7.2 Downgrades, Auxiliary Retarders, Bobtailing, and Winter Braking
Key Takeaways
- On downgrades, use lower gears as the main method of controlling speed and apply the brakes moderately or intermittently so they do not overheat; select the gear before starting down.
- A safe downgrade speed lets you stop at any time if a hazard appears, which often means driving slower than other traffic.
- Engine, exhaust, hydraulic driveline, and electric driveline retarders add braking control but are not for emergency braking and cannot meet sudden or large braking demands.
- Tractors with bobtail relays can cut primary service applications by as much as 80% when the trailer lines are empty, so an empty tractor needs extra following distance and gentle braking.
- A heavy load at the front of a trailer leaves the empty rear axles with the same application pressure but little weight, so the rear of the trailer can slide and swing.
Downgrades, Auxiliary Retarders, Bobtailing, and Winter Braking
Quick Answer: On a downgrade, gears are the main method of controlling speed. Choose the gear before you start down, and apply the brakes moderately or intermittently so they do not overheat. Travel at a speed from which you can stop at any time, which is often slower than other traffic. Auxiliary retarders (engine, exhaust, hydraulic driveline, and electric driveline) help control speed but are not emergency brakes. An empty tractor (bobtail) brakes very differently: bobtail relays can cut primary applications by as much as 80%. Uneven loads and winter roads demand extra care because traction, not brake power, decides whether the wheels lock.
Downgrades
Driving down mountain grades is a demanding skill. Alberta's guidance, from the course and the Commercial Driver's Guide:
- Get into the proper gear before descending. If you are in the right gear before the grade, you will not need to change gears on the way down.
- Use lower gears as the main method of controlling speed. Engine braking in a low gear holds the vehicle back without heating the foundation brakes.
- Apply the brakes moderately or intermittently to keep them from overheating. Riding the brakes lightly all the way down keeps them hot and invites fade. Pumping them wastes air (Section 3.3).
- Pick a safe speed. A safe downgrade speed is one that lets you stop at any time if you meet a hazard. Descending often requires slower speeds than other traffic, so do not let other drivers pressure you to go faster.
- Take extra care on unfamiliar grades and roads.
- Never pass another vehicle on a downgrade on a two-lane highway.
A common controlled-braking method. Many commercial driving manuals describe it this way: once you are in the right gear, let the vehicle reach your chosen safe speed. Make a firm application to bring it several km/h below that speed, then fully release so the brakes can cool while the gear and retarder hold you back. Repeat as needed. This matches Alberta's "moderately or intermittently" rule, and it keeps each brake application short and purposeful.
Signs, brake checks, and runaway lanes
- White signs are regulatory: they state requirements, not advice.
- Brake check pullouts carry information signs telling drivers what to inspect, and many also give information about the grade ahead. Use them to check your brakes, air pressure, and adjustment before you descend.
- Runaway lanes (escape ramps) exist for vehicles whose brakes have failed. Know where they are on the grade you are driving, and use one early rather than late if you lose control of your speed.
Auxiliary retarders
| Retarder | How it works | Notes |
|---|---|---|
| Engine retarder | Changes the exhaust valve timing so the valve opens at the top of the compression stroke, turning the engine into an air compressor that works against the driveline | Common on North American engines |
| Exhaust retarder | Closes off the exhaust pipe with a butterfly or sliding gate valve, like a stove-pipe damper, raising back pressure | Must be adjusted so back pressure stays within the engine's capacity; too much can float the exhaust valves open |
| Hydraulic driveline retarder | Rotor vanes pump oil into housing vanes, like a torque converter, resisting the driveline | Watch the oil temperature and turn it off when the oil gets too hot |
| Electric driveline retarder | An electromagnet resists drive shaft rotation, and heat vents through the rotor vanes and housing | Found mainly on transit buses, emergency vehicles, and waste trucks; activated by service brake applications |
Three limits to remember:
- Retarders help control speed but are not intended for emergency braking.
- They provide additional braking control but do not meet sudden or large braking demands. The service brakes still do that.
- The Commercial Driver's Guide says engine or transmission retarders should not be used where prohibited or under poor weather or road conditions. On a slippery road, a retarder working only the drive axles can make them lose traction.
Bobtailing
Bobtail means driving a tractor without a trailer. Highway tractors are built to carry heavy weight. Empty, with little weight on the drive axles, they can be dangerous even on good roads. On top of that, tractors with bobtail relays cut primary service brake applications by as much as 80% when the trailer air lines are empty, which limits drive-axle lock-up but greatly changes how the tractor stops. Standard advice for bobtailing:
- Allow extra following and stopping distance.
- Brake gently and early, and avoid hard applications.
- Slow down more for curves and slippery surfaces.
Brake balance and load distribution
The biggest braking challenge for combination vehicles is losing brake balance because the load is unevenly distributed. For example:
- A heavy load at the front of a trailer gives the drive axles good traction for pulling and braking.
- The brakes under the empty rear of the trailer receive the same application pressure as the drive axles, but without the load to hold them down.
- The result: the rear of the trailer can slide and swing.
The problem is magnified in multiple-trailer combinations. ABS can help maintain brake balance (Section 7.3). Loading evenly helps most of all.
Winter braking
- ABS or no ABS changes the technique. On icy roads, the way you apply the brakes differs with and without ABS. Know which you have (Section 7.3).
- Stay cautious even when the roads look good, especially when bobtailing.
- Tire chains: follow safe practices when installing them. Work off the travelled road, secure the vehicle, and chock the wheels.
- Parking in the cold: on a combination, keep the trailer charged and set only the tractor spring brakes so the trailer linings do not freeze to the drums (Section 4.2).
What does the Alberta course say about auxiliary retarders such as engine brakes?
Why does an empty tractor (bobtail) need extra care when braking?
A trailer is loaded heavily at the front and empty at the rear. What braking problem does the Alberta course warn about?