Here is a 3-D model of the tire carrier I am building. I am mounting it directly to the frame so I can change rear bumpers when I find one or build one I like. The tire on the model is a 31" but as you can see I designed the frame to easily handle up to a 33 and probably bigger. I have a rack I will mount to the top and sit above the tire and I will fab up some brackets to hold a Hi Lift and maybe a shovel.
The pivot is tubing with bushings pressed in and is greasable.
that's going to have to be some heavy duty pivot. all that weight on a single hinge is going to want to bend the inner pin. you might be able to get around it by making the pin longer. also, you could try making the pin like one of the hinges of the hood is set up. essentially you would cut a piece of pipe in three sections, two long ones equal length for each end and a shorter on for the middle. weld the center piece to the pin and then slide the two ends on the pin on either side of it and attach those to the the bumper. this way the pin spins inside the end of the pipe along with the frame of the carrier and the top and bottom ends help counteract the twisting force of the frame when it's in the open position. did that make any sense to anyone else?
You may want to take off the two outer uprights and shorten the top bar to shed some lbs. You can always branch off the two center uprights to mount cans or high-lift. And the rack you want to go on top will have enough support as long as you some small angle supports on the uprights under the rack. You will still need a beefy hinge set up. I will be re-doing my hinge, I under estimated mine bad!!! Oh well back to the drawing board
I wish I was good enough on these dang computers to do 3-D stuff like that. I am doing good to draw things out on a napkin while I am at McDonalds!!!
91' YJ,4.0 I-6(180,000 mi) 2" Rancho Add-A-Leaf kit, 1.5" shackle lift, 2" BL, 33x12.50 BFG M/T KM's, 15x8 589's, TJ flares, custom F&R bumpers w/ swing-away spare carrier & rocker guards, FULL ROLL CAGE, herculiner, Warn [B]XD9000i[/B] (IT'S ALSO MY WIFES... or was!!!)
Rig Rater score= 476RRv1.0
Its a dog-eat-dog world. and I'm wearing MILKBONE UNDERWEAR!!
I was a little concerned about the pivot as well but what I have come up with is a 1 1/2" bar turned 10/1000 under and the tube is machined to fit two 1 1/2" brass bushings in each end with a length of 5". I already have the pivot fabbed up. The bar rotates smoothly in the bushings and I have a grease fitting to lube it with. The bottom square tube of the swing will have a 1 1/2" hole in the one end for the bar to go through and get fully welded in place. This pivot "should not" flex. the flat bar that the tube is welded to will be welded to my Jeep frame making it independent of the rear bumper. Also the bar has a hole in the end for a cotter pin making it possible to remove the entire swing out leaving just the pivot point on the Jeep.
Keep the thoughts comming. No one is perfect and a team effort makes a good idea better!!!
Just some more background: I am basing my idea of of Garvin's tire carrier: Garvin Rack
The area where you bolt to the frame next to the hinge has to be double what you would normally do. Usually the stress/weight of the rack is spread across the entire bumper and at least two points where it mounts. Now you've cut it down to a comparably smaller area so the mounting needs to be ultra serious! .02
True about the stress, but the stress will be spread across the entire rear frame member. The rear frame member is basically a channel on its side, very strong. My pivot is welded to 3/8" thick steel plate 4" wide, this in turn will be welded to the end of the frame cross member, actually adding strength to it.
the weight of a 31 inch tire ranges between 40 and 50 pounds depending on width, so let's say it's 45, and the weight of the steel of the carrier will probably be what, another 30 or so? not sure what size stock you're using. so that means that you're carrying about 80 pounds on that single pivot. that tire has about a 15 inch radius and it's probably what, another foot to the pivot from the pivot side edge of the tire? so say about 2 and a half feet from the center of gravity to the pivot point? 2.5 x 80 = 200 ft-lbs of torque on that 4 inch weld. what's the sidewall thickness on the outer tubing of the hinge?
looking at the close up of the hinge part you posted, it seems that you're mounting the outer tube to the side of the frame channel? am i looking at that right? if that's the case, you may want to think about moving it to rear of the channel so that the tube faces front and back along the jeep, instead of side to side. the reason i say this, is because it will change the nature of the stresses in welds. normally you open the carrier so it points towards the rear of the jeep for the most part, maybe a little toward the passenger side. with the pivot mounted on the side of the frame rail, this position of the carrier will put the welds under a shearing stress that will want to twist the welds apart. generally, this is the type of stress any kind of fastener is weakest against. think of a screw or bolt, generally you twist the head off or snap the shaft twisting etc. you don't see one popping the head off because it was being pulled under tension, and you don't see it buckling because it was being pushed under compressive loads. moving the tubing to the outside will change the load to tension on the top and compression on the bottom of the welds with no stress in the middle. ideally anyway. in reality it will probably want to pivot at the base of the weld so the entire weld will likely be under tension. either way, you'll probably be able to resist much more stress under these conditions then with the tubing to the side. the general rule of thumb is that material is twice as strong under compressive and tensile loads then it is in shearing loads. if you need another example, take a pencil or pen and some sticky tack (or chewing gum) and stick the pencil to a flat surface. now try to remove the pencil, which represents the tube, by twisting it. the tack, which represents the weld, will give fairly easily this way since it's the only thing resisting the force applied. now try again by levering the pencil up from one end. if it's good tack, it should be a bit harder this way. the tack will want to stretch before it comes loose and the table or wall will act to help hold up the pivoting end of the pencil. anyway, I hope that made at least a little sense and that i didn't bore everyone to death. good luck!
The bushings are 1.5" long and the tube is machined on the inside to press them in. When I welded the pivot onto the plate I had the rod in place to help reduce the distortion. It did fine, rod turns really good.
Sentinal, The weld beads are on both sides of the tube contact with the 3/8" plate and the plate will be welded to the frame in both the vertical and horizontal legs. I good weld is stronger than the metal it is welding. I am sure the weight of the tire and other articles I mount on the rack won't bend or distort the 3/8" plate. In designing paving equipment I have developed a bad habbit of overkill (industry standard I guess )
But, I could be wrong, guess I'll find out when I get it together and stand my 200lb fat a$$ on it
Hopefully I will get the rest of it welded up in the next week and get some pics of it on my jeep.