By Rolf Lockwood. From highwaySTAR, October 2000.

Note: some things have changed since this article appeared in October of 2000. While the principles of the air disc haven’t changed, some specific products
have been superceded. See Product Watch for the latest air disc brakes. — ed.

Big trucks don’t stop on a dime like cars can do, but they can do it on about 15
cents with air disc brakes. They’re making a slow comeback, after many years in
which only one manufacturer offered them for sale. Now there are three.

Performance benefits have led European fleets to use air discs more and more often – almost all steer axles and 40% of drives currently – but here it’s going to take time. Prakash Jain, director of technical support, stopping systems at ArvinMeritor, says they’ll grow in popularity as governments mandate shorter
heavy-truck stopping distances. Truck buyers, eventually, may not have a choice.

First introduced in the late 1970s, air disc brakes were not an overwhelming success despite several proveable advantages. Pricey to begin with, parts access left something to be desired, as did service expertise and reliability. The industry said ‘no thanks’.

So while every brake supplier originally offered an air disc, they ultimately left the market to ArvinMeritor alone. The folks in Troy, Mich., haven’t gotten rich off disc-brake sales, though they’ve been selling them in modest quantities all along,
mostly to premium tanker fleets like Air Products.

[In 2005, Haldex Brake Systems and the joint venture between Bendix and Dana
also offer air discs. Surprisingly, a small Quebec company, NewTech Brake
Corp., is about to launch an air disc brake of its own. — ed.]

Brake buyers now have an increasingly wide choice of air-disc brakes to choose
from, and they’re far superior to those the industry rejected a decade and more ago.

WHY DISCS?
Its basic design means there are five key advantages to the disc brake on heavy
trucks:

Stopping distance – compared to the same tractor-trailer with S-cam brakes, AlliedSignal says its Bendix air disc can drag a 97,000-lb rig down from 90 km/h in a 49-ft shorter distance. That’s with discs just on the trailer, not the tractor. But put discs all around and add EBS (electronic braking system), and it says the gain is 75 ft. That’s about halfway to car territory.
Meritor agrees, saying that the combination of EBS and air discs can definitely
close the braking gap between cars and trucks – especially if truck brakes are
conceived as a single system and incorporate electronic actuation and air discs.

The company’s DiscPlus air disc brake is claimed to provide a 16% shorter
stopping distance compared to drums, incidentally.

Jim Clark, Dana’s chief brake engineer, says the typical tractor with drum brakes
on all corners will stop in 270 to 280 ft from 100 km/h. However, he says that by
increasing steer-axle brake torque (with disc brakes) to about the same level as
drive-axle braking torque (still with drum brakes), you can bring that distance down
to 200 ft or less. He’s achieved 209 ft in tests with that arrangement, and says a
disc brake with greater torque could break the 200-ft barrier. For a loaded
tractor-trailer, he figures a 10% stopping-distance reduction is achievable with the use of disc brakes just on steer axles.

Fade resistance – there may be nothing more frightening than to feel
your pressure on the brake pedal produce progressively less stopping power on a
downgrade. That’s what happens with repeated brake applications on ordinary
S-cams. As brake temperature rises, the drum expands away from the lining. The
brakes literally ‘fade’ away.

With discs, on the other hand, hot brakes mean the rotor is going to expand into
the pads, not away from them. Discs will dissipate the heat more effectively and just keep on working. And pedal pressure remains constant.

Improved stability – drivers usually like disc brakes because of their
shorter stops and fade resistance, but also because of increased stability.

There’s less likely to be a performance difference between individual disc
brakes, and if the left side of the vehicle is braking as much as the right side, the
result is a straighter stop. Bendix says an S-cam system can have side-to-side
variances in brake torque of up to three times as much as a disc setup.

Easy servicing – all disc brakes allow faster friction replacement than
S-cams, by three to five times. With Meritor’s DiscPlus, for example, the wheel is removed, the caliper swings out, and then the pad can be dropped in.

But replacing a rotor takes longer with most discs, so it’s more expensive than
switching a drum. It involves pulling the complete hub and rotor, which means
disturbing the bearing and seal as well. And that’s actually a bit of a step
backwards as the industry has been gradually moving toward unitized hubs.

Bendix has an interesting answer to that one, though. Its ‘split rotor’ technology
means you can change a rotor in five steps without even removing the caliper, let
alone hub and bearing. First you replace the pads with special wedges, then
apply the brakes at the pedal. This ‘cracks’ the rotor into two pieces which are then simply disconnected and removed. These rotors, going into production in
Europe this year, are actually said to resist distortion and accidental cracking in normal use.

Fewer parts – an air disc is simpler than a drum brake to start with, but in many cases most of the disc assembly used on one axle is the same as that used on others, from the steer axle on back. That often includes caliper, rotor, pad, and slide-pin/retainer.WHY NOT DISCS?
Going back to their first introduction 20-plus years ago, air discs have a hurdle to leap over like the one faced by antilock systems – a bad reputation.

One of the problems was rotor durability. They simply didn’t last. Rotors remain a
key engineering challenge, Prakash Jain says, the key being appropriate size to
fit the brake. New rotor families are being developed, he adds, to overcome that
problem of the past.

Another issue is friction life. No matter how you slice it, the disc brake’s pad is
usually smaller (except compared to 16×4 steer-axle linings) than an equivalent
drum brake’s lining and will thus wear faster. Built-in wear sensors will help
maximize pad life, but the difference in life expectancy can be significant.

If a disc-equipped tractor is mated to a drum-braked trailer, the drums will fade
sooner under heavy braking, so the disc brakes will do more work, thus wearing
faster. Fleets currently using disc brakes usually have both tractor and trailer
equipped with discs, which is the optimum arrangement.

It’s worth noting that, while North American tractor-trailers normally have only 20% of braking effort on the front wheels with 40% each on the drives and trailer wheels, European trucks divide the effort equally among the three axle sets. It’s a pattern we’re likely to follow as we accept discs over here.

One problem remains, but not for long. That’s the readiness of the market – in
terms of mechanic training and parts supply – to deal with disc-equipped trucks. It
won’t last because a tough lesson was learned last time out.

Along the same lines, will inspection and enforcement officials understand the
differences between disc and drums when it comes to checking pad thickness
and chamber stroke? Obviously a lot of training will be required. Prakash Jain
notes that some air-disc designs won’t allow for stroke measurement.

The biggest downside is the disc brake’s higher purchase cost, made worse by less lining life. That likely means that few fleets and owner-operators aside from
those in some niche applications will choose them without a legislative push. On the other hand, according to Anton Schneider, director of brake product marketing
at AlliedSignal, European fleets are being driven to discs because of lower
life-cycle costs. Simplified maintenance, he says, is the key.

While Jain says only 1% of North American heavy trucks are disc-braked at present, Schneider predicts that it will be 30% by 2006, but not on linehaul vehicles – that’s “a relatively long-term vision.”

It’s not a clear-cut thing, as you can see, but the performance of the air disc brake may bring a legislated mandate to go that way. In which case you’ll learn to love them.