Introduction: Motorized Bicycle Friction Drive

Friction drive bicycle

Designing the bike

I drew sketches of my proposed design of bike engine frame and friction drive. I drew front wheel drive ideas and rear wheel drive ideas.

I decided to go with rear wheel drive with an adjustable height of engine roller, this would allow for the engine to be engaged or disengaged, basically it is a form of clutch on a lever, it gives traction from the roller to the wheel, traction between back wheel and roller will be achieved by using a trampoline spring I salvaged which will pull the roller down to the wheel for best traction possible. When the lever is pushed downwards it will disengage the roller from wheel and can be locked in place so the bicycle can be cycled.

I used solids works to design each component of the frame. I changed the 3D image to plan and elevation. I then used my final designs as a guide for the lathe and milling machine.

To make this friction drive bike I decided to use an engine with a centrifugal clutch and fabricate a bracket to hold the engine over the back wheel.

Step 1: Machining the Clutch Housing and Threading the Shaft

I had to use the lathe to form the shape of the roller and clutch housing. I had to follow my SolidWorks plan and do peressice measuring. I had an inner diameter of 80 mm and an outer diameter of 86mm I used a hand tap to thread the centre point of the clutch housing to a depth of 10mm. I used a 10mm hand tap.

I used a 125mm long aluminium bar of 10mm diameter and used a hand die to thread 10 mm of the aluminium bar. I then could screw the aluminium bar on the clutch housing.

When the engine is running and roller is engaged the direction of the engine should tighten the bar to the clutch housing, I had made these two components with normal direction of threads but as the engine ran it loosened the clutch drum from the aluminium bar.

Therefore I had a second attempt and used reverse threads and it fixed that issue. To hold shaft in place I put a spot weld of weld on and machined the excess metal off.

Step 2: Machining the Rollers

I then made two different rollers, one 25mm diameter and one 30mm diameter. I presume that the 25mm roller is going to have faster acceleration and a slower top speed than the 30mm roller. The 30mm will have a higher top speed and slower acceleration than the 25mm roller.

I knurled them for better grip. I measured 100mm long of cylindrical steel and I milled a diameter of 10mm out of the centre of both 25mm and 30mm rollers, I knurled both rollers on the mill with the biggest knurling head for best grip on bicycle tyre.

Finally I milled 2 holes (top and bottom) of the roller for a screw to tighten the roller to the shaft. I had made little indents on the shaft to avoid slippage of the roller on shaft and to locate the screws and make the rollers interchangeable.

Step 3: Machining the Frame/ Engine Bracket

I used a box section 100mm x 100mm for the engine housing. I removed the strimmer shaft off the engine and took the measurements for the bolts that secured it on and marked them on the box section. I drilled 4 holes each 6mm diameter to secure the engine on.

I found the centre of the 4 bolts and I used a 6 mm drill bit on the mill as a pilot hole, then I used a 10 mm bit followed by a 13mm bit, the hole was then wide enough to use the boring bar. I then used the boring bar on the mill to widen the hole by 1mm and then I would adjust the boring bar again to further 1mm (a quarter turn on the boring bar adjustors). I continued until I got to a diameter of 33mm. as I neared the finished product I adjusted the boring bar to take off less than 0.5mm so I could monitor the finished product and avoid taking off too much and ruining the whole process.

I could then place my bearing of 35mm diameter into the hole. I repeated this process on the other side for the roller. In order for the second 35mm diameter hole to be parallel to the first one I turned the box section around in the mill and put the 13mm drill bill threw the pilot hole and assured myself that I was in the right location.

I cut away the 4th wall of the box section on the mill to complete engine bracket. I then followed my SolidWorks plan and drill the quick release holes and the point where it pivots.

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