Front-wheel drive (FWD) bike details
Contruction date: 2001
Wheelbase: 105 cm
Seat height: 50 cm
Weight: Much too heavy!
Weight distribution Front: 52% Rear: 48%
Wheels Front: 406 x 1.75 Rear: 630 x 1.25
Brakes Front: Magura Louise disk. Rear: Shimano Nexave
Transmission: Rohloff 14 speed hub gear.
Suspension: Mid-point pivot with hydraulic shockabsorber
Steering angle lock-to-lock: 75 degrees (while pedalling: 60 deg)
Click on the picture for a larger view.
The picture above shows the 2001 prototype fitted with Sachs
7-speed hub. Part of the frame and front fork have since been
Note: At the end of 2005 I built a new FWD bike with electric motor assist, especially designed for commuting. Details can be seen at the following web page:
The aim was to build a so-called "fixed bottom bracket" design in
which the chain twists as the steering is turned. I chose this
design because I enjoy the challenge of building it!
An important feature to be tested was the nearly vertical
steering axis. In fact, the angle of the steering axis, or rake,
on this bike is about 5 deg. "negative" (backward). The fork has
a backwards offset between the wheel spindle and the steering
axis to give a "normal" ground trail of 5 cm. This geometry
simplifies two design problems: routing the chain to
minimize steering torque, and fore/aft weight distribution.
A problem with front wheel drive bikes is the fact that
pressure on the pedals also exerts a steering effect. To
overcome this, one solution is to use rear-wheel steering, but
that introduces more problems than it solves. On a fixed
bottom-bracket FWD bike steering torque can be eliminated
because the pedals are attached to the frame as in a
conventional bike. However, the tension side of the chain must
run exactly parallel to the steering axis. If it does not, then
tension in the chain has the effect of turning the steering.
Ideally, the chain should run through the steering axis, but
this is practically impossible. A compromise is to route the
chain along the side of the fork.
Another set of FWD design parameters is concerned with the
fore/aft weight distribution.
Any vehicle that is used for hill climbing must have sufficient
weight on the driven wheel(s). If there is not, then slippage
will occur between tyre and road surface and traction will be
lost. On a rear-wheel driven bike this is less obvious because
the steeper the incline the more the center of gravity (CG)
shifts towards the driven wheel. Loss of traction rarely
occurs, except for example on wet roads. However, with a FWD
bike on an incline there is relatively less weight on the
driven wheel. To avoid loss of traction one has to position the
CG further forward than what is customary in bike design.
Keeping the CG low will also minimize the change in weight
distribution on an incline. Figure A below illustrates a
typical FWD low seat arrangement.
Routing the chain
Unfortunately, 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. This is because the pedals
must be a fixed distance from the seat depending on the length
of your legs. For example, if the pedals are positioned over the
fork the chain can run straight down the side of the fork, but
the rider will be sitting much too far back. If the seat were
positioned over the front wheel, then the chain would have to be
led back from the pedals, over an idler pulley and then down
along the side of the fork.
To maintain the proper relative location of the seat, fork and
pedals, use of an idler pulley to re-route the chain is therefore
An idler pulley bears a load proportional to the tension in the
chain and the angle the chain makes in passing over the pulley.
Bicycle chains, especially on recumbent cycles, run under very
high tension. Energy loss is proportional to the load on cogs,
chain wheels or pulleys the chain passes over.
An idler pulley on the tension side of a chain drive is generally
undesirable because it significantly increases energy loss
compared with an idler on the non-tension side.
Front fork design
Compared to a positive angle, a negative angle
positions the steering axis closer to the pedals yet brings the
front wheel closer to the rider. As a result it is possible to
route the chain close to the steering axis without the need for
passing the chain through a large angle over the idler pulley.
Figures A (positive angle) and B (negative angle) below
illustrate this. The steering axis is shown in red, and the
idler pulley in blue.
Below you can select a short video clip showing how
the chain twists with the steering as it turns from
lock to lock (note that this shows the new fork).
The following sources provide more details about the use of
positive versus negative steering axis design:
- My own summary
- The 2nd European Seminar on Velomobile Safety and
Design (1994) ISBN: 3-9520694-0-X included a paper by
Stefan Gloger on this topic.
Parallel side stick steering controls
It is apparent from the illustrations above that some kind of
remote steering linkage is required when the steering axis is far
removed from the rider. My solution was to use parallel side sticks
with the fulcrum located directly beneath the seat,
connected to the fork by means of rods.
After completion of the prototype and following some road tests,
I decided to put this bike into daily use. Time will tell if
I have really solved the design problems.
As of the date of this writing (2003), I have been commuting to and from
work about 20km daily for about one year. The following points
are worth noting.
Given the relatively large proportion of total weight on the
front wheel, the front brake does most of the work. The rear
brake locks up and skids very easily.
Initially, I used the drum brake that forms part of the Sachs
hub gear in the front wheel as seen in the picture at the top
of the page. For commuting in traffic the drum brake was
found to be inadequate. With the approach of winter and the
prospect of commuting in the rain, I decided to fit a
hydraulic disk brake.
The Sachs drum brake mechanism was removed and using a lathe I
machined the drum to accept an adapter plate on which to mount
the disk rotor. A fork with mounting point for the hydraulic
slave cylinder was made from a modified BMX fork. The finished
product is shown below.
Click on the picture for a larger view.
Later, a custom fork was built for this purpose.
The bike really consists of two sub-frames joined by a
horizontal pivot just beneath the seat, which is also
approximately the fore-aft location of the center of gravity.
A stiff coil spring is compressed when the weight of the rider
rests on the seat.
This mode of suspension was found to be adequate for road use.
It is especially effective for moderate size bumps like manhole
covers. Unfortunately it is ineffective for small, high
frequency irregularities such as the short stretch of cobbled
road I traverse each day!
The internal gear hub plus disk brake make the front wheel very
heavy (total weight of front wheel plus tyre = 3.4kg). This
mass has a high inertia that affects the steering at certain
speeds, resulting in a tendency to oscillate ("shimmy"). The
disturbance is only apparent when the steering controls are let
loose. At most speeds, however, I can ride long distances with
just one hand resting lightly on the steering. Control is
easily maintained at any speed simply by resting the hands
lightly on the side sticks.
The problem could be eliminated using a steering damper, or
better still, to re-design the geometry so that the wheel
spindle is not offset from the steering axis.
A discussion of these points can be found at Tony Foale's web
A standard Sachs 7 speed hub gearing is adequate for
commuting but not ideal. In combination with a triple chainring
the range was extended. Later, I decided the Rohloff 14-speed hub
gear provided a better solution that was worth the extra cost.
Front Fork Problems
The BMX fork on the prototype suffered serious damage after I
hit a pothole at 20 km/h in semidarkness early one morning. A
specially designed fork was built to replace it.
The full advantage of a recumbent over the conventional bike becomes
apparent when a fairing is used. A popular and very effective compromise
to the full fairing is the so-called "tailbox" semi-fairing. This
encloses only the rear half of the vehicle and improves the aerodynamics
behind the rider's torso, which is the widest part of the vehicle.
On my bike it was quite simple to attach such a fairing because the seat
frame has the same width as the rider's shoulders and is an integral
part of the bike's structure. The fairing shown below was made from
'coroplast' which is a very light yet strong white corrugated plastic
sheeting similar to corrugated cardboard (in this case re-cycled
election posters). I used several sheets joined by stitching
with nylon fishing line and pop-rivets. The fairing is self-supporting
with no additional framework inside it. Behind the seat there is
space for some luggage, and there is also a small luggage compartment
in the enclosed space between the rider's legs.
An advantage of the self-supporting coroplast structure is that I was
able to enclose the suspension pivot beneath the seat. The degree
of movement in the suspension is quite small, and the coroplast is
sufficiently flexible to accomodate that. The underside of the bike
now has a very pleasing shape that looks aerodynamically correct.
Click on the pictures for a larger view.
My impression is that the fairing does reduce drag, especially in the
strong headwinds that I frequently encounter.
It is difficult to determine the true aerodynamic efficiency of a
bicycle fairing, so I cannot make any strong claims to this effect.
Other factors have certainly made it worthwile: The large yellow
painted surface is conspicuous and greatly enhances visibility
of this vehicle in traffic. Motorists appear more inclined to
treat it as a vehicle and not simply as a two-dimensional object!
In rain, the fairing very effectively protects the rider from spray
that is thrown upwards by the wheels, and the general appearance of
the bike has been enhanced to the point where it almost looks 'cool'!
Final update (2006)
After about three years of daily service and with about 18000 km on clock, the bike has been put into retirement. Early 2006 I built a new FWD bike with electric motor assist, especially designed for commuting. Details can be found at the following web page:
Updated: December 2006