Cycle Zoo: Bikes for the 21st Century

Introduction

For many, degradation of the environment due to climate change, overuse of cars, and reliance on non-renewable resources are serious issues. It may seem impossible for us to help, but it isn’t. At least on a personal level we can change to improve the environment for ourselves and our kids. An example is using human powered or low-pollution transport when we can. This is healthy, worthy and fun.

In the early 1900s, engineers and scientists moved away from designing bicycles to motorcycles, cars and aircraft. The big challenges of this century are to do with the environment, and I hope that engineers and scientists will again take up cycle design. Designing, working on and building cycles is a small-scale, enjoyable process. It satisfies, and helps our personal and collective environments.

This book is about cycling in a broad sense — ‘human powered land transport’ — and starts where many bike books leave off. As well as bicycles, it discusses tandems, recumbent bikes and trikes, family cycling, load cycles and prototypes. There are many hundreds of machines available as human powered land transport and the introduction diagrams show cycles with their speed, load carrying capacity and cost. They are an attempt at ‘the big picture of cycling’ and should help you work out what sort of cycle you might want. Of course there are other criteria for human powered vehicles, such as size, what we are used to, what our friends like, off-road ability, passenger carrying, weight, appearance and colour, and these are important too. Most of all, have fun!

Although this book is divided into chapters, information is spread throughout. For example, tandems are discussed in the modular bikes chapter, trikes in the tandems chapter, and there is information on all cycles in the materials chapter. If a word or acronym looks difficult or unusual, I have included a glossary, and you might find it there. To simplify, I have mostly used inch wheel sizes: 16″, 20″, 24″, 28″. This is because a 20″ wheel has a 20″ (508mm) outside diameter and this is often used in calculations. Metric wheel sizes (say 700C) don’t define diameter because they depend on tyre size.

1Cycling Principles

Cycles depend on inventions and concepts including structural tubes, roller bearings, pneumatic tyres, levers, cables, symmetry and suspension. Laws of physics relating to mass and acceleration, pressure and area, wind and rolling resistance govern cyclists’ performances. Most of this cycling engineering was described in Archibald Sharp’s book Bicycles and Tricycles in 1896, and I am re-presenting some of it here.

This chapter also includes industrial design elements, because how people promote and perceive designs is vital to how bikes are made and sold. As Wiebe Bijker says, ‘A successful engineer is not purely a technical wizard but an economical, political, and social one as well.’

Mass, acceleration and gravity

The mass of a laden cycle (cycle, rider and luggage) is important because:

•Stopping or starting needs force proportional to mass. This means that, for the same effort, a light laden cycle will start and stop faster than a heavy one.

•Climbing needs force proportional to the laden mass and related to the slope. A combination of rider and cycle will have a slope angle that’s too steep and can’t be climbed. With all else equal, a heavier cycle will always stop on a shallower slope and be slower uphill.

•The mass of the cycle itself is only critically important when racing, or when picking it up to load into a car or onto public transport. Down hills, mass helps cycles go faster, but unfortunately we can’t have downhills all the time!

Structural elements

So cycles need to be light for acceleration and to be driven uphill without overexertion, and this needs sensible use of materials. For example, a bike made of solid steel bars would not use material efficiently, and would cost and weigh a lot. Hollowing out a 25mm round steel bar to 1 or 2mm wall thickness keeps much of its strength while reducing weight (1.1). The same principles apply to aluminium/titanium/glass and carbon-fibre-reinforced plastic tubes and square/oval/other shaped tubes. Tubular structures that have high strength-to-weight ratios don’t just occur in manufactured parts; they are in nature-engineered materials like bones and bamboo as well.

When a tubular beam is loaded to support weight, the depth of section affects performance and efficiency (1.2). Non-tubular beams such as ‘I’ beams (rotated ‘H’ beams) are often used in buildings and other structures that make sensible, sparing use of material.

Beams including triangle shapes (roof or bridge trusses) make strong and efficient structures. So conventional bike frames from tubes arranged in triangles are strong for their weight.

As well as structural principles to follow, there are ones to avoid. Sudden diameter changes and holes in tubes cause stress concentrations and eventual cracking. Workarounds include radiusing, tapering and reinforcing tubes as shown in 1.3.

Levers

A lever is a machine: it transforms motion by decreasing movement while increasing force or increasing movement while decreasing force (1.4). Torque (force × distance) remains equal. On cycles, levers wrangle power from what we can produce (pedal rotation by leg power at appropriate speeds) to what we want (forward motion for transport).

Levers can act with a central static point or fulcrum (1.5), and act through rods or chains (1.6). Cycle drives use both principles. A bike chain on sprockets is a continually acting lever, creating a higher angular velocity in the driven wheel than in the driving wheel.

A rod pressing on a second rod acts as a lever. The force and velocity at the contact point is the same on each rod, but the longer rod moves with less angular velocity but more torque due to length ratios (1.7). The continuously acting machine based on these levers is the meshed gear (1.7). These are used in hub (rear wheel centre) gearboxes using epicyclic geartrains and providing 2–14 gear ratios.

Some cycles have two chain drives, or a gear hub combining hub and derailleur gears such as the Sturmey Archer CS-RF3. The gear element ratios are multiplied to establish an overall ratio.

Pedalling, chains and gearboxes operate continuously. However, gearchanges and braking occur only intermittently, and are actuated through cables and levers. As cable inners are pulled, brakes and gears are actuated through tension, and a reacting compression is carried by cable outers and the bike frame. Cable inners can’t work in compression, and return to rest position is done by springs, stops and friction mechanisms (1.8).

The chain, and brake and gear control are the cycle’s push-and-pull components. They transmit power and control from the rider to the cycle.

Push and pull

Literal push-and-pull cycle components are described above, but in making cycles there is also figurative push-and-pull relating to demand.

If you are a bike maker, and a customer wants your product by a given date, there is motivation to complete on time and do a good job. A customer is ‘someone wanting it’, and it could be yourself preparing a bike for a race, making a bike for a friend’s birthday present, or a client paying money. Even if nobody wants the bike, creating a schedule or artificial ‘pull’ motivation makes sense. We all want to achieve things, and having completion dates helps.

A method of creating ‘pull’ for a product is patronage. This is where sponsors commission work or employ an artist or craftsperson for special works. This still exists, but in the world of unusual bikes there are now alternatives such as internet crowdsourcing to create pull.

Crowdsourcing is a method of empowerment, but the artist or craftsperson needs to sell their ideas and their ability to reproduce them to a selective audience. Bell Cycles make a very short front-wheel-drive cycle that was successfully crowdfunded (http://bellcycles.com).

So far this topic has only mentioned pull, but sometimes push production is necessary. ‘Push production’ means making for stock based on anticipated demand, not actual demand. This can be unavoidable in small-scale bike production. The aluminium section for my bike frames comes in 6m lengths. I can’t buy or obtain shorter lengths, so have no choice but to buy three frames worth for anticipated demand. Similarly, for me laser cutting involves design, obtaining quotes, obtaining material, delivering material and finally picking up products. The time and effort spent on parts make it worth buying extra parts for a small extra cost even if demand is low.

Suspension

Suspension is a cycle’s handling of bumps, and good suspension isolates riders from bumps and minimises energy loss and deceleration. Almost every cycle has variable suspension in the form of pneumatic tyres. Here are suspension options for comfort and less energy loss:

•Larger cycle wheels rise slower over bumps and are passive suspension (1.9).

•Isolating cycle parts gives reduced unsprung masses. Some mountain and folding bikes have full suspension; that is both wheels on small suspended sub-frames (1.10).

•Town bikes can have suspended seats, isolating just the rider (1.10).

•Riders who stand on the pedals turn their legs into springs over bumps. Arms have this sort of resilience most of the time (1.11).

•Isolation can be achieved by having long distances between wheels and the rider. Some recumbents with small front wheels use this suspension type. However, a lightly laden front wheel puts more weight on the back, so back-wheel suspension then needs consideration.

On recumbents, mesh seats and padded seats are part of suspension (1.12). Flexible frame elements give small but important amounts of suspension. Flexible frame elements include carbon fibre forks, seat and chainstays, and a titanium crossbeam in the Azub Ti-Fly trike (1.13).

In all suspension design, care must be taken to avoid bobbing, which is flexing of suspension elements during pedalling. It wastes energy and reduces speed.

Pressure and area

The pressure on a surface is the force acting on it divided by its area. Pressure and area help explain the comfort of a bike and how well it rolls.

Seating: Humans are a fragile collection of mostly liquid supported by bone and surrounded by skin. Rapid acceleration or deceleration or too much pressure causes pain — nature’s way of saying ‘stop the wounding’. For comfort, you need to reduce pressure by sitting or lying on something large and/or soft (1.14, 1.15). Soft surfaces change shape to better support the body and absorb acceleration or gravity force, and include gels, mesh seats, foams, chamois, cloths, leather and sheepskin.

On bicycles, the body is supported on the bottom, feet and hands by the saddle, handlebars and pedals. The percentage of body weight borne by bottom, feet and hands depends on the rider’s posture.

Upright riding on a roadster can cause the least body stress because weight is concentrated on seat and pedals, and the seat is a generous size and often suspended. Roadsters are usually ridden in street clothing.

A bent-over racing bike set-up is more likely to be painful because the saddle area is often small and hard, and there is increased neck stress and wrist pressure. Racing bikes are rarely ridden without padded shorts, and the bike has to be set up well to avoid pain when riding long distances.

On recumbents with reclined backrests, body weight is taken by the backrest and seat. The hands rest on the steerer but don’t bear much weight. Feet are usually clipped in to pedals, so legs hang from pedals rather than rest on them. Because the seat is large and pressures on the body are correspondingly low, the seat does not have to be particularly soft to be comfortable, but the rider’s back can get sweaty. Sitting on an open-weave mesh seat or using open cushion material such as Ventisit/ACS10 allows air to reach the back, and can help with this ventilation problem. A comparison of seat sizes on recumbent, and uprights is shown in 1.15.

Tyres: As tyres roll, they deform, leading to energy loss and deceleration. This loss is a large part of rolling resistance; and the higher the tyre air pressure, the lower the deformation and rolling resistance.

Bumps waste energy and speed, and hard tyres won’t absorb large bumps. Tyre types and pressures for best speed depend on road surface. On firm ground, tyre pressure is a compromise between high pressure/good rolling resistance and low pressure/suspension. Softer ground demands wider, lower pressure tyres. They have a large contact area and deform more but won’t waste energy ploughing the ground.

On hard surfaces, almost all cycle tyre tread wastes energy. It is best avoided for city and town cycling on bitumen roads. The heavy tread on inexpensive mountain bike tyres is particularly bad.

Wind resistance: Tyres plough tracks through soft ground. This work done by the bike on the ground slows it down. Less obvious but just as present is work done on the air by cyclists. Just as correct tyre size and pressure minimise ground disturbance, certain cycle shapes and profiles minimise air disturbance.

Cycles that cut through air easily give the rider/cyclist combination a low frontal surface area and a smooth, sleek shape/low drag coefficient. These properties are especially important at high speeds, so it should come as no surprise that the speed-record-breaking cycles of this world are enclosed recumbent speedbikes whose design emphasises aerodynamics at the expense of everything else.

Good aerodynamic shapes have rounded front edges, gradually tapering trailing edges, and are ‘clean’ without extra protrusions. Good aerodynamic shapes (sometimes described as ‘teardrop’) help a cycle travel fast whether small (frame tube, spoke, tyre rim) or large scale (overall cycle shape) Good aerodynamic shapes are ‘rounded in three dimensions’ and the ideal shape is something like an airship balloon, i.e. long and cylindrical, rounded at one end and tapering to a point at the other.

Symmetry

Mostly our bodies look the same right and left, so our mirror-image is almost the same thing other people see. But we know that inside, our heart is on the left, our liver on the right, the two sides of our brain are different, and we may be left- or right-handed, so are not symmetrical inside. Why is that? Symmetry is a way of saving energy, of improving aesthetics, of design simplification, and of reducing the number of systems occurring in a body or in a machine. So things are made as symmetrical as possible, but this has limits, and we notice when symmetry is unconventionally dispensed with or followed slavishly.

Cycle wheels have rotational symmetry. There are many other good examples of symmetry in cycling, and here are a few:

Bike lanes on hilly, busy Lennox Street near us break symmetry for sensible reasons. The street has dedicated bike lanes for hill ascents but no dedicated lane for descents. Looked at from above, it makes no sense, but on the downhills, cyclists can achieve car-comparable speeds, and not hold up cars when mingling with them. On the opposite side, uphill forces dominate cyclists’ motion, and they can’t travel without holding up cars, so the bike lane is provided.

In cycle bottom brackets and pedals, symmetry is followed slavishly, with left-hand threads on the left side and right-hand threads on the right, but only for a good reason. The pedals and the pedal axis bearings precess or loosen under bearing forces, requiring this thread arrangement.

In the same assembly, the bolts holding the cranks to their centre shaft are both right-hand threaded. A left-handed thread would serve no purpose; and it allows more parallelism and saves money and time to make and stock just one version of the bolt (1.18).

Brake actuators are often very similar on each