Words by Mike Levine, Mark Williams and Kent Sundling, Photos by Ian Merritt
40 mph to Zero Brake Test with 19,400 lbs Trailers
During the last HD Shootout, we performed 60-to-zero-mph brake tests to measure stopping distances from 60 mph with the trucks empty and with 2,000 pounds of ballast in their cargo boxes. Instead of repeating that test, this time we measured brake performance with the trailers attached.
Each truck was tested twice, side-by-side with the other trucks, in two tests. Tow/haul mode was on. The exhaust brakes on the Ram and GM trucks were enabled while towing. The Ford’s exhaust brake is automatically enabled when the truck is in tow/haul mode.
Because of the heavy weights involved, we tested stopping power from 40 mph to zero, but we changed things up a bit.
In the first round, the trucks were connected to the trailers via the 7-pin electrical connection, so the trucks’ integrated trailer brake controllers could manage the trailer brakes when we applied our foot to the service brakes inside the truck.
In the second round, we disconnected the electrical connection, so the truck brakes were forced to stop the entire rig — truck and trailer — without any assistance from the trailer brakes.
Why did we do this? The importance of testing trucks without the electrical connection to trailers demonstrates a common problem with 7-pin trailer plug connections. In the RV7-type connection, standard on all pickups today, the male-to-female connection isn’t locked tight. It depends on friction from a brass wedge sliding against a metal blade in 7 places to hold the plug in place. Over time, the plugs flex with truck movement, causing electrical arcing and voids that allow dirt and water in, which can lead to corrosion over time. Whether it breaks, corrodes or pulls away, if you lose connection from truck to the brake or ground wires in the trailer plugs, you lose the trailer brakes. This is why you should check your 7-pin connector often and regularly.
In contrast to our earlier, faster brake test with payload where the GMC Sierra 3500 excelled, this time the Ford F-350 had the best stopping power, followed by the Ram 3500. The Ford needed only 94 feet to stop compared with more than 100 feet for the Ram and Sierra.
Stopping distance grew dramatically when we disconnected the trailer connections, by an average of about 50 percent. The Ford still came to a halt in the shortest distance – 143 feet, about 20 feet sooner than the Ram (164 feet) and Sierra (165 feet).
Why did the Ford perform so much better in this brake test than the GMC and Ram trucks?
Even though Ford has the smallest brake rotors in the class (13.7 inches front and 13.4 inches rear compared with 14 inches for the GMC and 14.1 inches for the Ram), it appears to manage ABS more effectively with or without trailer attached. The lack of any "disconnected trailer brake" signal to the truck didn't seem to change what the yaw signal was to the Ford.
The GMC Sierra with trailer brakes connected seemed to give confusing signals to the ABS computer, not allowing full brakes as the truck slowed down, allowing it to roll forward to pass the stopped Dodge at slow speed.
Ford and GM use truck brake hydraulic pressure from the master cylinder to activate the integrated trailer brake controller, whereas the Ram’s trailer brake controller relies on an integrated accelerometer. This means Ford and GM don't require trailer movement to activate the brakes, while Ram measures the movement of the trailer to help manage brake application. All three brands’ integrated trailer brake controllers use accelerometer-based motion readings when the ABS computer takes over during hard braking.