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:
- A 250 watt boost to get me up those hills without slowing down to
a pace that becomes unsteady in heavy traffic
- A way to feel at least five years younger when you are nearing 60.
- A way to drive the rear wheel of a front-wheel drive bike
- Provides an electro-magnetic brake (see details on BionX
website), perhaps a small weight saving
Below is a closer view of the motor:
Click on the picture for a larger view.
Recumbent
Laid-back comfort!
- Safety: Why go head-first into a crash when you can go
feet-first?
Front-wheel drive
In my opinion, a recumbent bike logically requires FWD:
- A short chain: Less weight. Chain-line runs up, well clear of road
surface: Less maintenance
- One way to combine a sealed 14 speed internal hub gear (Rohloff)
with a sealed electric hub motor
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.
- Combines light weight and high rigidity.
- Space between the tubes could serve as a luggage compartment,
which is an important feature on a commuting bike
- Good opportunity to practice welding thin-wall CroMo tubing
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.
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:
- To maximize safety I moved the center of gravity (CoG)
sufficiently far back to allow 1g braking without lifting the rear
wheel. To achieve that, the CoG must be located below and behind a
line drawn at 45 degrees throught the contact patch of the front
wheel, as illustrated in the diagram below. It is clear that most
of the mass of bike and rider is below and behind the red line:
- While having only 37% of vehicle weight on the driven front
wheel if that slips, the driven rear wheel will almost never slip,
especially going uphill.
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.
- Having both wheels driven makes it easier to maintain the proper
relative location of the seat, fork, and pedals while also keeping
the line of the chain close to the steering axis
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.
Pedals
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).
- With short cranks the rider is required to pedal faster for
the same power output
- With short cranks plus extra power from an electric motor the
pedalling speed can remain normal while maintaining the
same total power, but the legs move less
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.
Seating
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.
- Low BB makes it easier to put your feet on the ground when
stopping and starting in traffic
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.
- Fat tyres on front and rear improve comfort without the need for
suspension
- Low rolling resistance, low weight kevlar-belted tyres optimize
performance
Lighting
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
Click on the picture for a larger view.
Click on the picture for a larger view.
Click on the picture for a larger view.
Click on the picture for a larger view.
Click on the picture for a larger view.
Click on the picture for a larger view.