Bike Cargo System

I have gotten tired of using Velcro (or hook and loop) straps, bungie cords, or zip ties to hold a milk crate to the cargo rack on the back of my bike. On a normal day I don't need a crate back there, and I don't like having it there unless it's needed. But installing it is (a) wasteful if using zip ties or (b) time consuming if using Velcro or (c) not very secure if using bungie cords.

To deal with this, I've designed a quick release system for securely mounting my milk crate to my cargo rack, or something substantially similar. This system has some built in adjustability to hopefully help this work with other types of bikes and racks that have the same basic construction.

Required Materials

x8 M6 x 20-25mm machine screws

x8 1.5mm finishing nails

x8 M6 nuts

x8 30mm washer (Amazon link)

x4 quick release M6 skewers (Amazon link)

x2 M6 heat set inserts

x2 M6 x 10mm machine screws

x2 15-20mm washer (Amazon link)

x1 Lowes Milk Crate (Lowes link)

Required Tools

M6 wrench or pliers (to hold the nuts and M6 skewers)

Hammer (to install the feet)

Hex key/Philips head screwdriver (to tighten the machine screws)

Marker/Tape

Print Settings and Material Suggestions

I would strongly suggest using a material that will hold up outside, for instance PETG or ASA. While I have run thousands of miles with PLA printed parts, I know that I run a risk that they will melt or deform under strain in hot weather. These prints are liable to see not-insignificant amounts of strain, so I'd strongly suggest a weather resistant material.

I would not print these in any material with less than 5 walls, 5 top layers, and 5 bottom layers at a minimum of 0.2mm layer height. At a 0.3mm layer height, I would print with 4 walls, 4 top layers, and 4 bottom layers.

I suggest printing with organic supports. While I would like to have no supports, in order to fit geometries, some support is required (on build plate only)

3D Printed Parts

1. Crate Foot
   If you use these, you will need four. These snap into the corners of the crate to keep the mechanism off the ground when not installed on the bike. Held in place with finishing nails.
2. Hold
   I suggest using four, but two is the absolute minimum. These are what hold around the rack.
3. Stabilizer
   If you use this, you will likely need only one. This needs to be installed at 90° to the holds. This keeps the crate from sliding off the rack.
4. Truncated Cleat or Full Cleat
   You will need one cleat per hold. These are designed to fit in any of the possible squares on the bottom of the milk crate. You can all truncated cleats or spread loads out a bit more with the full cleats. In my use, I have ½ truncated cleats and ½ full cleats.

Assembly Instructions

Part Prep

1. Remove all support material.
2. Install the heat set inserts into the stabilizer. 

Stabilizer Installation

1. Place the stabilizer over the cargo rack cross member, with the square sides up.
2. Line up the crate and press down, installing the crate in the desired position and lining the stabilizer part up with the crate's grid.
3. Use the 15-20mm washer and two short screws to hold the stabilizer in place, making sure the washers catch on the crate.

Hold Installation

1. Take a hold, two nuts, and an skewer, and install the skewer into the hold.
   1. Put a nut on the skewer and have it fully seated to the bottom of the threads.
   2. Insert the skewer into the hold, and out a nut on the end.
   3. Line the nut up on the end to be flush with the threads, such that when the pin is retracted the nut and the body of the skewer are flush.
   4. Line the skewer up such that the metal handle is facing directly away from the slot in the print, and hold it with a wrench.
   5. Tighten the first nut until the whole skewer is held firmly in place.
   6. If done correctly, the skewer should be able to be locked closed or pulled and then opened by turning the metal handle 90° either clockwise or counter clockwise.
2. Repeat for all of the holds you have.

Prepping the Crate

1. Take the crate and line it up how you'd like it to sit on the rack. Making sure the stabilizer lines up correctly. 
2. Using the tape or marker, mark out where the rack bars are.
3. Install a cleat with a press fit on the underside of the rack such that the holds can reach where the rack bar would be.
4. Take an assembled hold, two nuts, two washers, and two screws and install the the hold.
   1. The channel in the hold is designed for M6 nuts. You will need to hold the nuts while installing the screw through the washer, through the cleat, and into the hold/nut.
   2. Select a hole in the cleat such that the washers grab onto the frame of the crate, and loosely assemble.
   3. Select a second hole and repeat the above step, keeping things loosely held together. You want enough pressure that it doesn't fall apart but is still adjustable.
5. Repeat the previous step for all holds.
6. Open all the skewers, and install the milk crate on the cargo rack.
7. Close all the skewers.
8. Tighten all the screws down.

Optional Steps

Loctite

If you feel like the screws may vibrate loose, you can loctite them. I would suggest the following steps.

1. Remove the crate from the rack, and turn it upside down.
2. Loosen a single screw
3. Apply loctite
4. Tighten screw
5. Repeat steps #2-#4 for all screws, alternating what hold you loosen. Never loosen both screws for a hold at the same time.

Feet

I personally like the feet, as they keep the mechanism off the ground when the rack is not installed. The snap fit of the feet should hold them in place well enough for installation, but should not be relied upon for a long term attachment solution.

1. Remove the crate from the rack and turn it upside down.
2. Snap four feet into each corner.
3. On both sides of each feet is a 1.5mm hole. Hammer in a 1.5mm finishing nail. This punctures the plastic and permanently holds the feet in place.
4. Alternate to step #4, you can glue the feet. Turn the crate right side up.
5. Fill the rectangular openings above the feet with glue or epoxy and allow it to dry.

Possible Installation Variations

1. Forgo the cleats, just screw a wooden milk crate to the holds. If you do this, you will not need to print any cleats or the stabilizer. Use another bit of wood as a stabilizer replacement. Remember, the stabilizer is there to keep the crate from sliding off the back of the bike.
2. Remove the bottom of the crate and replace or augment it with plywood. Screw the holds to the new plywood bottom of the crate. Use another bit of wood as a stabilizer replacement. Remember, the stabilizer is there to keep the crate from sliding off the back of the bike.
3. Forgo the stabilizer, and use a rear light attachment to keep the crate in place. You can do this with either the plastic crate or the wooden one.

Tips and Tricks

• Remember, the stabilizer is there to keep the crate from sliding off the back of the bike. You will need something to do this job.
• The further apart the holds are, the more secure the crate.
• Most plastic crates bow in the middle a little. This can cause rocking. You can add rubber to the rack or the bottom of the crate to act as a bumper/compression fitting.
• For rack bars smaller than 10mm, you can add padding to the top of the hold where the bar meets the hold. Glue them in place and this will take up the slack.

Rack Compatibility

• Racks with 10mm or smaller diameter tubing.