Today's cart is also a trailer for pulling behind my beach bike, as shown in the photo, however, there is a handle for pulling by hand indicated later in this Instructable.
Link to bike project: http://www.instructables.com/id/EXTREME-FAT-TIRE-BICYCLE/
Step 1: Basic Frame
I figured what the overall dimensions I needed for my large cooler, tackle box and a large bucket for the basic frame rectangle.
The corners were clamped tightly, then drilled and screwed. This seems to be adequate for the connection after several years.
I added a length of angle across the frame near where the axle would go, for added stiffness.
Step 2: Handle
I ran the length of the frame, underneath and then used a conduit bender to get the curvature I needed.
I hammered the conduit a bit flat along the length under the frame so that it would have less of a tendency to roll, then drilled and screwed it to the frame at the front and back.
I used machine screws with wing nuts here so that the handle could be removed for transport.
I added two narrow aluminum straps from the handle to the front frame member as diagonal braces, also attached using machine screws with wing nuts.
Step 3: Platform and Axle
I suppose there are a lot of materials that can be used for the bottom of the cart, they just need to let sand and water through and be lightweight.
The axle is just a length of steel bar I got from the hardware store. Mine is actually a little smaller in diameter than the holes in my tire hubs, but I correct that some year. A little wobble doesn't affect the performance.
I just drilled holes in the side rails of the frame and slid the axle through. Note how the intermediate support also keeps gear off of the axle. Was that planned? I don't remember. But it works.
Step 4: Constructing the Cage/Basket
There are 4 vertical posts which also act as rod holders, made out of 1 1/2" ID pvc conduit.
The rail members are all 1" pvc conduit. These are straight lengths and prefabricated radiused corners. The radius sections have a flared flange one end, which receives the straight sections. Where no flare is provided, the conduit sections are connected with couplers. Fixed connections are bonded with pvc glue.
The 1" conduit side rails were coped with a bandsaw to nest snugly around the outside of the 1 1/2" posts. The side rails were then bolted to the posts with machine screws and lock nuts. A larger hole was drilled on the outside of the post to allow the bolt head and screwdriver to fit through so that the inside face of the post is the only part bolted to the side rail. This allows the post to remain clear for rod handles and other vertical elements you will carry in these tubes.
The radiused sections are bonded to the side rails. In the end you should have two sides, each consisting of 2 posts and 3 rails (using 2 radiused sections per rail). Cut slots in the bottoms of the posts with a circular saw, bandsaw or jig saw, to fit over the upturned leg of the angle frame rail. Fit the two side assemblies over the frame rail angles to determine the length of the short pieces that will joint the two sides together. You will need 3 small straight sections each at the front and rear of the cage. These do not get bonded if you want to break down the cart for transport. These short pieces fit into the flared ends of the radius sections.
Drill holes through the posts, such that they also go through the upturned angle frame rail. The post will be connected to the frame with long machine screws and wing nuts. Because the posts are secured at the bottom, the short connecting pieces between the radius corners will only need to be secured at the top rails. This is done by screwing pan head screws through the flared end and into the short section.
Step 5: Wheels
The axle was drilled to accept a retaining pin at the end. I then cut a piece of aluminum tubing to act as a spacer between the frame and the wheel. This was necessary to get the required tire clearance from the frame. A large washer was installed between the spacer and hub and another washer on the outside of the hub between the hub and retaining pin.
I did this setup on one side and then measured the projecting length of axle so that I could cut the other side and repeat the process over there.
Step 6: Auxiliary Handle
This was a piece of EMT that was smaller than the main handle and fit right inside. I bent it with a conduit bender, added an inner tube wrap for comfort grip and bolted to main handle with machine screws and wings nuts.
Step 7: Support Leg
It consists of a section of pvc conduit with the bottom end cut at an angle to insert into the sand.
The top was coped to fit the radius of the handle.
I cut a toe strap for bicycle toe clips and screwed each cut end to opposite sides of the support. When the toe strap is cinched down tight, it secures the handle to the top of the support leg very nicely.
make sure you cut the support leg long enough that you can insert at least 6-8 inches into the sand to provide a stable support.
Step 8: Cutting Board
I cut a circular hole in one corner of the top to fit a plastic container that the bait shop uses for clams. It seems to be a pretty standard plastic container size. Within the channel at the perimeter of the cutting board, I drilled some drainage holes and also a slot for my bait knife.
I secured a piece of aluminum angle cross member to the bottom of the board by heating screws and screwing them right into the plastic (like buttah).
Another piece of angle was secured perpendicular to the cross member after first hammering it to fit within the 1 1/2" ID pvc post
Step 9: Trailer Hitch
A 2" x 2" steel tube was welded to a swivel caster. The caster wheel was cut square and drilled to fit a hitch pin on my bicycle trailer hitch.