Chassis, Running Gear, Towing, and Hitches
This domain covers weight ratings, hitch setup, trailer brakes, wheel bearings, tires, and axles. Exam items are usually arithmetic you must do correctly: payload, tongue weight, and gross combined weight all matter, and the lowest rating in the chain governs the combination.
Ratings and hitches
- Know the terms: GVWR, GAWR per axle, GCWR, UVW, and cargo carrying capacity. Payload is GVWR minus UVW minus fluids and passengers.
- Tongue weight should be 10 to 15 percent of loaded trailer weight, with 12 to 13 percent a good target. A 7,000 lb trailer wants 700 to 1,050 lb on the ball.
- Hitch class rating must exceed both loaded trailer weight and tongue weight. Ball size must match the coupler exactly: 1-7/8, 2, or 2-5/16 inch.
- A weight distribution hitch returns load to the front axle and to the trailer axles. Measure fender heights before and after adjustment.
Running gear
- Repack bearings annually or every 12,000 miles. Follow the preload spec, fit new seals, use the specified grease. E-Z Lube axles still need annual inspection.
- Re-torque lug nuts to spec after 50 miles on a new or refitted wheel, with the wheel on the ground.
- ST tires are inflated from the load chart for the actual load, checked against the date code rather than tread depth alone.
Brakes and towing
- Set controller gain so the wheels nearly lock on gravel but do not lock on dry pavement. The breakaway switch needs a charged battery to work.
- Cross the safety chains under the coupler. The 7-way pinout covers ground, tail, left turn, right turn, brake, auxiliary, and reverse.
Worked example
A trailer has a 7,000 lb GVWR and the truck has 1,500 lb of payload. Driver and passengers weigh 350 lb, bed cargo 250 lb, and the hitch 100 lb, so 700 lb is used and 700 lb remains. Tongue weight at 12.5 percent of 7,000 is 875 lb, exceeding remaining payload by 175 lb. Options are lighter trailer loading, shifting cargo rearward within sway limits, or a different tow vehicle. Check GCWR as well as GVWR.
Traps
- Calculating tongue weight from dry weight instead of loaded weight.
- Using airbags or helper springs to mask an overweight condition.
- Overpacking bearings by hand until the seal is pushed out.
- Ignoring tire date codes on a low-mileage trailer.