Driveline Shunt
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Driveline shunt occurs when a
vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), wa ...
gives an abrupt jolt while coming on or off overrun or
freewheel Freewheel mechanism In mechanical or automotive engineering, a freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft. An ov ...
. It is caused when the
gearbox Propulsion transmission is the mode of transmitting and controlling propulsion power of a machine. The term ''transmission'' properly refers to the whole drivetrain, including clutch, gearbox, prop shaft (for rear-wheel drive vehicles), differe ...
or other transmission linkages do not immediately relay changes in engine output to changes in wheel speed. There is a very brief delay before the backlash is taken up in a sudden, abrupt manner.


Four scenarios

There are four scenarios where a shunt may occur: #The driver comes off the accelerator, the
engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...
quickly goes into overrun but there is a short delay before the vehicle feels as though it has been shunted forwards slightly. #The driver gets on the accelerator after the engine has been on overrun, there is a short delay before the vehicle feels as though it has been shunted backwards slightly. #The driver changes the
gearbox Propulsion transmission is the mode of transmitting and controlling propulsion power of a machine. The term ''transmission'' properly refers to the whole drivetrain, including clutch, gearbox, prop shaft (for rear-wheel drive vehicles), differe ...
direction to drive/reverse or vice versa with the
engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...
running (in an
Automatic Transmission An automatic transmission (sometimes abbreviated to auto or AT) is a multi-speed transmission used in internal combustion engine-based motor vehicles that does not require any input from the driver to change forward gears under normal driving c ...
or
Continuously variable transmission A continuously variable transmission (CVT) is an automatic transmission that can change seamlessly through a continuous range of gear ratios. This contrasts with other transmissions that provide a limited number of gear ratios in fixed steps. T ...
), there is a short delay before the vehicle feels as though it has been shunted backwards slightly. #The driver changes the
gearbox Propulsion transmission is the mode of transmitting and controlling propulsion power of a machine. The term ''transmission'' properly refers to the whole drivetrain, including clutch, gearbox, prop shaft (for rear-wheel drive vehicles), differe ...
direction to neutral (in an
Automatic Transmission An automatic transmission (sometimes abbreviated to auto or AT) is a multi-speed transmission used in internal combustion engine-based motor vehicles that does not require any input from the driver to change forward gears under normal driving c ...
or
Continuously variable transmission A continuously variable transmission (CVT) is an automatic transmission that can change seamlessly through a continuous range of gear ratios. This contrasts with other transmissions that provide a limited number of gear ratios in fixed steps. T ...
) but there is a short delay before the vehicle feels as though it has been shunted forwards slightly.


Reasons for occurrence

It can occur due to slack within automatic gearboxes, but it can also occur from slack in the other driveline linkages especially where the drive-line is long due to the
engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...
being the opposite end of the vehicle from the driven wheels. Due to the latter it can therefore occur in vehicles with a
manual transmission A manual transmission (MT), also known as manual gearbox, standard transmission (in Canada, the United Kingdom, and the United States), or stick shift (in the United States), is a multi-speed motor vehicle transmission (mechanics), transmission ...
. It has been known to occur even in very new and very high-end vehicles. With experience, most drivers simply allow for it and execute these torque transitions smoothly with finer control of the accelerator. Over time it is expected that engine management systems will increasingly do this on our behalf.


Engine surge

Driveline shunt should not be confused with engine surge in automatic vehicle transmissions where an engine "searches" for its idle speed whilst at very low road speeds, and causes unexpected surges in acceleration. This is a different phenomenon which can be mitigated by an experienced driver using
left foot braking Left-foot braking is the technique of using the left foot to operate the brake pedal in an automobile, leaving the right foot dedicated to the throttle pedal. It contrasts with the practice of using the left foot to operate the clutch pedal, lea ...
for such very low speed maneuvering.


See also

*
Automotive engineering Automotive engineering, along with aerospace engineering and naval architecture, is a branch of vehicle engineering, incorporating elements of mechanical, electrical, electronic, software, and safety engineering as applied to the design, manufactu ...
*
Drivetrain A drivetrain (also frequently spelled as drive train or sometimes drive-train) is the group of components that deliver mechanical power from the prime mover to the driven components. In automotive engineering, the drivetrain is the components o ...
*
Torque converter A torque converter is a type of fluid coupling that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the power ...
* Transmission


References

{{DEFAULTSORT:Driveline Shunt Driving techniques