FRONTLINE
FIRE DEPARTMENT TRAINING INC.

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TANDEM AXLE TRUCKS - SLIPPERY ROADS

With slippery road conditions in winter time, comes the problem of controlling tandem axle fire apparatus. Unless your department’s tandem axle apparatus are equipped with ABS sensors on each of the drive axles, it is still possible to get a lock up condition which could put the apparatus into a skid causing the driver to over re-act and apply the service brakes making the condition even worse. Understanding how the ABS works on a tandem axle vehicle is very important to the vehicle’s safe operation.
Most tandem axle trucks, including fire apparatus, have ABS sensors on only one of the rear axles. If the driver engages the retarding device on the truck to slow the vehicle without the use of the service brakes, the driving wheel (yes, the single driving wheel) could lock up and put the apparatus out of control unless that axle has ABS sensors on it. Power from the engine always goes to the wheel with the least amount of traction because it is easier to turn. By the same degree the engine’s braking force will always go to the wheel with the least amount of traction because it is easier to slow. If that wheel is on an axle without the ABS sensors it will either spin in a power situation or lock up in a retarding situation. By engaging the inter-axle lockers (not the differential lockers) the wheel on the axle without an ABS sensor is now mechanically connected to the opposite wheel on the axle that does have an ABS sensor and is now prevented from locking up because the ABS system automatically disables the retarder until all wheels are turning at the same speed again.
Any time that road conditions are less than favourable (dry pavement) always engage the inter-axle locker on a tandem or triple axle truck to prevent wheel spin or lock up. Engaging the inter-axle lockers also splits the load for one wheel to slow down or power up by one half thereby making it easier for the tires that are slowing or pulling the weight of the apparatus to do the job.

Posted on 14 Nov 2014
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