Stabiliser-chassis

The chassis consists of:
A solid-steel 17mm diameter, 50cm wide axle that rolls on a pair of 14" wheels at both ends.
It is a weight-bearing axle attached just behind the rear-wheel.
It provides both longitudinal-stability and shock-absorption.
A steel-tube frame that attaches this axle (rigidly) to the rear-wheel hub.

A mechanical, chain-driven steering mechanism that rotates this axle at half the rotation amount of the front wheel.
This mechanism consists of
1) a freewheel that is screwed onto the head-tube of the bicycle and
2) a hollow metal cylinder that is rigidly attached to the seat-tube.
A freewheel is screwed onto the top of this cylinder.
The 2 freewheels are connected via a bicycle-chain.
Another freewheel is screwed on to the bottom of the cylinder and trasfers torque to rotate the axle via 2 chain-driven rigid connectors (one for each side of the axle).
This chassis adds some extra drag and about 6kg weight (including the tires) to the bicycle.
It serves to stabilise the bicycle by making lateral balance easier and also adds shock-absorption to the rear wheels.
The impact of any bump is distrubuted across the surface-contact area of 4 additional wheels, and the wheels are space apart laterally (instead of just one rear-wheel).

Weight

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 (the seat-stay and the chain-stay) and adding a heavy iron triangular frame that is connected to the bottom-bracket at the base of the bicycle frame and rolls on a heavy solid-iron axle at the back.
A cargo-carrying frame or passenger-carrying frame is built on top of this triangular iron frame.

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 14kg (unladen).

Shock Absorption

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

Axles

The trailer consists of two axles.

1. A 88 cm 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 50 cm 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.
   

Structural elements

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).

17mm axle (wheel-pair sleeve design)

We bore the outer side of the sleeve to a 40mm diameter with a 12mm depth.
A 6203 bearing (40mm x 17mm x 12mm) slots into this bore.
The 17mm axle fits inside this bearing.
The total width of this sleeve is (12+12) 24mm.
We also use a bearing between wheels for the wheel-pair.
Since this bearing fits between wheels and we want to minimize spacing between the wheels in the pair, we only bore the sleeve to a 6mm depth on the bearing side (instead of 12mm).
The bearing slots halfway into one wheel's sleeve and halfway into the other wheel's sleeve.
These sleeves are therefore (12+6) 18mm wide.
We remove 2mm of material from the outside of both these sleeves, for an OD of 44.3 mm.
We create an addition sleeve (36mm wide) that is bored to 44.3mm.
This sleeve serves to align the 2 wheel-hub sleeves and hence the wheel-pair.

Sleeve design

We take a stock bicycle wheel, and remove the ball-bearing and inner ball-bearing holder attachments from the wheel-hub.

We add 1mm threads to both outer-extensions of the hub.
The OD of these extensions is about 35mm.

We use a 48.3mm OD seamless pipe (schedule 160) with a wall-thickness of 7.14mm (ID 34.02mm) to create sleeves.

We create a sleeve with an inner diameter of 35mm on one side.
We add 1mm threads to the inside of the sleeve on this side for 12mm.
This side of the sleeve is screwed into the outer-extension of the hub.

On the other side, it is bored depending on its usage.

20mm axle (single wheel sleeve design)

The bearing used in this case is 6004 (20mm x 42mm x 12mm).
A pair of single sided sleeves is created for each wheel in this case.

Lightweight bicycle-trailers

Several designs exist for very lightweight non-motorised or motorised trailers.

Cargo bicycle-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.

Cargo carrying tricycles

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