the legendary Jennifer Holt Bike

possible. Or not?

My friend Jan has sent me the following link:

http://blog.makezine.com/archive/2010/07/30mph_electric_mountain_bike.html

It is said that the bike is running on a 48V motor. The battery pack is said to consist of 144 3.3V lithium ion cells, which means that the overall capacity should be around 30Ah which is 50% more than the current eSpire, the benchmark in the market.

The single batteries have no charge balancing and no discharge protection but there is some monitoring of battery piles of 16 batteries, please look here

http://jennwork.homelinux.net/drupal6/node/2

The battery will probably not survive many hundreds of charging-discharging cycles. A replacement battery will cost around 1,800.00 EUR (including the controller) as compared with 900.00 EUR for the battery of the eSpire.

The gear box has only one single stage of said 2.91:1 between the motor shaft and the pedal shaft. We have measured the dimensions on the photos in the homepage, this is possible. The gear ratio between the front sprocket and the wheel sprocket is also larger than 1! The front sprocket is said to have 20 teeth while the rear sprocket is said to have 60 teeth. The overall ratio is then 8.59:1 which means that the wheel does one turn if the motor complete 8.59 turns. This means also that the rider cannot support the wheel with pedalling and also that the rider cannot reasonably paddle home when the battery is empty.

The motor has a max power of 15 kW! I consider it strange and even dangerous to put such a strong motor into such a small vehicle. You would need motorbike technology to cope with such a strong motor.

No wonder that the bike has an overall weight of about 130 pounds = 65 kg. I deducted this from the author who speaks about 100 pounds extra-weight as compared with a normal bike. The bike looks small but it is indeed heavy.

This vehicle will not run much further than 15 or 20 miles or so because of the bad efficiency that results from the bad design. The motor draws about 300 Amps under full power, resulting in an enormous loss when driven at low speeds. The motor, however, will definitely produce a large torque. The author writes:

"The ... large motor leads to insane torque. I had to set the current limit in the Alltrax controller to half its max value, otherwise it was impossible to keep the front wheel on the ground. As it is, you still have to be careful taking off, it will throw you over backward if you peg the throttle from a standstill. Acceleration is limited by the friction between the rear wheel and the ground, you can lean forward to keep the front down, but the reduced load on the rear means that the drive wheel will slip, even on dry pavement."

It is said that the builder, a girl named Jennifer, has already wrecked the bike and broken her elbow. No wonder, this bike is really dangerous.

A further thing. I suppose that the crank shaft has a freewheeling device for the cranks so that the crank shaft can be used both for driving the rear wheel with the cranks and and with the motor. The thing is that the crank shaft is turning with a speed of about 1,000 1/min at max speed becaúse of the 3:1 reduction. This means also that the front sprocket is turning with a speed of 1,000 1/min, not much different from a circular saw/buzz saw. The chain speed is very high, I shall calculate it soon. Did this girl think about what happens if the chain breaks one day?