This thing here is a bicycle operated vehicle for two persons, in larger scale also called beer bike. Bikers are facing left. Steering is done with one bike's steering handle. Braking is done by pressing a hinged arm to the ground.
It's made with a rear axle of a 4-wheel-drive car, which is connected by a motorcycle chain to an axis that reaches both bikes at the position where the rear bike wheel would normally be. Front wheels are motorcycle wheels. Chassis design is that of a tractor: The rear wheels are fixed and the front wheel chassis can be turned relative to the main chassis. Only one front wheel is steered, the other follows through trail. The vehicle cannot go backward, not even by pushing.
It should be easy to make a larger version with more bikes or to add peripherals like a cable winch, a generator or similar. This thing here here was built for kids driving offroad.
Step 1: Part List
The motorcycle chain is now called diff-chain. The long axle that reaches both bikes is now called collector axle. The differential transmission of the rear axle is called diff.
The parts are bundled to a few modules:
* The rear wheels are fixed on the main chassis
* The front wheels are hinged to the front chassis
* The diff-chain part holds the chain to the diff and is connected with one bike module, together called diffchain-bike module
* another standalone bike module
* the axle and cogwheels to connect them all
* and small things like chain protector, rear push bar, steering rod
Step 2: Design Ideas
Tensioning the motorcycle chain is simply done by moving the diffchain-bike module back and forth. Tensioning a chain of a bike with gears is done by the bike's chain tensioner, as for a bike without gears it's tensioned once and then welded at one point.
Transmission of the diff chain is done with 2 motorcycle cogwheels, one welded directly to the diff axle and the other one fixed with a keyless shaft-hub lock on the collector axle. Transmission of the bikes is done with a threaded 6-speed freewheel and a keyless shaft-hub lock as well.
Design of the front wheels is that of a bike, with 2 different steering trail values for each wheel, which can be changed by turning the fork by 180°. Other aspects of design include seat position which should be low but still high enough to give space to chain tensioners and pedals including feet on them. The brake needs space to go up and down, and it should not interfere with the pedalling space, expecially when driving over a bump which may make the brake jump upwards suddenly.
The main chassis' metal tubes, which were originally zink plated, are not welded from or near the bottom, which should be better against corrosion. They also got no holes (yet), but instead, an angle steel is welded to it to hold the top modules. This angle steel and the plain brake steel is welded with a gap of 2 mm for cutting it off easier in case of a mistake, for adjustion while fixing and it should also be better for corrosion (I believe).
Step 3: Cogwheels of the Bikes
Critical parts include the bike transmission cogset-freewheel combination. It must have a hole of at least the collector axle's diameter, here 20mm. Current freewheels try to support the smallest possible small-cogwheel outer diameter for fast speeds, which leads to small inner diameters as well. The older threaded 6-speed freewheel MF-TZ20 from Shimano is 22mm inside.
A 1mm seamless tube is used to fill the gap between the 20mm axle and the freewheel's 22mm bore. A lathed spacer ring with the inner diameter of the freewheel thread is welded to the freewheel. Actually it' a replacement for a threaded connection, which is more complicated to make. Mount all parts for welding to an axle for precise true run.
Step 4: Cogwheels of the Diff-chain
Cogwheels of the diffchain are made with motorcycle parts and a keyless shaft hub connection. The cogwheels and a spacer ring on the shaft hub will have to be lathed.
Step 5: Main Chassis
Another critical part is of course the rear axle with the differential. This is a Jeep 4-wheel drive axle, originally used with leave spings.
Check the direction of rotation and turn around the axle until it's correct. It may lie upside down compared to the car's mounting position. Also check the parallelity of the diff axle and the main frame like in the last picture.
Step 6: Front Chassis: Wheel Forks
Find motorcycle or moped wheels, back or front, and measure the axle. Lathe needed spacer rings and 2 disks of 5mm thickness for each axle. The wheel will be welded to the fork with these disks. Find out the distance on the future axle between the 2 disks and weld a fork with this value. Check the right angle between the (future, horizontal) wheel axle and the steering axle (upright). Also check that the middle of the wheel is under the middle of the steering axle. The middle of the wheel is not always in the middle of the wheel axle, because of braking disks or drive train parts. Welding the steering axle may require special care, like special elektrodes.
Step 7: Front Chassis: Connecting the Wheels
Make the front wheels about the same track width as the rear axle. Give the front wheels a head angle, for example 15°. In the same construction like on the pictures, with a 30mm square structural tube, this should result in a trail of about 12cm. Also you would get a trail of about 4 cm turning the wheel 180° around.
Check the parallelity of the steering axles by looking and measuring, with this kind of bracket like on the last picture, if you like.
With less trail, steering a turn needs less force, but the wheels tend to flatter, especially the right wheel in this case.
Step 8: Front Chassis: Hinge
Make the hinge between the front and the main chassis out of 2 telescoping tubes with a clear space in between. The clear space is provided by leaving 1mm between inner and other outer diameter. Overall length is about 70cm.
Let the wheel axle's connection line be more or less in the middle of the hinge and mount the 2 hinge parts to each front- and main chassis.
Step 9: Diffchain-bike Module: Diff-chain Part
Make 2 crossbars over the vehicle and 2 sockets for the pedestal bearings without drilling the holes. Also don't make the fixing angles for mounting the module on the main chassis.
Make the collector axle that the bearings hold around 20 cm above the main chassis top surface.
Step 10: Diffchain-bike Module: Bike Part "Sunny"
Weld toghether an Y shaped crossbar with rear cantilever to be laid on the main chassis. Weld a vertical bar under the saddle and 2 vertical bars near the handlebars, see bike instructions "Dorli" for details. Anyway, the 3 bars should have a certain height. When the bike is welded on the Y frame, the the rear axis of the bike should be 16cm over this Y frame (if the Y bar is a 40x40mm structural tube) or 20 cm over the main frame's top surface.
Check if the chain tensioner has enough space. Now weld the bike to the Y. Mark a position on the rear wheel axle socket, cut off most of it and replace it with the 2 sockets for the bearings. Again without drilling the holes. Also don't make the fixing angles for mounting the module on the main chassis.
Step 11: Diffchain-bike Module: Fusion
Put the 2 halves on the main chassis on the right position and weld them together with additional small bars. Put the collector axle (or a spare piece of it) in the bearings, adjust them if necessary and mount he bearings. Check the axle to be parallel to the main chassis.
Mount the needed cogwheels. Mount and tension the diff chain. Add the fixing angles for mounting the module to the main chassis.
Mount the chain tensioner for the bike chain somewhere near the collector axle, like on the bearing screw like in the picture.
Take a first ride (without brakes!).
Step 12: Standalone Bike Module
Make another Y plus clutter. This time a bike without gears is used, so no space for the chain tensioner is needed. the clutter at the rear that holds the bearings is mounted on the Y this time. The Bike gets 3 bars again, one under the saddle, and 2 near the handlebars. The last 2 are flat, thus allowing the following trick: If they are fixed to the Y frame, the bike can still be moved back and forth to tension the chain. The 2 last bars are not mounted immediately but the saddle bar and a helper bar is welded first, in the symmetry plane of the bike, painted blue in one of the pictures above. After that, standing upright, the 2 front bars are done and the helper bar is removed.
Use a ruler like the green angle bar in the same picture to indicate the (future) Y top edge base line. The wheel sockets should be 16cm above this line, and the 3 bars should end there, again if40x40mm structural tube is used.
Put all on the main chassis, insert the full collector axle, adjust the bearing sockets if needed, mount the axle. Choose a gear, mount the chain, mount the bike. If the chain is to broad for the cogset-freewheel, like it might be on a single gear bike, the big cogwheels have to be removed, sad but simple, last picture.
Step 13: Design Idea for Bike Modules
As an alternative, the bearings for the collector axle can also be mounted from the other side, from opposite the bike. This clears the space for the chain tensioner by design. It allows the bearing contruction to be separated from the bike construction and is charming the modular idea.
Consequently, all bearing sockets, also the diffchain's bearing sockets, should be mounted from the outside.
Step 14: Brake
Find a plate like this luggage carrier, maybe add 4 nuts, and fix a part of a tire. Add 4 thick plates with a hole near the bottom to the main chassis. Put a screw through it. This is half of the hinge. The other half is another plate, not that thick, where the brake cross beam and diagonal beams are mounted to.
This brake here also has an upper end-stop, which it is pulled up to and which it can't go beyond (last picture, taken from below, at the first screw).
Add a bungee cord to hold the brake up.
Step 15: Steering
Steering is done with a bar from the first bike ("Sunny") to the left front wheel. Add a construction that provides a mounting point (a screw here) for the steering bar at this bike. In the handlebar's center position, it should be in the symmetric plain of the bike (in the middle). The same applies to the wheel's side mounting point for the steering axle (screw).
Step 16: Chain Protector, Push Bar
Add a protector at least of the diff chain. It's strong and can't be stopped easily.
A handle bar near the rear end is not necessary but useful, especially when pushing the vehicle is necessary.
Remember to leave or make holes in bars for water to escape. Paint or protect against corrosion. Don't throw your neck out when driving and have fun!