WinBFC is a Windows-based, stand-alone computer program that analyzes the braking/stopping performance and characteristics of a heavy truck equipped with air brakes.   This program allows for typical S-cam and the new generation air disc brake systems.  It does not include the logic to analyze wedge brakes or older disc brake technology. The equations used in WinBFC assume the brake is an S-cam type brake or an air-disc brake with a lever/multiplier arm.  The general concepts and workflow were described by Heusser (SAE 910126).  It is strongly suggested the user of WinBFC be familiar with this paper, specifically Revision B, available from the Society of Automotive Engineers Make sure to get Revision B of this paper.

Generally, the braking/stopping performance for a heavy truck involves the following:

  • Determine the coefficient of friction (COF), or mu value, for vehicle tires on roadway of interest
  • Determine the weight at each wheel position
  • Calculate the brake force available at each wheel, by multiplying the COF times weight on that wheel
  • Determine the brake parameters
  • Measure the pushrod stroke at 90 psi and ambient temperature (cold stroke)
  • If appropriate, adjust the stroke by applying equations which estimate the effects of heat (above 300 degrees F)
  • Calculate the dynamic stroke (stroke increase as a result of a long brake application)
  • Using air chamber pressure/stroke/force charts, determine the pushrod force from air chamber
  • Using the pushrod force, determine the brake force applied at the tire/road interface
  • Compare the attempted brake force to the brake force available (step 3 above), use the smaller value
  • Total the brake force for each wheel on the vehicle
  • Divide the total brake force by the total weight of the vehicle, resulting in the deceleration rate in g's.

To calculate the brake forces and vehicle performance, the following data are required for each wheel position (all dimensions are in inches, weights are in lbf):

  • Air chamber size
  • Slack adjuster length (S-cam) or Lever multiplier (air disc)
  • Cold stroke
  • Brake drum Diameter (S-cam) or Radius of pad center sweep arc (air disc)
  • Tire rolling radius
  • Weight on the wheel

In addition, there are data that generally are the same for each wheel position so it can be thought of as being a value for the vehicle. This data can be entered for the whole vehicle, but in the WinBFC program these data can also be changed specifically for each wheel position. This data includes:

  • Effective coefficient of friction
  • Air pressure applied to brake from the treadle valve (brake pedal)
  • Brake temperature
  • Brake lining coefficient of friction (mu)
  • ABS air pressure adjustment

Finally, when ABS systems are installed on the vehicle it is important to know whether there is a wheel speed sensor at each wheel location. Oftentimes, on a 3-axle tractor-truck there will only be one axle on the back of the vehicle which has an ABS sensor (there would be one on each side). For example, on a heavy truck with air bag suspension the rear axle (axle 3) typically has the sensor and axles 2 and 3 are controlled by the sensors on axle 3. On a heavy truck with leaf suspension, it is common for axle 2 to have the sensors controlling both rear axles. In WinBFC, the user can specify where ABS sensors are located.

Win-BFC is available for $399.  The Win-BFC installer can be downloaded here.  Once executed, the program will walk you through the installation and licensing process.