Cargo carrying bicycle-trailer

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Background

Load-carrying trikes are quite common in India.
A basic design (shown in picture) has been in use, unmodified for more than a hundred years. The chassis of this tricycle is created by taking a roadster bike, removing the rear wheel and the rear part of the frame (behind the seat-post) and adding a heavy iron triangular frame that is connected to the bottom-bracket at the base of the bicycle frame and a heavy solid iron axle at the back.  

The weight of such a trike is about 60kg. It can be used to pull loads of 500kg but at very slow speeds (3km per hour). This design has been used both for passenger transport (2 people) and for transporting loads.

Our weight target

We are designing a lightweight trailer starting with the same bicycle frame as was used in the trike design. It is a trailer attachment to a bicycle and not a trike. It weighs about 10kg.

Since the trailer is being designed for speed, we try to keep the weight of the load close to the center line of the trailer and we focus more on longitudinal stability than lateral balance. 

Longitudinal Stability Axle

The primary axle of the trailer provides shock absorption and serves to stabilize the weight of the load behind the rider.

It is as narrow as possible and has 4 heavy-gauge 14" wheels in 2 pairs.

Lateral Balance Axle

When a cargo-carrying frame and axle is attached to the trailer-chassis, the cargo-carrying axle which is wide and lightweight, has a low pressure contact with the ground and serves to balance the load laterally. 

We use heavy duty wheels for the longitudinal stabilizer and lightweight wheels for the lateral balancer. Additional shock absorbers could be added.

A modern commercial trike design (shown in the picture)

removes the rear-wheel and replaces it with a 2-wheel axle at the same location. Hence, it has the riding characteristics of a bicycle and the additional stability and load-carrying capacity of a tricycle.

We target a bicycle-attachment type design with similar ruggedness and durability characteristics.

The trailer consists of two axles.

1. A 1m wide forward axle with a 16in bicycle wheel at each end. This axle is designed to maintain lateral-stability. It is made with a 20mm outer diameter seamless steel pipe (2.5mm wall thickness) and weighs about 1.5kg.
2. A 60cm wide rear axle with a pair of 14in bicycle wheels at each end. It is a load-bearing axle. It is made with a 16mm diameter solid steel rod and weighs about 1kg.

All structural elements (for pulling, steering, strengthening and frame-building) are made using rectangular  cross-section (40mm x 8mm), hollow (2mm gauge) steel beams. Weight is about 1.6kg/m.

These beams are strengthened at their attachment points with iron plate segments. A 4mm thick iron plate segment fits inside the beam and two 2mm thick plate segments sandwich it from the outside. The 3 plates and beam section are aligned, bored through with 2mm holes and riveted. (This  keeps weight low while providing strength where needed).

Several designs exist for very lightweight non-mototorised trailers

The most popular design of cargo-carrying bicycle-trailers is by Carla Cargo.

They can carry a weight of upto 200 kg and have a special brake design for the trailer that prevents it from running into the bicycle -- overrun brakes.

They are typically configured with a DC motor powering the trailer.

Workmancycles has been producing commercial tricycle designs for several decades. They are still typically non-motorised.