Tip up and flip-over backpack class III hitch rack

[andrewwynn]

Been wanting one of these for years but never found one that did all the things I wanted it to do:

➀ work right side up
➁ flip upside down for use as a table or seating
➂ stow vertically when not used
➃ useable in vertical mode (think soft item like tent rolled up)
➄ can stand in front to access cargo area through open window
➅ can open tailgate partially when stowed vertically
➆ no obstruction to tailgate even when loaded
➇ not wobbly loaded or not

I'm sure there are a couple more.

I could not find a piece of 2" square tubing to use for my base so I just bought a 2" trailer ball for $15 on sale at harbor freight.

I already bought the backpack rack also HF for about $60. I've seen on sale for $50.

I spent a solid day doing computer models to determine how I wanted to make the movable connection and about 5-6 hours drilling the few holes and fine tuning how I would assemble it.



Stowed vertically


In the normal use position


Video of me changing from stowed to use mode and back in mere seconds


Close up of the magic connection. Using a pair of off the shelf conduit strut bracket and drilling holes in just the right spot I use 1/2 inch bolts to hold the brackets and a hardened pin to hold the whole thing on.

By precisely locating the two through holes for the horizontal and vertical locations, I can achieve a tight enough fit that the rack will stay vertical with negligible wobble and will work right side up or upside down also with negligible wobble.

To flip upside down requires pulling the ball hitch and flipping the combined unit. To change from go mode to stow mode only requires pulling the 1/2 inch pin, pulling the backpack rack off and re attaching with the pin in the back position.

I have yet to add the hardwood that will convert it into a proper deck / work table, also I can attach my portable vice to the back. I plan to add a removable square extension out the back so the vice won't interrupt the table surface.

Here is the backpack fully utilized: fits all four of my Milwaukee 26" toolboxes. No longer have to put them inside.

If I'm only using two they fit sideways perfectly and clear the tailgate. They actually work well as a raised work platform as the tool boxes are designed to be a pipe-vice for cutting round stock like conduit or pvc pipe.

More photos to follow when I get the next stage done. I'm planning to out threads on the pull pin so I can hand tighten and remove the last bit of wobble.

Anybody wishing to copy me let me know I can post the how to on the tricker parts.


Another view


Last but not least: to remove the wobble that all removable hitches have, I drilled a hole in the side of my receiver and built this threaded rod:



Which I can twist in by hand and it holds the hitch ball absolutely still. No more kathump when going over bumps but more importantly since the rack is 5 feet wide: removes the 1" side to side and nearly 1" up and down wobble of the rack itself. Annoying to say the least but when trying to use as a work bench just awful. With this trick "problem solved"

-awr


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[srmmmm]

One might consider this as an option too:

Ultra-Tow Steel Folding Cargo Carrier — 500-Lb. Capacity, 60in.L x 20in.W | Receiver Hitch Cargo Carriers| Northern Tool + Equipment

Not bad for $120

[X53Jay4.8is]

[QUOTE=andrewwynn;1044510]Been wanting one of these for years but never found one that did all the things I wanted it to do:

➀ work right side up
➁ flip upside down for use as a table or seating
➂ stow vertically when not used
➃ useable in vertical mode (think soft item like tent rolled up)
➄ can stand in front to access cargo area through open window
➅ can open tailgate partially when stowed vertically
➆ no obstruction to tailgate even when loaded
➇ not wobbly loaded or not

I'm sure there are a couple more.

I could not find a piece of 2" square tubing to use for my base so I just bought a 2" trailer ball for $15 on sale at harbor freight.

I already bought the backpack rack also HF for about $60. I've seen on sale for $50.

I spent a solid day doing computer models to determine how I wanted to make the movable connection and about 5-6 hours drilling the few holes and fine tuning how I would assemble it.



Stowed vertically


In the normal use position


Video of me changing from stowed to use mode and back in mere seconds

Close up of the magic connection. Using a pair of off the shelf conduit strut bracket and drilling holes in just the right spot I use 1/2 inch bolts to hold the brackets and a hardened pin to hold the whole thing on.

By precisely locating the two through holes for the horizontal and vertical locations, I can achieve a tight enough fit that the rack will stay vertical with negligible wobble and will work right side up or upside down also with negligible wobble.

To flip upside down requires pulling the ball hitch and flipping the combined unit. To change from go mode to stow mode only requires pulling the 1/2 inch pin, pulling the backpack rack off and re attaching with the pin in the back position.

I have yet to add the hardwood that will convert it into a proper deck / work table, also I can attach my portable vice to the back. I plan to add a removable square extension out the back so the vice won't interrupt the table surface.

Here is the backpack fully utilized: fits all four of my Milwaukee 26" toolboxes. No longer have to put them inside.

If I'm only using two they fit sideways perfectly and clear the tailgate. They actually work well as a raised work platform as the tool boxes are designed to be a pipe-vice for cutting round stock like conduit or pvc pipe.

More photos to follow when I get the next stage done. I'm planning to out threads on the pull pin so I can hand tighten and remove the last bit of wobble.

Anybody wishing to copy me let me know I can post the how to on the tricker parts.


Last but not least: to remove the wobble that all removable hitches have, I drilled a hole in the side of my receiver and built this threaded rod:


Which I can twist in by hand and it holds the hitch ball absolutely still. No more kathump when going over bumps but more importantly since the rack is 5 feet wide: removes the 1" side to side and nearly 1" up and down wobble of the rack itself. Annoying to say the least but when trying to use as a work bench just awful. With this trick "problem solved"

-awr


Awr is the hitch on the vehicle the OEM one for the X5 e53 that you are using or is it an aftermarket unit. If aftermarket what is the brand and model?

[JCL]

What is the weight of the four toolboxes, and the rack itself? Looks to be about a 24" offset from the hitch pin to the CoG of the boxes and rack.

[andrewwynn]

@srmmmm: that makes model was inspiration for my design but won't work upside down and I get a bonus tow ball in addition to less wobble. It's a decent value for a canned flip up.

@X53: oem receiver. Also you used "quote" vs reply so my entire first post is duplicated.

@JCL total weight about 120# for the tool boxes. Original rating of 500# on the carrier but I would derate to 400# just in an abundance of caution. 1/2 inch bolts grade 5 have 10,200 # sheer strength through the threads and there are two of them. I didn't measure the torque moment yet but pretty confident the bracket would fail before the bolt and its hefty 1/4 inch. Even basic steel is 40ksi means 10,000# force to start to deform.

Fortunately I'll likely never load more than 200# but will be doing some measurements to determine the force multiple on the steel pin and the bolts.



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[JCL]

Originally Posted by andrewwynn

@JCL total weight about 120# for the tool boxes. Original rating of 500# on the carrier but I would derate to 400# just in an abundance of caution. 1/2 inch bolts grade 5 have 10,200 # sheer strength through the threads and there are two of them. I didn't measure the torque moment yet but pretty confident the bracket would fail before the bolt and its hefty 1/4 inch. Even basic steel is 40ksi means 10,000# force to start to deform.

Fortunately I'll likely never load more than 200# but will be doing some measurements to determine the force multiple on the steel pin and the bolts.

While the carrier may have a rating of 500 lbs, the OE receiver has a rating of 600 lbs applied at a maximum horizontal distance of 8" from the pin. Estimating 24" for the CoG from the pin with the carrier, the rating would therefore be 200 lbs at 24", just due to the moment arm. That includes the both the carrier and the tool boxes. It does not consider any additional derate for a bouncing load.

Edit: Out of interest, I looked up the maximum tongue weight that BMW publishes for the OE E53 receiver for off road use. It is 330 lbs at 8" from the hitch pin. In my mind, the reduced rating is most likely due the effects of a bouncing load, not commonly seen with a ball mount trailer due to the inertia of the trailer. One can decide if that rating is appropriate or not, but it does illustrate that BMW have calculated what the receiver is designed for (considering the mount to the vehicle, not the receiver itself, IMO).


Jeff

[andrewwynn]

@ JCL: some pretty good points except if you look at how I load my tools, they are very CG forward. about 14" or less from the hitch pin (i'll measure at some point but from the photos, the 2" ball appears about 8" from the pin; the spacing of the bars in the rack is just about 4" center (16 inch front to back).

The pin is a reference for 'the consumer' and by no means the torque axis. based on how far the mounts go into the car, it would be fair to consider the center of the main cross bar the 'axis' for torque calculations. (the entire assembly is robust enough to consider nearly solid); solid enough the effective torque center could easily be forward of the crossbar because of the amount of body work that becomes part of the mount. In fact, when i do my measurements I will use the vertical plate as the torque axis until i would be shown why that would be at fault.

I didn't measure but it's a 2" bar at least; it's at least 2-3" forward of the hitch pin, so.... if my CoG is 12" behind the pin that means 15 from the axis, and using 600x11 as the rated torque, divide 6600 ÷ 15 = 440; so for a CoG in the front 25% of the 16" rack, I would be quite comfortable in saying 400# as mentioned earlier.

If i extended the CoG to center, than the ratio is 600x11÷19 or roughly 350#, only double what i'll every be loading. (other than the static load of 4-5 people sitting on it at a tail gate party). (which the hitch won't even notice; if it can handle 600# on a road or 350# off-road it'll handle double that static easy).

It is a very good idea to 'pull out the slide rule' occasionally and make sure common sense prevails, but i'm not the least bit concerned that my 120# of tools will be even within 33% of the capability of the rack/hitch/mount.

I 'only wish' i could apply 'off road' to the equation. Even so. 350 x 11 ÷ 19 = 200#, so the rack 'as is' with a CoG centered front to back would handle 200# 'off roading' I'm good with that.

I *always do the math*. I'm quite confident i'm nowhere near the 50% safety margin I will usually design around, and as I approach the 2:1 ratio i'll get more serious about measuring details.

A rough estimate of the original design before mod: 13" Cog from the pin = 16" to torque center means 6600÷ 13 = 507 #; imagine that. rack is rated 500# coincidence, notsomuch, i would argue a good case of using the crossbar as the torque axis making sense. (and most certainly not the hitch pin).

-awr

[andrewwynn]

Did some measuring. 19" from axis to CoG for my main load. 4" inside from pin = torque axis (my estimation: it's where the main crossbar bolting plate is)

600x12 = 720 ft. · lb torque divide by 19" and it is: 378. Darn close to my "seat of pants" 400.

Thanks for bringing the detail to my attention. I've seen plenty of bumper mounted external cargo kits at a 15° angle because they don't realize they are applying 1000-2000 ft·lb of torque to something inside.

The flip up adaptation adds 4.5 inches. The center of the rack is 24 inches vs 12 to the rated 600 means 300 is a perfect value for the rack evenly loaded.

Without the added length the # works out to 370#

Off-road: 350 becomes 175# (again thanks for bringing to my attention but doesn't apply to my $5000 worth of Milwaukee tools that will be driven to a job on smooth paved roads)

I only lose 70# capacity for the flip up convenience.

One last calculation; hopping off the end: 2 1/2 ft x maybe 250# impact a mere 750 ft•lb of torque.

Assuming at least 2:1 3:1 safety design factor and 600 ft•lb rated the hitch should be able to handle impacts on the order of 1500 ft•lb (load transitions, pot hole induced shock etc). The fact they derate to 350# for off road indicates maybe closer to 2:1 design margin or 1200 ft•lb torque rating (if the axis is where I think it is: sure makes calculation easy being 12" from the rated 8" from pin)

-awr


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[andrewwynn]

Figured out a way to perfectly fit my smart strap 1500# model. Going over two at a time and down around the center bar in the middle. Crazy secure. I can even make it "stoplight secure" (eg no grab and dash of my tools when I stop at a red light) by adding a cable tie to the smart strap.


Added flags in the corner. Somebody already bumped into the rack in a McDriveThroigh. The flags are about the height of typical headlight. Helpful to me because I can see them in my backup lens


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[JCL]

I'd agree the torque axis (for calculations relating to the mounting point strength) is at the front of the hitch crossbar.

There are other things to consider relating to why BMW rated it as they did. The limit could also relate to the rear axle loading (a multiplier over tongue weight due to the offset from the rear axle, with a corresponding lightening of the front axle). I would also consider the effects of a bouncing load, even on smooth roads, since a carrier acts differently than a trailer coupler (a trailer has a large rotational inertia, and can't apply a longitudinal twist).

My only point was to consider that the receiver wasn't designed for other than a point load at 8"; I have also seen many bent hitch receivers due to the use of cargo carriers (but specifically to the E53, only in photos).