I.      Introduction and Background
II.     General Terms and Abbreviations
III.   Experimental Propulsion Systems for PA's and Other Advanced Vehicles
IV.    A Short Guide to Batteries for Electric Vehicles
V.    Common Questions to Constructing and Maintaining a PA

                                        compiled by Eric Peltzer  -  www.peltzer.net

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A power assist is any vehicle with any number of wheels that is primarily or partly human powered, and assisted by a small motor. An electric bicycle is probably the most common such vehicle. Power assist enthusiasts are typically interested in simple human-scaled transportation that is earth-friendly, fun, non-intimidating, and affordable. Transportation that either extends the range of their bicycling, allows them to arrive at work without needing a shower, or allows them to leave the car at home more often.

Many people are somewhat familiar with old-style mopeds, which are technically power assists. However they are too heavy to really be pedaled more than to start the engine. Mopeds are really lightweight scooters with pedals and limited top speeds which allow them to qualify in most parts of the world for lesser registration and insurance classifications.

Many people feel that a more useful set of pedals is important. This means the vehicle must be light enough to pedal meaningfully, either with or without the motor energized. Practically speaking it should weigh less than 75 lbs. or 35 kg and of course the lighter, the better. A power assist is not meant to entirely replace human power. The exact amount of assist desired, however, is really up to the individual.

Today electric vehicles are becoming more popular, but there are still internal combustion engine (ICE) power assists that perform well. They both have their adherents, but really they each have their place as their strengths and weaknesses are in different areas.

ELECTRIC ASSISTS  -  today's practical electric vehicle

The practical electric car has become a kind of Holy Grail and is the subject of continuing intense research and legislative debate. With the current shock in fuel prices and growing alarm over greenhouse gas emissions, it's becoming a world wide imperative. Electric drive's extreme simplicity and reliability, silent and vibration-free operation, no tailpipe emissions, and extremely high energy efficiency are further reasons for it's attractiveness. While the auto makers make abortive efforts to electrify a full size auto, many hobbyists have come to the more reasonable conclusion that electric bicycles make a lot more sense. They work and they're practical and affordable today.

Many electric enthusiasts have built their own vehicles using low voltage 24 or 36 volt DC motors, 2 or 3 12-volt batteries, speed controllers, and various transmission methods to get the power from the motor to the wheel. Motors range typically from 250 watts to 750 watts. (746 watts equals 1 hp). AC or brushless motors are also being used in greater numbers. AC and brushless motors are simpler and cheaper but the speed controller is more complicated.

Most of these PA's are bicycles or trikes, either normal or recumbent.

Electric assists seem to be more suited to short around town commuting and errands rather than long tours, as their range is seldom more than 15 to 20 miles. They use a few pennies of electricity per charge, they never have trouble starting, they are simple and reliable, and they don't emit anything. But the best part is the silence. All you hear is the wind.

Ready made electric bicycles have been made for years, but the availability and popularity seem to spike when gas prices climb, and ease off again after the shock wears off. Many companies that once made electric bikes abandoned the effort, only to be resurrected again  The Zap, Currie's US ProDrive, the Wavecrest, and Lee Iacocca's EBike are some of the better known examples that have come and gone. Schwinn and Giant have offered electric models, only to discontinue them. Currently, Lashout, Wilderness Energy, and Crystalyte make motor kits for bicycles, and some major bike makers are again offering factory electrified models.  They are true "assists" in that the batteries and motors have very limited power (much less than the usual moped.) With 300 to 400 watt motors, they go about 17 mph on flat ground and cover 10 to 20 miles (with pedaling) before needing a recharge.  The better kits and bikes use simple and compact hub motors and even advanced batteries are becoming available. The future increasingly looks electric.

INTERNAL COMBUSTION ASSISTS  -  How does 160 mpg grab you?

Hugh Currin's ICE power assist recumbent
Some PA enthusiasts want to go on long bicycle tours and use a very small and unobtrusive assist motor to help their heavily loaded touring bikes over the long hills. Or they just need a vehicle with a longer range. The assist helps them extend their range and their feeling of self-sufficiency without feeling like they should have taken the station wagon. Touring is a very pleasant, back-to-basics way to see the countryside. Many touring enthusiasts conclude that small internal combustion engines are the way to go because they have a longer range and lighter weight than battery electrics.

Many people also feel that electric drive is just not good enough yet. Though clean and quiet, and generally though of as more environmentally friendly than internal combustion engines, many people feel that the easy refueling and phenomenal gas mileage of a small ICE makes it still a good choice for a bicycle motor. Many ICE bikers report engine-on mileage in the hundreds of miles per gallon.

Then there is the two-stroke versus four-stroke consideration. Two stroke piston engines are light and simple, but as they use gas mixed with oil, they have historically been fairly gross polluters, even if the mileage is high. Newer four strokes are better in this respect, however even tiny four strokes may have higher particulate exhaust than most new cars as they don't have catalytic converters and other sophisticated clean burn technology. 

The internal combustion engine is far from finished as a motive force. They are getting cleaner and more efficient, and it is also possible to power them with cleaner fuels, such as compressed natural gas, propane, alcohol, hydrogen, and, apparently, even vegetable oil. Will the current gas shock of 2008 see a resurgence in tiny clean motorized bicycle's? Wouldn't bet against it.


Not an exhaustive list but should get you started. For other abbreviations see also the battery discussion following.

PA         Power Assist - power-assisted human powered vehicle
EV         Electric Vehicle
HPV      Human Powered Vehicle
ICE       Internal Combustion Engine

EBIKE, ELEBIKE, EPED  -  terms for an electric motorized bicycle 

BRUSHLESS DC is one of the two types of motors commonly used on bicycles. They are generally considered superior to brush or PMDC motors, but there are excellent motors of each type.

ELECTRATHON not PA's, but a type of ultalight EV used in competitions usually with three or four wheels and no pedals. Electrathon racing sets class rules, and the race is to see how far the vehicle can go in one hour. Thus it is really a test of efficiency aimed at improving the state of EV technology and getting normal people interested in EV's. Many of the same components, motors, chargers, etc. are ideal for use in PA's.

HUB MOTOR  an electric motor that doubles as the hub of the vehicle's drive wheel. Ferdinand Porsche demonstrated electric hub motors on a vehicle at the turn of the century. Today they are being embraced as great compact solution to driving a PA. Heinzmann makes some of the most popular, being used on the Ebike (Lee Iacocca's promotion.) But others are also available and in the works, such as the Rabbittool Ex-Bike motor, and the Taiwan-made TopRun motor used in a number of ebikes.

MOPED a type of motorized bicycle. The term moped is generally used for such european vehicles made by Puch, Motobecane, Piaggio and Peugeot, that were popular in the 70's and 80's. They are really lightweight scooters going about 30 mph (48 km/h) that were too heavy to actually pedal. The low-geared pedals were really just there to qualify legally as a moped and to start the engine. A typical moped had an integral 50cc two stroke gasoline engine, weighed about 150 lbs. and made about 2 hp. They did not look like bicycles, more like light step-through scooters with larger wheels.

PEDELEC electric bicycle that must be pedaled. The electric assist is only activated by pedaling. You cannot just twist the throttle and go. In some areas a pedelec is a legal distinction from a throttle controlled electric, is more restricted in speed and power, and as a result is subject to less stringent license and registration rules. In most cases, the motor is designed to cut out above a certain speed.

PMDC or PM DC Permanent Magnet Direct Current, usually referring to a type of motor, and occasionally refering to the controller that will operate this type of mot. This is the kind of motor that usually has brushes and a commutator.  These motors usually come in 24 to 36 volts for small electric vehicle use. See the discussion of brusless vs brushed motors below.

PWM  Pulse Width Modulation - a method used in modern electric motor speed controls, employing high frequency power pulses of varying lengths of time. This effectively and efficiently controls speed. Since voltage is always at full, there is less loss due to resistance. Much more efficient than using a rheostat speed control, which varies the actual voltage, but uses a variable resistor which is basically a big heat sink.

RECUMBENT  or 'BENT for short. A bicycle where the rider is reclined back and low to the ground for better aerodynamics. Recumbents are also more comfortable to many who dislike the narrow wedge saddle and hunched over riding position of a regular bike. Recumbents tend to be faster on flat ground. On hills they do not offer as much of an advantage as they are generally a little heavier than other bikes. Some people also feel their riding position and steering is awkward. However they have a loyal and growing following.

REGEN or REGENERATIVE BRAKING  Some electric motor speed controllers are designed to turn the drive motor into a generator when the throttle is turned down. This acts as an "engine brake" and can help reclaim energy back into the batteries. This only occurs under braking or when driving downhill. Due to air and tire friction and system efficiency losses, a fairly small percentage of energy expended can be re-converted back into battery charge, but every little bit helps.

TRACTION MOTOR and TRACTION BATTERY motors  batteries intended specifically for powering vehicles or very heavy work loads. These motors tend to be much different from other kinds of electric motors. 

FWD  front wheel drive
RWD  rear wheel drive

This is not a course in basic electric power but:

A basic formula that is very useful in electric vehicle discussions: power = volts x amps
or W = V x A.


ELECTRIC   Battery powered is not the only way to supply power to an electric motor. Fuel cells, hybrid systems, solar power and even flywheels can be viable sources of energy storage and generation.

Another issue with electrics is that they are generally recharged off the electric grid, which may actually be a coal or oil or gas fired generator. Thus they are not necessarily completely clean. This is another reason to find alternate ways to charge the batteries or power the motor with some other source.

I.C.E.  Internal Combustion Engine - the normal gasoline or diesel engine. Includes gasoline (petrol), diesel, two stroke or four stroke, piston, rotary, etc. Any engine which actually burns or ignites fuel in a combustion chamber. Does not include steam or Stirling cycle engines, which are external combustion engines. The internal combustion engine is far from finished as a motive force. They are getting cleaner and more efficient, and it is also possible to power them with cleaner fuels, such as compressed natural gas, propane, alcohol, hydrogen, biodiesel, algae-produced gasoline and diesel, etc.

FLYWHEEL  A rotating wheel has inertia, which makes is a useful repository of energy. A number of researchers and companies have proposed that flywheels spinning in a vacuum and suspended by magnetic bearing could be a good way to store energy that could later be fed to an electric generator and thence to a traction motor.

HYBRID  A drive system combining a fuel-burning engine/generator, and an electric motor. A hybrid is intended to maximize the efficiency of an internal combustion engine by allowing it to run only when necessary, while an electric traction motor actually drives the wheels. A hybrid may also have batteries to store energy so that the engine runs as seldom as possible. There are many possible configurations of hybrid systems. For instance, a gas turbine engine supplying power to a flywheel storage system which then powers the electric motors has been studied.

An advanced hybrid system running on a clean fuel could conceivably even compete with a battery electric for efficiency and overall environmental impact.

However, a hybrid system on a lightweight bicycle seems problematic, since it is complicated, requiring both an ICE and an electric drive system. Also, a small ICE alone on a bicycle is already extremely fuel efficient, so it would seem there is little to be gained.

FUEL CELL  A fuel cell generates electric power from fuel by direct chemical conversion. Fuel cells have generated power on the space shuttle and for satellites. They are now being hotly researched by the major car companies for use in electrically driven vehicles. The fuel cell would replace conventional rechargeable batteries. They use fuel such as hydrogen, methanol, or even gasoline. The simplest fuel cells use pure hydrogen. They do not burn the fuel, and the by products are just water vapor and heat, making them much cleaner than ICE powered cars. The fact that they run on fuel means that they would not suffer the limited range and long recharge times of battery EV's. A demonstration fuel cell bicycle PA has been shown by Manhattan Scientifics. However cost and fuel availability are still large practical hurdles to be overcome.

SOLAR  Uses photovoltaic cells to power an electric motor. Probably the ultimate clean energy source. With the advent of cheaper and more powerful photovoltaics, it is possible to power a vehicle this way, however, most bicycles are small and do not have enough room for a large horizontal array of solar cells. A solar array on a bike trailer is one possibility. Obviously, any solar technology is dependent on strong sunshine.

However, since an electric bike has a fairly small battery pack, it is definitely possible to use solar cells to re-charge the batteries, which a few  enthusiasts already do. Thus it is actually possible to have real-world, practical solar-powered personal transportation right here and now.

STIRLING A Stirling cycle engine. This is a type of external combustion engine using a heat differential principle to power a piston. It is related to a steam engine. Some think they would make good power sources for PA's since they are simple and quiet and more efficient than other fuel burning engines, However, they tend to be heavy and somewhat impractical. To my knowledge, few vehicles of any kind have ever been powered by a Stirling.


The following is a brief introduction to batteries with more of an eye to suitability for PA's and other small electric vehicles.

Batteries for electric vehicles are much more problematic than the motor. Electric motors are already small, quiet, powerful, reliable, and reasonably priced. Batteries, however, are heavy, expensive, and energy poor. Batteries will have to be improved dramatically before electric vehicles can truly compete on functional and economic terms with ICE powered vehicles.

Luckily research into advanced batteries is reaching a fever pitch. Still, at this time advanced batteries are expensive, which is why many EV's today continue to use lead acid. PA's however, being the smallest EV's, could conceivably afford a really great battery pack, if cost were the only distinguishing factor. It's not, however.

Batteries for EV's need to be evaluated basically on four criteria: cost, power density; current delivery, and life cycle.


Ah  Amp Hours. Most batteries' capacities are rated in amp hours, or how many amperes of current they will supply for an hour at the rated voltage before going "dead". This may also be thought of in terms of how many hours the battery will supply the given number of amps.

DEEP CYCLE A rechargeable battery designed to be repeatedly discharged fully and recharged without being ruined. These are the only batteries suitable for use in a PA vehicle or any EV. Automotive starting batteries are not deep cycle.

DISCHARGE RATE  - C/20, C6, etc. The current rate at which a battery's amp-hour rating has been ascertained. This generally expressed as the number of hours until the battery has been fully discharged. For instance C/20 or C20 means the battery full charge has been run out over a 20 hour period. (C stands for the theoretical capacity of the battery.) See also Peukert's Equation.

A 12 volt 10 Ah battery will supposedly supply 10 amperes of current for an hour, or 5 amps for 2 hours or 1 amp for 10 hours. However, most batteries (including lead acid) will supply more amp-hours if the current draw is lower. So manufacturers are tempted to pump up the numbers by rating their batteries at low current draws. However, most electric vehicles require high currents to drive the motors. In short, the stated Ah rating is often misleading for electric vehicle purposes.

For instance, the Hawker Odyssey lead acid AGM battery model PC545 is rated at 13.9 Ah. However, this is its C/20 capacity - if it is discharged over a 20 hour period. If it is discharged in half and hour, it will only supply 9.3 Ah.

D.O.D.  Depth of Discharge. A 20% DOD means that 20% of the battery's  power capacity has been used. Most rechargeable batteries are capable of more cycles if they are not discharged as deeply in each cycle. A quality lead acid deep-cycle battery may be rated at 400 charge cycles at 100% DOD, while it will give 500 charge cycles at 80% DOD. The same battery will last almost indefinitely if it is seldom discharge past 20% DOD.

In short, a battery will last much longer if you can avoid running it dead every time you use it. This is more true for some chemistries than others. The exception is NiCad chemistry; these are better off being almost fully discharged regularly.

LIFE CYCLE  Since batteries are the costliest part of an EV, this is one of the critical factors in choice of battery. An EV used on a daily basis and completely drained of power will need it's batteries replaced in less than 2 years.  All deep cycle batteries are capable of a certain number of charge-discharge cycles before "wearing out." or significantly loosing energy capacity. The generally accepted number is about 400 discharges to at least 80% DOD (see below). Any less than this is generally not acceptable for commercial viability.

POWER DENSITY or SPECIFIC POWER  A term which basically means the power capacity of a battery in relation to its weight. Technically, it is expressed as watt-hours per kilogram. This is easy to calculate. For instance, a 12 V lead acid battery rated at 13 amp-hours and weighing 6 kilograms would have a power density of:
  (12 V x 13 Ah) / (6 kg)  =  26 Wh/kg

For example, flooded lead-acid batteries generally have about 25 wh/kg, the latest advanced lead-acid designs claim about 50 wh/kg, and newer battery technologies such as NiMH and Li Ion are in the 80-135 wh/kg range.

SELF DISCHARGE  the tendency to which an idle battery will lose its charge. Normal lead acid batteries, for example, will go dead due to self discharge in about 6 months. Advanced lead acid batteries will stay charged for years. NiCads will go dead in about a month.


LA  Lead Acid battery. The most popular chemistry type used in amateur and smaller electric vehicles such as bicycles and golf carts, due to very reasonable expense, reasonable life expectancy, and high current delivery. However, the low power and high weight has also been the principal reason for the limited range of most electric vehicles. Electric vehicles do not typically use high tech batteries for the simple reason that EV's need LOTS of juice, far more than any laptop or cel phone. This means lots of batteries. Lead acid is by far the cheapest battery construction and so there you have it.

All of these are types of lead acid batteries:

It should also be mentioned that lead acid batteries may have a little more life left in them. For instance, a company called Firefly Energy has patented a kind of carbon graphite foam material that both drastically lightens lead acid batteries and makes them less likely to degrade.  The idea is that lead acid batteries could be nearly as good as lithium ion, but far cheaper.

Li Ion
  Lithium Ion - batteries for high energy, high expense, and lower continuous current capacity, making them not as popular yet for EV's, though this is where all the research and action is starting to coalesce. It's widely considered that lithium may be the only battery chemistry with the potential to radically improve electric vehicles and make them widely available and practical. Lithium Ion and the related Lithium Polymer are some of the most intensively researched chemistries for future battery storage. They are now widely used in cell phones, laptops, and cameras. Most people feel that, for this chemistry to become established in EV's, it must be substantially improved, made in much larger cell sizes, and its cost must come down tremendously. General Motors, Toyota, Hyundai, and other car companies and battery start ups are pledging to put lithium batteries into production vehicles (so called 'plug-in' hybrids) by 2010.  The Tesla Motor Co. is currently selling a (very expensive) battery electric sports car that use lithium batteries. The Tesla actually uses thousands of Super-C size battery cells wired up into large battery packs, the same basic battery cells that power laptop computers. The car costs $90,000 but it is possible to buy one.

Lithium batteries have had some safety problems. This is due to the fact that older technology mostly used electrodes made of cobalt and other materials which can overheat and even cause fires. The newer technology that will go into EV batteries will almost certainly eliminated these problems by using safer materials.  Some of the notable variations on lithium chemisty are known as lithium polymer, lithium phosphate, lithium nano-phosphate, lithium iron phosphate or LiFePo4. 

NiCad  Nickel Cadmium batteries. NiCads have been popular for many years in smaller rechargeable devices because of good power to weight, extreme depth of discharge capability, high current delivery, reasonable cost, and long life cycle if they are properly charged and maintained. They also maintain voltage better when almost completely discharged.  However, specific energy is not that much greater than lead acid, while they are significantly more expensive. They must be charged carefully. They also use cadmium, a toxic heavy metal, though they are 99% recyclable. Some recent PA's have used these, such as the ElecTrek. However, NiMH seems to have largely taken over from NiCad, because of better performance and cheaper  materials. NiCads have become relatively unusual in recent years. 

NiMH  Nickel Metal Hydride batteries. The phenomenal popularity of the Toyota Prius and other hybrid cars is largely thanks to it's NiMH batteries. These batteries started out in electronics and cel phones, before being replaced by lithium ion in those devices. NiMH batteries are suitable for hybrinds because they can give very high currents both in powering the vehicle and in being recharged. Just as importantly, as long as they are not discharged too deeply, they can last a very long time. This is key: Toyota decided they needed to warrant their Prius batteries for ten years. They can do this by programming the car to never allow the batteries to be discharged more than 50%. The penalty for this, of course is that the vehicle is always carrying around twice as much battery as it really needs. Couple this with the fact that the batteries are still expensive, and you can see that NiMH, for all it's success, is far from the ideal solution to the needs of electric vehicles. It may be that improvements to NiMH batteries may give them more life in the market. But the expense of nickel is a limiting factor in how cheap the batteries will ever be.

NiZn  Nickel Zinc - a chemistry made for a time by Evercel, promising twice the power-to-weight of lead acid, with similar life cycle, at a reasonable cost. Said to have high specific power even at high current rates, which would be a great improvement over lead acid. Models were starting to become available specifically for bicycles, scooters, and marine trolling motors. Seeming to be a very promising chemistry for EV's, it suffered when Evercel ceased production. 

There are many other possible chemistries being researched. Which ones will actually become economically viable is hard to say. Or will cheap fuel cells make battery EV's obsolete? Only time will tell.



As I haven't tried every available kit, it wouldn't be good for me to give the thumbs down to anyone, or the thumbs up for that matter. However, that being said, some of the kits seem to offer a good cost/performance combination. Note that I have no affiliation with any of these companies or any site that may advertise here.

  • BionX offers a hub motor kit with a choice of 250, 350, and 500 watt motors. The use a NiMH battery pack.  Kits range from $1,100 to $1,800. BionX seems to be a popular kit fitted to a number of ready-to-ride electric bikes.
  • eZeebike has a compact 400 watt motor and a small lithium battery pack. $1300.
  • Crystalyte makes a series of hub motors and kits. Offered in 400 watt, 600 watt, and 750 watt models with various speeds and wheel sizes, many dealers offer a wide range of kits and battery packs to suit the size of wheel and the power and top speed requirements of the rider. The larger motors can be geared to exceed 30 mph. Smaller kits start around $450 and the most powerful kits go for over $1000.  Adding lithium batteries is an additional $1100.
  • Wilderness Energykits look quite similar to the Crystalyte kits, a Chinese made hub motor. Newer kits are brushless, older ones are brush motors. They are not available in quite as many power levels as the Crystalyte, but the standard 600 watt motor should be quite sufficient, and the price is reasonable starting around $400.
  • Lashout has been discontinued, but there are still Rayos bikes being made with this motor system.
  • Currie USProDrive was once and extremely popular kit that could be bolted to nearly any bike but is discontinued. Parts and motors are still available but are getting expensive.


There are so many electric bikes now that an exhaustive list is difficult to maintain. They typically cost between $650 and $1500, though high end models can go for far more. For more powerful motors, rear wheel hub motors are recommended, however with motors of lower power, under 400 watts, front drive works well enough. Here are some of the more notable models available in the US, in no particular order:

Folding electric bikes are also getting more attention as good short range commuters than can be stored in a closet or car trunk, and even taken on the bus or the subway.  Most folding electrics these days use one of the lighter motor kits adapted to a folding bike from Dahon, Brompton, Xootr, or some less well know Chinese and Taiwanese folding bikes.



Generally rear wheel drive is best for bicycles and light weight vehicles. If you want good hill-climbing, rear wheel drive will give much better traction as weight transfers to the rear wheel on uphill slopes. If you live in a rainy climate RWD is also preferable as front wheel slides are more dangerous than rear wheel slides. The more powerful your motor is, the better off you are with RWD, as more torque will tend to interfere with steering and slippage will be a problem. Acceleration also acts to transfer weight to the rear.

FWD is alright where the motor is not very powerful (up to about 400 watts.) also, if you are on flat ground and acceleration and performance are less of a priority, FWD is workable. FWD can be simpler and cheaper to build since it will not interfere with the chain and derailleur of the pedal system. 


Traditionally, EV traction motors have been brushed direct current motors. These are also known as PMDC or permanent magnet motors.  Increasingly, though, we are seeing AC or 'brushless DC' motors being used and sold. What is the difference?

Well let's first say that brushed PM motors are extremely reliable, quiet, efficient, and able. It is largely a myth that you need to replace the brushes in a well engineered vehicle motor. The rpm is not that high and the brushes typically last for years, and they are generally quite easy to replace. Your vehicle may wear out before the brushes.

That being said, an AC motor and a brushless motor have no contact parts at all and should require absolutely no service ever. A brushless DC motor is, really, just a variation on an AC induction motor, the kind most household appliance use to run on alternating single phase house current. A 'brushless DC' motor is simply meant to run on DC, with a mandatory external controller which converts battery DC electricity into multi-phase AC power. A multi-phase AC setup is even more efficient than single phase AC motors. You might look these up in Wikipedia or something as it's really too complicated to go into here.

A brushless motor can be smaller, simpler, and lighter than an equivalent power DC motor. That is the main advantage. Brushless motors have no commutator in the motor, which makes them simpler. So the motor is smaller and cheaper.  However, the controller for a brushless motor is more expensive and complicated. But there is still an overall weight savings because electronics don't weigh very much, motors do. 

One drawback with brushless motors is that you need to carefully match the motor with it's motor controller. Generally these would be made by the same company. This is not the case with DC brush motors; you can almost always get a general purpose PWM controller of the appropriate voltage and power rating, and it will work with any PM DC motor of matching voltage. Many motor companies do not even make controllers, and vice versa.


Connecting the motor to the drive wheel is one of the most challenging problems of constructing a PA. The motor drive has to co-exist with the pedal drive, and with two drive systems on a small vehicle, it gets crowded quickly. There are almost as many solutions as there are builders. Basically the most popular solutions use either friction drive on the bicycle tire, a timing belt, a chain, a vee belt, a j-belt, or a combination of these in a multi-reduction setup.

A chain would seem to be the most logical choice for a drive train, yet most electric motors turn 3000 RPM or more. A chain is not happy running on a shaft of more than about 1500 rpm. It will be very noisy at 3000 rpm, negating one of the nicest things about an electric motor, the quiet.

Belts are strong and quiet, but a long belt tends to stretch. One solution is a short belt driving a second shaft which then connects with a chain to the rear wheel. This double reduction is a also a good way to get higher gear reductions. However, this second 'jackshaft' needs to be set in ball bearings and solidly aligned with the motor shaft, something beyond the construction capabilities of some hobbyists.

The actual connection to the drive wheel can also be problematic. Bicycle wheels and hubs do not generally accommodate a second drive pulley or sprocket easily. A rear hub made for mounting a disc brake on the left side may be used to mount a specially modified chain sprocket or belt pulley. However, there is also the consideration of having a freewheel on the motor drive. (See freewheel discussion below.)

Basically, take a look at the pictures of what other people have built to get some ideas.

Using a hub motor built especially for bicycles solves the transmission problem, and these are increasingly available separately or as kits with batteries and controller, etc. However, these are internally geared, and changing the gearing is not possible. Many PA constructors would like to be able to choose the gearing based on motor power, hill climbing ability vs. top speed desired, etc.


Most motors must be geared down to turn a vehicle wheel. Electric and ICE motors typically turn anywhere from 2000 to 7000 rpm, while the bicycle wheel turns at normal speeds about 200 to 300 rpm. A 26" bicycle wheel, for instance turns 264 rpm at 20 mph.

The normal gearing methods are via chain and sprockets of different diameters (similar to the normal pedal chain and derailleur). However, greater reduction is typically needed. Anywhere from 5:1 to as much as 25:1 gearing reduction may be needed, depending on motor speed, wheel diameter, top speed desired, etc. (See below for a method to calculate your gearing requirement.)

Gearing is one of the most difficult problems for any power-assist builder or designer. Many simple assists just use a friction drive roller running off the motor shaft itself onto the surface of the vehicle tire. The small roller and the large diameter of the wheel give an instant reduction of as much as 25:1. The ZAP electrics and many intermittent-use ICE assists use this method. It is tried and true and cheap and simple. However, it is prone to slippage in the wet and wears the tire out more quickly, and the power transmission efficiency is not high, which means the electric batteries will run down more quickly and some power is lost.


A freewheel is used on the rear wheel hub of almost all bicycles. It basically locks in one direction of rotation, while spinning freely in the other direction. It enables the bicycle to continue to move forward while the pedals stop rotating.

Many people mounting a motor onto their bicycle want to use a freewheel on the motor as well, so that they can pedal the bike when the motor is off without the motor spinning around, which would greatly increase the effort required. Moreover, it's very economical to shut the motor off and coast, this can really extend the range of an electric vehicle. In fact, this is probably better than having regenerative braking, especially on a small light vehicle like a bike that doesn't brake often anyway.  At any rate, you have to choose one or the other, as a regenerative system requires a fixed connection to the rear wheel; obviously, a freewheel would prevent it from working at all.

A problem with fitting a freewheel to a bike motor is that all pedal chain drive systems are built to work on the right side of the bicycle, therefore all bicycle freewheels are built to work on the right side also.

Most people would prefer to put the motor drive on the left side, separate from the pedal chain. However, very few wheel hubs have any provision for mounting anything on the left except spokes, and even if you could thread a freewheel onto the left side, it would be freewheeling in the wrong direction.

One alternative is to mount the freewheel on the motor rather than on the wheel. However, this means that the drive chain and/or belt is always spinning around when the vehicle is in motion even if the motor shaft is stationary.

Another alternative is to mount a clutch on the motor or on the wheel. This is also a problem since there are not many clutches ready made for this kind of application.

ICE engine enthusiasts also have another problem. They would prefer to bump start the motor (meaning no freewheel can be used) but would also like to have the motor completely out of the loop when not in use. This means either using a clutch, or just using friction drive directly on the tire so that the entire motor swivels on it's mount away from the wheel when not in use.


The ideal traction motor for a lightweight vehicle is hard to come by, and they are generally not cheap. It should be a permanent magnet DC motor of at least 12V but preferably 24V or 36V. Higher voltage means lower operating current and smaller, more efficient speed controllers. It should have a shaft supported at both ends by ball bearings. A no-load speed of not more than 4000 rpm is best. The lower the speed, the less you will have to gear it down. Continuous power should be in the range of 400 W to 750 W. 600 W is a pretty ideal combination of good power, but no so much that you will burn through your batteries in ten minutes.

It is actually getting to the point where the electric add-on kits for bicycles actually have some of the best motors. (This should not be surprising, yet it has not always been the case.) The Crystalyte and Wilderness Energy hub motors are often available as a motor and controller only, and can be pre-spoked to a front or rear wheel of your choice, resulting in various sizes and gearing options.

For a good inexpensive motor for experimenting and building your own system, a 36V, 500 watt, 2500 rpm PMDC motor weighing just under 9 lbs is available from evparts.com for less than $60. (Part #MT5132). They also have 24V 350 watt motors for lower speeds (and even lower cost.)

If more power is your reason for building your own, you may want to go with something in the 750 to 1000 watt area. Cloudelectric has a selection of motors from $80 to $170 that would be very suitable according to your budget and power desired. A very interesting choice is their 'Motor 36 Volt 1000 Watt' model that has an integral freewheeling clutch. This can also be run at 24V to get 535 watts of power. A powerful and extremely rugged choice is the Scott  PM DC 1hp  24V (750w) motor available from cloudelectric.com for about US$360. At 17 lbs. it makes for a rather heavy bike but it is very rugged and powerful (it actually puts out over 2.5 Hp peak.) In 2008 Cloud EV/Scott introduced an improved and somewhat lighter motor called the "Electrathon" from Cloudelectric.com for about $400. It puts out closer to 1000 watts and is a bit lighter at 14 lbs.  Increasingly expensive and powerful alternatives in this power range would be the Mars, Etek and Lemco motors but we are now getting into the $600-1000 plus range for many of these.


It is probably also wise in a FAQ to say what motors are not suitable for a small electric vehicle. Lots of people have asked about the following:

Automotive starter motors are not good, though it has been done. They get very hot, the brushes will wear out quickly, their shafts are often not supported by ball bearings, and they are simply not designed for continuous use. They are designed for very brief bursts of high power. 

Cordless drill motors are not good. They are noisy, get hot, are not designed for continuous use, and the gear train is not heavy enough for the inertial loads of a vehicle (even a very light one.)

Electric trolling motors are not good. They are designed to be liquid cooled and the shafts are not designed to take an axial load, only a thrust load.

Electric winch motors are not good. They are geared down too much and are noisy and not intended for continuous use.

Model aircraft motors are not good. They run at very high RPM (like 10,000) and would require a multi-step gear reduction or gearbox. It has been tried, but the complication, expense, weight, and noise of such a gearbox negates any cost or weight savings that might be had by using such a small and inexpensive motor. They also need lots of forced air to keep them cool. In an aircraft this is easy, but on a bike you would need a fan.


Hugh Currin has build a wonderful and simple PA using a Honda string trimmer motor, which is a 22cc four stroke ICE. It is the recumbent pictured at top. Anyone thinking of doing something similar should visit his informative site..

A good motor for a PA is between 20cc and 50cc. Most of them turn about 6000 rpm so a pretty good speed reduction will be needed on the order of 15:1 or greater.

Leaf blowers and string trimmers seem to be the most popular and reasonably priced sources for small ICE engines. Post hole diggers are also a possibility, and many of these also come with suitable gear reductions.

Of course it's best to encourage the use of the cleanest motor possible. Four strokes are generally much cleaner and quieter than two strokes. A small two stroke motor can put out significantly more pollutants than the average automobile.

www.azdirtwirks.com sells a Honda GX31 engine mated to a 5:1 gearbox. It is intended as a power drive for a Viza scooter. This apparently sells for about $350.  The gearbox alone costs about $84.

A 5:1 gearbox will still need additional gearing to drive a bicycle. (See gearing calculations below.) Perhaps another 5:1 reduction will be necessary, however, this is achievable with just a single chain or belt drive to the rear wheel.

Many people have thought about using the ICE units sold for large model airplanes. As these would require an additional cooling fan and cowling, a better muffler, and a good gearbox, this would seem to be more trouble than it's worth.


As stated above, a 5:1 gearbox is available from Dirtwirks for a reasonable price. This is a continuing source of investigation, so any info on other cheap and suitable gearboxes is always welcome.

Gearboxes tend to be expensive, and unless the gearbox is already mated to the motor, you will need to find a way to connect the two. Using a timing belt is a good way to go as they are fairly quiet and efficient. A chain would run too fast and be noisy. However in this case you might as well just use a belt drive double reduction utilizing a jackshaft (or belt and chain combination) which can easily achieve a 25:1 reduction, no gearbox needed.


Worm gear speed reductions are fairly cheap, compact and quiet, and can achieve upwards of 20:1 reduction in one stage. However they have two drawbacks which have been the subject of no little discussion.

One is that they really do not like to be driven from the backside. In fact, most high-reduction worm gears simply will not turn when attempting to rotate the output shaft. This locking nature is actually a natural "parking brake" and has been exploited as a safety feature on some wheelchairs, winches and overhead hoists. On a bike or PA, this means that trying to pedal against a non-energized motor will be impossible without a freewheel or clutch to disconnect the motor drive.

Secondly, the worm principle calls for a spinning worm gear sliding against the teeth of a pinion gear, and the sliding action produces some frictional power losses. So worm gears tend to be less efficient than spur gears or helical gears, which do not slide against one another. This is more of a factor on electric PA's and less so on ICE setups which generally have some power to spare. Worm gear losses can reportedly be 10% or more depending on the reduction ratio and speed.

With these factors in mind, however, it is still possible to design a perfectly workable PA with a worm gear. Some PA constructors have use electric wheelchair motors which incorporate integral worm gear with good success.


The practical answers:

Most ready-made electric bicycles travel 17 mph using a 300 to 400 watt motor.  A 500 watt motor will have extra power in reserve for heavier loads and mild hills. A 600 to 750 watt motor will perform extremely well and climb steeper hills.  Anything larger will be heavy, expensive, drain your batteries really fast, and may well be illegal on roads without a motorcycle registration.

A popular EV motor used in bike conversions and electrathon racers is the Scott 24V 1 hp motor. It puts out 750 watts continuous power and about 2000 watts peak power. It will generally power an 80lb bicycle up to around 30 to 35 mph.

The smallest electric motor used on PA's is about 250 watts. This is good if low cost, low power consumption and/or lots of pedaling is desired. Higher power requires bigger and heavier batteries, heavier power transmission components, and beefier speed controls, all of which get more expensive fast. The battery and consequent weight involved to power a motor of more than 750 W starts to become so heavy that human power starts to become irrelevant.

It is also important to note that most electric motors are rated at continuous power, or the power the motor is designed to supply without overheating. ICE engines, on the other hand, are generally rated at peak power. Some electric bicycles makers and motor makers, however, will quote peak power. Generally, peak power of an electric motor is two to three times that of continuous power, so this can be misleading and a source of confusion. Make sure you know which it is you are using. For instance, an electric motor of 400 W continuous power may have peak power in the neighborhood of 1000 W, giving better torque and peak power than a 1 hp ICE (750 W peak.)


This is an interesting question for those trying to figure out how powerful an assist motor they need or want. These figures are from general consensus, having taken the statements of various experts and sources. It seems that an average adult in good shape can put out about 100 watts on a continuous basis. Perhaps about 200 to 300 watts continuously for an hour working up a sweat. Maybe 500 watts in a 30 second burst.

A bicycle racer can put out 600 watts continuously for an hour. This is about what Lance Armstrong puts out in order to do a one-hour time trial at an average speed of about 30 mph. A sprint racer can probably do over 1000 watts in short bursts. You also have to keep in mind that only very highly conditioned athletes are capable of this, and that bicyclists can sustain higher continuous high power output than most other athletes because they are being cooled very efficiently by a continuous blast of air.

This is all borne out empirically. A heavy electric bicycle with a 300 watt motor will go about the speed of a human-power only bicycle pedaled hard by a healthy adult (17 mph.)


Many people wonder if they can, for instance, add another 12V battery to their 24 V system and get a 36 volt system with greater power and speed. This is not actually as foolhardy as it sounds, though keep in mind that the motor controller will need to be changed or altered for the higher voltage, and the battery charger will also need to be changed. Do not attempt this unless you really know what you are doing

However, a high quality DC motor can generally sustain running at higher than rated voltage. In fact, many electric traction motors are designed to be run at different system voltages. Usually the continuous power ratings are adjusted to reflect the higher heat that will need to be dissipated.

Motor rpm on a DC motor is generally directly related to voltage, so increasing the voltage from 24 to 36 volts (50%) will also increase the no-load shaft speed by 50%. A gearing change may be needed to compensate for this.

The motor will produce more heat, so continuous current and therefore power should not be expected to increase by 50%.

Also remember that the batteries should all be identical, and adding a new 12 V battery to two older 12 V batteries, which may well have lost some charge capacity, could very easily cause all the batteries to fail sooner due to charge imbalance.


I will give a short practical answer rather than the theoretical formulas. There are many electric bicycles out there and the consensus of power and speed relation is fairly consistent.

Let us assume that you have taken good advice and found a motor of at least 300 watts and not more than 750 watts.

A 400 watt motor is generally used to propel a 50 lb. electric bicycle up to about 17 mph on flat ground. However, it will climb steep hills rather slowly. It might be re-geared for 10 mph top speed, in which case it will climb more efficiently. A typical compromise might end up being 13 mph. If you can hook up a 400W motor through a multiple gear system, it will perform better.

A 750 watt motor will propel a bicycle up to about 30 mph. It will climb better if geared to about 17 mph. A compromise might be to gear it for 22.

It really depends up the terrain you will using it on, your top speed expectations, how heavy your batteries are, how heavy the rider is, how much pedaling you expect to do etc.


You need to determine what top speed you want to travel on flat ground. You also need to know your wheel diameter and motor shaft speed.

THE 'UNGEARED SPEED' METHOD.  An easy and flexible way to ascertain gearing is to figure out how fast your vehicle would go if the the motor were hooked up directly to the drive wheel with no gearing whatsoever. We can call this the 'ungeared speed' and it's a very useful number to calculate and write down. 

With the ungeared speed determined, you can simply divide this by the top speed you desire for your PA, and you have your gear ratio.

So let's take an example and calculate our ungeared speed. For instance, say my electric motor goes 3000 rpm (no load maximum). I have standard 26" bicycle wheels - I measured the actual circumference of the rear tire to be 79.72". That is, each revolution of the wheel, the vehicles travels forward 79.72". The following is the formula used. Divide the top three terms by the bottom three terms and you will see that all the units cancel out to leave miles per hour.

(3000 revs/minute) x (79.72 inches/rev) x (60 minutes/hour)
(12 inches/foot) x (5280 feet/mile)

= 226.5 mph UNGEARED SPEED

So this is my ungeared top speed. (Apologies to all the metric users out there, however you get the idea.) 

A generic version of this formula:

(motor rpm) x (tire circumference inches)      =  UNGEARED SPEED MPH

Now I can establish and change gearing easily for this example:

For 20 mph top speed:  226.5 / 20  =  11.3:1 gearing

For 30 mph top speed:  226.5 / 30  =  7.55:1 gearing

Of course the actual top speed may well be slightly less. This is because a motor's rpm listed is the "no load speed" and the vehicle will have some load. However the actual load on flat ground at the sub 30 mph speeds we are considering is not that great, maybe a five or ten percent difference.

Also, I can figure out top speed for a given gear ratio. Say I have an 11 tooth belt pulley on the motor, and an 80 tooth pulley on the rear wheel. Divide the driven wheel by the drive wheel to get the ratio. Then divide the ungeared top speed by the ratio to find the geared top speed.

80 / 11 = 7.27: 1            226.5 mph / 7.27 = 31.2 mph top speed

If you don't have toothed sprockets or pulleys just divide the diameter of the driven wheel by the diameter of the drive wheel. If you have a 6" belt wheel being driven by a 2" belt wheel on the motor, the gear ratio is 6 / 2 = 3:1.


If you have more than one gear reduction, for instance a primary belt drive and a secondary chain drive, you should multiply the two together to get the total reduction. Do not add. For instance, a 5:1 primary reduction driving a 4:1 final reduction is 5 x 4 = 20:1.  It is not 9:1.


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