Many drivers notice one peculiarity immediately after switching from a manual to an automatic transmission. At a traffic light or in a traffic jam, it's enough to release the brake pedal, and the car starts moving slowly forward, even though the accelerator pedal remains untouched. If reverse gear is engaged, the same thing happens in the opposite direction.
With a manual transmission, there is no such behavior in a similar situation: with the clutch depressed, the car remains stationary. In an automatic transmission, this phenomenon is called "creep."
The reason lies not in the accelerator pedal settings or in accidental car movement. This mode is directly related to the design of the automatic transmission and was intentionally incorporated by engineers.
What is creep
Creep refers to the ability of a car with an automatic transmission to start moving slowly on its own after releasing the brake, even if the driver does not press the accelerator pedal.
Typically, the speed of such movement is approximately in the range of 1 to 5 km/h. This is enough for the car to gradually move forward on a flat road, and sometimes even overcome a slight incline.
The effect is especially noticeable in heavy city traffic. The driver takes their foot off the brake — and the car smoothly picks up a small speed.
It is important to understand that creep is a specific feature of automatic transmission operation. Manufacturers did not accidentally allow such traction transfer — on the contrary, its characteristics are part of the overall car control concept.
The main reason is the torque converter
To understand the origin of creep, it is necessary to look at how the engine connects to the transmission.
In a manual transmission, this is handled by a friction clutch. When the pedal is pressed, it disengages the engine and transmission, completely stopping the transfer of torque. An automatic transmission uses a different device — a torque converter.
Its basis consists of two main elements:
- a pump impeller rigidly connected to the engine's crankshaft;
- a turbine impeller connected to the transmission shaft.
Between them is a working fluid — transmission oil.
While the engine is running, the pump impeller continues to rotate even at idle. It creates a fluid flow that acts on the turbine impeller. Thus, torque is transmitted from the engine to the transmission not through a rigid mechanical connection, but through the working fluid.
And here lies the key to the puzzle.
The torque converter does not have a full disengagement mode, analogous to a depressed clutch in a manual transmission. Even when the engine is running at minimum RPMs, the pump impeller continues to move oil. As a result, a small amount of torque is transmitted to the turbine impeller.
This then goes through the transmission to the wheels. That's why the car starts moving slowly after releasing the brake.
In other words, the engine continues to transmit a small amount of thrust even when the driver is not pressing the accelerator pedal at all.
Why a manual transmission stays put
In a car with a manual transmission, the torque transfer mechanism works fundamentally differently.
Here, the engine and gearbox are connected by a friction clutch. When its discs are pressed against each other, torque is transmitted from the engine to the transmission. When the driver presses the clutch pedal, the discs disengage, and the connection between the engine and gearbox is interrupted.
This results in two essentially different states:
- clutch released — engine transmits torque to the transmission;
- clutch depressed — engine and gearbox are disengaged.
Therefore, a driver stopped at a traffic light can depress the clutch and hold the car with the brake. At this moment, the engine continues to run, but its torque does not reach the wheels.
If the clutch is kept depressed and the brake is released, the car will remain stationary.
It is precisely this complete disengagement of the engine and transmission in a manual gearbox that a classic torque converter automatic transmission lacks, preventing the car from completely ceasing to move after releasing the brake.
Why is creep even necessary?
At first glance, creep might seem like just a side effect of the design. In reality, it provides quite specific benefits to the driver.
First of all, a small constant torque facilitates starting on an incline. When the driver moves their foot from the brake to the accelerator pedal, the car receives a small forward thrust. This partially compensates for the effect of gravity and reduces the likelihood of rolling backward.
This is especially useful in situations where there is another car behind.
The second important function manifests itself in city traffic jams. Constantly pressing and releasing the accelerator at a speed of a few kilometers per hour is inconvenient. Thanks to creep, the driver can control movement almost with just the brake pedal.
The algorithm is extremely simple:
- released the brake — the car started moving slowly;
- lightly pressed the brake — speed decreased;
- released the pedal again — the car continued to move.
This mode makes moving in heavy traffic smoother and more comfortable.
There is another reason directly related to the transmission design. Theoretically, engineers could create a mechanism that would completely disengage the engine and gearbox at idle. However, this would require complicating the design and could negatively affect the smoothness of the automatic transmission's operation.
Therefore, a small constant torque through the torque converter turned out to be an acceptable solution that simultaneously provides smoothness and ease of control.
Not all automatics behave the same way
However, it is incorrect to assume that absolutely any car with an automatic transmission will necessarily creep forward with the same intensity.
In cars with CVT variators and robotic gearboxes, the effect may be felt much weaker. This is especially characteristic of some robotic transmissions with a dry clutch. In such designs, the nature of movement can be additionally regulated by electronics.
A separate situation arises with modern hybrids and electric vehicles. There, the familiar sensation of creep is sometimes specifically simulated by software so that the driver gets a familiar car reaction when releasing the brake.
There is no physical necessity for such behavior there: the electric motor is capable of completely stopping torque transfer and starting movement when the control system dictates it.
Finally, relying on creep as a full-fledged way to hold the car on a slope is not advisable. On a long uphill or downhill, the forces acting on the car may be stronger than the thrust generated by the transmission at idle.
Therefore, in such conditions, the car must be held with the brake or the parking brake must be used.
Where the famous "creep" comes from
Thus, the movement of a car after releasing the brake on a classic automatic transmission is a direct consequence of the torque converter's operation. While the engine is idling, the pump impeller continues to move the transmission fluid, and through it, a small amount of torque is transmitted to the turbine impeller.
This torque then reaches the drive wheels through the gearbox, and the car begins to move slowly.
In a manual, the principle is completely different. Pressing the clutch pedal completely breaks the mechanical connection between the engine and the transmission. Therefore, the car can remain stationary even after releasing the brake — until the driver himself starts to release the clutch and transmit torque to the wheels.
It turns out that the "creep" familiar to owners of cars with automatic transmissions is not a malfunction or an accidental side effect. It is a natural result of the design of a classic automatic transmission, which also makes starting and moving in traffic jams noticeably more convenient.