Influencing the track….

Axle ratings are posted on the door label. These values represent the maximum capacity for the axle, based upon the lowest rating of the contributing components. At times, this nominal capacity published for any single component can be influenced by the assembly. This is commonly true of hubs.

Aluminum and iron hub rating are dependent on the wheel offset, both on the steer and drive axles. Aluminum steer axle hub ratings can also be influenced by presence of a lift axle. Typically, iron hubs on steer axles are not affected the same.

The list of components which effect the rear axle rating are the axle, hub/drum, suspension, tires and wheels. Any one of these can reduce the rating. The key to overall rear axle ratings is of course the axle, but more importantly, iron or aluminum hubs coupled with wheel offset.

When specifying wide base wheels/tires (Super Single) on the drive axle, the offset makes a tremendous difference. It is important if changing wheels/tires on an as-built chassis to take these into considerations. The data below identifies the hub ratings for tandem drive axle sets.

• Aluminum using a wheel with 2-inch offset = 38,000 lbs.

• Iron using a wheel with 2-inch offset = 40,000 lbs.

• Aluminum using a wheel with 0-inch offset = 46,000 lbs.

• Iron using a wheel with 0-inch offset = 52,000 lbs.

Dip, but hold the chips

Trailer angle dip refers to the point when the trailer top corner hits the top of the sleeper. To assist with envisioning this, think of a tractor being towed by a wrecker, while the trailer was still coupled to the tractor. The squeeze effect presented at the top of the semi-combination junction represents what we refer to as dip angle.

Dip angle must be calculated to ensure that on undulating ground there will be no interference.

The inputs are influenced by the trailer design, coupled with the height of the day cab (with or without roof fairing), or sleeper height (with or without roof fairing). The taller the roof heights and the smaller the trailer gap, the smaller the actual clearance angle becomes.

But this is not limited to trailer characteristics, the load also can be a consideration. Loading a flatbed can depend on where the load is positioned on the trailer, and how tall it is at the corners. If the load measures 13’ 6” (from the ground), then it is treated much like a van trailer. When spec’ing flatbeds, it is important to provide negative 5th wheel settings to allow for loads that are positioned forward of the trailer nose. This loading style is frequent when weight distribution is not a concern.

Engineering assumes a 5-6 degree angle for calculating Dip Clearance. The tangent of the angle multiplied by the load/trailer height above the 5th wheel plate will give the inches of clearance you will experience. Any positive number provides that you will have clearance.

Avoid sticker shock

It is important to have a sicker in the cab door jamb that indicates this vehicle complies with California and U.S. emissions as opposed to just U.S., if the vehicle will be registered in California. The engines are no different, regardless which state they operate. It is just much easier to register a vehicle in California with this identified on the vehicle.

If you are building a stock vehicle, it doesn’t hurt to chose California Registry so that the sticker denotes the vehicle meets regulations for that state. Only the label changes, so this provides a level of flexibility to the customer. Some OEs may provide the verbiage about California compliance as standard, be certain to verify.

The CARB sticker is for idling, do not confuse it with California Registry compliance. If your customer needs to idle more than 5 minutes, several states require a CARB compliance sticker on the side of the vehicle. It can be added after build but is a nuisance plus several hundred dollars. As with California Registry, nothing changes about the vehicle, its only an identifier.

See the link for regulations on emissions and how they drive today’s offerings.

The proposed standards would become 2.5% more stringent every year from model years 2021 to 2027. The proposed program would reduce CO2 emissions and fuel consumption for these vehicles by about 16% beyond Phase 1 when fully phased in.

https://www.nhtsa.gov/staticfiles/rulemaking/pdf/cafe/Detailed-fact-sheet.pdf