I've got 2x3x1/4" HSS that I'm fabricating the bar out of. I'm using a 1989 Dodge Dakota like the one in the attached photo as inspiration. This isn't a cage, just a loop to help keep the truck off the ground if it ever rolled somehow in an on-street accident. I'm planning to wrap the roll bar in sheet metal and create a hard top that is removable like a lid (think ferrari) so that the windshield can still fold down.

I have a hole cut in the floor to pass the HSS through so I can mount it to the frame and I've been trying to devise a way to connect to the frame that won't transmit vibrations and will allow some flex so it doesnt hit the inside of the sheet metal surround.

My best idea at the moment is a bushing with 9/16" Grade 9 bolt. I ran some numbers and the bolt wont be the weak point. I like this bushing in 3":

https://www.ballisticfabrication.com...ith-dom-sleeve

See attached sketch I did to mock it up.

I'm looking for feedback. I'm absolutely overthinking this but thats what I do! I'm wondering if anyone sees a problem with my idea or an easier way to achieve this?
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File Type: jpg convert truck.jpg (74.2 KB, 6 views)
File Type: jpg Screen Shot 2021-04-20 at 7.07.08 PM.jpg (4.1 KB, 4 views)
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Old April 21st, 2021, 07:52 AM
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Unfortunately I wasn't able to open up the thumbnail or the attached image, only the link to the bushing.
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Old April 21st, 2021, 08:34 AM
benjamintx@gmail.com benjamintx@gmail.com is offline
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Let's try this.

Here is a link showing a Dodge Dakota:

https://www.hagerty.com/media/car-pr...t-convertible/

And here is a link to my design:

https://imgur.com/a/ZlMZkZi
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Old April 21st, 2021, 09:05 AM
teking teking is offline
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Bushings to frame is the way to go. I've also seen welding plates to the floor send bolting through the cab so you don't have to cut big ass holes in the floor. Basically sandwich the floor between two plates and bolt them together. This is good if you or a future buyer wants to remove it. You are only left with the bolt holes. Drawback here is as the frame flexes so does the floor pan but the plates help prevent cracking.

I might also consider mounting to the outside of the bushing instead of on top if you are concerned about frame flex. Especially during offloading. As the frame flexes you want the bar to move up and down.

Sent from my SM-G781V using Tapatalk

Last edited by teking; April 21st, 2021 at 09:13 AM.
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Old April 21st, 2021, 10:20 AM
benjamintx@gmail.com benjamintx@gmail.com is offline
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Thanks, Teking.

The up and down movement makes sense, does this sketch reflect what you meant by mounting to the side of the bushing?

https://imgur.com/a/VSOKvk4

I considered the two plate idea but couldn't find a way to predict how much the frame would move relative to the cab and whether it would cause problems for me in the cab floor. It sounds like you have seen it done successfully? Do they put any kind of rubber pad between the plate to help with stresses and vibrations or just sandwich the sheet metal between two plates of different sizes?
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Old April 21st, 2021, 12:52 PM
teking teking is offline
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THIS ...... The bar goes off to thr side of the bushing to act kinda like a lever instead of straight in from the top with all the force straight down.. view in FIREFOX if you cant see the pics
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File Type: jpg rollcage.jpg (118.4 KB, 9 views)
File Type: jpg rollcage2.jpg (162.7 KB, 8 views)

Last edited by teking; April 22nd, 2021 at 03:14 AM.
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Old April 22nd, 2021, 06:34 AM
benjamintx@gmail.com benjamintx@gmail.com is offline
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Got it, thank you!


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Old July 21st, 2021, 01:31 AM
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Hmm. I would like to add positive constructive criticism.

I like your tube choice- it's overkill, but that's a good thing.

Google "miata roll bar". That's a shrimpy little car that barely weighs anything, and they almost without exception have
1) cross bracing
2) forward/rear bracing.

Yes, that Dodge came from the factory with no bracing at all, just a picknick basket handle sticking up. LINK
The cheapest solution the beancounters could squeak past DOT regs doesn't seem like something to copy, IMHO.
Check here for some much newer factory cage failures.
https://irate4x4.com/threads/roll-cage-failures.50590/

I think those bushings look like a terrible idea. In fact, they look just like this gate hinge:


I understand the isolation benefit, but the other side of that is that your "cage" is now a free-floating structure, and gets no reinforcement from the frame. Those tabs and bolts won't add up to anything in a wreck. The cage needs to be rigidly anchored to the frame, and the seats need to be suspended from the cage, creating a lifepod. The sheetmetal body is disposable wrapping paper.

Needs more triangles! Squares turn into parallelograms. That big upside-down 'U' can crush sideways, and pivot forward and back on it's hinges. Imagine you blow a tire on the highway, catch a ditch, and are sliding down the road on the rollbar at 55mph. What keeps that bar from flopping back into the bed?

At minimum, it needs a diagonal crossbar left to right, and door bars next to the seats, triangulating forward to the frame.

A couple more links:
https://irate4x4.com/threads/minimal...designs.46225/
https://www.pirate4x4.com/threads/no...1067505/page-2

I get that you're not racing or rockclimbing, but highway speeds with kids are no joke either.

For serious no-nonsense engineering advice, you might post up on https://irate4x4.com/. It's got Ultra4 racers, pro fabricators, and a bunch of obnoxious trolls who know their stuff.

Anyway...there's my opinion!
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