Loading and Weight Distribution +4British Columbia · CDL · English

How cargo weight is distributed affects vehicle handling and the life of tires, frame, springs, axles and bearings. A poorly distributed load can overload an axle or set of tires, stress the frame, cause permanent damage and steering misalignment. Improper loading can also make a load dangerously top-heavy.

• ✓A poorly distributed load can overload an axle, stress the frame and cause steering misalignment
• ✓Never place a very concentrated heavy load against the cab as it may bend the frame and overload front tires

Place a very concentrated heavy load near the rear, on its long side, partially over or just ahead of the rear axles for proper tire loading. Block the load securely to prevent forward sliding. Load evenly so equal weight is on all rear tires to prevent frame twisting and axle/bearing overload. Never place a load on one side (overloads springs/tires) or on the tailgate (severe strain).

• ✓Place concentrated heavy loads partially over or just ahead of the rear axles with longest side on the floor
• ✓Distribute equal weight on all rear tires to prevent frame twisting and axle overloading
• ✓Never load items on the tailgate as it strains equipment and can cause collisions
• ✓Securely block loads to prevent them from sliding forward

Weight distribution requirements vary by vehicle type. Five-ton freight trucks are designed for about 90% of cargo weight on rear axles and 10% on the steering axle. Cement mixers and dump trucks: about 70% rear, 30% steering. Tridem drive trucks must have combined steering axle weight of at least 25% of tridem axle weight (40% for twin steering). Trailers should carry about 45% on trailer axle, 45% on drive axle, 10% on steering axle.

• ✓Five-ton freight trucks: ~90% rear axle, 10% steering axle
• ✓Cement mixers/dump trucks: ~70% rear axles, 30% steering axle
• ✓Trailers: ~45% trailer axle, 45% drive axle, 10% steering axle
• ✓Tridem drive truck steering axle must be at least 25% of tridem axle weight (40% for twin steering)

The Commercial Transport Act sets limits for commercial vehicle lengths, widths, heights and weights. You must obey all Ministry of Transportation signs limiting dimensions or weight regardless of the licence or permit you carry. Temporary additional weight restrictions can be posted at certain times of the year.

• ✓The Commercial Transport Act sets limits for length, width, height and weight
• ✓Obey all dimension/weight signs regardless of licence or permit held
• ✓Temporary weight restrictions can be posted seasonally

Know the height of your vehicle and its load at all times. The maximum allowable height for a commercial vehicle is 4.15 m unless the driver has an oversize load permit. Highway signs before underpasses and tunnels give overhead clearances; overhead check bars may be installed to test clearance.

• ✓Maximum allowable height for a commercial vehicle is 4.15 m without an oversize permit
• ✓Know your vehicle and load height at all times
• ✓Highway signs before underpasses/tunnels give overhead clearance heights

The maximum legal width of a vehicle and its load is 2.6 m unless the driver has an oversize load permit. Certain items may extend up to 10 cm beyond the sides without counting toward overall width: anti-splash/spray devices, load securement devices, ladders, dangerous goods placards, glad hands and air connectors, electrical/hydraulic connectors, and clearance lights. Side mirrors may extend up to 20 cm; more than 20 cm counts in overall width. Loose hay/straw/fodder may be loaded to a maximum of 3.1 m overall width.

• ✓Maximum legal width of vehicle and load is 2.6 m without an oversize permit
• ✓Certain items may extend up to 10 cm beyond sides without counting in width
• ✓Side mirrors may extend up to 20 cm before counting in overall width
• ✓Loose hay/straw/fodder may be loaded to a maximum 3.1 m width
• ✓Side mirrors must always extend beyond your load even when oversize

Commercial vehicles are licensed according to gross vehicle weight (GVW). GVW is the combined weight of the vehicle and its load. GVWR (gross vehicle weight rating) is the manufacturer's rating of the vehicle and maximum load it should carry. Licensed GVW is the combined weight of the vehicle and maximum load it's licensed to carry or tow. The minimum licensed GVW of most commercial vehicles is 1.5 times the net weight of the vehicle.

• ✓GVW is the combined weight of the vehicle and its load
• ✓GVWR is the manufacturer's rating of vehicle plus maximum load
• ✓Minimum licensed GVW of most commercial vehicles is 1.5 times the net weight of the vehicle
• ✓Watch for maximum allowable weight signs when approaching a bridge

Always be aware of your vehicle's length and the length of your load, especially when negotiating turns on narrow roads and in alleys. Leave enough clearance to avoid striking poles, parked cars and buildings. The overall length for reefer vans or trailers may be extended to include the refrigeration or heating unit.

• ✓Be aware of vehicle and load length when turning on narrow roads and alleys
• ✓Reefer van/trailer length may be extended to include the refrigeration or heating unit

The simplest air brake system consists of a compressor with a governor, air lines, a reservoir, a brake pedal (foot valve), and foundation brakes. These components work together to store compressed air and convert it into braking force.

• ✓A compressor pumps air and a governor controls the compressor
• ✓A reservoir stores compressed air and the foot valve directs air to the brakes
• ✓Foundation brakes include brake chambers, slack adjusters, brake linings and drums or rotors

The air brake chamber converts the force of compressed air into strong mechanical force through the pushrod and slack adjuster. It consists of a flexible diaphragm clamped between two steel housings, plus a pushrod, plate assembly and return spring. When air is admitted, the diaphragm inflates and pushes the pushrod out.

• ✓The air brake chamber converts compressed air force into mechanical force via the pushrod and slack adjuster
• ✓Pushrod force is governed by air pressure and effective diaphragm surface area
• ✓When the slack adjuster is at about 90 degrees to the pushrod it provides optimal leverage

The foundation brake is the assembly at each wheel operated by the air brake system. The most popular type is the s-cam drum brake. Air pressure moves the pushrod, rotating the slack adjuster and s-cam, forcing brake linings against the drum to create friction and stop the wheel.

• ✓The s-cam drum brake is the most popular foundation brake type
• ✓Air pressure moves the pushrod, which rotates the slack adjuster and s-cam to press linings against the drum
• ✓Brake shoe return springs keep the linings away from the drum when air pressure is released

The governor stops the compressor from pumping air once a set pressure is reached (unloading stage) and restarts it when pressure drops. This controls system pressure within a working range.

• ✓Governors typically unload the compressor at about 125 p.s.i. (max varies 105–135 p.s.i.)
• ✓The cut-in/cut-out range should be approximately 20 p.s.i. (138 kPa)
• ✓The governor must restart the compressor if pressure drops below 80 p.s.i. (552 kPa)

Steel tanks called reservoirs store compressed air. A safety valve on the first reservoir protects against over-pressurization. The supply reservoir (wet tank) collects water vapour, oil and carbon that form sludge, so drain valves are used to remove it.

• ✓The safety valve (pop-off valve) is normally set to vent excess pressure at approximately 150 p.s.i. (1,034 kPa)
• ✓If the safety valve relieves pressure, the governor or compressor needs service by a qualified mechanic
• ✓Most manufacturers recommend draining reservoirs daily to remove sludge and water
• ✓Stay out of the direct path of compressed air when draining reservoirs

Pressing the brake pedal (foot valve treadle) applies the air brakes. It is a foot-controlled pressure regulator; you press farther, not harder, to apply more braking force. Holding it in position maintains constant pressure and automatically replenishes small leaks. Releasing exhausts application air to the atmosphere.

• ✓To apply more braking force you press the foot valve farther, not harder
• ✓Holding the treadle in one position keeps delivered air pressure constant and replenishes small downstream leaks
• ✓Maximum brake application cannot exceed the pressure in the reservoirs

When the foot valve is depressed, air flows from the reservoir to the air chambers, pressurizing them so linings contact the drums. When released, an exhaust port opens, air escapes, and return springs retract the pushrod, slack adjusters, s-cams and linings.

• ✓Applying the foot valve directs air to the chambers to contact linings with the drums
• ✓Releasing the foot valve opens an exhaust port so chamber air escapes to the atmosphere
• ✓Return springs move the pushrod and s-cam back and brake shoe return springs retract the linings

Dual air brake systems, used since the mid-1970s, use a dual foot valve that acts as two independent control valves operated by one pedal. This divides the system into primary and secondary sections, each with its own supply, delivery and exhaust, so a controlled stop is possible if one section fails.

• ✓The dual foot valve has primary and secondary sections; primary is closest to the pedal and often operates drive axle brakes, secondary usually operates steering axle brakes
• ✓Most dual systems use three reservoirs: a supply reservoir and two service (primary and secondary) reservoirs, each filled through a one-way check valve
• ✓If one section totally fails, the driver can still make a controlled stop with reduced braking power

The air brake chamber converts the force of compressed air into strong mechanical force through the pushrod and slack adjuster. The slack adjuster acts as a lever, providing leverage (force multiplication).

• ✓A type 30 chamber at 100 p.s.i. (690 kPa) develops a pushrod force of 3,000 pounds
• ✓The slack adjuster acts as a lever to multiply force, and also adjusts for brake wear
• ✓The air brake chamber consists of a flexible diaphragm clamped between two steel housings, plus pushrod, plate assembly and return spring

The foundation brake is the brake assembly at each wheel. The most popular type is the s-cam drum brake, where air pressure moves the pushrod, rotating the slack adjuster and s-cam to press linings against the drum.

• ✓Air pressure moves the pushrod, causing the slack adjuster to rotate the s-cam, pressing linings against the drum to create friction
• ✓The slack adjuster is used to adjust brakes to compensate for brake lining and drum wear
• ✓Brake shoe return springs keep the linings away from the drum when air pressure is released

The compressor takes in atmospheric air and compresses it, pumping it through an air line to a supply reservoir. It runs whenever the engine is running and operates in intake and compression strokes.

• ✓The compressor runs as long as the engine is running
• ✓The compressor must build pressure in the reservoirs from 50 to 90 p.s.i. within 3 minutes at fast idle (1,000–1,200 r.p.m.)
• ✓On the intake stroke air is drawn in; on the compression stroke the inlet valve closes and discharge valve opens to feed compressed air to the reservoir
• ✓Check the air filter and compressor oil supply regularly; check belt tension

The governor controls when the compressor pumps air. When enough pressure builds, the governor causes the compressor to unload; when pressure drops, it restarts compression.

• ✓Governors are usually set to unload the compressor at about 125 p.s.i. (max varies 105–135 p.s.i.)
• ✓The range between cut-in and cut-out should be approximately 20 p.s.i. (138 kPa)
• ✓Vehicle safety standards require the governor allow the compressor to resume compressing if pressure drops below 80 p.s.i. (552 kPa)

The foot valve (brake pedal treadle) applies the air brakes and acts as a foot-controlled pressure regulator, allowing you to select the application pressure needed for gentle or rapid stops.

• ✓You don't press harder for more braking force; you press farther to increase application pressure
• ✓Holding the foot valve in one position keeps the delivered air pressure constant
• ✓The maximum brake application will not exceed the pressure in the reservoirs
• ✓Releasing the foot valve exhausts application air through exhaust ports to the atmosphere

Air brake systems apply much greater force than hydraulic braking systems, which is needed to cope with the heavy loads of commercial vehicles. They are also more tolerant of small leaks that could cause failure in a hydraulic system, because the compressor generates more compressed air as needed.

• ✓Air brakes apply greater force than hydraulic brakes to handle heavy commercial loads
• ✓Air brake systems are more tolerant to small leaks because the compressor replenishes air
• ✓Air brakes are capable of stopping heavy commercial vehicles safely

Air can be compressed into a smaller space, increasing its resistance and creating pressure that converts to mechanical force to apply brakes. Pressure is measured as force per unit area, such as pounds per square inch (p.s.i.) or kilopascals (kPa).

• ✓10 pounds of force on one square inch equals 10 p.s.i. or 68.9 kPa
• ✓The greater the air pressure, the greater the force exerted to apply the brakes

The force at the wheel to stop is much greater than the force applied to the brake pedal. The slack adjuster acts as a lever, providing force multiplication (leverage), and also adjusts for brake wear. Heavier trucks and buses need more mechanical advantage than cars.

• ✓The slack adjuster acts as a lever to multiply braking force and adjusts for brake wear
• ✓Leverage is a form of force multiplication needed for heavy vehicles

Air chambers are made in a range of sizes to match axle capacity so no axle is under- or over-braked. A common Type 30 chamber has 30 square inches of effective diaphragm area and produces strong force at system pressure.

• ✓A Type 30 chamber applied with 100 p.s.i. (690 kPa) develops a pushrod force of 3,000 pounds
• ✓Air chambers range from Type 9 (9 sq in) to Type 36 (36 sq in) to match axle capacity
• ✓Normal stops usually use less than 20 p.s.i. (138 kPa) of application pressure

Many new air brake systems use long stroke brake chambers, which have a longer pushrod stroke than standard chambers. They can be visually identified by specific markings.

• ✓Long stroke chambers have a longer pushrod stroke than standard chambers
• ✓Long stroke chambers can be identified by square-shaped inlet ports and/or trapezoid-shaped name tags on a clamp bolt

The compressor takes in atmospheric air, compresses it, and pumps it to a supply reservoir. It is mounted on and driven by the engine (gears or a belt) and runs whenever the engine runs. It operates on intake and compression strokes like a car engine.

• ✓The compressor runs whenever the engine is running
• ✓All trucks use piston-type air compressors with one, two or four cylinders
• ✓Air leaves the compressor at over 204°C (400°F) at 120 p.s.i. (827 kPa)
• ✓The compressor must build pressure from 50 to 90 p.s.i. within three minutes at fast idle (1,000–1,200 r.p.m.)

Air brake systems use greater force than hydraulic systems, are more tolerant to small leaks, and can safely stop heavy commercial vehicles. A compressor generates additional compressed air as needed.

• ✓Air brakes use much greater force to apply the brakes than hydraulic systems, needed for heavy loads
• ✓Air brakes are more tolerant to small leaks because the compressor generates more air as needed
• ✓Air brakes generate more braking force than hydraulic brakes

Air can be compressed into a smaller space, increasing its resistance and creating pressure. This pressure is converted into mechanical force to apply the brakes. Pressure is measured in p.s.i. or kPa.

• ✓10 p.s.i. equals 68.9 kPa; 100 p.s.i. equals 690 kPa
• ✓The greater the air pressure, the greater the force exerted on a surface

Many new air brake systems use long stroke brake chambers, which have a longer pushrod stroke than standard chambers and can be identified by their shape.

• ✓Long stroke chambers have a longer pushrod stroke than standard chambers
• ✓Long stroke chambers can be identified by square-shaped inlet ports and/or a trapezoid-shaped name tag on a clamp bolt

Air chambers come in a range of sizes to match braking force to axle capacity. Even though system pressures are 100 p.s.i. or above, much lower pressure is used for normal stops.

• ✓Type 30 clamp type chamber has 30 square inches of effective diaphragm area and is most common on drive and trailer axles
• ✓Chambers range from Type 9 (9 sq in) to Type 36 (36 sq in) to match axle capacity
• ✓Normal stops usually use less than 20 p.s.i. (138 kPa) of application pressure

To prevent overheating, compressors are cooled by circulating engine coolant or by air cooling. Oil lubricates and helps cool the moving parts, usually from the engine oil supply. Clean air supplied through an air filter is required for proper operation.

• ✓Compressors are cooled by circulating engine coolant or by air cooling
• ✓Check that the compressor has sufficient oil supply for lubrication and cooling
• ✓The air filter must be checked regularly to ensure it is not clogged
• ✓Check belt tension: if it presses in more than double the belt width, adjust it

In a worst-case line rupture causing total loss of primary reservoir pressure, the secondary reservoir is protected by the one-way check valve. The low-air warning must activate when any reservoir falls below 60 p.s.i. Only the steering axle brakes apply, stopping distances are longer. If the secondary system fails, only rear axle brakes work.

• ✓The low-air warning system must activate when pressure in any reservoir falls below 60 p.s.i. (414 kPa)
• ✓With primary failure, only steering axle brakes apply and stopping distances are longer
• ✓If the low-air warning activates, stop immediately and do not proceed until repaired

Air pressure is unreliable and illegal for parking because any air leak could cause a rollaway. Regulations require parking force to be maintained by mechanical means and be unaffected by loss of air pressure. The most common is the spring parking brake; the second is the safety actuator.

• ✓It is illegal to park a vehicle using only air brakes
• ✓Parking force must be maintained by mechanical means and be unaffected by loss of air pressure
• ✓The spring parking brake is the most common type; the safety actuator is found on some buses and coaches

Spring parking brakes use a powerful coil spring (1,500 to 2,000 lbs force) in a chamber piggy-backed onto the service brake chamber, mounted on rear axles only. They apply and remain applied by mechanical spring pressure, not air pressure. Air compresses the spring to release; if air is lost, the spring applies the brakes.

• ✓Spring parking brakes are mounted on rear axles only, not steering axles
• ✓The coil spring can exert 1,500 to 2,000 pounds of force
• ✓Spring parking brakes are held off by air pressure and apply when air is lost
• ✓Spring parking brake assemblies should only be serviced by qualified personnel due to extreme spring pressure

Spring brakes are normally applied and released with the parking brake control valve (usually a yellow button) on the dashboard. Pushing in releases them; pulling out applies them. If air pressure falls below approximately 60 p.s.i., they may drag, and at 20 to 45 p.s.i. (138 to 310 kPa) they may automatically fully apply.

• ✓Spring brakes are controlled by the dashboard parking brake control valve (usually yellow button)
• ✓At 20 to 45 p.s.i. (138 to 310 kPa) the spring parking brakes may automatically fully apply
• ✓Below approximately 60 p.s.i. the spring brakes may begin to drag

Always release spring parking brakes before making heavy service brake applications. When spring brakes are applied (up to 2,000 lbs), a heavy service brake application adds air force (up to 3,000 lbs more, totaling 5,000 lbs). This compounding can damage slack adjusters, s-cams, mounting bolts, rollers, shoes and drums. Light applications under 30-40 p.s.i. are not harmful.

• ✓Always release spring parking brakes before making a heavy service brake application
• ✓Compounding adds spring force and air force, potentially reaching 5,000 lbs and damaging brake components
• ✓Light applications of less than 30 to 40 p.s.i. (207 to 276 kPa) to prevent rolling are not harmful

Most spring parking brake chambers can be manually released (caged) with a release bolt, used only by mechanics during repairs or to tow a vehicle after total air loss. Always block the wheels before caging. Once caged, there is no parking brake force at that wheel. Call a tow truck to move a caged vehicle.

• ✓Caging spring parking brakes should only be done in an emergency (e.g., to tow after total air loss)
• ✓Always block the wheels before caging spring parking brakes
• ✓Once a spring brake chamber is caged, there is no parking brake force at that wheel

In dual systems, a two-way check valve supplies the parking brake control with air from the reservoir with the highest pressure (called blended air). The valve has two inlet ports and one delivery port with a free-floating shuttle. This prevents spring parking brakes from automatically applying if one reservoir loses all pressure.

• ✓The two-way check valve draws air from the reservoir with the highest pressure (blended air)
• ✓It ensures spring parking brakes won't automatically apply if there is a total loss of air in either reservoir
• ✓Can be tested by draining the primary reservoir and checking secondary pressure doesn't bleed back

Used on many buses and highway coaches, safety actuators use a one-way locking mechanism instead of a coil spring, with two diaphragms (service and parking) and a separate parking reservoir. Pulling the dash control applies air and engages the lock. They cannot be compounded. They only apply automatically if the parking reservoir loses pressure.

• ✓Safety actuator parking brakes use a locking mechanism, not a coil spring, and cannot be compounded
• ✓If more than 4 p.s.i. (27.6 kPa) is lost from the parking reservoir, the parking brakes will not release without a heavy service brake application
• ✓Safety actuator brakes only apply automatically upon loss of parking reservoir pressure, not service reservoir pressure

Some dual air vehicles have an optional spring brake modulator that senses primary system pressure loss and exhausts air from the spring parking brakes in proportion to the service brake application, letting the driver control spring force to help the front brakes make a controlled stop. All vehicles must meet Canadian Motor Vehicle Safety Standards for emergency stopping.

• ✓A spring brake modulator senses primary system pressure loss and exhausts spring brake air in proportion to the service application
• ✓All vehicles must have adequate braking force even with a partially failed air system per Canadian Motor Vehicle Safety Standards

A Class 1 licence allows operation of semi-trailer trucks and all other motor vehicles or combinations of vehicles except motorcycles. The minimum age is 19. A Class 1 licence covers all lower commercial classes.

• ✓Class 1 covers semi-trailer trucks and all combinations except motorcycles
• ✓Minimum age for Class 1 is 19
• ✓Class 1 knowledge test includes all air brake material for that class

A Class 2 licence permits driving buses, including school buses, special activity buses and special vehicles. Trailers or towed vehicles may not exceed 4,600 kg, unless neither the bus nor trailer has air brakes. It also covers any Class 4 vehicle. Minimum age is 19.

• ✓Class 2 allows buses, school buses, special activity buses and special vehicles
• ✓Towed vehicles may not exceed 4,600 kg unless bus and trailer have no air brakes
• ✓Minimum age for Class 2 is 19

A Class 3 licence permits trucks with more than two axles such as dump trucks and large tow trucks, but not buses transporting passengers. Trailers may not exceed 4,600 kg unless truck and trailer have no air brakes. Also permits tow cars towing any weight, mobile truck cranes and Class 5 vehicles. Minimum age is 18.

• ✓Class 3 covers trucks with more than two axles (dump trucks, large tow trucks)
• ✓Trailers may not exceed 4,600 kg unless truck and trailer have no air brakes
• ✓Minimum age for Class 3 is 18; does not include a bus transporting passengers

Class 4 unrestricted permits buses with maximum seating of 25 (including driver), taxis, limousines, ride-hailing vehicles, and ambulances. Class 4 restricted permits taxis, limousines, ride-hailing vehicles (up to 10 including driver), ambulances, and special vehicles seating not more than 10. Both require minimum age 19.

• ✓Class 4 unrestricted allows buses with seating up to 25 including the driver
• ✓Class 4 restricted limits taxis/limos/ride-hailing to 10 persons including the driver
• ✓Minimum age for both Class 4 categories is 19

A heavy trailer endorsement (code 20) for Class 4 or 5 allows towing trailers exceeding 4,600 kg provided neither truck nor trailer has air brakes. A house trailer endorsement (code 07, formerly code 51) allows towing recreational trailers exceeding 4,600 kg provided neither has air brakes. Minimum age for both is 18.

• ✓Heavy trailer endorsement (code 20) allows towing over 4,600 kg only if neither unit has air brakes
• ✓House trailer endorsement (code 07) applies to recreational trailers over 4,600 kg with no air brakes
• ✓Minimum age for both endorsements is 18

You must hold a full-privilege Class 5 or equivalent licence before applying for a commercial licence, heavy trailer endorsement or house trailer endorsement.

• ✓A full-privilege Class 5 or equivalent must be held before applying for a commercial licence or trailer endorsement
• ✓Driving without the correct class of licence may void insurance coverage in a crash