Lightweight Bicycle-trailer
Introduction
We are producing a lightweight bicycle-trailer.
The trailer attaches to the bicycle's seat-post and the rear-wheel's axle.
It will be able to pull loads of about 120 kg at a speed of about 10 km/hr (6 mi/hr).
Its wider axle is about 1m wide and rests on 16" wheels,
while the narrower axle is about 66 cm wide and rests on paired 14" wheels.
The trailer adds about 70 cm to the length of the bicycle.
A cargo-carrier can be attached to trailer-frame
using vertical support beams attached to rear-wheel spindle, the wider axle and the narrower axle,
and an additional attachment at the seat-post and the top of the head-tube.
We plan to start getting the design evaluated and tested in October
Illustrations
4 Design Considerations
Weight
Background
Load-carrying trikes are quite common in India.
A basic design 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.
Longitudinal Stability & Lateral Balance
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
The trailer also has a secondary axle which is wide and lightweight. It has a low pressure contact with the ground add serves to balance the load laterally. It also helps turn the trailer.
Shock Absorption
We use heavy duty wheels for the longitudinal stabilizer and lightweight wheels for the lateral balancer. Additional shock absorbers could be added.
Ruggedness and Durability
A modern commercial trike design is shown in the picture above.
Such a design 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.
Axles, Wheels, Structural Elements
Axles
The trailer consists of two axles.
- 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.
- 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.
Wheels
We modify a stock 14in or 16in wheel in the following way:
- The spindle and ball-bearing housing are removed, leaving just the hub that binds the spokes.
- A sleeve is made using a 48.3mm outer diameter, 7.14mm wall thickness (schedule 160) seamless steel pipe section about 24mm long. This section is bored (about 12mm deep) to fit outside of the hub at one end and at the other end it is bored out (about 12mm deep) to fit a sealed ball-bearing. 2 such sleeves are made, one for either end of the hub. Each sleeve weighs about 50g.
- For the double wheel case, 3 ball-bearings are used for a pair of wheels. One goes between the 2 wheels and one each goes on the outsides of the wheel pair. The interconnecting sleeve is a slightly different design. It's actually 2 sleeves made the same way but only 18mm long. They are bored only 6mm deep on the ball-bearing side to fit half the ball-bearing. Their outer diameter is also reduced by 6mm. The 2 sleeves are fit into both sides of the ball bearing's outer casing. An additional outer sleeve of 36mm length and about 3mm remaining gauge encases these 2 sleeves. This design serves to align the wheel pair while providing an inter-wheel spacing of 36mm.
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).