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This page discusses the concept of a suspended power arm that would utilize the high power to weigh ratio of hobby engines to add power assist to almost any bicycle. I am proposing an open source design phase where enthusiasts will attempt to bring the idea to life, and test it in the field. I will be doing the same as an example for further discussion. The light weight, and high power of high performance hobby engines, coupled with the power arms suspension damping should allow motorized bicycles to enter a new sport, stadium racing. The same indoor tracks used for indoor motoX could be used for motorized extreme mountain bikes, with their riders adding pedaling power to win. My time with the FM128 Bicycle Engine has been an awakening, and I give that product a lot of credit for my decision to buy into the moped namespace. I plan to give back by kicking off an open source product, I want anyone to make these things, in what ever form they arrive. The Morini Motori FM128 has shown me that life for bicycle engine enthusiasts has thus far been relegated to over weight, over sized, and underpowered cycle motors that are only useful on smooth surfaces. Even the FM128 at 6 kgs can sustain only moderate bumps and tiny jumps, and I know that contradicts what I have written elsewhere on the web but, in that case I was making a point about the 128's ability to sustain abuse, albeit at the expense of a wheel rim. The 128 is supplied with its own hub you have laced up to suit the rim size you want, and you can also have the wheel laced in a cross spoke pattern which will handle the extra torque.The transmission is a wafer that fits into the gap to the left of the wheel hub, and the 128s mounting also allows it to swing around the axle, perfect, but it has a frame location mount that prevents this, wrong. First bike I put it on broke the frame at this point within 3 months, why? every time I went over a bump, the inertia would race up through the axle, through the spokes, and into the engine, which was rigid, so the energy carried on and was absorbed by the frame with a jolt to the rider. I Improved the unsprung weight problem by fitting a 2.5 inch wide high profile down hill tire, that has enough cushioning in it to take out most bumps even on dirt roads, but not jumps. I also improved things dramatically by allowing the engine to swing around the axle, and suspending it with a piece of shock cord to the seat. This allows the engine to absorb impacts by swinging around the axle.

I was tempted to try and market the 128 and its type globally, but even over 100 years after the first guy bolted an engine on to a bicycle, we still haven't got it right, and it frustrates me that I can't go out and buy the product that I can see in my mind. Ironic considering the market consists of about 4 billion existing bicycles, and another 250,000 new ones everyday. So thus presents a challenge, a niche in a global market. To exploit it would be logical, using modern manufacturing technology it should be possible to put 1 hp at the back wheel for about 1 kg using a 400 gram 6.5 cc helicopter engine generating 1.62 bhp. allowing for transmission losses should put about 1 hp at the back wheel, for a weigh penalty of just over 2 pounds.

I've been working on the idea, a power arm that would swing mount on the axle, on the drive side of a disc ready rear bicycle wheel hub. The final drive would be a toothed belt back to a secondary free wheel dog clutch. The primary drive would be plastic gears. A robust, dry centrifugal clutch would kick things off from the crankshaft of any generic model airplane, or preferably, model helicopter engine. The product I have in mind is designed to interface a commonly available hobby power unit from 3.5cc, up to an this amazing 4 h.p monster, (realistically 15 cc would be sufficient for sprightly performance on a mountain bike). It's configuration would resemble the hour hand of a clock at 2 o'clock. The power arm would be a thin alloy profile body milled out of thick alloy plate, and housing the bearings for the primary and secondary transmission. At the outer end to the left, an adjustable engine mount, designed to accommodate almost any hobby engine with bearers on the crankcase, (which is 99% of them). The ideal engine for long life would have the crankshaft supported on ball bearings. The engine would mount with its axis parallel with the wheel axis, it would face into the wheel on a radius of about 10 inches. The engine drive boss and spindle would poke through the body plate, where it would drive a light centrifugal clutch. The reduction would take place through plastic gears with the final drive being toothed belt. If you can now go back to the clock and imagine the spindle imposed on the rear axle, the power arm is at 2 o'clock, it also includes a suspension arm which sits at 8 o'clock opposite and under the drive arm to mount the suspension arm below the left swing arm of the bike would depend on the weight of the engine. A small damper would link the shock arm to the frame, thus minimizing the problem that has held bicycle engines back for over a century, "UNSPRUNG WEIGHT"!!! In the middle of the arm is the axle pivot, I can see this part being made of Teflon to be a sliding fit over the axle.

This product is ideal for model engine makers, the technology is the same. I know model engines have a short life, but I flew RC model planes for years, and have heard of engines reaching 1000 hours. Gas conversion would be sensible and relatively simple in most cases, but for absolute weight efficiency, as in racing, Methanol based fuel would be used. Anyway all that is a long way off, but this is where it starts. I'm going to build the power arm! a prototype to test the concept. I believe it could all be done with model helicopter gears and belt pinions, with a one off final toothed belt pulley that would mount on any generic disk brake hub, (where the disk bolts on). In time a product like this could become a standard bicycle accessory, and as an Internet product its superb, having the ability to be delivered globally in the mail.

Thanks for reading

Iain Mackay - Moped.com