Clinometer - (Tilt Detection): Notes

The clinometer identifies the vehicle's inclination and determines transverse and longitudinal acceleration.

Fig 1: Identifying Clinometer - (Tilt Detection)

Courtesy of BMW OF NORTH AMERICA, INC.

The clinometer consists of three air bubble floats with LED transmitters and photo transistor/ receivers. Two of the floats are positioned on opposing angles of 52° to the horizontal axis of the vehicle.

As the vehicle starts to rollover sideways the air bubble will move across the transmitter/ receiver circuit when the angel passes 52°. This will signal the electronics to deploy the roll bars. The critical angle for rollover on the E36 convertible is 62°.

Fig 2: Identifying Critical Angle For Rollover On Convertible

Courtesy of BMW OF NORTH AMERICA, INC.

The third air bubble float is positioned at an angle of 72° to the longitudinal axis of the vehicle. If the vehicle starts to rollover end to end, the sensor will cause deployment of the rollover bars.

If the vehicle slips sideways into an obstacle (i.e.: the curb) the transverse acceleration is retarded rapidly.

Fig 3: Identifying Third Air Bubble Float

Courtesy of BMW OF NORTH AMERICA, INC.

The effect of the vehicle hitting the curb will force the air bubble in the float downwards, before the vehicle starts to rollover. In doing so the air bubble can pass the transmitter/ receiver circuit and trigger the deployment of the bars.

This requires an acceleration force of at least 1.28 "G" lasting for approximately 80ms.

"G" SENSOR  - detects vehicle's loss of contact with the road surface

If the vehicle becomes airborne, and the rate of acceleration is 1 "G" or more, the effective weight of the vehicle is zero. At this point, the clinometers can no longer detect transverse acceleration with respect to the gravitational pull. In this situation, the "G" sensor will act as the triggering device for the roll bars.

The "G" sensor consists of a reed contact, magnet and spring assembly. As long as the vehicle is in contact with the road surface, the spring does not have enough tension to overcome the weight of the magnet and gravity.

Fig 4: Locating "G" Sensor

Courtesy of BMW OF NORTH AMERICA, INC.

However, if the vehicle becomes airborne, and weightlessness occurs, the spring will force the magnet up which opens the reed contact and triggers the deployment of the roll bars.

A time period of approximately .3 seconds with a "G" force of approximately 0.9 or less is required before the roll bars will deploy.

Fig 5: Identifying "G" Sensor Opening

Courtesy of BMW OF NORTH AMERICA, INC.