SAE Tow Ratings - Tow Ratings Finally Pass the Sniff Test
Preposterous as it may seem, until recently there was no legitimate means of comparing the trailer-tow rating advertised by one truck manufacturer with claims issued by competitors. That created a game of leap-frog. With the introduction of every new large truck, the proud manufacturer would announce some new high in the number of pounds that could be hauled or towed. That would prompt one or more competitors to pause for a few months before announcing revised tow ratings for their products which would of course return their progeny to the top of the towing heap.
But after years of effort, the SAE's tow vehicle trailer rating committee is finally ending the farce. The SAE's Surface Vehicle Recommended Practice J2807 spells out in precise terms a procedure for determining two important ratings: the maximum permissible gross combination weight (GCWR) for a tow vehicle and its trailer and the maximum permissible trailer weight rating (TWR).
After three years of effort by the three domestic manufacturers and representatives from several of the Japanese brands, a standard was approved in April 2008 and scheduled for 2013 model year implementation. Some companies have already begin phasing in the more realistic J2807 tow ratings for their new models.
There are five engineering characteristics that strongly influence any tow vehicle's performance:
- The engine's power and torque characteristics.
- The powertrain's cooling capacity.
- The durability of the powertrain and chassis.
- Handling characteristics during cornering and braking
- The structural characteristics of the vehicle's hitch attachment area.
Standard J2807 spells out test procedures and performance requirements that must be meant for a manufacturer to assign a maximum tow rating to a particular vehicle. While various trailer configurations are suitable for these tests, the towed unit must provide a minimum specified frontal area starting with 12 square feet for a TWR below 1500 pounds, ranging to 60 square feet for a TWR exceeding 12,000 pounds. There are also specifications for how the trailer's load is distributed on its axle(s) and how the attachment tongue is configured.
One major change from past practice is what the SAE committee defines as Tow Vehicle Trailering Weight (TVTW). Unlike the past, a driver, a passenger, optional equipment purchased by at least one third of the customer base, and hitch equipment are now included in this calculation along with the base weight of the tow vehicle. Raising the TVTW figure automatically lowers the maximum permissible GCWR and TWR figures.
Acceleration Performance Requirements
The tow vehicle must meet these level road performance criteria to merit a particular TWR:
- Acceleration from zero to 30 mph in 12.0 seconds or less in vehicles with a single rear wheels.
- Acceleration from zero to 30 mph in 14.0 seconds or less in vehicles with dual rear wheels.
- Acceleration from zero to 30 mph in 16.0 seconds or less in vehicles with dual rear wheels and a GVWR over 13,000 pounds.
- Acceleration from zero to 60 mph in 30.0 seconds or less in vehicles with single rear wheels.
- Acceleration from zero to 60 mph in 35.0 seconds or less in vehicles with dual rear wheels.
- Acceleration from zero to 60 mph in 40.0 seconds or less in vehicles with dual rear wheels and a GVWR over 13,000 pounds.
- Forty to 60 mph passing acceleration in 18.0 seconds or less in vehicles with single rear wheels.
- Forty to 60 mph passing acceleration in 21.0 seconds or less in vehicles with dual rear wheels.
- Forty to 60 mph passing acceleration in 24.0 seconds or less in vehicles with dual rear wheels and a GVWR over 13,000 pounds.
Grade Launch Requirements
The tow vehicle must be capable of repeatedly moving from rest for a distance of 16 feet on a 12-percent grade in both forward and reverse directions. Five such launches must be accomplished within five minutes in each direction.
To merit a particular TWR, a vehicle must be capable of maintaining a minimum cruising speed while climbing the grade at Davis Dam on state roads 68 and 163 in Arizona and Nevada. This 12-mile-long run originating in Bullhead City, Arizona, involves grades that vary between 3- and 7-percent with an average over 5-percent. During this test, the minimum acceptable ambient temperature is 100-degrees F. and AC systems must be operating on the maximum cold setting with no recirculation and the blower at the highest possible setting.
Single rear wheel vehicles must be able to maintain an average of at least 40 mph on this grade. Dual rear wheel vehicles are required to maintain 35 mph or more here. Dual rear wheel vehicles with a GVWR over 13,000 pounds must maintain at least 30 mph.
To pass these hot-ambient-temperature, steep-grade challenges, there can be no vehicle component failures, no warning lamps, and no diagnostic codes alerting the driver. In addition, the tow vehicle cannot lose any engine coolant. The vehicle under test must be equipped with the lowest numerical axle ratio available from the manufacturer.
Standard J2807 specifies that an understeering handling attitude must be maintained up to at least 0.4g cornering without a weight distributing hitch. With a weight distributing hitch (which transfers vertical load from the tow vehicle's rear wheels to its front wheels), an understeering attitude must be maintained up to only 0.3g cornering.
The test vehicle and trailer must stay within a 11.5-foot wide traffic lane during stopping tests. The parking brake must be capable of holding the rig on 12-percent up and down grades.
Stopping distance requirements from an initial 20 mph without use of trailer brakes are:
- In 35 feet or less with a TWR of 3000 pounds or less and no trailer brake requirement.
- In 45 feet or less with a TWR of 3000 pounds or less and a trailer brake requirement.
- In 80 feet or less for TWRs above 3000 pounds.
To assure that the tow vehicle's structure is capable of towing a particular trailer load, standard J2807 specifies that no more than 5 degrees of permanent angular deformation at hitch attachment points is acceptable. Also, the highest experienced trailer hitch attachment force must be withstood for five seconds without significant loss of load (no structural deflection).
The SAE towing committee purposely defined the scope of this standard not to include brake fade and durability aspects related to the tow vehicle such as the endurance of chassis, powertrain, suspension, and brake components. Other SAE standards and each manufacturer's own internal requirements instead address these towing issues.
If all requirements specified in J2807 are met, the tow vehicle manufacturer may state the following: This model meets or exceeds the tow-vehicle trailering requirements of SAE International per SAE J2807. All manufacturers are strongly encouraged to use this test for tow ratings beginning with the 2013 model year. Some brands that have tired of the leap-frog game -- Toyota for one -- have already commenced use of this worthwhile trailer towing advancement.