Cycles of all sorts

Dynamo systems are probably the most widely-used worldwide, although they are uncommon in North America. They provide a self-sufficiency which is valued by their users: no batteries to recharge or replace, and permanently fitted to the bike. (Note: bicycle dynamos produce AC, and so are more correctly referred to as alternators or generators, but the word 'dynamo' has stuck in universal usage). There are three main types: hub dynamos are built into the front hub, and are generally the most efficient; bottle dynamos attach to a fork leg and are rotated by a small wheel in contact with the tyre sidewall, they are generally easiest to obtain and cheapest; bottom bracket dynamos bolt between the chainstays behind the bottom bracket and are powered by a roller against the tyre, these are easy to fit and do not wear the tyre sidewall.

Dynamos are generally limited to 0.5A output, nominally 3 W at 6 V, although 12 V dynamos are available and the best nominal 6 V dynamos can produce 6 W at speed, as detailed here. A dynamo behaves as a constant-current device, not constant voltage; this means that the voltage can exceed the capacity of the lamp at speed, causing failure. Historically this was considered a nuisance but these days lamps and dynamos often incorporate zener diodes to prevent it and/or the dynamo can be carefully designed to "saturate" beyond a certain voltage to protect the lamp (saturation is a feature of all permanent magnet generators). Good dynamos can achieve efficiencies of up to 70% (i.e. under 5W of the rider's output is diverted to produce 3W of electricity) and provide full output at surprisingly low speeds, often 4 to 6 mph is sufficient for full brightness.

To compensate for their limited output, dynamo headlights have good optics which focus the limited amount of light in a narrow beam that lights up the road directly in front of the bicycle; this can be seen in Andreas Oehler's side-by side comparison of beam patterns.

To compensate for the fact that dynamos produce no power when stationary the best lights (and some dynamos) have a stand light facility, usually a single blue-white LED powered by a capacitor which will run for around five minutes. It is now common practice to use all the dynamo's output to power the front light; the alternative is a 2.4 W headlight and a 0.6 W tail light. Most good dynamo rear lights now use LEDs instead of incandescent lamps.

Some riders consider the output of dynamo lighting insufficient. Others report that it is adequate to see on dark roads at speeds up to about 20 mph (single 3 W) or more if a 12 V system is used. Whether the output is sufficient or not will be a personal choice.

Hub dynamos are generally considered the best but require a wheel to be rebuilt if retrofitted (your existing rim can usually be reused). Some bikes are available with hub dynamo lighting systems as a factory option; this can be very cost-effective.

Advantages of dynamo lighting

• Unlimited burn time
• Always there
• High-efficiency optics that are designed to maximize the available power for the lamp (optimised for road use)
• Reliable (although see below for issues with bottle dynamos)
• Environmentally friendly and cheap to run
• Unattractive to thieves

Disadvantages

• Bottle dynamos are noisy, can slip when wet, and wear the sidewall of the tire
• Without standlights, when stopping the lights are off
• Limited power output
• Cut out at very low speeds (below walking pace)
• Difficult to remove where theft / vandalism are a problem
• Cheap dynamos produce noticeable drag
• Halogen bulbs run by a dynamo have a relatively short service life (around 100 hours)