NHTSA to apply new high-speed braking rules - street smarts - National Highway Traffic Safety Administration

Diesel Progress North American Edition, Feb, 2004 by Jim Winsor

While EPA'S 2007 diesel engine emission rules make most of the engine headlines these days, there are some other big changes ahead which will have significant impact on heavy trucks in the same time frame. The biggest, perhaps, is the yet-to-be-announced 60 mph brake performance rule from the National Highway Traffic Safety Administration (NHTSA).

Later this year NHTSA is expected to start rulemaking which will require substantially shorter stopping distances for heavy trucks and tractor trailers.

Truck manufacturers and brake suppliers are anticipating this and engineering is already underway. The rule is likely to require manufacturers to upgrade today's S-cam air brake systems, especially front brakes, to get more braking torque.

More powerful front brakes, whether larger S-cams or the adoption of air discs, will significantly change braking dynamics and vehicle handling. There will be much more weight transfer to the front axle during heavy (panic) braking. This will require beefier front suspensions and steering gear.

These changes are likely to come at the same time as the 2007 model engines, which must comply with EPA'S super-low N[O.sub.x] and particulates rule. The latter will require exhaust traps (filters) for the first time, another big-ticket item to add to 2007-model price tags. Truck users, fleets and owner operators alike, are just now learning that the engine/brake/chassis changes all hitting at the same time will raise the price of a Class 8 chassis at least $10,000 and perhaps $15,000. This is a huge hit, one which heavy truck buyers will undoubtedly resist by delaying new purchases just as they did in 2003.

But the big surprise, which doesn't have a time deadline yet, is what I call NHTSA'S 75 mph surprise. At the SAE Truck and Bus meeting last November in Fort Worth, Texas, NHTSA'S Duane Perrin, chief of the Crash Avoidance Division, Office of Vehicle Safety Research, told the largely engineering audience what his agency has learned from testing big rig brakes above 60 mph. This was a first and caught many by surprise.

As Perrin showed charts and graphs illustrating ongoing truck-brake research, up popped a slide with the subhead "75 mph." It read as follows:

* Stopping distances increase with the square of speed.

* In-stop (brake) fade is more likely from higher speeds.

* 42 states have legal speed limits for big trucks above 60 mph.

* Nine states now have 75 mph legal speed limits for big rigs.

As his presentation progressed, we saw charts comparing tractor stopping distances from 75 mph with traditional S-cam brakes, air disc brakes and with electronically controlled braking systems (ECBS). The graphs showed clearly that the air discs with their greater torque input and fade resistance, can stop a big rig or heavy straight truck in one-third less distance.

Following presentations showed fade resistance comparing S-cams with air discs both from 60 and 75 mph. As brakes heated up during a series of 17 stops, fade got progressively worse on the S-cams and more than doubled stopping distances by the 17th stop. With air discs, there was a slight distance increase starting at the 10th stop but no change in distances below that.

The unanswered question was what is the likelihood of a big rig having to make emergency (panic) stops from 75 mph? There's no denying the facts that drum-type brakes expand rapidly from high heat. Expanded drums require linings to move out further to meet the dram. This, in turn, requires more push-rod travel and more air needed in each brake chamber. It's a losing battle.

Perrin wouldn't say new rules were in the works for 75 mph brake performance, but he did indicate NHTSA would be gathering more data at this higher speed. In talking with brake engineers afterwards, there was no question in their minds that NHTSA is clearly on a 75 mph path. The question is, how soon? A 75 mph stopping performance requirement will clearly drive a change over to air disc brakes. It's only a matter of time--and costs!

STREET SMARTS IS A MONTHLY COLUMN DEVOTED TO THE ON-HIGHWAY ENGINE MARKET. JIM WINSOR IS EXECUTIVE EDITOR OF HEAVY DUTY TRUCKING AND DIRECTOR OF MARKETING FOR NEWPORT COMMUNICATIONS.