Where to begin?
Four basic problems to overcome: Weight, Power, Charging, Drag
Biggest weight issues can be broken down as Frame, Batteries, Drive system (see Power
). The best that money can buy is to invest in good mass-produced battery technology, and for that I suggest LiPo. Bang for buck, it has the highest power density per gram in the smallest package possibly. Keep your weight down and you’ll need less power and can travel farther!Power:
To keep it simple, purchase a well-matched motor and controller, then link this to the driven wheel(s) how you please (chain, belt, shaft). There is enough information on this forum and in other technical areas to sort out a good reliable power system to get you started. Understand that to go from 0-20 mph will require nearly double to go from 20-30, and 30-40, etc. This makes the calculation for power required to reach and to hold 70 mph difficult because much depends upon weight and drag. I could provide a theoretical value of the amount you could need, but then it’s my opinion and worth about that much. I would survey the thoughts of fellow members on their power consumption at velocity to build a profile, then figure your needs to be in the upper end on your first attempt.
The second part of the Power requirement is to determine the type of terrain: For 20 miles, is it hilly, flat, blustery, or calm? Is it rural roads, freeways, city, or mixed? All of these factor in towards starts and stops and drag; starting from a dead-stop uses the most power with hill-climbing or bracing wind second. A good conservative ratio is to double the range and factor backwards; calculate battery requirements 70 mph for 40 miles.
Another approach might be to analyze the horsepower for the replacement bike: You suggested using a frame from a 125 cc motorbike. Crudely, and depending upon where you read, the ratio of cc to hp ranges from 15-25 cc : 1, therefore if we take the worst-case, 125 cc / 25 = 5 hp, or it can mean 125 / 15 = 8.3 hp! Pick a value, then match a motor/controller to meet the spec, and calculate backwards to determine the battery size. Without getting lost in a bunch of math, let’s play conservative and suggest 8 hp is a reasonable figure for a lightweight eMoto. 8 hp roughly equals 6 kW. Because we have limited the range to 40 miles at 70 mph, we don’t need 6 kW for the whole hour: (40/70) * 6 = 3.43 kWh.
I developed a spreadsheet that suggests the Wh/kg can range between 150-180; obviously we want the higher value. Let’s be optimistic and use 180 Wh/kg; 3430 / 180 = 19 kg, or about 42 pounds. Cost per kW varies between $360-700; let’s pick $500 after shipping and tax; 3.43 * 500 = $1715 estimated cost for your batteries. Wow this looks great on paper, but I don’t think these values are correct. Baptism by Fire (read: experience) is the only way to know for sure, however we have some place to start and I really think that there is a lot of room to add more batteries. I am not going to bother with voltage and current since that will depend completely upon the motor/controller.Charging:
Solve the battery capacity problem and the charging issue will fall into place. The good news is that the battery is sufficiently small enough that you could charge at home without any problems, and there are a plethora of configuration possibilities to choose from.Drag:
At 70 mph, this will hugely impact how much power you will consume. Simplistically, you want to be as slick as possible, like a bullet in the wind, crouched down and nestled into the body of the bike for the best slipstream effect. The reality is this is difficult to achieve for the casual commuter. There are threads dedicated to aerodynamics and it’s worth your time to delve deeply into the matter. Your ambitions though are reasonable and achievable because the required distance is so very short. Perhaps in this case, drag could be overcome by adding more battery capacity and call it good.
You have been given a concise brief on how to proceed; the world is your oyster. I am building my own contraption too and so I share. Let us know how it goes.