Editing Manual transmission (section)

===Shafts===
A manual transmission has several shafts with various gears and other components attached to them. Most modern passenger cars use 'constant-mesh' transmissions consisting of three shafts: an ''input shaft'', a ''countershaft'' (also called a [[layshaft]]) and an ''output shaft''.<ref>{{cite book |last1=Bosch |first1=Robert |title=Automotive Handbook |date=2004 |publisher=Bentley Publishers |isbn=978-0-8376-0333-9 |page=741 |edition=6th |url=https://books.google.com/books?id=zsRHPwAACAAJ |access-date=10 March 2020 |language=en}}</ref>

The ''input shaft'' is connected to the engine and spins at engine speed whenever the clutch is engaged.<ref>{{cite web |title=How Manual Transmissions Work |url=https://auto.howstuffworks.com/transmission2.htm |website=www.howstuffworks.com |access-date=13 March 2020 |language=en |date=1 April 2000}}</ref> The ''countershaft'' has gears of various sizes, which are permanently meshed with the corresponding gear on the input shaft.<ref>{{cite web |title=Basic Anatomy – How The Manual Transmission Works |url=https://www.gotodobbs.com/blog/how-manual-transmission-works/ |website=www.gotodobbs.com |access-date=13 March 2020 |date=29 April 2017}}</ref> The gears on the ''output shaft'' are also permanently meshed with a corresponding gear on the countershaft; however, the output shaft gears are able to rotate independently of the output shaft itself (through the use of bearings located between the gears and the shaft).<ref>{{cite web |title=How It Works: Manual transmissions |url=https://driving.ca/auto-news/news/how-it-works-manual-transmissions |website=www.driving.ca |access-date=13 March 2020 |language=en |date=12 September 2018}}</ref> Through the use of [[Shaft collar|collars]] (operated using the ''shift rods''), the speed of the output shaft becomes temporarily locked to the speed of the selected gear.<ref>{{cite web |title=Manual Transmission Basics |url=https://www.edmunds.com/car-technology/manual-transmission-basics.html |website=www.edmunds.com |access-date=13 March 2020 |language=en-us |date=25 April 2001}}</ref> Some transmission designs—such as in the Volvo 850 and S70—have two countershafts, both driving an output pinion meshing with the front-wheel-drive transaxle's ring gear. This allows for a narrower transmission since the length of each countershaft is halved compared with one that contains four gears and two shifters.

The fixed and free gears can be mounted on either the input or output shaft or both. For example, a five-speed transmission might have the first-to-second selectors on the countershaft, but the third-to-fourth selector and the fifth selector on the main shaft. This means that when the vehicle is stopped and idling in neutral with the clutch engaged and the input shaft spinning, the third-, fourth-, and fifth-gear pairs do not rotate.

When neutral is selected, none of the gears on the output shaft are locked to the shaft, allowing the input and output shafts to rotate independently. For reverse gear, an idler gear is used to reverse the direction in which the output shaft rotates. In many transmissions, the input and output shafts can be directly locked together (bypassing the countershaft) to create a 1:1 gear ratio which is referred to as ''direct-drive''.

In a transmission for [[longitudinal engine|longitudinal engined]] vehicles (e.g. most rear-wheel-drive cars), it is common for the input shaft and output shaft to be located on the same axis, since this reduces the [[torsion (mechanics)|torsional forces]] to which the transmission casing must withstand. The assembly consisting of both the input and output shafts is referred to as the ''main shaft'' (although sometimes this term refers to just the input shaft or output shaft). Independent rotation of the input and output shafts is made possible by one shaft being located inside the hollow bore of the other shaft, with a bearing located between the two shafts.

In a transmission for [[transverse engine]]d vehicles (e.g., front-wheel-drive cars), there are usually only two shafts: input and countershaft (sometimes called input and output). The input shaft runs the whole length of the gearbox, and there is no separate input pinion. These transmissions also have an integral [[Differential (mechanical device)|differential]] unit, which is connected via a pinion gear at the end of the counter/output shaft.

<gallery caption="Gear selection in a constant-mesh transmission" mode="packed">
Image:Manual transmission clutch First gear.PNG|First gear (blue, to back)
Image:Manual transmission clutch Second gear.PNG|Second gear (blue, to front)
Image:Manual transmission clutch Third gear.PNG|Third gear (purple, to back)
Image:Manual transmission clutch Fourth gear.PNG|Fourth gear (purple, to front)
</gallery>
<gallery mode="packed">
Image:Manual transmission clutch Reverse gear.PNG|Reverse (green engaged)
Image:Manual transmission clutch Neutral.PNG|Neutral (all disengaged)
</gallery>