Due to the transfer of weight forward during a hard brake event, the trailer axle unloads by...

Due to the transfer of weight forward during a hard brake event, the trailer axle unloads by several thousand pounds. The drive axles behave similarly but by about half the rate of the trailer. The steer axle, remarkably, assumes most of the transferred weight and can load the axle and the tires briefly up to 18,000 or 19,000 pounds. 

Photo: Jim Park

I consider brakes the single most important system on a truck. Yet few drivers or even fleet maintenance personnel have ever had the opportunity to see what brakes can really do. I had such an opportunity recently and came away with huge respect for a system many of us take pretty much for granted.

Earlier this summer, Meritor invited me to spend a couple of days putting various combinations of air brakes through their paces under controlled conditions on a test track stopping pad. It was a rare privilege to do this, and it’s something I wish every driver could experience.

The purpose of the exercise was to compare the feel and performance of drum and disc brakes, and drums and discs in different configurations on the tractor. We tested three combinations, starting with a full-drum setup featuring Meritor’s Q+ RSD (reduced stopping distance) drum brakes on the tractor axles. Next was the all-disc combination, with Meritor EX+ air disc brakes on the steer and drive axles. Finally, we tested a split setup, with the EX+ discs on the steer axle and the Q+ RSD brakes on the drives. The trailer was equipped with 7-inch drum brakes, type 30 chambers, and Meritor’s bread-and-butter MA212 friction.

I went into this exercise expecting the disc brakes to outperform the drum brakes in most respects, but I came away with a different opinion. Their performance was nearly equal in all respects except the fade tests. The drum brake stopping distance did increase as the brakes got hotter, but by a lesser margin than I had expected.

Mark Ugo, senior test engineer for brakes at Meritor, was in the cab with me during the test runs, explaining what was occurring in each case. Thanks to Mark, I learned much about brakes and braking in the few hours we spent on the track.

The trucks used for this test were off-the-shelf Freightliner Cascadias with the company’s...

The trucks used for this test were off-the-shelf Freightliner Cascadias with the company’s standard brake offerings. There was nothing special about the brakes used in the test.

Photo: Jim Park

75,000 pounds full stop

The tests consisted of a straight run into the stopping area in a standard 12-foot-wide lane. A pair of orange cones marked the starting point where I was supposed to apply the brakes. Another pair of cones marked the spot where I could come to a stop. A formal FMVSS 121 brake test is done under much stricter conditions with professional test drivers.

We started each series of tests with five low-energy stops from 35 mph with a light to moderate application pressure, maybe 10-20 psi — the kind of stop most drivers would make on a city street. Predictably, there was no discernible difference in performance or handling between the three configurations. The drum and disc trucks stopped in the same distance, give or take a few feet, but that probably had more to do with the way I applied the brakes and when I applied them. I was just eyeballing the start marker.

The stops done with disc brakes on the steer axle had a more automotive feel to them and applied more “smoothly.” Drum brakes, on the other hand, applied a bit more aggressively at first because of their self-energizing nature. At moderate speeds and with fairly gentle applications, the difference between the discs and drums was barely noticeable.

From there we went to full pressure applications (100-120 psi) at 35 mph. Here, the difference between discs and drums was more obvious. For one thing, we started to get some ABS activity on the drive axles of both the disc- and drum-equipped trucks.  

At some point, a sufficient amount of torque will lock up the wheel regardless of the type of brake producing it. With antilock braking systems, as soon as a wheel locks, the system releases the pressure at that wheel, allowing it to rotate again before immediately reapplying the brake. If lockup reoccurs, the ABS releases the brake once again.

“The amount torque required to lock a wheel is a moving target dependent on the friction between the tire and pavement, which is further dependent on the tire tread (lug or rib, low-rolling resistance or traction, tread depth, etc.), the weight on the tire, the coefficient of friction of the road surface, and of course the condition of the brakes,” Ugo explained. “Very generally speaking, application pressures in the 60-75 psi range are needed to lock a drive or trailer wheel and trigger an ABS event on dry pavement.”

We had an old-fashioned analog accelerometer on the cab that showed the G forces experienced in these stops. The moderate stops produced a force of 0.5 G, while the full-pressure stops hit 0.8 G. That’s more than enough to dislodge an unbelted driver from the passenger seat or launch any loose stuff in the cab toward the windshield. Things such as laptops, suitcases, unsecured TVs, etc., would become projectiles and could do some serious damage if they took flight.

Here’s something for flatbed haulers to consider: The minimum performance standards for cargo securement devices referred to in the cargo securement regulations require those devices to withstand forces of 0.8 G forward and 0.5 G laterally and to the rear. Based on how the full-application stops threw us tight against the seat belts, 0.8 G forward is a pretty tall order.

Performance-wise, both the disc, drum, and disc/drum combinations produced similar stopping distances. All produced ABS activity on the drive and trailer wheels. Even after five full-pressure stops from 35 mph, there was little discernible fade in the drum systems. Again, I’d call it a wash at this point; the drums were hanging right in there with the discs.

This screen grab from video footage hardly does justice to the impact of a 0.8-G stop from 60...

This screen grab from video footage hardly does justice to the impact of a 0.8-G stop from 60 mph with a full load in less than 200 feet. 

Photo: Jim Park

100+ psi, 60 mph

Some interesting things began to happen when we punched the test into high gear with full-pressure applications from 65 mph. Like before, we made at least five stops with each configuration, and the brakes became hotter each time, which changed the dynamics of each stop. A test like this comes close to simulating the effect of braking on long downhill grades.

The long and short of it was the all-disc tractor consistently had the shortest stopping distances, followed by the tractor with drums on the drives and discs on the steer. Stopping distances for the drum/drum tractor were longer overall, but by only about 15-20 feet.

Temperature readings taken from the brakes at the end of the five runs showed the steer-axle drum brakes got the hottest, reaching 420 degrees in one sample. Most of the drive-axle samples were in 300-350 range, while the trailer drum brakes remained the coolest at about 260-275 degrees. Those temps were taken from the exterior surfaces of the drums and rotors; the temperature of the brake linings and pads would have been substantially higher.

What was unusual for me in this test exercise was feeling ABS activity on the steer axle brakes.

That wouldn’t have happened prior to the RSD rules, but I felt ABS activity on both the disc-brake-equipped steer axles. The drum brakes reacted early in the stop and then the ABS activity subsided, while the higher-output disc brakes continued triggering ABS events almost until the truck came to a stop.

The general point to all this is that much of the increase in braking force that was required by the RSD rules comes from the steer axle — either the beefier 16.5x5-inch drum brakes with type 24 chambers (up from 15x4-inch with type 20 chambers) or the disc brakes.

That was something I had never experienced. The steering wheel jerked a couple of inches right and left, but the truck tracked perfectly straight through the stop. On a few occasions, after I got used to what was happening, I loosened my grip on the wheel and let it steer freely — the truck stayed perfectly straight. This ABS activity is perfectly normal under the circumstances, but it might be a bit shocking to a driver who doesn’t know what to expect.

As for stopping distances between the disc- and drum-equipped trucks, the all-disc truck stopped in the shortest distance, with the distance decreasing with each stop as the rotors got hotter. Amazingly, one of the stops, the fourth on I think it was, was less than 200 feet — that’s an amazing 50 feet short of the RSD requirement. The disc/drum truck was pretty consistent in its stopping distance over the five high-speed, high-pressure stops, with the steer-axle discs apparently countering the slight fade from the trailer and drive axles.

The all-drum truck did take a bit longer to stop as the drums warmed up, but interestingly during the third stop, once the brake linings “came to life” as Ugo put it, it came up with a shorter stop than the previous two. The remaining two stops got slightly longer as the linings had to reach a little further for the expanding drum, pretty much as expected.

My take-away

This exercise expanded my knowledge of brakes and the dynamics of braking immensely, thanks to the conversations Mark Ugo and I were having in the cab. That served only to deepen my respect for modern braking systems.

As for the debate over discs versus drums, I can say that each has its merits and drawbacks. If I was spec’ing a truck with a lot of mountain exposure, it would have discs for sure. As a side note, I’ve seen video footage of the Alpine brake testing Meritor does in the Swiss Alps, where the brake rotors on the truck glow cherry red but the truck still stops. That would weigh heavily on a spec’ing decision.

The maintenance advantages are worth considering, too. Even though the upfront cost would be higher, a 30-minute pad change would certainly come cheaper than a standard brake job. Of course, the idea that very little that can go wrong with a disc brake goes a long way at CVSA Level 1 inspection time, too.

I would also be perfectly happy with RSD drum brakes at all wheel positions, but I would probably lean toward having discs on the steer axle. Having just witnessed drums going head-to-head with discs and coming out a wash (except in fade), if my exposure to extreme driving conditions were normal or minimal, that slight difference wouldn’t matter much.

The other significant take-away is the brake lining material. When the OEMs list a brake system as standard, they usually demand a significant margin of improvement on the RSD-required 250-foot stopping distance. Ugo says Meritor tests its brakes for a 20-25% margin, and I expect the other major brake manufacturers do something similar. Much of that margin comes from the friction material formulation. At reline time, I’d be spec’ing OEM linings. Sure, they are more expensive, but that 25-foot margin could make all the difference between a crash, a bump, or a near miss.

And finally, if every driver and maintenance manager had the chance to experience what I did over those two days at the test track, I think they’d all have a much deeper appreciation of the need for proper brake maintenance. If you flip to the back few dozen pages of the Commercial Vehicle Safety Alliance’s Out of Service Criteria handbook, you’ll find some mind-boggling examples of brake system neglect and damage. I simply cannot imagine trying to stop one of those trucks under the conditions we tested Meritor’s brakes under at the track.

About the author
Jim Park

Jim Park

Equipment Editor

A truck driver and owner-operator for 20 years before becoming a trucking journalist, Jim Park maintains his commercial driver’s license and brings a real-world perspective to Test Drives, as well as to features about equipment spec’ing and trends, maintenance and drivers. His On the Spot videos bring a new dimension to his trucking reporting. And he's the primary host of the HDT Talks Trucking videocast/podcast.

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