maidbloke

Random thoughts of a Maidenhead man

Homemade bike cargo trailer


Posted in Cycling on Wednesday 1 July 2020

I made a cargo trailer for my bike. Here's how I did it.

First take a look at the video which gives you a tour, tells you a bit about it and goes for a test ride.

And now in more detail:

Goal

The main idea was to be able to cycle to my nearest superstore (Sainsbury's, 5km away), load it with a week's shopping and return home. I wanted it to have a lid, mainly to stop items jumping out when going over bumps - not easy to notice as the trailer is behind you. I also wanted to keep the lid flat and obstruction-free in case I ever want to transport cargo that won't fit in the trailer, e.g. wood board for another project; it can lay on top and be strapped down.

Limitations

My side gate is quite narrow - it had to fit through that. And I have no metalwork skills or tools - only woodwork. So the project needed to be either completely wood based or I needed to start with an existing metal frame. I decided on the latter.

Beginnings

I got the idea of using a child's trailer as the basic framework from this YouTube video. Basically strip off the fabric and build a wooden base and sides on it. I lurked on eBay for a while sussing out second hand children's bike trailers. I decided on the Halfords single child trailer model because there were normally a few available and was narrow enough to fit nicely through my side gate. I just had to wait for one to become available at a decent price. One did.

Stripping it down was quite easy and I ended up with a basic rectangular frame with the 2 wheels and a bent arm which connects to the rear axle of the bike.

Above: Two things to note. The trailer is designed to tow to the left, presumably to keep it closer to the kerb on the road. You need to remember this when lining up for narrow passages. Secondly that's the narrow gate it has got to get through!

Plan

I measured the frame and came up with a rough plan.

The plan was to have a wooden box centred over the wheels. The wheels are not centred within the frame, they are towards the back of the frame. This means the box will overhang the frame at the back. The advantage of this is it is easier to balance loads over the trailer wheels and not have too much weight on the trailer arm. To have the wheels centred means a wooden box length of 1080 mm if I line the front of the box up with the front of the frame, which seems logical.

The width of the frame is 475 mm so that fixes the width of the wooden box. The height of the tallest shopping bag I have is about 400 mm so I thought I would make the height of the box 475 mm to match the width; why not make it large.....seemed logical to me. So that gives us our external box dimensions.

Capacity

The capacity of the box is around 244 litres. Measuring a standard Sainsbury's trolley is not easy because it is not a regular shape but I reckon it is around 79 x 47 x 50 cm which is around 186 litres. So my trailer, with the lid shut, is 30% bigger than a full Sainsbury's trolley.

With an area of 108 x 47.5 cm, my trailer's capacity is in the same ballpark as the cargo bikes shown in this cargobike cargo space comparison blog post.

Materials

I planned to use hardwood plywood as it is easy to cut, strong and durable. It comes in several thicknesses. I first considered using 12 mm for the whole thing as I could then screw it all together without using any internal strengtheners. However I decided it would be too heavy using this material and also the sides and top did not need to be that thick as they have to cope with less force than the base. I eventually decided on 9 mm plywood for the base and 5.5 mm plywood for the sides and top to try to keep the weight down. This meant that I needed strengtheners inside the box to both give more rigidity to the sides and give something thicker to screw in to. My strengtheners were 18 x 34 mm planed softwood timber.

Construction

Let's get sawing!

Above and below: The frame has bolts sticking up through it at the back. I drilled 2 holes in the base to enable the base to lay flat on the frame.

Above: One side of 5.5 mm plywood with strengtheners added.

 

Above: Strengtheners were glued in place as well as screwed, to give extra rigidity.

Above: Time for the 1 m long piano hinge to be stuck to the lid, no screwing because at 5.5 mm the ply is too thin for screws. The Gorilla Glue I used was excellent.

Above: Groceries hold down the glued hinge while it dries. And yes, some glue did leak to the hinge itself and seized it but I was able to free it.

Above: I used scraps of wood to secure the box to the frame. See next photo.

Above: View of the underside, one of four frame grippers. The thicker piece of wood is slightly smaller than the depth of the metal frame (seen on the left). The thinner piece of wood (left over 9 mm plywood) is screwed in to the larger piece and grips the frame tightly. No glue here in case I have to remove the frame in future.

Above: Maiden voyage! It works!

Above: Woodstain varnish applied.

Above: Inside the box, showing the central vertical strengtheners. These were added for rigidity and so I could mount a cleat hook centrally to the outside.

Above: Finished. Three cleat hooks added each side for bungee cords for extra luggage on top and to keep the lid shut and not rattling.

Above: First trip to Sainsbury's.

Above: A trolley load of groceries loaded with room to spare, around 35 kg.

Costs

Item From Cost £
Halfords single kid's trailer eBay 41.00
5.5 mm thick hardwood plywood 2440x1220 mm (sides and back) Homebase 24.00
9 mm thick hardwood plywood 1829x607 mm (base) Homebase 23.50
Planed smooth timber 4 off 18x34x2400 mm (strengtheners) B&Q 9.76
Gorilla glue Homebase 12.50
Woodstain Homebase 21.00
1 m long piano hinge eBay 8.19
Cleat hooks 6 off eBay 9.72
Screws, wood scraps already in stock   0.00
TOTAL   149.70

Specification

Weight of box 11 kg, weight of frame 5 kg. Total weight of trailer 16 kg.

External size of box 475 high x 475 wide x 1080 mm long.

Approx capacity 244 litres (30 % bigger than a Sainsbury's trolley).

Weight carrying capacity at least 35 kg.


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