6.2 Brake Stroke, Adjustment Limits, and Stroke Indicators

Brake Stroke, Adjustment Limits, and Stroke Indicators

Quick Answer: A brake chamber delivers its best force in the first part of its stroke, and its force drops to zero as the push plate reaches the end of the chamber. Properly adjusted brakes work in the upper half of the available stroke on the course's force graph. Out-of-adjustment brakes work in the lower half, and you cannot feel the difference when driving. For manual slack adjusters, the optimum free stroke is 1/2 inch (13 mm), and 3/4 inch (19 mm) means readjust. Measured with a full service application, the maximum applied stroke is 1 3/4 in (45 mm) for a Type 20, 2 in (51 mm) for a Type 30, and 2 1/2 in (64 mm) for a long-stroke Type 30.

Stroke versus force: why adjustment matters

The student manual uses a graph of brake application force (vertical) against pushrod stroke (horizontal):

• At the left, the pushrod is fully retracted and the brake is released.
• As the stroke increases, force rises. Properly adjusted brakes use the upper half of the available stroke, the high-force part of the curve.
• As the linings wear, the stroke grows. Brakes that are out of adjustment use the lower half of the curve and apply with less force.
• As the push plate reaches the end of the chamber, application force drops to zero.

The dangerous part is that drivers may not notice the loss under normal conditions. You can "feel" that the brakes are good and still have them fail when more is asked of them: a heavy stop, a long downgrade, or a load that has heated the drums. Brake adjustment cannot be felt when driving, so it must be checked.

Heat makes an out-of-adjustment brake worse. Hot drums expand away from the linings, so the pushrod must travel even farther for the same application. Brake fade comes partly from this expansion and partly from the reduced friction between hot linings and drums. A brake that was near its limit when cold can run out of stroke when hot.

Free stroke (manual slack adjusters)

Free stroke is how far you can move the slack adjuster by hand (or with a pry bar) before the linings contact the drum, with the brakes released.

Automatic slack adjusters often feel loose when pulled by hand but are still within the required applied stroke. Do not judge an automatic adjuster only by how loose it feels. Section 6.3 explains how to maintain and confirm automatic adjusters.

Applied stroke: the enforcement method

At a Level 1 CVSA inspection, enforcement officers measure applied stroke:

1. With the wheels chocked and the spring brakes released, the brakes are held in a full service application. Partial applications do not push the pushrod far enough to give a true reading.
2. The pushrod's travel from the released position to the applied position is measured at each brake.
3. Each reading is compared with the maximum for that chamber type.

Notice the pattern: for these mid-sized chambers there is about 1/2 inch (13 mm) of reserve between a properly adjusted maximum stroke and the point where the chamber bottoms out. The maximum strokes are out-of-service standards, so a stroke at the limit is not "fine". It should be shorter. Brakes should not be too tight either, or they will drag when released.

Identifying long-stroke chambers. Long-stroke chambers are marked "LS" and are recognized by square delivery (inlet) ports. They have a longer limit, so misidentifying one leads to a wrong adjustment judgement. Long-stroke and standard chambers are not interchangeable.

Brake stroke indicators

Many pushrods carry a stroke indicator: knurling painted red, or green, yellow, or red tape, that shows an over-stroke condition when it appears outside the chamber. The Alberta course states that brake stroke indicators are required and must be retrofitted if missing. Canadian Motor Vehicle Safety Standard 121 has required them on air-braked vehicles manufactured on and after May 31, 1996.

How the adjustment rules are enforced

• Daily trip inspection: under the National Safety Code Schedule 1, a pushrod stroke of any brake beyond the adjustment limit is a major defect. The vehicle may not be driven until it is fixed.
• Roadside: officers use the applied stroke method above, and out-of-adjustment brakes are a leading reason for out-of-service orders.

Slack adjuster angle

When the brake is applied, the slack adjuster arm and pushrod should be close to a right angle (90 degrees). This is where the lever turns the most pushrod force into cam torque. As the stroke grows, the arm swings well past 90 degrees, and more of the pushrod's force is wasted. That is one more reason long strokes mean weak brakes.

Automatic slack adjusters at a glance

• An automatic slack adjuster that is out of adjustment is a sign of a problem, such as worn parts or a failing adjuster, and the vehicle should go to a repair facility.
• The course teaches an emergency adjustment procedure for getting the vehicle safely to a repair facility. It is not routine maintenance, and doing it routinely damages the adjusters (Section 6.3).
• Vehicles built with automatic slack adjusters cannot be retrofitted with manual slack adjusters.