Front-wheel drive (FWD) bike details

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 improved.

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:

Design

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.

  1. Steering torque

    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.

  2. Weight distribution

    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 a necessity.

    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).

Steering geometry

The following sources provide more details about the use of positive versus negative steering axis design:
  1. My own summary
  2. 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.

Road use

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.

  1. Brakes

    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.

  2. Mid-Point Suspension

    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!

  3. Steering

    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 site:

  4. Gearing

    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.

  5. 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.

Fairing

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.

Construction

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.

Performance

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