I wanted to kit out a nice size toolbox for carrying tools and projects to/from my local makerspace. I used an accessory strap looped around my seat post and the handle of the toolbox to prevent the toolbox from sliding backwards. I used a larger, luggage strap to hold the toolbox firm to the rear bike rack. This system proved to be not quite adequate. The toolbox had a tendency to slide and fishtail while I was cycling. I decided modding the toolbox would be the easiest way to get it firmly mounted to the rack.
- (6) hex bolts - I used 3/8" diameter, 1.5" long bolts
- (6) nuts (matching diameter)
- (12) flat washers (matching diameter)
- electric drill
- drill bit (matching bolt diameter)
- (3) mini-packs of Sugru
- ruler or measuring tape
- fine tip marker or grease pencil
- adjustable wrenches and/or socket wrenches
I decided to go with a 23" Stanley Fatmax toolbox. The structural foam is makes it lighter than a metal toolbox and easier to mod. It's pretty unlikely you'll have the same toolbox and bike rack combo that I do so rather than sharing exact measurements I'll just give you general tips and guidelines.
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Step 1: Figure Out the Placement of the Toolbox and Bolts
You'll need four bolts that will slide over the edges of the bike rack. This will keep the toolbox securely in place when the straps are holding it down. Make sure you select bolts long enough to make it through your toolbox and have enough coverage on the side of the rack. If the bolts are too short they might not stay in the proper place if you hit a pothole or bump. You will also want to position the bolts close the front and back of the rack but not so close that those darn potholes still won't cause you troubles.
As you can see in the photo showing the bottom of the toolbox I have placed four of the bolts at very close to the corners of the toolbox. The other two bolts are in a position where they are just over an inch from the end of the rack.
The only downside to my choice of toolbox is that it is tall enough that it doesn't fit under the seat and therefore doesn't slide all they way to the front of the rack.
This is a good time to employ the popular tip: Measure twice, cut once. I actually place my bicycle upside down on a table and place the toolbox under the rack so I could get a good view of where things should line up. Even with a black toolbox I was able to make out measurement markings with a black marker. A grease pencil probably would have been better though. If your toolbox has an uneven pattern on the bottom like mine you will also want to take into account the size of the washers. You can tell from the photo that I neglected to do that and had to skip the washer on one of the bolts.
Step 2: Cut the Holes and Install the Bolts
You're about to drill holes into your nice toolbox. Are you sure you don't want to go check the measurements again? I have botched enough projects to know that it never hurts to take the extra time to measure and line things up properly. My toolbox didn't exactly fit under a drill-press so I just used a cordless drill for the holes. Place a washer on the bolt then insert it through the hole from the inside of the toolbox. Place the second washer and then finger-tighten the nut. You'll want to repeat this for all six bolts. Then you can place the toolbox on the rack to make sure it fits as you expect. If everything looks good then use a socket and wrench to tighten up the bolts.
Step 3: Sugru to the Rescue
I realized that the bolts were likely to scratch up my shiny, new bike rack and maybe scratch surfaces when I bring the toolbox inside. Sugru (www.sugru.com) is a great solution to this problem.
I used half of a mini-pack of Sugru for each bolt. Knead the Sugru to expose it to air and activate it. Flatten the Sugru out so it is a bit taller than the bolt. Wrap the Sugru around the bolt and fold the top over. Sugru has a working time of 30 minutes so you should be able to get all the bolts down well within that time. This is important because I found it helpful to place the toolbox right-side-up and place it down on the table. You don't want to force it down but the weight of empty toolbox should be fine. This will flatten the Sugru out at the end of the bolts and ensure they are all the same height so you don't have a wobbly toolbox when you place it down on a workbench. You should then flip the toolbox upside-down again and leave it for 24 hours for the Sugru to completely cure.
Step 4: Take It for a Test Drive
After the Sugru has hardened now you can put it on the rack and ensure that everything fits as it should. Play it safe by taking the empty toolbox one or two test-drives to make sure it is going to stay securely in place.