- I have neither driven nor worked on tractor-trailers IRL; I’m a gamer. Please correct any mistakes I have made.

Types of braking:

• Wheel brakes: hydraulic-fluid-operated or compressed-air-operated. Abbreviated as hydraulic brakes and air brakes.
  
• Trailer braking = trailer-wheel braking
  
• Retarders = transmission braking
  
• Engine braking = Jake braking

Wheel, retarder, and engine brakes exist on semi-trucks (aka tractors); only wheel brakes exist on trailers. Default ATS & ETS2 wheel brakes behave the same as hydraulic wheel brakes on real-life cars and small trucks: pushing the brake pedal simultaneously brakes each and every wheel on the vehicle (tractor & trailer).

IRL hydraulic wheel brakes don't serve tractor-trailers well.
1) Wheel brakes (both hydraulic and air) heat up quickly and fail because of the tremendous weight,
2) Air lines are suited for continually coupling and uncoupling trailers. Imagine doing that with hydraulic lines. (See comment by KKJ below.)
3) If a trailer with hydraulic brakes decouples from a moving tractor, the trailer rolls free until something stops it. Air brakes automatically self apply when the hoses uncouple or break.

(Air-brake technology originated in trains for several reasons including 1) continual coupling/decoupling and 2) air brakes automatically apply when air pressure is lost.)

IMO SCS includes hydraulic wheel-brake behavior as default so the games are playable by non-truckers. When players have learned basic game play, they can use advanced braking features, which greatly enhances immersion.

Great job, SCS !!

IRL both tractors and trailers have wheel brakes operated with compressed air; coupling a trailer includes connecting air lines between tractor and trailer.

To the gamer/driver the most obvious difference between driving with hydraulic brakes versus air brakes is pumping a hydraulic brake pedal is often desirable when stopping, but pumping an air brake pedal causes rapid air-pressure loss that locks the brakes (thus the warnings).

In the game, braking cancels cruise control.

If you enable air brakes in game, learn to use constant but varying pressure on the pedal; do not pump your brake pedal.

When a trailer is coupled to a tractor, tractor and trailer wheel brakes are applied simultaneously when the brake pedal is pressed. Trailer brakes may also be applied independently with a control in the tractor (assigned key); applying trailer brakes independently can prevent jackknifing by pulling the trailer straight if they're applied quickly enough in slippery conditions.

In the game, trailer braking cancels cruise control.

I assign the space bar to the trailer brake, and keep my finger near it when driving in slippery conditions especially when the trailer can start swinging around the tractor when tractor brakes are applied (e.g., curved, wet entrance/exit ramps). Some mods also provide slippery conditions (e.g., Grimes winter, promod's (in Iceland), Dalton and Elliot highway, . . . ).

Retarders are an optional component of transmissions. They’re used most commonly to reduce tractor-trailer acceleration down hill. Retarders are not powerful, yet they enhance a tractor’s ability to haul heavy cargo because retarders slow acceleration of heavy loads downhill without using wheel brakes. (Every time wheel brakes are applied, they get hotter; when they get too hot they fail (known as brake fade, which I sadly haven't experienced in SCS games). Wheel-brake avoidance keeps them cool and useful.)

Cruise Control:
- In the game, when set at lowest strength, retarders do not cancel cruise control. At more than lowest strength, they inhibit acceleration and prevent use of cruise control (a frequent new-player question).
- Also, when CC is set, and you accelerate past the setting, then release the accelerator, the retarder will engage to help slow the truck down to setting.

I typically use automatic retarder at lowest setting so it always helps control downhill acceleration.

Engine braking (EB) uses engine compression to powerfully and quickly decrease engine rpm; when the tractor is in gear, the drive axle(s) (not steering axles) are slowed. Truckers use Jake brakes (invented by Clessie Lyle Cummins around 1930 and manufactured by Jacobs Vehicle Systems) in several creative ways: search the internet to find out more. Engine brakes were invented because wheel brakes fail under heavy braking requirements like long downhill/mountain runs. Engine braking is exceptionally powerful!!

KEY ASSIGNMENTS
- "Engine Brake" - activates engine brake manually when Game Preference "automatic engine brake" box is unchecked
- "Engine Brake Toggle" - provides ability in-game to change between automatic and manual engine braking without needing to visit Game Preferences to check or uncheck "automatic engine brake"
- "Engine Brake Increase" - transmissions have 0-3 strengths depending on how they're coded
- "Engine Brake Decrease"

How EB use affects Cruise Control:
In the game, EB can be set to automatic or manual.
- automatic: EB engages when cruise control is not set and the throttle is released.
- automatic: EB engages when cruise control is set, you have accelerated past the setting, and the throttle is released; cruise control is not cancelled; engine braking helps reduce speed to the cruise-control setting.
- manual: EB engages when the assigned key is held down and the throttle is not pressed, whether or not cruise control is set.
- manual: if EB is engaged while cruise control is set, cruise control is cancelled.

I use engine braking selectively (automatically only when I expect paved and dry road conditions. Using engine braking during the wrong conditions can be catastrophic: experiment.

- EXHAUST BRAKES also exist on tractors IRL; I don’t know much about them except they are another type of braking system.

- I have neither driven nor worked on tractor-trailers IRL; I’m a gamer. Please correct any mistakes I have made.

- I always use real automatic transmissions in game (no shifter). I imagine using a shifter will make these advanced braking options even more fun. Search the intenet for engine braking for more info.

- IMO, game physics should be changed a bit (increasing tire slip, and simulating brake fading)* to truly appreciate the value of all these brake types. With increased tire slip, understanding how to use these various brake types becomes NECESSARY when driving slippery areas such as downhill in quarries during heavy rain, northern Iceland in the snow and ice (promods)., and Dalton Highway in the winter (ATS map mod).

* player-written mods exist that increase wheel slip. I (an amateur modder) have found no way to simulate brake fade.)