These terms refer to the acceptable overload value of the steer, drive axles and respective suspensions temporarily encountered when lift axles are raised (off road condition). When engineering calculates these values, they account for the acceptable weight limit to occur at 5 MPH or less. We all know there is nothing to regulate this speed on site.
The generally accepted overload is 1.2- 1.5 times the axle and suspension nominal ratings.
To calculate the vehicle’s actual overload, determine ground loading per Local, State or Federal Bridge formulas to determine max GVW (Gross Vehicle Weight). Then determine how much weight will be applied to the steer/drives with the balance on the pusher/s and or tag. Either use the Z-Factor Consultation weight distribution calculators, or OE Sales Tools.
All OE’s calculate creep using different manners, so check with the specific Application Engineers to determine the actual values. Engineering will limit the weights on the chassis depending on their process. The chart above shows examples of creep ratings for common components. Detroit does not publish creep ratings for their steer axles, check with Engineering, or dealer, for details.
This factor must be considered when adding an aftermarket lift axle as well. Remember the weight of the lift axle/s becomes payload when they are lifted, so these add to the creep weight as well as the payload they were supporting.
There are no steering gears in the chart because they have no creep rating (see chart). This means that the published nominal value is the maximum weight allowed. If you spec the steering gear/s to match the steer axle, then it will be undersized for any site work where the lift axle is not on the ground. For example, if you have a single steering gear rated for 16K and a steer axle rated for 16K, but also have a pusher, then the steering gear is insufficient to handle the added creep loading, so steering can become “heavy” off road.
General steering gear ratings are included in the chart. Check with manufacturer for exact values. Other steering components may affect the rating as well.