Braking Comparisons From 75 MPH
You can noodle the min./max. numbers in the table if you're so inclined.
Jim Winsor
Executive Editor
Stopping Distances From 75 mph By Type Of Brakes
(Tractor with unbraked trailer) |
| |
Average (in ft.) |
Maximum |
Minimum |
Standard S-Cam
16 x 4 in. front.......
16 x 7 in. rears |
........540........ |
...563... |
...482 |
High Output S-Cam
16 x 5 in. front.......
16 x 7 in. rears |
........387........ |
...406... |
...377 |
Air Disc front
S-Cam..................
16 x 7 in. rears |
........353........ |
...372... |
...342 |
|
Air disc front brakes teamed with S-cam rears give the shortest stopping distances, but high-output S-cam brakes on all axles aren't far behind.
NHTSA, as I discussed in my January column, has growing interest in improving stopping distances for big trucks traveling 75 mph. That's 15 mph faster than the speed used in data traditionally gathered by the agency.
NHTSA's interest grows from the fact that 42 states currently have legal speed limits for big rigs above 60 mph and nine states have a legal 75. Forgetting the posted limits for a minute, I think you'll agree that, except in congested traffic, it's not unusual to see 18-wheelers buzzing along at 75 or more in most states. This doesn't make it right; it's just my observation. My guess is NHTSA looks at it the same way.
Duane Perrin made NHTSA's first presentation on the matter at the Truck and Bus SAE meeting last fall. He's the chief of the Crash Avoidance Division, Office of Vehicle Safety Research for NHTSA and is the agency's "point man" on heavy vehicle research.
In my previous column on this subject, I discussed brake fade comparisons based on data from Bendix testing, which concluded that in a series of 17 stops from 75 mph, stopping distances more than doubled with S-cam brakes due to brake fade as they heated up. With air disc brakes on all axles, there was no noticeable fade until after 10 stops.
My conclusion was that if NHTSA comes out with tough stopping minimums from 75 mph, the trucking industry might have to switch to air disc brakes. That may, or may not be true depending how such a rule is written. If I painted a biased or slanted conclusion, it was unintentional.
Take a look at the stopping comparisons with three different brakes as shown in the table above. You get a different picture. The data comes from test work done by brake engineers at ArvinMeritor and sent to me recently by Paul Johnston and Prakash Jain.
There's no question that a rig with air disc front brakes and regular 16 x 7 inch S-cam rears give the shortest stops by nearly 200 feet over standard S-cams in a worst-case situation (the maximum distance columns).
Now take a close look at the high output S-cam brakes. These have the wider 5-inch front brake shoe teamed with 16 x 7 inch high output S-cam rears. This brake package has more aggressive linings than standard brakes for only a modest up-charge in costs, I learned. They're available from many tractor OEs today. While stops aren't as short as with the air disc fronts, there's only an average increase of 34 feet over air disc fronts but an average of 153-foot shorter stops compared to today's standard S-cam brakes.
You can noodle the minimum/maximum numbers in the table if you're so inclined. I find the differences in the three brake packages very significant, especially when you consider costs.
