Collision Investigation and Understanding Brake Failure

We often find in crash reports the phrase that “the brakes failed”.

But what does this mean, and how does an Accident Investigator/ Collision Reconstructionist go about establishing whether brake failure has been one of the contributing factors to the crash?

After the recent horrific crash at an intersection at Pinetown, we decided to approach well-known accident investigator Stan Bezuidenhout with questions to provide some clarity and insights on brake failure as a contributing factor to vehicle crashes.

We raised the following questions:

What are the most important differences between the braking system on trucks and the brakes on a normal passenger vehicle?

The most important difference between "car" and "truck" brakes include the following:

  1. Car brakes are essentially in a permanent state of non-engagement. There are no brakes until you apply foot pressure. This pressure is amplified by the brake booster (brake system components) and converted into pressure applied via two cylinders to a pair of brake "pads" that clamp down on the brake disk. Essentially, this means that there are no brakes until brakes are actively applied. The car brake system contains a fluid (brake fluid) that is used to transfer pressure from the operator's foot to the brake callipers.

  2. Truck brakes essentially consist of TWO systems: Park (or Spring) brakes and Service (or Foot) brakes. The truck brake system has a cylinder that contains a strong spring and a vacuum chamber (booster). This is connected to a push-rod which is, in turn, connected to an arm (slack adjuster) which twists a shaft with a cam (S-Cam on it). The S-Cam causes two brake SHOES to expand (open up) and contact with the inside of a brake drum. So, a truck brake system would be permanently ENGAGED if there was no external influence. A truck brake system works with a vacuum. If there is NO VACUUM, the springs would actuate the push-rod, which will lever the slack adjuster, turning the S-cam (shaft), engaging the brakes. When vacuum is applied, the push-rod (end) is "sucked back" and then the brakes are released. Then - every time a driver tries to "break" the vacuum would be released and the brakes would be applied. You can "essentially," say that the truck brake system (air brakes) work in "reverse" to car brakes.

When we hear that the “brakes have failed” - What does this actually mean?

  1. In the case of car brakes, this could mean a number of things, including:

    1. There is a "leak" in the brake pipes, causing brake fluid to escape (leak out) and this would mean that there is no longer enough fluid to transfer pressure.

    2. Seals could damage, resulting in some of the pressure "Escaping," reducing brake efficiency.

    3. The brake linings (brake pads) could become worn, requiring more and more pedal actuation to engage brakes.

    4. Brakes could overheat, resulting in poor brakes or even the brake fluid boiling or breaking down and reducing brake efficiency.

  2. In the case of air brakes, it gets a bit more complicated. The following things can lead to a "Failure:"
    1. If there is ANY fault (leak) on the vacuum system and the vacuum is lost, the spring brakes will "actuate" and stop the vehicle.

    2. If there is any fluid leak (bearing grease, etc), the friction surfaces can be compromised (made too slippery) and the vehicle could lose braking efficiency.

    3. If the Slack Adjuster is not set (or adjusted) properly, the push rod might not be able to push the slack adjuster far enough to result in actuation.

    4. If a booster is failing, operators sometimes remove the pipes, jacks off the brakes (this can be done) and seal the pipe so that the rest of the brake (boosters) can work; but this can lead to total brake failure due to overheating.

    5. If the S-Cam turns too far, it can "flip over" and it then stops having any effect on brakes, while "everything looks fine." This is very dangerous and renders the brakes ineffective.

As an expert witness in court - are there specific challenges from the prosecutor/defence lawyer you have come to experience with regards to testimony on brake failure?

One of the biggest issues with regards to brake failure is the actual towing and recovery. When tow companies want to tow a vehicle that was involved in a collision and the brake system has been compromised, they need to "jack off" all the brakes.

This then forces the brake system, to compress the spring (brake) and release the wheel brakes.

If this is done the original condition of the brakes cannot be checked effectively and therefore testimony becomes harder.

Also - if the truck (horse) is totally destroyed, the efficiency of the vacuum pump and regulators cannot be determined reliably. This means that SOME components of the brake system might remain unexamined - limiting the accuracy of testimony.

Which brake components are the most likely to “fail”?

In cars, it would be the brake pads (themselves), the master cylinder (main cylinder near the foot-brake that applies initial pressure) and the slave cylinder (the cylinder at the wheels that actuate the calliper) seals. But, as brake fluid gets older it also starts to deteriorate, and this also reduces braking efficiency.

In air brake systems the slack adjuster angles, vacuum (pressure) regulators and load sensors (not set properly), as well as worn brake linings, are the most common causes of failure.

When you are requested to investigate a crash with alleged brake failure - what do you specifically look for and what are the indicators that there has indeed been brake failure?

Drivers of cars must ALWAYS become alert and have their brakes seen to if:

  1. The brakes become "spongy."

  2. The brakes work fine, but "fade" and become less effective at the time (normally after increased use).

  3. They find that there is a "scraping" sound when they brake.

  4. The vehicle pulls to either side while they are braking.

  5. They have to "pump" the brakes to stop.

Drivers of trucks and buses must become alert and report faults when:

  1. They have to wait too long for the spring brakes to release (after starting up).

  2. The pressure (or vacuum - there is a gauge in most vehicles) fluctuates at times.

  3. They can hear the brake system "unloading" intermittently while they are driving.

  4. They smell or see smoke from any wheels or brake system while driving or stopping.

  5. They are unable to stop effectively at any time.

  6. Their trailer brakes lock up when they are loaded or partially loaded when they stop.

  7. They hear any mechanical knocking or banging sounds while driving or braking.

What would you suggest as the most important steps for the trucking company and passenger vehicle driver to do to avoid brake failure?

Drivers should have their vehicle tested and checked REGULARLY.

Drivers should report BRAKE SYSTEM ISSUES IMMEDIATELY (see those above) and PUT IT IN WRITING.

If you experience ANY issues with braking, hear any funny sounds, see any fluids run out anywhere (at wheels) or if you can see brakes are hot (steam, smoke, etc), REPORT IT or HAVE IT SEEN TO immediately.

How important is training on vehicle maintenance for the trucking company - van the accident investigator play an important role in preventative measures as well?

No person should be allowed to work on a vehicle brake (or any other safety) system who is not a qualified mechanic/technician.

What role can the accident investigator/truck brake expert play in crash prevention?

We conduct regular technical fleet audits for clients. This having been said, these are NOT (mere) roadworthiness checks.

We examine a vehicle completely and our evaluation is so strict that we even "fail" new vehicles.

As crash reconstruction experts, we look not only at the functional elements but also the theoretical/legal ones.

We look at any and all items that could, might, would or may (one day) contribute to a collision.

With our Technical Fleet Audits, we assign a fault gravity value for each element we are able to predict as a possible cause or factor in COLLISIONS.

From this, we produce a chart, showing the components or aspects that need IMMEDIATE care (vehicle must be removed from service), urgent (fault/element must be repaired as soon as vehicle returns), serious (Fault must be repaired at next service) or general (it should be attended to at the next major service).

By being made aware of all the elements and seeing a chart (with red indicators) also enables a fleet owner to IMMEDIATELY evaluate the quality of his/her fleet and to determine how urgently they need to act.

Our system is so effective that we have seen clients recall and immediately repair 84 vehicles in three days (working overnight).

By having us do an independent technical fleet audit and acting on our recommendations, our clients are able to show (in the very unfortunate event of a collision) that they are SO SERIOUS about road safety, that they have appointed an independent EXPERT to examine all their vehicles and that they took immediate steps to eliminate all possible risks.

Stanley S Bezuidenhout
Forensic Road Transport and Risk Expert
Crash Guys International

Also view:

Brakes/ Braking and Road Safety

Accident Investigation and Road Safety