Homebuilt LWB recumbent inspired by Groundhugger XR2

Since i was young i've had the idea to build my own recumbent bike. Over the years I have looked at various designs but dreaded the technical challenge. I also didn't like the exaggerated lying position because it looked too uncomfortable. Eventually i came across a design that made you sit more upright, namely the XR2 Groundhugger. Based on this design i designed and built my own bicycle.

To reduce costs and make things technically easier i decided to make the frame from 40 x 40 x 2 mm square steel tubing. And to use second-hand “donor” kidsbikes for parts. For the wheels, rear fork, bottom bracket, front fork, V-brakes, rear derailleur and the front headset I used a second-hand "MBM FatX 20 inch" kidsbike.

Fatbike kidsbike

The frame of this bike is made of mild steel making it easy to weld. The “fat bike” tires provide more resistance but are comfortable and look nice. I bought a cheap bicycle for the rear headset. I don't remember the brand, but it is important that the bicycle is made of mild steel so that it is easy to weld.

Dismantling the kidsbike

The next step consists of disassembling the bicycle and cutting loose the bottom bracket, rear fork and headset.

The separated rear fork.
The separated rear fork.
The separated and disassembled bottom bracket
The separated and disassembled bottom bracket. I later removed the frame remains that were still attached to the bottom bracket with a flap disc.
Losgeslepen balhoofdbuis kinderfiets
The separated and disassembled head tube. Cleaned up with a flap disc.

OSB mold

To align everything properly and make it easy to weld, i made a mold from OSB boards. Assuming that these boards are square and level, i drew a line in the middle on which i aligned the frame of the bicycle. Laying the board on a flat surface, like a smooth garage floor, will help everything line up straight and square.

OSB mold
OSB mold
Front wheel in mold.
The front wheel and front fork aligned perpendicularly and clamped by OSB boards.

Fixing the front fork angle

Fixing the front fork angle.
The angle of the front fork relative to the floor is 71 degrees. To adjust the front fork, i drew the angle on a piece of wood and aligned the fork to it.
Front wheel fixed with tie wraps.
Then i secured the fork with zip ties.

Aligning the frame tube

Frametube mold.
The bottom of the frame tube should be 31cm from the floor. I mounted two OSB boards with a height of 35cm and a notch of 40 x 40 mm in the middle to the bottom board.
Frametube.
This way the frame tube can be aligned in the middle at the correct height.

Frametube bend

The place and angle where the frame tube bends up towards the front head tube is determined by the position of the bottom bracket. The center of the bottom bracket should be 45cm from the ground and +- 27cm behind the rear of the front tire so that your feet do not touch the front tire or the ground while pedaling and there is still room for a possible fairing around the bicycle.

The bend can be made in the frame tube by cutting the correct angle from the the top of the tube without cutting all the way through and bending and welding it after.

Frametube kink.
Frametube bend

Welding the frame tube to the head tube

The frame tube can then be welded to the front head tube.

head tube.
I copied the curve of the head tube onto the frame tube and cut it out with an angle grinder.
head tube reinforcement.
After i welded the head tube to the frame i used a piece of flat bar to reinforce the weld.

Rear fork

rear fork reinforcement.
To mount the rear fork to the frame tube, i first welded two 10cm pieces of square steel tubing to it.
rear fork reinforcement.
And interconnected them with a piece of square steel tubing.
welding the rear fork to the frame tube.
After this i welded the rear fork to the frame tube. The rear wheel can be adjusted in the same way as the front wheel so that everything aligns properly. The angle of the frame tube relative to the rear fork can be measured by keeping the bottom of the rear fork horizontal.
rear fork reinforcement triangle.
I further reinforced the place where the rear fork is welded to the frame tube with a triangular piece of steel.

Bottom bracket

bracket.
To mount the bottom bracket, i first cut a bracket from a piece of square steel tubing.
bracket cut out.
bracket welded.
And welded it onto the frame tube.
bottom bracket welded.
After this i leveled the bottom bracket tube and welded it onto the bracket.

Seat

seat
The seat is made out of 9mm birch plywood. I searched for an image of a chair on Google images and traced and enlarged the side view of the chair in Photoshop. I then printed this out and used it as a template to cut out two vertical parts.
seat parts.
The two vertical parts are 40mm apart so that they fit exactly around the 40mm frame tube. They are connected by horizontal pieces that taper.
The complete seat.
The completed seat
Seat with foam rubber
I covered the seat with foam rubber, but i would have preferred to use a denser type of foam because the foam rubber is too soft.

Steering tube

steering tube.
To install the steering tube, i first mounted the seat, bottom bracket and pedals so that I can determine the correct distance and height of the handlebars. The handlebars should be at a comfortable arm's length and my knees should not touch the bottom of the handlebars.
rear head set.
I aligned the rear headset so that the stem runs horizontally. This headset comes from the second donor bike. The front fork has been sawn off just below the fork tube.
steering tube.
The steering tube is welded to the frame.
complete frame.
The completed frame.

Steering rods

Steering rods
The handlebars are connected to the front fork via two steering rods. There is a piece of M8 threaded rod through both fork tubes to which suspension eyes are screwed.
rear steering rod connection.
The holes through which the threaded ends pass through the stem or washer of the front fork must be wider than the threaded end so that you have sufficient play to adjust the headset. (Adjust the headsets before tightening the nuts and eyes on the threaded end.)
Steering rods
The steering rods consist of aluminum tubes into which I have glued a piece of threaded end on both sides with epoxy resin. The steering rods are connected to the suspension eyes via ball joints.
rear head set.
The completed steering system.

Guide pulleys

To guide the chain under the seat, you can use ready-made chain guide pulleys. The cheaper option is to make the pulleys yourself by cutting a groove into some old inline skate wheels.

making chain guide pulleys
I did this by clamping the wheels with two pieces of plywood to a piece of threaded rod and placing the whole thing in a drill press. I then cut out the groove with an angle grinder.
mounting the guide pulleys
The guide pulleys are mounted through the frame with a piece of threaded end and a threaded bushing to align them at the correct distance. I bent brackets from aluminum strips so that the chain cannot run off the pulleys.

Bicycle stand

buckstand mounting point.
At the bottom of the frame, just before the bend, i welded a plate with a piece of M10 threaded end to which a bike stand can be mounted.

Final assembly

For the final assembly, i reused the cassette, rear derailleur, V-brakes and brake levers from the donor bike. The front chainring of the children's bicycle only had 24 teeth, so i replaced it with a new chainring with 60 teeth, including new cranks and pedals. I bought a new 6-speed shifter for the 6-speed rear derailleur. The brake and gear cables are of course all new. It might have been nicer to guide the cables through the frame, but for practical reasons i secured them to the frame with tie wraps.

Completed recumbent

Blueprints

Because i made the frame on the fly i don't have a design for it but i measured everything later and made a sketch of it. Because i am 175cm tall myself, the frame may be a bit too small for taller people. However, the seat can be moved back about ten centimeters. The dimensions on the sketch are in mm.

Blueprints recumbent frame.
seat crosspieces.
The seat crosspieces are placed every 30mm and are 65mm deep. The widest part of the seat is 250mm and tapers to 80mm for the backrest and 150mm at the front of the seat.
zitting zijaanzicht.
Seat side view 52 x 48.5cm

Demo

It takes some getting used to, but the bike is pretty stable. Because it's a LWB (long-wheel-base) design, the turning circle is a bit large. But compared to a normal bicycle, the sitting position is very comfortable.

Fairing

I am currently building a fairing that i designed myself in Blender. The fairing will be made from polyester/fiberglass laminate. Below is a render of the Blender design:

stroomlijn ligfiets