Two-wheel drive (2x2) pedelec bike details

Click on the picture for a larger view.

Background and Design Objectives

The aim was to build a commuting bike with electric motor assist that would be lightweight, safe to use in traffic, comfortable, and low in maintenance. Previous experience with a front-wheel drive (FWD) recumbent had shown these objectives could be well served by that type of design.

  • Details of my first FWD bike and how I built it After more than 20000 km on the previous bike, and getting to the age of 57, my needs had shifted even further towards more safety, less weight, and greater comfort. The electric motor assist was considered, not as a luxury, but as a way to extend my ability to be car-free and happy.

    However, the addition of an electric motor and battery would introduce a very significant weight penalty, and the bike I had was already too heavy at 27 kg. Being car-free also means having no garage: Access to my house is by a narrow footpath and up a steep narrow flight of stone stairs! I found the extra 7.5 kg too big a penalty, but instead of foregoing the electric power assist, I decided to build another bike that would be designed specifically for this purpose. The challenge to keep the total weight below 27 kg was to be part of the fun.

    Design features

    Electric motor

    The BionX system is widely considered one of the best in terms of power to weight ratio and manufacturing quality. For me, the following points were also important: Below is a closer view of the motor:

    Click on the picture for a larger view.


    Laid-back comfort!

    Front-wheel drive

    In my opinion, a recumbent bike logically requires FWD:

    Space frame

    Instead of the mono-tube frame common on recumbents, I chose the so-called "space frame" consisting of many widely-spaced small diameter tubes. I chose 1/2 inch square tubing of .9mm wall thickness. The bare frame plus fork as shown below, required 10 m of this tubing, but total weight is only 5.5 kg. Although the structure is rigid as required (especially to twisting forces resulting from steering and pedalling) the CroMo steel is more elastic than mild steel or aluminium, which allowed me to have some spring in the seat-back and fork tubes without additional suspension, thus saving weight.

    (space frame picture) Click on the picture for a larger view.

    Weight distribution on a 2x2 bike

    An important set of FWD design parameters concerns the fore/aft weight distribution. The need to keep weight on the front wheel for good traction conflicts with the need for safe braking performance. My first bike suffered an unhappy compromise between these two conflicting demands: There was not quite enough traction for the variety of conditions you encounter when commuting all-year round, while the hydraulic disk brake had sufficient power to cause the bike to lift a rear wheel or even to flip right over.

    Having both wheels driven frees one from these constraints:

    Routing the chain

    The need to avoid steering effects from the chain pulling on the front wheel leads to some design complications discussed on the web page dealing with my first FWD bike: The most simple solution is to route the chain parallel and very close to the steering axis, but on a conventional FWD bike the requirement to have the chain lying close to the steering axis conflicts with the need to have the rider sitting close to the front wheel. A much simplified diagram below illustrates this point. An idler pulley on the tension side of a chain can also be avoided:

    Figure (A) is conventional FWD low racer, (B) is the design used in my own previous bike, and (C) is the design of the new 2x2 bike (where yellow shows the driven wheel). To picture the weight distribution the approximate CoG is marked in red. The steering axis is shown by a dotted red line. The normal positive rake angle of the steering allows use of above-saddle steering, whereas the negative rake angle requires under-saddle steering or else an indirect linkage.

    Below you can see the how these ideas were put into practice.

    Click on the picture for a larger view.

    Click on the picture for a larger view.


    Despite the electric motor, pedal power remains the primary driving force on this bike. To optimize the electric power assistance, very short pedal cranks (135mm) are used, but at the same time the gearing is changed so that the Gain Ratio is the same as for longer cranks (165mm on my previous bike). The advantage of this is that with less strain on the pedals aerobic fitness can be maintained.

    Pictures above show the bike fitted with a pair of experimental carbon-fibre cranks that are slotted to be adjustable, allowing me to find the optimum length. Below is a picture showing the custom built 135mm cranks that I made once the best length had been found:

    Click on the picture for a larger view.
    A standard 170mm crank is shown for comparison. The chainwheel is 48 tooth, and on the hub there is a 16 tooth sprocket. Cruising on a level road, this gearing is just right for the 11th speed of the Rohloff hub, which has a direct (1:1) ratio.


    With shorter cranks, it was also possible to lower the height of the pedals (or bottom bracket) without touching the front tyre. A small front wheel served the same goal. A low BB is not considered optimal for performance but the safety advantage in traffic was more important. In keeping with this, the seat angle was also raised (as seen in the diagrams above). Aerodynamics suffer, but safety and comfort are improved.


    It makes sense to power accessories off the main battery. Instead of using conventional bike lights, I assembled a set of LED's to operate directly off the 24volt supply. Eight of these in series just nicely make up 24v, and for front and rear I used 40 altogether. They were set at different angles so that instead of throwing a beam they give a highly conspicuous array that is visible from all angles for safety in traffic. The picture below was taken in daylight, darkened slightly to increase the contrast.

    Road use

    So far, after 20 months of daily use in commuting to work (more than 9000km), it seems that most of the above holds true: The new bike is less efficient than the previous one in terms of aerodynamics and seating position, but comfort and safety in traffic are greatly improved. Despite being unsuspended, the ride is softer than the previous bike (which had mid-point suspension). It is also my impression that with the steering above the saddle I can maneouver the bike better at low speeds, and it is much easier to walk the bike along the narrow footpath leading to my house. The (relatively) light weight is simply a delight!

    Electric motor usage

    Regarding the electrics, my first battery needed replacement after about one year and less than 300 charge-discharge cycles. This was a disappointment because it is claimed that a NiMh battery can give about 500 charge-discharge cycles. Roughly speaking, I have paid about one ZAR per kilometer (about seven kilometers per US$). This is less than the cost of running a car, but enormously expensive compared to cycling on human power alone! Perhaps the cost can be justified in terms of the experimental nature of this project?

    For the last six months I have been measuring the power consumption by means of a power meter (see the Drain Brain) to check my battery usage. It appears that I require on average about 90 watt-hours (or about 3.6 amp-hours at 24v) per one-way trip of 19km. That is about half the capacity of the BionX battery. It is reasonable to expect 500 charge-discharge cycles as claimed by the manufacturer especially if the battery is not being fully discharged each time. By carefully monitoring usage and charging at the end of every trip, I may get longer service out of the present battery.

    For the technically-minded, I have been using about 4-5 watt-hours per kilometer. At 20km/h that is about 100 watts power assistance over and above my pedalling efforts.

    Updated: January 2008