Tires, Tires, Tires!

Tires have a significant impact on the performance of any vehicle. This is especially true for 4x4 vehicles and is, in fact, a critical issue for full-time, four wheel drive vehicles. The following information regarding tires is important for owners of vehicles with the following transfer case models:

BW4404 BW4405 BW4406 BW4410   
BW4411 BW4472 BW4481 NP/NV247
NP/NV136 NP/NV149 NP229  NP/NV249

These transfer cases make use of a viscous coupling. This internal component is critical to the operation of the transfer case and is sensitive to tire-related issues.

The viscous coupling is the transfer case component that transmits power from the center drive shaft to the front differential, and eventually to the front wheels. Inside the coupling there is a silicon gel that reacts to sudden changes in force. This characteristic causes the gel to nearly solid when exposed to the shear force caused by the input and output shafts rotating at different speeds. When the front and rear wheels turn at different RPMs, the shear force raises the temperature and viscosity of the silicon get inside the viscous coupler. Under these conditions, the silicon becomes nearly solid which causes plates inside the coupler to engage. The result is that power is transmitted from the center drive shaft to the front output (that is, the front wheels).

Once the front and rear wheels begin to rotate at relatively the same RPMs again, the gel reverts to liquid (making it less solid). The result is that the front output disengages and the vehicle is again powered primarily by rear wheel drive.


Tires – Tires – Tires
All four tires must be of the same manufacturer, make and model.
Replacement of all four tires at the same time, not just the front or rear tires, is necessary in order to keep the Transfer Case and its co-working parts running smoothly and is necessary to keep your warranty valid.
AWD and Full Time 4WD Vehicles require EXTRA Attention to Tire Size! All 4 tires must be NO MORE THAN ¼ inch Circumference Difference! Any more than that and ALL TIRES must be replaced! Even new tires MUST be measured! Incorrect size and/or uneven tire wear will damage the Viscous Coupler and/or the Differential and WILL VOID ANY AND ALL WARRANTIES!

Typical Service:
- Check inflation pressure at lease once a month (including the spare).
- Have the tires rotated every 6,000 Miles.
- Rotation time also serves as a good opportunity to have the tires balanced. Unbalanced wheels can cause rapid wear of shock absorbers and struts, and wheel balance can change as a result of normal tire wear. On full-time four-wheel drive vehicles, a difference of only ¼ inch between the outside circumference of the front and rear tires can cause extensive damage to the transfer case or center differential. Rotating the tires to keep their sizes equal is critical on these vehicles, and replacing all four tires at the same time, rather than just the front or rear tires, is highly recommended.
- Uneven or accelerated tire wear may indicate an alignment problem.
- Have your car’s alignment checked at least once a year.