Additional Gearing

For vehicles that haul heavy loads there are two considerations for gearing: Startability and top end speed. When the equipment has been delivered you do not want to gear a vehicle that is gear bound below road speed limit, however when loaded these vehicles need to pull from a stand-still. 

To accomplish both you will need some form of additional gearing that provides a broad range of reduction. This can be accomplished out of most factories with a 2-speed rear axle or an auxiliary transmission. 

2-speed rear axle: Acceptable for GCWR up to around 160K. Includes a dash valve that switches between gear sets, you specify two rear axle ratios. The downside is cost, weight, and the absence of locking axles.

Auxiliary transmission: Additional 2-4 speed transmission multiplying gears. Requires driver skill since many units accept only 17,500 ft lbs. of torque.

If you are working with a heavy application and they do not want a 2-speed rear axle or auxiliary transmission, you might consider DT planetary hubs.  These are NOT factory installed or endorsed; however, it is another concept for gearing reduction.  They provide a 3 to 1 reduction at the hub and use an air shift to revert back to 1 to 1.  In essence, take the axle ratio X 3 to get the effective result.  For the DT reduction mode, the speedometer indicates the truck is doing 90 MPH, however the truck is only traveling 30 MPH… plus the odometer is racking up miles 3 times as fast.  

Here is a reduction example: A 4.11 rear axle ratio X 3 (reduction factor) would equal 12.33 ratio with DT hubs, as compared to an AT1202 auxiliary w/4.11 which would equal 8.38.  The APO1750-4C auxiliary w/4.11 would equal 9.74. The hubs are very effective.

Driveline speed should also be taken into consideration as this much reduction could cause vibration issues.  Additionally, the hubs would have to be removed to replace brakes.  You will not be able to increase the factory sales order GCW rating, this is strictly an aftermarket gearing reduction solution. And now...

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Believe It OR Not

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