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Cage Design Questions

Discussion in 'Builds and Fabricators Forum' started by 47v6, Nov 7, 2016.

  1. Nov 7, 2016
    47v6

    47v6 junk wrecker! 2020 Sponsor

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    I need some framework in my mind about building a cage so I can think about it. I guess the standard is DOM tube,.120 wall, 1.75 OD. Would this be used throughout? How about the front hoop?

    The Price of CrMo might be prohibitive, but can the OD of the tube be reduced for similar strength? This might be of particular interest in the front hoop because I have limited space in my 2a.

    The hoops, 3? one front, one middle and one rear? When looking at my random roll bar from whenever, the top bend is not 90*, its more like 80* . It might look nicer if it tapered at an angle toward the top and increase the rigidity of the structure because it triangulates a bit? I am not building a jungle gym, but want something I can count on to work as designed.

    Front hoop and sight. On a 2a there is plenty of times I have to either look over the top of the windshield or turn my head sideways to look out the front window to see stop lights. What have some of you done to make sure your sight lines are not completely obstructed by the front hoop. I have already gotten rid of my underseat tank and sunk my seat a bit to help this. 1.5 OD CrMo tube?

    What about hitting your head on a cage? How do you mitigate that? Short of making everyone wear 5 point harnesses with HANS devices, how best to design one to limit head impacts.

    Seat cradles. I really hate high back bucket seats in 2a's, but low back seats do nothing for safety. Would it make a lick of difference to build a cradle for the seats attached to the cage and use low back ones with a stock style rear seat? Would using a stock style seat completely negate the reason for building a cage? I need a back seat for kids, so what is a nice way of designing a cage to protect the passengers?

    Tying in the frame. If I build a cage, I plan on tying it into the frame. I would weld outriggers to the fame to the cage attachment points. Should I cut a hole through the tub and bolt directly to the outrigger or use poly isolators, or sandwich the tub floor. I can think of a strange but possible scenario where the tub and frame could separate, but not the cage. This could also cause problem if a seat cradle was not attached to the cage.

    I am not building a race car, don't plan on doing ROCK CRAWLING!:rofl:, and am looking at this for safety if someone were to run into me on the street or a roll over on a trail. If I do build it though, I want it to be able to withstand whatever emergency its called for while not being heavy, obtrusive or a PITA.

    There is a lot to answer and talk about in this post. I have a lot to learn and others will hopefully benefit too.
     
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  2. Nov 7, 2016
    ITLKSEZ

    ITLKSEZ Volvophilic

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    Oooh! I have opinions but no time to type! I'll chime in tomorrow.

    :watch:
     
  3. Nov 7, 2016
    tarry99

    tarry99 Member

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    Maximum protection without any inconvenience? Sorry.......turn the page.


    Briefly 1.625 .083 Chmo will get you similar strength as 1.750" DOM x .120 wall..........Basic cage design is all about attachment points , structural joint integrity and lateral supports , The cage is normally there for protection but also to enhance chassis strength and integrity..........
    Site picture is always compromised........once you get used to it the picture becomes clearer. Get the hoop above your head behind you and the seat back ............still a good Idea to have head rests if you use low-backs attache something to the bar behind you........down tubes running from the main hoop above your head forward along the body sides can be an issue with head clearance & impact , that's why we put padding in those area's............door bars are important for side impact as well as rear facing bars to add support to the main hoop..............I like seat frames attached to the cage ..........and once you get used to the cage you'll be using your seat belts more often to keep you in place..........and that's really important to understand that if you are unsecured in the seat and your head comes in contact with the roll bar during an accident the damage to your head may be greater than the actual accident..........so lot's of things to put in proper prospective.
    For your Kids , securing the seat or seats as mentioned above and providing a cocoon similar to what's forward for them to be safe in is also important.

    As far as mounting , I like using the method I once showed on here by mounting the cage to outriggers and using the same poly Isolator's between those mounts that the body is also mounted on..........This Keeps the body and cage moving in the same moment and will also allow the body to be attached to the cage if your concerned about separation. Personally , I think the factory mounting system of a Jeep body is pretty antiquated both in locations used and how it's supported.......there are much better systems to keep the components together...........

    Unfortunately planning safety for the unknown accident is a tough task.

    That landing pad off the side bars is just below the front down tubes of the cage a poly isolator would be there between the body and cage.
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
    Rear cage mount inside wheel tub
    [​IMG]
     
  4. Nov 7, 2016
    GraySkies

    GraySkies Always late, never finished...

    Western Washington
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    Keep in mind also that straight bars meeting at 90 degree angles will not have sufficient torsional rigidity in a violent rollover situation. Gussets can help with this (lots of designs out there, both plate and tubular), but that help will be somewhat limited. Better would be to contemplate how you can add more triangles to your cage design (both front-to-rear and side-to-side). All of these things are compromises where visibility is concerned, but the protection offered by giving some weight to these considerations could very well save your life someday.
     
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  5. Nov 7, 2016
    47v6

    47v6 junk wrecker! 2020 Sponsor

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    Terry,
    Thanks for the pics and the advice. I really like the idea of integrated outrigger/ rock guard/steps. I am very concerned with the cage and heads impacting it during an emergency that can cause death or serious injury. It would be of paramount importance to try and mitigate that possibility as well as can be engineered.

    I ALWAYS use a 3 point harness and if I had provision for that in the rear, I would make my kids wear it too. Right now I have lap belts. I bought all new belts, USA manufactured. In installing a cage I would certainly integrate 3 point belts in the rear into the design if at all possible.

    In looking at the pics that Terry posted, the outriggers are bolted into place. I am sure those would be welded in. The front part of the cage doesn't have a hoop, but 2 down tubes and a cross bar. That is important for me because my windshield folds out and the only provision I have for wiper motors is top mounted.

    Another idea for the front down tubes would be to outboard them. I have stretched my body a foot, so a top is not going to happen, therefore to make my front down tube come down on the outside of where the body truncates inward might save valuable foot area in the tub. Similarly in the rear of the jeep, the hoop or down tubes for the cage can directly bolt to the rear crossmember while providing a point to mount a spare tire that would be significantly stronger than the body. (No tailgate).

    Without concern for a soft top, making the cage might produce not just a safety improvement, but allow accessories to be more easily attached (spare tire) and possibly provide removable sun protection.

    The next step in to NOT design a jungle gym. Minimal tubes, maximum strength. Remember, this is for a predominantly street driven vehicle where the frame is already made of 2x4 tube 3/16 wall and most of the tub is constructed of 16 and 11 gauge steel or thereabouts.

    The more I think about this, the more I think it is at least as complex as building the frame. The last thing I want to make is some big ugly thing that I hate.
     
  6. Nov 7, 2016
    tarry99

    tarry99 Member

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    Do your homework , before you proceed.......no two vehicles are the same when building one...........thinking ahead in the design and how & where you can tie things into more central locations vs multiple looks and functions much better .........go down to Home depot and get some 1.5" PVC water pipe just as some mock up material and you'll get a better Idea where and how things will fit.........You'll probably need to rent a bender or find someone that can bend the main hoops to keep those as tight as possible.
     
  7. Nov 7, 2016
    BobH

    BobH Member 2020 Sponsor

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    I saw a custom header company that sends you a PVC kit and you mock up the headers from that. When you are finished, you send that back to them and they build the headers from that. You could do the same for your cage. I have seen PVC benders that heat the plastic.
    Then you could send all that to someone with a bender to have your tubes cut and bent.
     
  8. Nov 7, 2016
    tarry99

    tarry99 Member

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    That is a traditional way for a roll cage in a race car............down tubes that are bent down along the dash and into the floor.........and in that case there are two cross pieces that tie the cage together.........the top is mounted back far enough that wiper motors are of no consequence and the bottom can either be welded in slightly up / down from that location or made removable if need be..........both bars are necessary to keep some integrity to the cage either from side impact or going over.

    Or another way to attache the down tubes to a common dash or cross bar which also supports the front strut suspension......look at the door bars how they intersect for strength......not quite done yet in that picture that's a SFI certified 25-1 car good for 200 mph.

    [​IMG]
     
    Last edited: Nov 7, 2016
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  9. Nov 7, 2016
    47v6

    47v6 junk wrecker! 2020 Sponsor

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    In the tube frame chassis Terry pictured above it uses dissimilar OD tube. It looks like there is primary tubes that are used for protection of the occupant and smaller OD used for the structure of the car itself. The race car chassis I have seen are more heavily and crudely built models that what is pictured above, but I am sure they are designed in accordance with the definite probability of another car running into them at speed, (oval track stuff from the 80s/90s)

    I have done a google image search. A lot of the stuff out there looks like jungle gym and seems to be built for for fashion than function. I am not of the idea that more tube is always better.

    I dont think I would design a cage for my 2A without 2 front down tubes and 2 cross braces for sure. The middle hoop really needs some sort of cross bracing, otherwise it seems to me that it could collapse. Unfortunately in the designs i have looked at make it seem as if the people in the back are in a "cage" so to speak. I wonder how I could design something that felt a bit more open between the front and back. I have no intention of 5 point belts...
     
  10. Nov 7, 2016
    tarry99

    tarry99 Member

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    All the main chassis and roll cage is 1.625 x .083 CrMo , all the other CrMo tubes can be of various sizes and wall thickness depending on use and area within the car , but still held to the specifications as set forth in the SFI spec sheets ...........when finished you call a tech inspector.....he comes over for a fee and measures the OD...........all tubes that are CrMo are striped marked from the factory....if they have been wiped off the inspector can check the wall thickness with and electronic gauge. CrMo tubing has different wall thickness's than mild steel.

    You can use mild steel of various sizes for different bars also...................

    With the body on it
    [​IMG]
     
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  11. Nov 8, 2016
    ITLKSEZ

    ITLKSEZ Volvophilic

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    Normally, with a stock frame, I'd warn of stresses from outriggers causing breaks and cracks due to twisting and vibration suppression, but with your tube frame, much of that worry is eliminated.

    Jeeps are a little different than race cars when you think about where the stresses are/leverage is during a rollover or crash. A car is harder to flip, but easier to roll. A jeep and its cage are inherently taller and have a flat side to flop against, so by nature a tip-over is easier, but a full rollover is harder. A car's rotational center of weight is actually higher within the vehicle than a jeep with a full cage. Think: Jeep = square Weeble-Wobble. I've been in 9(?) tip-overs in a jeep (a few of them were really violent), but only one complete rollover(s), and it involved a lot of speed and a steep hill. Otherwise, a Jeep generally seems to prefer to slide on its side, or maybe tip up onto the corner of the front hoop and flop back down.

    Because the top edge of the cage is taller and further from the mount than it would be in a car, it will produce more leverage against it's mounting points when stressed. Again, this might not be an issue with your tube frame, but it causes a real issue when building a serious cage in a stock jeep, in which case it is vital that cage mounts get tied to the frame in locations that either have full-frame (top to bottom) crossmembers in place, or other steps have been taken to alleviate frame twisting. Just for kicks, the next time you have a body off a stock frame, find a way to clamp or tack a piece of 2x4" tubing or similar to the side of the frame where a front hoop would mount and stand on it around 16" out. The frame deflects so much, your foot will drop over an inch. and that is only 200#, not 2000# or more. That front hoop is vital for safety, and its mounting point is just about at the weakest point on these old jeeps due to a lack of a crossmember in that area.

    You definitely want some sort of isolation between the frame and the cage, especially if you plan on mounting your seats to the cage. Your fillings will thank you. Poly bushings are harder and squeakier than I prefer, so I'm turning mine out of rubber cow mat (recycled tires).

    The argument between 4130 and DOM will go on for eons. IMO, DOM is plenty strong for our intents and purposes. The 1/4" or whatever of saved space isn't worth the extra price and temperament of 4130. Plus, I was taught that 4130 needs heat treatment after welding at thicknesses above 1/8", which is fine for the tubing body, but what about the mounting plates?
    Good Q&A on 4130: TIG Welding Chrome-Moly Tubing | Lincoln Electric

    If you build your seat mounts into the cage, would you be able to get your seats any lower?

    One major point that is often overlooked with shoulder-style 3-pt belts is the style of belt buckle itself. There are standard "sliding" belts where the clip can slide easily along the fabric, then there are "locking" belts that have a cam system (like the old-style lap belts) that allow you to pull and tighten down on your lap. This style is much-preferred in older conversions where an auto-tensioner (tied into the airbag system in newer cars) isn't included. It can really keep you tightly in your seat; the retractable ratchet mechanism only has to worry about the upper half of your body, rather than your whole body. I know our '03 Dakota has sliders up front and locking buckles in the rear jumper seats.

    As far as cage design goes, the sky is the limit. Do what you think looks good. Triangulate if/where you can, gussets are a good idea too. Think about where the impact will happen, and try to build the strength accordingly. Like has been alluded to, you can only plan so far; risk will aways be there and only a test will find the weaknesses. All we can do is find that middle ground between "good enough" and "monkey bars". For my old racing inspection, at least one diagonal bar behind the occupants was recommended. That, combined with a diagonal bar across the top opening and sturdy frame mounts really makes a cage strong enough to withstand most (self-inflicted) impacts without folding. We also ran side bars too, which makes it a little difficult to enter/exit Dukes brothers style, but it really strengthened the cage and made you feel invincible.

    I'll post some pics later.
     
    Last edited: Nov 8, 2016
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  12. Nov 8, 2016
    ITLKSEZ

    ITLKSEZ Volvophilic

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    I know it is't back-seat friendly, but I tied my rear bars directly to my frame on this one to avoid outriggers. The cage also goes in through the engine compartment and is a front and rear shock mount as well as a motor mount. It sandwiches the grill and mounts again to the front. I had a cross-bar behind the seats that had bars down to the frame, and a diagonal bar from the DS upper corner to the PS frame mount location. That thing felt so good to rip through the woods. Stiff, allowing the soft suspension to do the work.

    [​IMG]

    Seat mounts and battery. (Ignore all the tanks. Poor man's nitrous. (y))

    [​IMG]
     
  13. Nov 8, 2016
    47v6

    47v6 junk wrecker! 2020 Sponsor

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    Do some underwater jeeping with those scuba tanks? AHaha:rofl::rofl:
    But seriously, this is really great info. I didnt really think about the amount of leverage placed un the frame rails with an outrigger in the event of a rollover or flop over. As far as a seat cradle. I can make it as low as I like. No underseat tank no more! A cross bar behind the rear seat is a good idea and might be able to be incorporated into a center console. To make it strong though it needs triangulation. That can also be an issue with head impacts in a lowback seat idea. Connecting right to the frame in the rear seems like a good idea and the length of outriggers from the frame rails need to be of a minimum length.

    Great stuff guys.
     
  14. Nov 8, 2016
    tarry99

    tarry99 Member

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    Leverage?..............all chassis have some form of outriggers or brace's that extends away from the main frame rails on both sides........... the additional width is what increase's the integrity , rigidity and strength of the body and chassis as a unit , with the cage also adding some additional strength.....................Jeep chassis are only about 30" wide ...........it would be of little benefit to just secure the roll bar some how to the narrowness of the narrow frame rail..........with your rectangular tubing chassis there is no issue.

    Yes , there could be a question of leverage coming from a single member attached to an un-boxed OEM C channel frame rail that was not near a place where a cross member or other cross frame structures reside........but that ingenuity in building , attaching or adding to the structure to secure additional strength in given areas should never be taken for granted and should always reflect upon the abilities of the individual fabricator to understand the dynamics of cancelling stress while also applying it as needed to firmly secure the roll cage properly on any chassis. And if a person is not capable of that then perhaps he should take it to a shop.

    The benefit of the sliders are many fold if properly connected in this instance. One they keep the body away from obstacles and also allow for a pivot point against an immovable object , they also serve as a cage mounting platform to distribute load and stress along a much greater area. And perhaps in your case most importantly if you drive it on the street they just may keep cars or other objects from entering the drivers compartment.

    You mentioned about welding the braces directly to the chassis and I like the ability to unbolt the stuff , just in case it gets damaged or a later modification may be needed.

    Outriggers if attached properly are of no consequence here and more than needed than not . One negative may be changing the routing of exhaust if you choose to go under the door exit.

    I said it would not be easy...........just got to put the thinking cap on , do some planning and look at the overall picture. Your a smart guy you'll figure it out.
     
  15. Nov 8, 2016
    Rich M.

    Rich M. Shoe pusher

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    I made outriggers. The 1/4" wall is way overkill but its what I had. The plate is 6x6 attached to the outrigger. 3/4" horse stall mat on top. Dont have a pic but the front hoop floor plate is 4x4. Grade 8's through. Glass body, gotta have it. Synergy bushings on order and 1.5 Dom to make the supports for the rear hoop ( in the wheel well, going to be very similar to the pic Terry posted) I made rocker guards out of 1.5" that telescope into the out riggers IMG_20161016_171033_192-1-1.jpg
     
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  16. Nov 9, 2016
    ITLKSEZ

    ITLKSEZ Volvophilic

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    The problem with that is, the windshield is only as wide as the body, so the cage would either be wider than the windshield, or bend out around the body. It looks odd, IMO.

    [​IMG]

    We're talking about a wimpy C-channel frame that was designed to carry a 1/4-ton load on springs that were located directly under the frame rails, and even still, after years of use, the frames are cracked from the inherent twisting it caused. I think Willys knew this; the widest outrigger they went with unsupported was a body mount that was 1 1/2" out from the frame. (Motor mounts were longer, but at a boxed frame section.) What I'm getting from your statement is that you feel comfortable prying sideways on a cage-width outrigger, and that it somehow increases the rigidity of the frame?

    It is one thing if the cage tube is welded to a wide, sturdy flange that is bolted directly to the outrigger; In that design, in order for the frame to flex, the tube and/or plate would have to flex or bend.
    But when incorporating a bushing of some sort, as we were recommending, a pivot point (the bushing) is being introduced, and that outrigger is going to be allowed to flex independently of the cage, and yes, leverage will be an issue on a stock frame if no other steps have been taken to alleviate frame twisting, as I stated in my post. Frame cracks (or worse) will follow. If you want to alleviate that issue with a giant rock slider, that might be fine for you. I'm just trying to get some info out there that I've learned by a little research and a whole lot of brutal trial and error - through tons of abuse, actual rollovers, and putting the stamp of approval on things that work and modifying things that don't.
     
  17. Nov 9, 2016
    tarry99

    tarry99 Member

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    Thousands of Jeeps are running around right now with the wimpy C-channel frames you mention...........that's a given , and yes I'm sure that several are cracked and stressed........Having said that it still does not stop the competent fabricator from installing a safe cage design on wimpy frames by either adding boxing or additional bracing if needed while still addressing the normal concerns of either having the down tubes and main hoops either bolted directly into sheet metal below or welded or bolted to outriggers that are fashioned off the wimpy chassis.

    Direct welding is also fine in some applications , but that method does not allow for any movement or the ability to remove later and further complicates how you block off the holes created for the tubes while still keeping water and debris out...........

    In the race cars we tightly fashion are removable panels around cage components in a way that still allows for D-Zus button removal but of course being water tight is not necessarily mandatory. ( see pictures below)

    I've found that in Jeeps , but not race cars the method of sandwiching the cage down tubes with plates between the body and the outrigger below using poly body mounts that have a 90% memory compression factor while also using the very same poly mounts for the tub which allows for the cage and tub to move in the same moment all the time.........thus cancelling any cracks in the metal floor that would be inherent otherwise..............in the event of a roll over , Yes they will compress some and perhaps may still damage the sheet metal but will still offer compatible strength and integrity if welded directly. Any issue with flex to the frame as you mentioned above I really don't see that as an issue as long as the basics are covered.

    I have installed many with excellent success and in some cases just provided a cup at each side to contain the poly shock absorber..............

    I'm not here to create a controversy or educate anyone who wants to attache things to there Jeep.........just here to show proven methods followed by sound fabrication.

    If your uncomfortable about frame twisting as you seem to be then by all means figure out a way that works for you.

    Here is how we position our sheet metal around the cage
    [​IMG]
    [​IMG]
     
  18. Nov 9, 2016
    ITLKSEZ

    ITLKSEZ Volvophilic

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    PM sent.
     
  19. Nov 9, 2016
    Tumbleweed

    Tumbleweed Senior member

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    If you put a behind the seat cross bar in, especially If you want to mount a shoulder belt to it. Consult Simpson or Mastercraft or similar websites for the height recommendations. It is critical to place it at the right height to your shoulders so you don't crush your spine or snap your neck. SCCA may be another site to check.
     
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  20. Nov 9, 2016
    jpflat2a

    jpflat2a what's that noise?

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    I don't have a bone in this, but a competent fabricator can make the front cage and follow the windshield and go straight down the dash to the floor on a 2A 3A etc. with the proper bends. Gee..wait a minute...that's how mine is; no wonder I like that design.
    Floor space/foot room is a non issue.
    No need to go outside the body. That's just......looks terrible.
    Like in all those old photos I posted in other threads of roll overs, factory stock roll bars will work fine; add a cage if you wish.
    Some of you are trying very hard to re-invent the wheel in my opinion.
    To each his own.
     
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