With Phase 2 of CMVSS 121 looming in 2013, focus is on brakes, torque, balance, temperature
Jan 1, 2012 12:00 PM, BY RICK WEBER
HOW is Canada going to meet the CMVSS 121 compliance date of August 2013?
It's an “interesting problem,” according to Eddy Tschirhart, who moderated a CTEA panel to discuss the issues of stopping distance and trailer balance.
“There are a lot of makes and models customers can choose from, and there are many different trailer manufacturers in the world,” he said. “The ironic part of it is that rarely, if ever, do the power-unit and trailer people get to talk to each other. You also have other variables. Various slack-adjuster lengths. Various operating systems in trailers.
“On top of it all, a lot of people may hodgepodge it, buying Brand X for part of it and Brand Y for something else. We could have all drum brakes across tractor and trailer, all disc brakes, or a mixture. We are pretty unique in the way things have worked over the years.”
Phase 1 of CMVSS 121 was enacted in August 2011 and mandated a stopping distance of 250 feet for standard 6x4 tractors of 59,600 lbs GVWR and below (which was approximately 70% of the total build), with a 14.6k front or less with 45k rear tandem or less. That was a reduction of 30% from the old stopping distance of 355 feet.
Phase 2, which goes into effect in August 2013, calls for 250 feet for 59,600 to 70,000 lbs; 310 feet for above 70,000 lbs; and 250 feet for all 4x2 tractors.
Here's a look from four different perspectives:
New Tricks for Old Trucks
Chuck Eberling, principal engineer of vehicle braking systems, Bendix
Eberling said the mandate will lead to more powerful brakes on steering axles, something that the industry previously had tried to avoid because of fear that the front brakes would lock up. With today's antilock brake systems, however, that should no longer be a concern.
“You'd think that it'd be a simple approach: one brake would work for everybody and every application,” he said. “The reality is that everyone has taken a different approach. Some say, ‘I want to have a 10% margin on all six stops and I want every vehicle to meet that. Others might say, ‘I want to go with 255 feet and one stop.’ Most OEMs require an additional 10% safety margin (225 feet).
Truck manufacturers and brake suppliers are at work now on tests for meeting the requirements.
“There are joint field tests with respect to wear. We have a much more capable front brake, but do we have the mass, if you will, to absorb that initial energy we're putting into the front axle?”
He said 30% shorter stopping distances cause wheel ends to provide brake force that replicates the dynamic load distribution.
“The weight rating combined with load transfer can dictate that the brake torque produced by the front wheel ends must exceed that of the rear,” he said. “In general, no one wants to increase the mass of the front. Everyone wants a lower-cost, small-as-it-can-be front brake so it reduces the overall weight and cost of that application. That means we have some challenges in making sure we don't overwork that front brake based on the mass that's available to us.
“The industry has a goal to meet with the lowest-cost, lowest-weight, and minimal-performance impact. These factors may drive pneumatic system changes (pressure modifiers). CMVSS 121 has historically caused the current wheel ends to be sized for static load distribution. That's not going to be the case anymore. We definitely have to size for the dynamic load. The fleets' biggest concern regarding required stopping distance (RSD) is the potential impact to compatibility and wear balance.”
Most tractor OEMs desire a minimum 10% margin, resulting in a more dynamic load transfer.
To meet RSD criteria, lower brake torque on drive axles is desirable on many applications.
Minimum brake output could be higher than desirable, as established via dyno fade and recovery portions of 121.
It's desirable for drive and trailer axles to produce similar brake output for thermal/wear balance.
He said the ramifications of Phase 2 are very complex.
“Is ABS going to be required on auxiliary axles just to meet stopping distance or to prevent customer issues with brake tire wear?” he said. “There's not a good way to get there. There are systems to put on, but they are quite cumbersome. With a trailer ABS system today, you can only add one additional axle with the current architecture of tractor ABS systems and have ABS on that axle. Beyond that, you're adding a trailer system to the vehicle if you want more axles.
“With vehicles capable of higher decelerations, one concern I've always had is with cargo-restraint systems. I believe that actual requirements are probably adequate, but how often are they certified, enforced, utilized in practice?
“Will the effectiveness of RSD brake solutions be retained in-service or will they be mandated? To me, that's a travesty if that does not happen. If we don't have the carry-through of the brake force that was established by this law in service, then we've not done what NHTSA intended.”
Brake Balance Tractor to Trailer
Greg Dvorchak, Hendrickson International
He said there are three factors in balance:
“With load transfer and larger front brakes, this may be possible, but a challenge. It is very likely that the days of one friction material across all axles are over.”
“With the introduction of the new linings for the 2011 regulations, it is too early to predict life cycles of the new materials. This will be an evolving/learning process.
“New regulations are forcing maximum tire-to-ground utilization. This will make temperature balancing while maximizing brake output a challenge.”
He said CMVSS 121 increases load transfer to the steer axle on tractors and forces more steer-axle brake torque. It forces the brakes on the axles at the front of the combination vehicle to take more advantage of the increased tire frictional forces.
“The shorter distance is specific to an overloaded test condition on a high-friction test track. The new test is not representative of the real-world, typical on-highway conditions. Compared to existing configurations, the steer-axle brakes will require a significant increase in torque level and the drive-axle brakes may need to have slightly more torque output to meet the track testing requirements.
“The new test track requirements are to load the vehicle to 4500 lbs above the GVWR, which is significantly higher than the maximum legal load. The typical tractor would be tested at 56,500 lbs where it legally could only weigh 46,000 lbs. The test track surface is also a higher friction surface than the average public road surface.”
He said that for optimum tractor-to-trailer balance, it is not recommended that trailer torque output be increased.
“The axles at the rear of the combination vehicle are already near the maximum tire friction utilization during hard braking, so additional brake torque cannot be utilized,” he said. “This was validated by testing performed by NHTSA in 2003, completed in 2005, and the data presented at SAE COMVEC in 2006.
“In applications where empty trailer weight is very low, the fleet should consider to continue to use the standard 20K, 16.5 × 7 brake to maintain today's torque level in order to limit the propensity for wheel lock when empty.
“If high performance tractor brakes are spec'd, high performance equivalent-sized trailer brakes should be spec'd, but this should be done with caution. When the tractor is configured with air disc, we recommend matching the trailer brakes with disc brakes.”
Active Vehicle Safety Systems
Jeff Groshong, Daimler Trucks North America LLC
He said tractor/trailer temperature balance is a common concern, with an example of poor brake balance being excessive steering-axle temperatures.
There are new steer-axle knuckles for all suppliers, with a change from a 5/8" to ¾" inch fastener to meet Phase 1. New designs are needed to handle higher torque loads of Phase 1. Preliminary data indicates no further changes are needed to meet Phase 2.
“In Phase 2, we are faced with extremes,” he said. “The heavy haulers have a long wheel base, heavy axles, and unique loading. Are there enough wheels ends to apply brake force? Higher torque demands may call for an increase in the use of disc brakes.
“The light 4x2 tractors have the shortest wheel base, light axles, and only four wheel ends to apply brakes. Higher dynamic steer axle loads may call for an increase in the use of disc brakes.
“With heavier tractors, the good news is that a longer stopping distance of 310 feet is allowed; multiple axles equal lots of wheel ends to put brakes on; lots of tires to spread the brake torque among; and if necessary, only the two rear most non-liftable and non-steerable axles require ABS, and non-ABS controlled wheel ends could be allowed to slip.
“The bad news is that they are unique, special-use vehicles. Brake systems must be designed both for the specific application and also their ability to meet FMVSS 121. Unique special customer order vehicles may not be available for compliance testing.
“With lighter tractors, the good news is that they are lighter than the heavy tractors. The bad news is that they still have to meet the 250-foot requirement. Tractors with only have two axles only give us four brakes to work with. It may require ‘super brakes’ on steer axles. What to do about the potential for steer-axle wheel lock? And some configurations may not be available.”
He gave a word of caution for designers: Make sure the brakes required for compliance are also suitable for real world service. Torque levels required to meet high deceleration compliance stops may change the overall tractor/trailer brake balance. Trailer brakes are not changing — only the tractor brakes will change. High torque bias to the steer axle may be required for compliance that may not always provide the preferred front/rear proportioning.
He said the recession created some fallout as some key suppliers no longer exist.
“Work that was done needed to be re-started all over again,” he said. “In some cases, extensive field test data already completed was lost. Anti-copper is not just a California phenomenon any longer as more states sign on.
It may require replacements for some of today's popular brake linings/pads.
“There are also EPA-required changes. Extra weight added to some vehicles cause different front/rear weight distributions. With the potential for rules on fuel economy/ GHG emissions, lower rolling resistance tires are likely.”
Dick Radlinski, Link Commercial Vehicle Testing Inc
He said brake compatibility — the ability of tractor and trailer brake systems to work together to produce desired combination vehicle performance — is important because if it isn't there, and either the trailer or tractor brakes overheat, there is the possibility of excessive brake fade and wear, or even brake fire.
- Pneumatic timing balance
“You would want the tractor and trailer applying brakes at roughly the same time.”
- Brake force balance
“So you don't have the axle locking up or cycling ABS prematurely when you try to make a moderate or hard stop.”
- Energy balance
“So all brakes are running at a temperature where they don't wear excessively.”
He said the goal is to have the brakes applied as quickly as possible, and have the tractor and trailer timing “matched.”
“The reality is, there is some sort of delay,” he said. “Because the trailer is well behind the tractor, it takes time for the pneumatic signal to get back to the trailer. The typical delay between the tractor reaching maximum brake force and trailer is 1/10 to 2/10 of a second.”
A lack of compatibility happens because:
“There are two separate industries, and many times they don't talk to one another, and that can result in differences in braking characteristics.”
“There are many component suppliers. You can buy vehicles with different valves, brakes, and ABS systems. In many cases, tractors and trailers are out on the road that have never been in a testing environment.
“Fuel-efficient trucks roll easier. As trucks become more fuel efficient, drivers have to make a light application of brakes to control speed on a relatively low-percent grade. As a result, they're operating at very low pressures. If you don't have good threshold pressure balance at these low pressures, you get situations where one set of brakes is doing all the work.”
© 2013 Penton Media Inc.
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