Professor Pivot answers your Technical Questions
ELECTRIC bikes

What is VOLTAGE and which Voltage is best?
Voltage can be thought of as the pressure or strength of electric power. All things being equal (see AMPS below), the higher the voltage the better, because high voltages pass more efficiently through wires and motors. Very high voltages (100+ volts) can give you a nasty shock because they also travel through people rather well, but the sort of voltages found on electric bicycles (12 - 36 volts) are quite safe. As a rule, a 12 volt system is fine for low-powered motors, but more powerful machines work better with 24 or 36 volts.

What are AMPS?
Amps can be thought of as the volume or quantity of electric power. To aid this analogy, the flow of amps is called the current, as in the flow of a river. Unlike a river, though, the speed of the current is fixed - only the volume varies.
The maximum flow of amps in a bicycle drive system can vary from 10 to 60 or more. A current of 60 amps requires thick wiring and quite substantial switchgear.

What are WATTS?
Once we know the voltage (or pressure) and current (or volume), we can calculate the power, or wattage by multiplying the two figures together. The number of watts in a system is the most important figure of all, because it defines the power output. A few examples:
The Zap motor draws 20 Amps x 12 Volts = 240 Watts
The Giant Twist Lite draws 15 Amps x 24 Volts = 360 Watts
The Powabyke draws 20 Amps x 36 Volts = 720 Watts
The Curry Drive draws 40 Amps x 24 Volts = 960 Watts
Despite having a fairly low voltage, the Curry is the most powerful motor, followed by the Powabyke and the Twist, with the Zap coming in last. It's impossible to calculate the power without knowing both the number of amps and volts. Large machines, like cars, trains and trucks have their power measured in the same way - usually as kilowatts, or units of 1,000 watts. The old-fashioned 'horsepower' unit is the equivalent of about 750 watts.

I thought the legal limit was 250 Watts in the UK?
Well, yes and no. The legal limit refers to the continuous power output, whereas the figures above are for absolute maximum power. Most motors can give maximum output for a minute or two, but they'd melt if asked to do it all day - just like a cyclist. Obviously, maximum power is more useful than continuous power as a guide to the way a bicycle will climb a hill. Look at the spec of bikes on sale and you may see 200 watts, 250 watts or (illegally) 400 watts. These figures are only a rough guide to the true maximum power output.

How many watts do I need?
As a general rule, a cyclist can produce several hundred watts briefly, and one hundred watts for a reasonable length of time. To be really useful, a motor needs to produce another 100 Watts on a continuous basis, with peak power of at least 400 watts. Just to confuse things, our measurements are of power consumption - losses in the motor and drive system mean that the power output to the wheel can be much lower.
If you expect the motor to do most of the work, especially in a hilly area, you'll want a peak consumption of 600 watts or more. On the other hand, if you prefer gentle assistance, a peak of 200 watts may be enough.

How big a battery do I need?
The capacity of the battery is usually measured as the amount of current it can supply over time (defined as amp/hours). However, this is useless on its own, because you'll need to know the voltage too. By multiplying the two figures together, we get watt/hours - a measure of the energy content of the battery. Unfortunately, it isn't that simple... but you didn't think it would be, did you? In practise, you're unlikely to get results that match the stated capacity of a battery, because battery capacity varies according to the temperature, battery condition, and the rate that current is taken from it.
Lead/acid batteries are tested at the '20-Hour' rate. This is the number of amps that can be continuously drawn from the battery over a period of 20 hours. However, an electric bicycle will usually exhaust its battery in an hour or two, and at this higher load, the battery will be much less efficient. So the figures for lead/acid batteries tend to look optimistic.
On the other hand, Nickel-Cadmium (NiCd) batteries are rated at a 1-Hour discharge rate, so although the stated capacity of a NiCd battery might only be half that of a lead/acid battery, performance on an electric bicycle will be much the same. Nickel-Metal Hydride batteries (NiMH) are measured at the 5-Hour rate, so their performance tends to be somewhere between the two.
The capacities of typical bicycle batteries vary from Powabyke's 504 watt/hour giant (36 volts x 14 amp/hours) to the tiny 84 watt/hour pack on the early SRAM Sparc kit.
It's best to choose a package that will provide twice your normal daily mileage. It's difficult to guess the mileage from the watt/hour capacity, because actual performance depends on the bicycle and motor efficiency, battery type, road conditions, and your weight and level of fitness.

How can I measure the efficiency of an electric bike?
We measure overall efficiency by dividing the watt/hours used by the battery charger by the mileage achieved, giving a figure of watt/hours per mile. This varies according to the terrain, the weight and riding style of the rider and the type of battery and charger, but our figures are measured in exactly the same way for each test, so they should be comparable, bike against bike. The best we've seen is 8 watt/hours per mile, and the worst is 32... Typically, an electric bicycle will consume 10 - 20 watt/hours per mile. So a big battery like the Powabyke's will give a range of between 15 miles (doing all the work in quite hilly terrain) and 50 miles (a joint effort in flat terrain). This is fine for most uses, although it's a big, heavy battery. As a general rule, medium-sized NiMH batteries on lightweight bikes give the best results: the Giant Twist runs for more than 20 miles on a 156Wh battery, and the faster Ezee Sprint more than 25 miles on a 324Wh battery. Small units, such as the Panasonic WiLL, give a maximum range of 5 - 10 miles.

Do electric bicycles recharge when you coast downhill?
With the exception of the Canadian BionX, the answer is generally NO. Taking into account wind-resistance, road friction and so on, there's surprisingly little energy left over for recharging the battery, even before generator and battery losses are taken into account. In most systems the motor coasts when you ride downhill, but those that don't (mainly electric scooters) are capable of putting back only 15% of the power absorbed climbing the hill. Regenerative systems do have their advantages though - mainly in reducing brake wear and over-heating.

Which battery type is best?
Lead-acid batteries are cheap and easily recycled, but they are sensitive to maltreatment and have a limited life. Weight for weight, Nickel-Cadmium gives more capacity, but it's expensive and the cadmium is a nasty pollutant and difficult to recycle when the battery fails. The life is greater, which tends to compensate, but disposal problems mean that Nickel-Cadmium is being phased out. NiMh is theoretically more efficient still, but these batteries can be expensive, and because the capacity is measured at the more generous 5-Hour rate, the advantage is not what it appears to be. Our experience is that NiMH offers little, if any, improvement in range. They are, however, easier and safer to dispose of when they eventually fail.
Most modern lightweight bikes come with Lithium-ion (Li-ion) batteries. These are more weight-efficient than the other types, and are supposed to have a longer life, but can do some odd things. Charging and discharging must be carefully controlled to prevent the cells going into terminal meltdown, so chargers are packed with electronics, as are the batteries. Costs are coming down rapidly and fires are now rare(!), so Li-ion looks destined to become the default battery. Lithium-ion Polymer (usually called Li-pol) doesn't really offer any performance advantage in terms of weight or range of Li-ion, but it's safer, and promises to become even cheaper once the technology becomes mainstream. Very few bikes offer these batteries yet, and no-one really knows what the life of the batteries will be.

Which charger is best?
Swings and roundabouts here. Batteries do not take kindly to fast charging, although NiCd and NiMH are more tolerant than lead-acid, which can start fast, but prefers a long tapering charge thereafter. A fast (sub four hour) charger makes a great difference to the flexibility of an electric machine. You can, for instance, travel for the full range in the morning, recharge while visiting a friend, and run home in the afternoon. No lead-acid charger can do this, although the best NiCd, NiMH and Li-ion chargers will. For basic commuting, an overnight charger is safest and kindest to the battery, but if you expect to push a high daily mileage, you'll need something faster.

Should I choose a brushless motor?
Broadly speaking, there are two types of electric motor -
Direct Current motors - simple but comparatively heavy and inefficient, and
Alternating Current motors - smaller, lighter and more efficient over a broader speed range

Generally speaking, Direct Current motors have brushes to transfer power into the rotating bit and Alternating Current motors do not. However, most of the brushless motors fitted to electric bicycles are a hybrid of the two types, often called 'Hall Effect'. These are not quite as clever as a full Alternating Current motor, but do away with the brushes, so they should be more efficient and more reliable than the straight Direct Current type. Hall Effect motors are now almost universal on electric bikes, so don't concern yourself too much with these categories. But don't rule out Direct Curent brushed motors! They may have mechanical brushes, but they're mercifully short of complex electronics.

What should I look for?
We've put together an electric bike specification wish-list below. At the present time, there are no machines that win in every category, but the closer yours gets the better. If the salesman is unable to provide all the answers, or starts blustering or attempting to blind you with science, we'd recommend looking elsewhere. A good shop should be able to provide most of the figures in a straightforward and honest manner, but some are quite incompentent:

Weight Less than 30kg (66lb)
Price Less than £600
Maximum assisted speed Not less than 15mph (legal maximum)
Peak power More than 300 watts
Power consumption Less than 10 watt/hours per mile
Range** More than 20 miles
Battery type NiMH or Li-ion (Nickel-Metal Hydride or Lithium-ion)
Replacement battery price Less than £100

** You'll need to verify this for yourself - manufacturers figures are universally dubious

 A few other pointers: If you are expecting to tackle very steep hills (in excess of 17%, or 1 in 6), we'd recommend a Crank Drive motor. This type puts power through the rear gear system and can be fine-tuned to suit almost any environment. It's the best system if you can afford it. The more common Hub Motor effectively has only one gear, and although some are very powerful, it will prove less efficient in a really hilly area. For most other purposes a hub motor is fine, but avoid Friction Drive unless you intend to make light use of the bike. The roller and/or the tyre tend to wear out in a few hundred miles.

LATEST NEWS (July 2004): Electric Bicycle Legislation
It appears the UK Department for Transport is now clearly stating that electric bicycles can supply power without being pedalled. Is this the end? Not quite. The legal position of bikes caught between the UK limit of 200 watts output and the European limit of 250 watts output is still ambiguous. Our advise is to ignore this purely technical detail.

LATEST NEWS (June 2003): Electric Scooter Legislation
Electric and petrol-powered micro-scooters are not, and never have been, road legal in the UK. Recent court rulings have imposed heavy penalties on users, treating the machines as small motorcycles in law. Thus, riding an electric scooter in a public place can result in prosecution for riding without insurance, MOT, tax, and a BS-standard motorcycle helmet, resulting in hefty fines and (according to a number of recent cases) points on your driving licence. Consequently, we are unable to recommend an electric scooter. The electric bicycles below are treated as conventional bicycles, but riders must be over 14 years of age.
Our advice is not to buy any of these machines.

LATEST NEWS (20th July 2003): Electric Bicycle Legislation
Under Europe-wide type approval legislation passed in June 1999, electric bicycles that did NOT need to be pedalled for the motor to operate were banned, with manufacturers being given three years to comply (June 2003). This has now been extended to 9th Novemenr 2003.
To remain exempt from motor vehicle legislation, an electric bicycle must comply with the following:

1) power no more than 250 watts rated output
2) motor must stop when you stop pedalling
3) motor power should reduce as you reach the maximum speed (25kph)
4) maximum weight 40 kilos
5) bike complies with existing United Kingdom pedal cycle standards.

Rule 2) is new and applies to bikes manufactured after June 2003. However, the exact standards covering the way power is cut off when the pedals stop turning has not yet been written and will probably not be produced until the end of 2004. To further confuse matters, the former UK legislation (almost identical, but with a 200 watt power limit and no requirement for the motor to stop when you stop pedalling) will remain in force, bringing UK law head to head with newer EU law. This older legislation will apply to self-built vehicles and those from small importers that are not covered by the type approval legislation. To summarise:

• All machines imported or manufactured prior to June 1999 remain legal
• Machines imported or manufactured between June 1999 and June 2003 may need to be adapted to prevent their use in non-pedal assist mode
• Machines imported or manufactured after June 2003 must comply with the new rules (but the rules may remain open to interpretation for at least another year).
• Self-built machines, and those imported in small numbers are exempt from the changes and may be used in non-pedal assist mode.

If you're still confused, you're not alone. Recent clarification of the rules has raised almost as many questions as it has answered. Some machines, such as the Powabyke Euro range, can be switched between the two modes of operation - it is not clear whether these will remain legal or will need the switch disabled. And how small will a manufacturer/importer need to be to skip the regulations all together? And no-one seems quite sure whether the new rules will apply to kits fitted to conventional bikes. The Department for Transport is currently unable to answer these questions and has now suggested that it will be for the courts to clarify the law. The legal profession suggests the ball is firmly in the Department's court.
Our advice is to buy any machine you wish, but check that the retailer is willing to make any necessary changes to the specification should these be required by law within a reasonable period.

For more electric bike pages, try:
Why Choose an Electric Bike?
UK Electric Bike Price Guide
Electric Bike Buyer's Guide
Glossary of Terms
Road Tests

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