What it the "Ball Bank Indicator" and what does it measure?

'What Is' type questions related to highway safety, accident reconstruction and vehicle simulation
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MSI
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What it the "Ball Bank Indicator" and what does it measure?

Post by MSI »

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 following is from Simulation Models of Vehicle Dynamics by McHenry from the TRB Circular "Beyond the Green Book"
  • The “discomfort factor" is representative of the resultant acceleration that an occupant experiences during a recovery maneuver (Fig.4 see below).
  • 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".
ball bank.jpg
ball bank.jpg (59.21 KiB) Viewed 1539 times
Sept 2021 update: replaced the figure with one at better resolution
      • NOTE: the lateral component of gravity is Sin(PHI)
        • where PHI is the vehicle roll angle/roadway slope angle
        • and there may be a difference between the vehicle roll angle and roadway slope due to suspension response of the vehicle, but generally approximately the same
      • NOTE2: for small angles, generally less than 15-20 degrees, the sine of the angle is approximately equal to the angle in radians
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MSI
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Re: What it the "Ball Bank Indicator" and what does it measure?

Post by MSI »

Article posted today by Wade in the NAPARS Facebook page called
  • Today's tool: Ball bank indicators.
    Also sometimes called "Slopemeters" (and made by a company of that name), Ball bank indicators (or bankmeters) are mentioned in this "curve speed selection" article from FHWA. A lateral acceleration that gets a 16 degree rise in a bankmeter is equivalent to TAN(16°) = 0.28g
    Chapter 3. Methods for Establishing Advisory Speed
We've also added some detail from that reference for quick reference:
  • The ball-bank indicator must have a precision of at least one (1) degree (i.e., provide readings to the nearest whole degree). Indicators with less precision (e.g., 5 degree increments) cannot be used with this method. The indicator should be installed along the center of the vehicle in a location that is easily accessed and in the recorder's field of view. The center of the dash is the recommended position because it allows the driver to observe both the road and the indicator while traversing the curve. The type of mounting apparatus needed may vary; however, the ball-bank indicator should be firmly mounted so that it cannot move while the test vehicle is in motion.
  • Ball-Bank Indicator Method determines the advisory speeds in the field using a vehicle equipped with a ball-bank indicator and an accurate speedometer. The speedometer should be check using a calibrated radar gun or other method. The simplicity of construction and operation of this device has led to its widespread acceptance as a guide to determine advisory speeds for changes in horizontal alignment. Figure 12 shows a typical ball-bank indicator.
    ball bank.png
    ball bank.png (90.05 KiB) Viewed 1840 times
    Figure 12 – Ball-Bank Indicator
    Photo of a mechanical device that measures a curve with a ball bearing that moves left or right in a manner similar to the bubble on a level.

    The ball-bank indicator consists of a curved glass tube which is filled with a liquid. A weighted ball floats in the glass tube. The ball-bank indicator is mounted in a vehicle, and as the vehicle travels around a curve, the ball floats outward in the curved glass tube. The movement of the ball is measured in degrees of deflection, and this reading is indicative of the combined effect of superelevation, lateral (centripetal) acceleration, and vehicle body roll. The amount of body roll varies somewhat for different types of vehicles, and may affect the ball-bank reading by up to one degree, but generally is insignificant if a standard passenger car is used for the test. Therefore, when using this technique, it is best to use a typical passenger car rather than a pickup truck, van, or sports utility vehicle. Also, the ball-bank indicator test is normally a two-person operation, one person to drive and the other to record curve data and the ball-bank readings, especially if advisory speeds are being determined for a series of curves.

    The Ball-Bank Indicator method requires two people in the vehicle and multiple runs through the curve to get the correct advisory speed. In addition, reading the ball-bank indicator to determine the maximum degree of lean can be subjective.
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MSI
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Re: What it the "Ball Bank Indicator" and what does it measure?

Post by MSI »

Wade added some more info on Ball Bank to the NAPARS Facebook Page as follows:
  • Quoting from an online article on the topic The Manual on Uniform Traffic Control Devices (MUTCD) 2009 edition, in section 2C.08, [5] determines the criteria for using a ballbank indicator as following:
    • 16 degrees of ball-bank for speeds of 20 mph or less
    • 14 degrees of ball-bank for speeds of 25 to 30 mph
    • 12 degrees of ball-bank for speeds of 35 mph and higher.
    By comparison, AASHTO’s “Green Book” [6] concluded that speeds on curves that do not cause driver discomfort are designated by ball-bank readings as shown below:
    • 14 degrees for speeds of 20 mph or less
    • 12 degrees for speeds of 25 and 30 mph
    • 10 degrees for speeds of 35 to 50 mph.
    To find their intended lateral acceleration (g), we take the tangent of the angle:
    • Tan(14°) = 0.249 g
    • Tan(10°) = 0.176 g
    At higher speeds, people are more sensitive to lateral acceleration, hence the guidance for even gentler curves. I think it's a matter of risk (though many drivers don't think of it consciously): The higher the speed, the higher the penalty if something goes wrong, so ease off as speeds increase.
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