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Roll Cage Hoop X-bracing design


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Currently occupied with thoughts of Roll cage design for my Disco Bobtail. Looking at what is out there commercially it seems that one diagonal brace on the main hoop is considered by some to be sufficient to prevent shear, while most now seem to come with a X brace. My problem is that the design I have ended up with means that any cross brace will fall directly over my rear window which is

a. very useful in challenge events to look out of and

b. the only conceivable means of escape in an emergency

I have made a few sketches of some ideas I have had, please feel free to comment/critisize them. I guess I want max strength with min wieght and highest possible safety factor (don't we all)

This is how it looks at present. The hrizontal and all the lower structure is 60mm box with appropriate 6mm strengthening. The hoop is 48 tube/pipe (3.2 mm wall?). X brace will be made from same stuff as hoop. Window opens on drivers side. All views are from rear of car.

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Thiis is the standard X with its obvious problem

normal X

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I think this solution should be as strong as the normal x but stand to be corrected

corner braces

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A neat solution? But involves lots of steel and lots of wieght

second hoop

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A few others just for comment

design 4

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design 5

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design 6

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And finally my personal favorite. Anyone think of any good reasons why this is not suitable?

design 7

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Cheers for all your input

Mark

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Hi, regarding your question at the end: its not great cos its not really helping achieve what you want. All those vertical bars aren't doing much to stop it collapsing like a parallelogram...

Here is my suggestion: (depending on the exact scale of everything) - be prepared to lose a little at the corners (or adapt the shape below to 'just' miss the window) of the window and just put in a V from the centre to the two upper corners like this:

post-139-1176725334_thumb.jpg

Its mass efficient and will do a pretty good job with the shear problem.

Most of the diagrams just have far too much bar in. The diamond shape (3rd pic) and 'design 6' are reasonable alternatives though.

Cheers, Al.

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Al's design looks spot-on, except the bottom of his red beams don't brace TO anything - I'd move them apart as far as the vertical members below them (which becomes design 6 but lighter - are you expecting heavy point loading in the middle of the roof? I hope not).

Colin Chapman identified that the strongest spaceframe will always be the one with beams only in tension or compression, not bending. If you brace to a member between nodal points, there will always be some bending to resist the tension or compression in that member so: minimise the nodal points, you'll lighten your structure.

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sorry to butt in- but im a little confused the bottoms of the diagonal need to go to the chassis rails, this is what gives the strength as it is fully triangulated, if the ends of the tube finish at the start of the trayback shelf then if the car rolls the compression force will force the trayback mounts apart.

A picture is worth a thousand words, damn it.

If you do this then isnt the rear window kept clear??

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sorry to butt in- but im a little confused the bottoms of the diagonal need to go to the chassis rails, this is what gives the strength as it is fully triangulated, if the ends of the tube finish at the start of the trayback shelf then if the car rolls the compression force will force the trayback mounts apart.

A picture is worth a thousand words, damn it.

If you do this then isnt the rear window kept clear??

cage.jpg ??

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Cheers for all the replies so far. Food for thought. I will take a photo ASAP so the reality is a bit clearer. The sketches above are not to scale and looking at it again tonite it would appear that Als suggestion, modded to Turbochargers spec and possibly incorporating additional diagonals below the tray as in Landyluke/Gavs design may be the way to go. Had enough of it for tonite though, my head hurts. Another look tomorrow evening me thinks

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Cheers for all the replies so far. Food for thought. I will take a photo ASAP so the reality is a bit clearer. The sketches above are not to scale and looking at it again tonite it would appear that Als suggestion, modded to Turbochargers spec and possibly incorporating additional diagonals below the tray as in Landyluke/Gavs design may be the way to go. Had enough of it for tonite though, my head hurts. Another look tomorrow evening me thinks

post-1427-1176758945_thumb.jpg

Hi Mark...

Any pics of how she is coming along?

Lawrence

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thanks landmanluke, now i can draw on it, these are my ideas

the first is structurally stronger and would be the way i would build it- infact it will be the way at some point (waiting to see what yours turns out like first) I really like the look of this, but depends on the actual dimensions on the disco, as you could make the all the diagonals symmetrical through the cross of the main diagonal. sweet

the second is more practicable and still ties in all the bars. very common is comp safari racing

(i get excited at the oddest of things)

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post-2168-1176765870_thumb.jpg

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If you do MSA events and are scrutineered you must have a cross brace.

The Blue Book gives a lot of detail on cages and is worth a read.

I've seen a lot of cars after heavy impact and the MSA have developed cage rules after failures etc.

You can I believe homologate a cage but as a one off its not practical.

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LandyManLuke has the answer. The only one to consider IMHO.

Triangles are your friend.

Bracing to the top of the tub will just mean the cage above the tub will stay nice and square but the tub will lozenge in a side impact.

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Two further options - for thought.

Make the X out of steel cable (like thin winch cable). You would need to pre-tension it - but that will serve as well as a solid X as the legs of the X are not doing much in compression.

You could even make the cables out of Dyneema. Then if you need to escape, you can cut it with a knife!

Si

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Make the X out of steel cable (like thin winch cable). You would need to pre-tension it - but that will serve as well as a solid X as the legs of the X are not doing much in compression.

You could use the stainless steel cable used for yacht rigging, 1x19 form would be suitable. Sta-lok fittings would solve the problems of getting terminals machine swaged.

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Hi Gav

a few pics of the build process over on Difflock

http://forum.difflock.com/viewtopic.php?t=8061

Not taken photos of cage yet as its still a work in progress (hence this thread!) I will take some when it looks a bit more "finished" so that the full integration with the tray and chassis is clear.

Cheers

Mark

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Two further options - for thought.

Make the X out of steel cable (like thin winch cable). You would need to pre-tension it - but that will serve as well as a solid X as the legs of the X are not doing much in compression.

You could even make the cables out of Dyneema. Then if you need to escape, you can cut it with a knife!

Si

You can always rely on Si to think outside the box... :)

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Two further options - for thought.

Make the X out of steel cable (like thin winch cable). You would need to pre-tension it - but that will serve as well as a solid X as the legs of the X are not doing much in compression.

You could even make the cables out of Dyneema. Then if you need to escape, you can cut it with a knife!

Si

Or if you want to stay all tube why not think about thin wall T45 for cross braces? Very strong and very light (but not so much on the pocket...)

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Quick update

well i've started and the pics below show current state of play. The hoop and X braces are fixed now as are the outer arcs of the tray. All else can be moved and added too. The Tray is open to the rear to accommodate the spare and to make winch servicing a bit easier (having experienced this on a full tray. The final pic shows a mock up with most of the structural steel sort of in place. Have a look, tell me what you think, and then call me a fool as I carry on doing it my way anyway :lol: :lol:

Thanks for all input so far, it REALLY has been helpful.

Mark

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Durability may be a consideration if you start using wire rope or rope for ties!

Just for info, yacht insurance demands 5 yearly renewal for racing yachts, 15 years for cruising yachts.

yacht rigging (typically 6mm) exists in a salt water environment (granted, with few or no 'abrasives') and is subjected to a load/unload cycle somewhere in the region of a tonne or so, day in day out. you could easily go up to 10 or 12mm and it'd be extremely strong.

1.5m of wire Vs 1.5m of tube - weight saving?!

I reckon it'd do ok in a roll, it'd be cheap and easy to replace afterwards too.

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