'''原動機付自転車'''（げんどうきつきじてんしゃ）とは、日本においてエンジンの総排気量が50cc以下の、二輪車および三輪車のこと。一般に'''原付'''として知られる。

== 名称 ==
200px|thumb|right|自転車補助エンジン「カブ-F」
thumb|200px|right|自転車とは一線を画す性能をもつスクーター（ホンダ[[ホンダ・トゥデイ#スクーター|トゥデイ）]]
thumb|200px|right|ジャイロX
thumb|200px|right|YAMAHA Passol-L

自転車に小型のエンジンを付けた乗り物が原動機付自転車の起こりである。日本でも第二次大戦後大いに流行し、既存の自転車にとりつけるエンジンキットも市販されていた。大手オートバイメーカーであるホンダ等も、最初はこの種の製品を販売していた。これらのエンジン付き自転車（ペダル付きバイク）は、モペットと呼ばれており、日本国内では衰退しているが、西欧の諸国では、いまも製造販売されている。

一方、現在の原動機付自転車はむしろ、小型のコミューターとしての側面が大きい。スズキ・チョイノリのような特殊な例を除き、現在の市販車両の多くは自転車が持つ軽快な機動性よりも、乗り心地やホンダ・カブに代表される実用的な積載性と経済性、スクーターのように加速力や便利な機能（ヘルメット収納スペース、セルスターター、自動チョーク、自動変速機等）を追求している。またレジャーバイクなるものも存在し、代表的な市販車としてホンダ・モンキーは乗用車のトランクスペースに積載可能でありカブ系エンジンを搭載した超小型バイクである。バイク史の変遷をたどれば、1980年代のバイクブーム時代は100km/h近くに達する最高速を誇るいわゆる「ゼロハンスポーツ」が鎬を削った。その一方で日本では道交法から制限速度は時速30kmとされ、自動二輪車に比べ制限速度の制約が大きく、二人乗り禁止、交差点で二段階右折が必要とされる場合があるなど、実用面における原動機付自転車は自転車とオートバイの中間的な位置付け若しくはオートバイとは異なる位置付けにされることがある。現在の市販車は、速度上限装置を設けメーカー自主規制にて60km/h以上の速度が出せないような構造になっているのが一般的である。

このため、自転車の語を外した略称として'''原付'''（げんつき）という語がよく使われる。さらに自転車の俗称であるチャリンコを略したチャリと合わせて、「原動機付チャリンコ」の意味で、俗に'''原チャリ・原チャ'''と呼ばれることがある。報道では「ミニバイク」と呼ばれることが多い。また、三輪のものは'''原付三輪'''（げんつきさんりん）や'''スリーター'''と呼ばれホンダ・ジャイロなどがこれに当たる。

原動機付自転車は運用する法律によって排気量の定義が異なる。後述するように、道路交通法では50cc以下が原付であるものの、道路運送車両法などでは125cc以下を原付と定義しており、この場合、50cc以下を「原付一種」、50cc超125cc以下を「原付二種」と呼ぶ。（詳しくは小型自動二輪車参照）。

== 日本の法律上の定義 ==
日本の法律上の定義は道路交通法・道路運送車両法・道路法・高速自動車国道法等により複数の区分がある。

=== 道路交通法での原付 50cc以下 ===
'''50cc以下（ミニカーを除く）'''を'''原動機付自転車'''とする(道路交通法等)
# 「原動機付自転車 内閣府令で定める大きさ以下の総排気量又は定格出力を有する原動機を用い、かつ、レール又は架線によらないで運転する車であつて、自転車、身体障害者用の車いす及び歩行補助車等以外のものをいう。」（道路交通法第2条第1項第10号）
# 「道路交通法第二条第一項第十号の内閣府令で定める大きさは、二輪のもの及び内閣総理大臣が指定する三輪以上のものにあつては、総排気量については〇・〇五〇リツトル、定格出力については〇・六〇キロワツトとし、その他のものにあつては、総排気量については〇・〇二〇リツトル、定格出力については〇・二五キロワツトとする。」（道路交通法施行規則第1条の2）
# '''原付三輪'''（道路交通法施行規則第1条の2における「内閣総理大臣が指定する三輪以上のもの」）はさらに以下の要件を満たすものであり、それ以外はミニカーとなる。
#* 「車室を備えず、かつ、輪距（二以上の輪距を有する車にあつては、その輪距のうち最大のもの）が〇・五〇メートル以下である三輪の車及び側面が構造上開放されている車室を備え、かつ、輪距が〇・五〇メートル以下である三輪の車」（平成2年12月6日総理府告示第48号）

=== 道路運送車両法などでの原付 125cc以下 ===
'''125cc以下'''を'''原動機付自転車'''とする(道路運送車両法、道路法、高速自動車国道法等)
# 「この法律で「原動機付自転車」とは、国土交通省令で定める総排気量又は定格出力を有する原動機により陸上を移動させることを目的として製作した用具で軌条若しくは架線を用いないもの又はこれにより牽引して陸上を移動させることを目的として製作した用具をいう。」（道路運送車両法第2条第3項）
# 「道路運送車両法第二条第三項の総排気量又は定格出力は、左のとおりとする。<br/>一 内燃機関を原動機とするものであつて、二輪を有するもの（側車付のものを除く。）にあつては、その総排気量は〇・一二五リツトル以下、その他のものにあつては〇・〇五〇リツトル以下<br/>二 内燃機関以外のものを原動機とするものであつて、二輪を有するもの（側車付のものを除く。）にあつては、その定格出力は一・〇〇キロワツト以下、その他のものにあつては〇・六〇キロワツト以下
# 前項に規定する総排気量又は定格出力を有する原動機付自転車のうち、総排気量が〇・〇五〇リツトル以下又は定格出力が〇・六〇キロワツト以下のものを第一種原動機付自転車とし、その他のものを第二種原動機付自転車とする。」（道路運送車両法施行規則第1条）

right|thumb|A bike equipped with an after market electric hub motor conversion kit, with the battery pack placed on the rear carrier rack.

A '''motorized bicycle''', sometimes referred to as a '''light electric vehicle''' or '''LEV''', is a bicycle with an attached motor used to assist with pedalling. Generally considered to be a vehicle, sometimes as a motor vehicle or a class of hybrid vehicle, motorized bicycles are usually powered by electric motors or small internal combustion engines. Some can be propelled by the motor alone if the rider chooses not to pedal, while in others the motor will only run if the rider pedals.

Some early motorized bicycles were powered by batteries and better storage density, the electric motor has recently seen an increase in popularity.

Motorized bicycles are distinguished from motorcycles by being capable of being powered by pedals alone if required. The actual usage of the pedals varies widely according to the type of vehicle. Those known as mopeds mostly have pedals for emergency use or because of legal requirements and these are not normally used. Those known as power-assist bikes have the pedals as the main form of propulsion with the motor used to give a bit of extra speed, especially uphill. Many motorized bicycles are based on standard bicycle frame designs and technologies, although the modifications to the design to support motorization may be extensive.

In countries where there is a strong bicycle culture (notably in Asia), the motorized bicycle is particularly popular; in 1996 Shanghai had 370,000 motorized bicycles and 470,000 other vehicles.<ref>"China: Exhaustive effort to cut vehicle emissions", ''China Business Information Network'', 10 April, 1996. pg. 1, via Financial Times Information Limited.</ref>

==History==
<!--Reason: It may be confusing to readers, and it doesn't help that there is no reference for the first sentence in the section "Other Motorized bicycles". In fact, we include information on quadracycles and tricycles and the horseless carriage which is a bit of a divergence. Further references and clarifications are, I believe, needed to help clarify this idea. ...Sorry, I don't see a problem here sufficient to warrant this notice.-->

:''For history of the bicycle in general, see History of the bicycle''
:''See also Timeline of motorized bicycle history''

The two-wheeled pedal powered bicycle was first conceived in Paris in the 1860s.  By 1888 John Dunlop invented pneumatic tires and the chain drive making possible the safety bicycle, where the two wheels had the same diameter.

==Electric bicycle history==

In the 1890s, electric bicycles were documented within various U.S. patents.  For example, on 31 December 1895 Ogden Bolton Jr. was granted  for a battery-powered bicycle with “6-pole brush-and-commutator direct current (DC) hub motor mounted in the rear wheel.”  There were no gears and the motor could draw up to 100 amperes (A) from a 10-V battery.[http://www.google.com/patents?id=DIVIAAAAEBAJ&dq=552,271]

Two years later, in 1897, Hosea W. Libbey of Boston invented an electric bicycle () that was propelled by a “double electric motor.” The motor was designed within the hub of the crankshaft axle.[http://www.google.com/patents?id=ZiZEAAAAEBAJ&dq=596272]  This model was later re-invented and imitated in the late 1990s by Giant Lafree electric bicycles.
  
By 1898 a rear wheel drive electric bicycle, which used a driving belt along the outside edge of the wheel was patented by Mathew J. Steffens.  Also, the 1899  by John Schnepf depicted a rear wheel friction “roller-wheel” style drive electric bicycle.[http://www.google.com/patents?id=F2JHAAAAEBAJ&dq=627,066]  Schnepf's invention was later re-examined and expanded in 1969 by G.A. Wood Jr. with his .  Wood’s device used 4 fractional horsepower motors; each rated less than ½ horsepower and connected through a series of gears.[http://www.google.com/patents?id=BFhrAAAAEBAJ&dq=3,431,994]

thumb|390px|right|A ''Google Trends'' chart showing an increase in the use of the term "ebike". 

Torque sensors and power controls were developed in the late 1990s. For example, Takada Yutky of Japan filed a patent in 1997 for such a device.  In 1992 Vector Services Limited offered and sold an electric bicycle dubbed Zike.[http://www.zikebike.com/about.htm] The bicycle included Nickel-cadmium batteries that were built into a frame member and included an 850 g permanent-magnet motor. Despite the Zike, in 1992 hardly any commercial electric bicycles were available. It wasn’t until 1998 when there were at least 49 different bikes. Production grew from 1993 to 2004 by an estimated 35%. By Contrast, according to Gardner, in 1995 regular bicycle production decreased from its peak 107 million units.
Some of the less expensive electric bicycles used bulky lead acid batteries, whereas newer models generally used NiMH, NiCd and/or Li-ion batteries which offered lighter, denser capacity batteries. The end benefits usually varied from manufacturer; however, in general there was an increase in range and speed. By 2004 electric bicycles where manufactured by Currie Technologies, EV Global, Optibike, Giante Lite, Merida.

By 2001 the terms, E-Bikes, power bike, pedelec, assisted bicycle and power-assisted bicycle where commonly used to describe electric bicycles.  E-bike, according to Google, is a term that has increased in trend. This term generally referred to an electric bicycle which used a throttle. Pedelec is a European term that generally referred to an electric bicycle that incorporated a torque sensor and/or a power controller that delivered a proportionate level of assist and only ran when the rider pedaled. Finally, Assist Bicycle is the technical term used to describe such a vehicle and Power-Assisted Bicycle is used in the Canadian Federal Legislation, but is carefully defined to only apply to electric motor assist, and specifically excludes internal combustion engines.

==Other motorized bicycles==

The origins of the motorized bicycle can be traced back to the latter part of the 19th century when experimenters began attaching steam engines to stock tricycles and quadracycles. This moved into attempts to fit the newly-invented internal combustion engine (ICE) to the bicycle form.<ref>"Horseless Carriages", ''The Times'', 21 May 1896, pg. 6, via The Times Digital Archive</ref>

right|thumb|[[Félix Millet's 1897 motorcycle, showing the common ancestry of motorized bicycles and motorcycles. Note the radial engine built into the back wheel.]]

Development diverged into two distinct streams: motorcycles, which are powered solely by their engines, and motorized bicycles as defined above. The closeness of the two forms in early years is demonstrated by Félix Millet's machines of 1892/93 and on. These had both pedals and an ingenious fixed crankshaft radial engine built into the back wheel. Within a few years motorized bicycles and motorcycles were recognisably divergent, with for example early motorcycles being longer, heavier and with a markedly different riding position from that of a contemporary pedal cycle. Later, development forked again with the advent of mopeds, small motorcycles fitted with pedals that can be used as a starting aid but which cannot, practically, be ridden under pedal power alone. This development appears to have been largely in order to exploit ambiguities between the regulatory framework for bicycles, powered bicycles and motorcycles - in jurisdictions where pedals were not required to meet the legal framework they were often simply omitted on otherwise identical models.

left|thumb|Derny motor-pacing cycle (motor mounted in frame, drive via chain to rear wheel).

In the case of motorized bicycles, too, there were soon two parallel streams of development: motor assistance as an addition to existing machines, and purpose-built motor-assisted bicycles like the Derny and VéloSoleX, with stronger frames and sometimes with only token ability to be wholly human powered. In these cases some assert that the product is more formally a motorcycle or moped than a motorized bicycle, and some jurisdictions also take this view.

Modern motorized bicycles follow both trends, with conversions being applied by hobbyists as well as commercial manufacturers. Hub motors in particular facilitate after market conversion, being built into the wheel and not requiring modifications to the drivetrain or frame, as well as having a low centre of gravity. Converting bicycles or tricycles has proven useful for some people with physical disabilities such as arthritis. The strength of tricycles is that they will balance even while stationary, but some people find it harder to ride a tricycle and claim it lacks agility. Portability is also compromised compared to bicycles.

The modern electric bicycle is true to the concept of a pedal bicycle with assisting propulsion, being ridable without power. Batteries have finite capacity, which means that the hybrid human / electric power mix is much more likely to be emphasised than is the case with an ICE. Electric bicycles are gaining acceptance, especially in Europe and Asia, in response to increasing traffic congestion, an aging population and concern about the environment. electric or hybrid vehicles &ndash; is also increasingly common.

Motorized bicycles' popularity has waxed and waned largely in response to local regulatory requirements. For example, the French "vélomoteur" could be ridden by young riders without need for a license, making it very popular during the 1960s and 1970s.

Autocycle manufacturers were well established in countries such as Britain and Australia before the second world war, but the hiatus of the war appears to have set the market back, although the American bolt-on Whizzer continued until 1962. The motorized bicycle saw a resurgence of popularity in Britain during the 1950s and such bolt-on motors as the Cyclaid and the Cyclemaster motor wheel saw brief periods of immense popularity. The Cyclemaster, which was a hub motor which could be fitted to an ordinary bike,  started at 25cc (painted black), but later the size went up to 32cc (painted grey). Elsewhere in Europe the motorized bicycle continued to be popular. The Italian, Vincenti Piatti had designed a 50 cc engine for driving portable lathes and this was also used to in the form of the Mini Motore to power bicycles. Piatti later licensed the design to Trojan for production in Britain as the Trojan Minimotor. Production of The French VELOSOLEX began in 1946 and continued until 1988. After French production ceased, the VELOSOLEX continued to be produced in China and Hungary. In 2003 production ceased in Hungary. Today production continues in China and has restarted in France. Velosolex America is the company that markets the VELOSOLEX worldwide.

Currently there are several companies manufacturing aftermarket Internal Combustion (IC) motorisation kits for Bicycles. These include both 4-Stroke and 2-Stroke. Notably there was also a Compression-Ignition engine kit produced using an 18 cc variable head engine - this was made by Lohmann in Germany. 

Current manufacturers include Golden Eagle Bike Engines using a rack mounted belt drive, Stanton who use a rack mount with a chain drive geared transmission , and various similar kits using 49/60/70cc 2-stroke engines made to have the engine centrally frame mounted in a position echoing that of Motorbikes (again generally using chain drive, or in the case of Whizzer, a belt drive. Companies marketing the latter types include Dax, Spooky Tooth Cycles and Kings. These generally have a top speed of between  and  (using aftermarket tuning and higher gearing ratios).

==Naming and legal status==

The legal definition and status of motorized bicycles varies by jurisdiction. Legal terms for motorized bicycles include "Power Assisted Bicycle (PAB)" (Canada), MOPED, "Electrically assisted pedal cycle (EAPC)" (United Kingdom), or (commonly) "electric bicycle", frequently abbreviated as "ebike". In comparison some custom designs of electric bike have a range of up to  and a maximum speed of +.

While generally not known it is possible to register a Cyclemotor or motorized bike for legal use on the UK's roads. If the machine is from a known manufacturer such as Rudge or Francis-Barnett this is a fairly simple procedure. It becomes much more complicated if this is not the case with current law requiring an SVA (Single Vehicle Approval) test for each individual machine.