What it the "Ball Bank Indicator" and what does it measure?
Posted: Fri Jun 19, 2009 9:45 am
The term "Ball Bank Indicator" refers to an inclinometer that is used for the specific purpose of determining safe (uniform advisory) curve speeds for horizontal curves.
- It measures the overturning force (side friction), measured in degrees, on a vehicle negotiating a horizontal curve - whether it is an isolated curve, multiple "S" curve, or a ramp to/from a freeway.
- Typically used by the Department of Transportation and other advisory transportation agencies.
- Some FHWA research McHenry Software did with Jack Leisch & Associates (JEL) in the 80's was on highway curve design, etc and included an investigation of what the ball bank indicator actually measures.
- The “discomfort factor" is representative of the resultant acceleration that an occupant experiences during a recovery maneuver (Fig.4). The use in the evaluation of cornering maneuvers of a hard mounted accelerometer measurement of lateral acceleration of the vehicle does not include the effects that the vehicle roll angle has on the occupant. For example, a vehicle in a normal cornering maneuver rolls in a direction opposite to the turn (I .e., rolls positive, or right for a left hand turn & visa versa) and therefore the “discomfort” which would be experienced is greater than the calculated lateral acceleration (i.e., V2/R). Curves are super-elevated to reduce or reverse the magnitude of the vehicle roll angle to reduce the “discomfort factor" felt by the driver and occupants. Many of the earlier geometric design criteria standards were based on experimental measures of a ball bank indicator during cornering maneuvers which essentially measured the same resultant acceleration as the HVOSM calculated "discomfort factor: The 'discomfort factor- output of tbe HVOSM illustrated the problems associated with cross-slope breaks on highway curves in HVOSM Studies of Highway Cross Slope Breaks on Highway Curves, which was also prepared as a part of the JEL study. In that study the problems associated with cornering on an adversely super-elevated curve (i.e.. cross-slope break) were demonstrated by examination of the “discomfort factor." Subsequent recommendations were made to reduce excursions onto the shoulder breaks which may result in unacceptable levels of driver “discomfort".