Design Considerations
Weight
Longitudinal Stability & Lateral Balance
Longitudinal Stability & Lateral Balance
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.
Longitudinal Stability & Lateral Balance
Longitudinal Stability & Lateral Balance
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 (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.
Axles and Structural Elements
Axles
Structural Elements
Structural Elements
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.
Structural Elements
Structural Elements
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).
Wheel and bearing design
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.
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.
20mm axle (single wheel sleeve design
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.
Prototype and Specification
Prototype
We produced our first prototype of this trailer on October 31st, 2025.
It was rideable, but very difficult to turn.
We added a prototype rear wheel turing mechanism and functionally validated it on November 20th.
Specification
We are producing an app to specify the design called the Specifier.
A demo is planned for Jan 2nd.
Review of existing designs
Lightweight bicycle-trailers
Several designs exist for very lightweight non-mototorised 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
Cargo carrying tricycles
Workmancycles has been producing commercial tricycle designs for several decades. They are still typically non-motorised.