# Stiction

Parts originally copied from Wikipedia.

Stiction is an informal contraction of the term "static friction" ("μs"), perhaps also influenced by the verb "stick".

Two solid objects pressing against each other (but not sliding) will require some threshold of force parallel to the surface of contact in order to overcome static cohesion. Stiction is a threshold, not a continuous force.

## Experiment to Demonstrate Stiction

Place a wooden block near one end of a long wooden board. Leaving the far end of the board on the floor, lift the end with the block until the slope of the board is sufficient for the block to begin sliding downward without being pushed. Then lower the board slightly.

Placing the block again at the top, it will not begin to slide on its own. However, it will begin and continue to slide if given a small initial push. The push adds the necessary force to overcome stiction. Once the block is moving, it no longer requires the larger force.

## Stiction at Work

### Stiction and Model Trains

Stiction can appear in the motor and drivetrain, as the amount of torque required to get moving is greater than that required to maintain motion. Digital Command Control's multifunction decoders use PWM or Pulse Wave Modulation to drive the motor. By application of full voltage to the motor in pulses at varying time periods, stiction can be overcome and low speed operations made realistic.

The Kickstart function can be used to tune the decoder's response during low speed operation.

### Stiction and Automobile Driving

Stiction is also the same threshold at which a rolling object would begin to slide over a surface rather than rolling at the expected rate (and in the case of a wheel, in the expected direction). In this case, it's called "rolling friction" or "μr".

This is why driver training courses teach that if a car begins to slide sideways, the driver should try to steer in the same direction as the slide with no brakes. It gives the wheels a chance to regain static contact by rolling, which gives the driver some control again. An overentheusiastic driver may "squeal" the driving wheels trying to get a rapid start but this impressive display of noise and smoke is less effective than maintaining static contact with the road. Many stunt-driving techniques are also done by deliberately breaking and/or regaining this rolling friction.

A car on a slippery surface can slide a long way if the driver "locks" the wheels in stationary positions by pressing hard on the brakes. Anti-lock brake systems use some means of detecting this dangerous situation automatically, then interrupt the braking, giving the tires a chance to re-establish the higher resistance of stiction many times per second in rapid succession. Anti-lock brakes can be much more effective than cadence braking which is essentially a non-automatic technique for doing the same thing, though human beings quickly adapt to the same level of risk.

### Stiction and Computer Maintenance

In the context of hard disk drives, stiction refers to the tendency of disk read/write heads to stick to the platters, preventing the disk from spinning up and possibly causing physical damage to the media. Some hard drives avoid the problem by not resting the heads on the recording surfaces.

Stiction is also known to cause read/write heads to stick the platters of the hard drive due to the breakdown of lubricants which coat the platters themselves. In the late 1980s and early 1990s as the size of hard drive platters decreased from the older 8" and 5.25" sizes to 3.5" and smaller, manufacturers continued to use the same calendering processes and lubricants that they had used on the older, larger drives. The much tighter space caused much higher internal operating temperatures in these newer smaller drives, often leading to an accelerated breakdown of the surface lubricants into their much stickier components. When the drive was powered off and would cool down (say at the end of the day when a user went home and shut off their PC), these now-broken-down lubricants would become quite viscous and sticky, sometimes causing the read/write heads to literally stick to the platter.

The common solution to this problem was the counter-intuitive move of taking the affected drive out of the host system, striking it gently, but firmly on the side against a desk or something as laterally as possible and then re-install it in the host system. This would break the heads free of the goop long enough to power the system back on, have the drive spin up and recover whatever data could be retrieved off it. While the data was retrieved, the machine would be left on constantly so that the heat from the drive's internals would keep the decaying lubricants in a liquid state.

### Stiction and Amateur Astronomy

Additionally, the term has come into use in amateur astronomy circles to describe a characteristic of Dobsonian mounts. These mounts can resist initial movement by the user, making it difficult to track an object in the sky. Breaking this resistance requires enough force to cause the observer to overshoot the object.