Editing Straight-twin engine (section)

== Design ==
Compared with [[V-twin engine]]s and [[flat-twin engine]]s, straight-twins are more compact, a simpler design and cheaper to produce.<ref name="ICE_TnP" /> Straight-twin engines can be prone to vibration, either because of the irregular [[Firing order#Firing interval|firing interval]] present in 180° crank engines or the large uncountered reciprocating mass in 360° crank engines.  Inline-twins also suffer further from torsional torque reactions and vibration.<ref name="heavy" />

=== Crankshaft angle ===
[[File:Straight-twin engine with different crank shaft angles.gif|thumb|Animation with different crankshaft angles]]

The most common [[crankshaft]] configurations for straight-twin engines are 360 degrees, 180 degrees, 270 degrees,<ref name="ABM1911" /><ref name="MotorcycleWorks" /><ref name="feel" /> and less common are 90 degrees.

; 360 degrees
In an engine with a 360 degree crankshaft, both pistons move up and down at the same time. However, the firing interval is offset between cylinders, with one of the cylinders firing during the first crankshaft rotation and then the other cylinder in the following rotation. This set up results an even 360 degree firing interval unlike other crank configurations in inline twin engines. The 360 degree engines can use a single ignition system for both cylinders, using a [[wasted spark]] system.

The imperfect [[primary balance]] is as per a single-cylinder engine of equivalent reciprocating mass. Early engines attempted to reduce vibration through counterweights on the crankshaft; however, later methods also included [[balance shaft]]s and a separate weighted connecting rod. Compared with a single-cylinder engine, the more frequent firing interval (360 degrees compared with 720 degrees) results in smoother running characteristics, despite the similar dynamic imbalance.

From the 1930s, most British four-stroke straight-twin motorcycle engines used a 360 degree crankshaft,<ref name="JAE1971" /> since this avoided the uneven intake pulsing of other configurations, thus preventing the need for twin carburettors. In the 1960s, even though Japanese motorcycles mostly switched to 180 degree crankshafts for engines sized from 250 to 500&nbsp;cc, various smaller and larger engines continued to use a 360 degree crankshaft. Vibration was less of an issue for smaller engines, such as the 1965 [[Honda C92, CB92, C95 Benly|Honda CB92]] and 1979 [[Honda CM series|Honda CM185]]. Larger engines, such as the 1969 [[Yamaha XS 650]] and 1972 [[Yamaha TX750]], often used balance shafts to reduce the vibration.<ref name="balls" /> The later 1978–1984 [[Honda CB250N/CB400N]] engines also used a 360 degree crankshaft, as does the 1989 [[Yamaha XTZ 750|Yamaha XTZ750 Super Ténéré]]. The 2008 [[BMW F series parallel-twin]] motorcycles also use 360 degree crankshafts, with a third "vestigial" connecting rod (acting as a counterbalance) and a rev limit of 9,000&nbsp;rpm to reduce vibrations.<ref name="HolmstromBMW" /> In 2009 [[Fiat]] launched [[Multiair]] inline twin car engines that use 360 degree crankshaft which relied on balance shafts to reduce the vibrations.

; 180 degrees
In an engine with a 180 degree crankshaft, one piston rises as the other falls. In a four-stroke engine, the firing interval is uneven, with the second cylinder firing 180 degrees after the first, followed by a gap of 540 degrees until the first cylinder fires again. The uneven firing interval causes vibrations and results in a 'lumpy' power delivery. A 180° engine also requires a separate ignition system for each cylinder.

Perfect [[primary balance]] is possible with a 180 degree straight-twin engine; however, the design creates a [[Couple (mechanics)|rocking couple]] which requires use of a balance shaft to reduce the vibration. A 180 degree straight-twin engine has a [[secondary balance|secondary imbalance]] (similar to an inline-four engine); however, the lower reciprocating mass means that this often does not require treatment.

A 180° crankshaft engine suffers fewer pumping losses than a 360° twin, because displacement of the crankcase is relatively unchanged as the pistons move.

In the 1960s, Japanese motorcycle manufacturers favoured the use of 180 degree crankshafts, since the increased smoothness allowed higher [[revolutions per minute|rpm]] and thus higher power outputs. For example, the 1966 [[Honda CB450]] 180 degree crankshaft engine has a similar power output to contemporary British 360 degree crankshaft engines, despite having a smaller displacement of 450&nbsp;cc compared with 650&nbsp;cc.<ref name="WFMCutts" /><ref name="Turner_Threat" /><ref name="ClassicMac" /> Both the 1973 [[Yamaha TX500]] and the 1977 [[Suzuki GS series|Suzuki GS400]] had a 180 degree crankshaft and a balance shaft. Since 1993, most Honda straight-twin motorcycle engines use 180 degree crankshafts.

[[Two-stroke engine]]s typically use a 180 degree crankshaft, since this results in two evenly-spaced power strokes per revolution. The [[fundamental frequency]] of vibration is twice that of an equivalent single-cylinder engine; however, the amplitude is halved.<ref name="MotorBoatingOct1964" /> Two-stroke engines that do not use a 180 degree crankshaft include the 1972 [[Yankee (motorcycle)|Yankee]].

; 270 degrees
In an engine with a 270 degree crankshaft, one piston follows three quarters of a rotation behind the other. This results in an uneven firing interval where the second cylinder fires 270 degrees after the first, followed by a gap of 450 degrees until the first cylinder fires again. This is the same pattern as a 90 degree [[V-twin engine]], and both configurations have a similar 'pulsing' exhaust sound as a result. The pistons in a 270 degree straight-twin engine are never both stationary at the same time (as per a 90 degree V-twin engine), thereby reducing the net momentum exchange between the crank and pistons during a full rotation.

An imperfect primary balance is created in a 270 degree straight-twin engine, due to a combination of free force and rocking couple; a balance shaft is often used to compensate for this. The secondary balance of a 270 degree engine is perfect; however, the configuration does result in an unbalanced rocking couple.

The first production 270 degree straight-twin motorcycle engines were fitted to the 1996 [[Yamaha TRX850]] and [[Yamaha TDM]].<ref name="MotorcycleWorks" /><ref name="CW2007" /><ref name="feel" /> Later examples include the 2009 [[Triumph Thunderbird (2009)|Triumph Thunderbird]], 2010 [[Norton Commando#961 Commando|Norton Commando 961]], 2012 [[Honda NC700 series]], 2014 [[Yamaha MT-07]], 2016 [[Triumph Thruxton|Triumph Thruxton 1200]], 2018 [[Royal Enfield Interceptor 650|Royal Enfield Interceptor 650 & Continental GT]] and 2019 [[BMW F900R]].<ref>{{Cite web |title=The new BMW F900R and F900XR. |url=https://www.press.bmwgroup.com/canada/article/detail/T0302311EN/the-new-bmw-f900r-and-f900xr?language=en |access-date=2025-05-26 |website=www.press.bmwgroup.com |language=en}}</ref> This architecture is proving popular among manufacturers, which are upgrading models that were previously equipped with other engine types, such as the 2016 [[Honda Africa Twin]] (formerly a [[V-twin]]), 2023 [[Honda CB600F|Honda Hornet]] (formerly an [[Straight-four engine|inline-4]]) or 2023 [[Suzuki V-Strom 800|V-Strom 800]] (the older design being equipped with a [[V-twin]]).

; 90 degrees
In an engine with a 90 degree crankshaft, one piston follows quarters of a rotation behind the other. This results in an uneven firing interval where the second cylinder fires 90 degrees after the first, followed by a gap of 630 degrees until the first cylinder fires again.

Probably [[CFMoto]] on year [[2030s]] use this crankshaft

=== Main bearings ===
Each cylinder in a straight-twin engine has a separate [[crankpin]], unlike V-twin engines which can use a common crankpin for both [[connecting rod]]s. Most vintage British straight-twin motorcycle engines (such as Triumph, BSA, Norton and Royal Enfield) had two [[main bearing]]s.<ref name="BargerRide" /> Beginning in the late 1950s, most Honda straight-twin engines had four main bearings.<ref name="EveryHonda" /><ref name="FrankHonda" /> Subsequent straight-twin engines had four or occasionally three main bearings.<ref name="balls" />