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Now that antilock braking systems are the law on new air-braked
vehicles (March,1997 for tractors, March,1998 for trailers and buses, and March,1999 for
hydraulically-braked vehicles), it’s reasonable to ask whether ABS will have an
effect on tire performance. So far, the news is good. |
How does ABS affect tire performance?
As far as panic situation braking is concerned, ABS may be able to greatly improve it.
The fact is, the idea behind ABS is to improve the performance of
the braking system and improve vehicle control by eliminating wheel lockup.In what way?
When you apply the brakes, as you put more and more power into them, the braking forces
that reduce vehicle speed increase as well.
Up to a point. And that point depends on the ability of the tires
to grip the pavement. As long as they can do that, the tires will keep turning.
If you keep increasing braking power, eventually, you’ll
reach a peak. But beyond that point, the brake shoes lock against the drums, in turn
locking the wheels, and the tires start to slide over the pavement instead of rolling over
it.
And you don’t want that to happen.
Why is that?
There are three reasons: First, friction between road and tires is far less when the
wheels are locked and the tires are sliding. So, you’re not getting as much stopping
power when you’re in a skid. That can mean longer stopping distances.
Second, and perhaps most important, in a skid, your ability to
control the vehicle is greatly reduced. Tires that are sliding provide very little
lateral traction, which is essential to keeping vehicle motion in a straight line or
providing steering control.
Third, if the wheels are locked and the tires are sliding along
the road, you may be grinding flat spots into the tread surface of your tires.
How does ABS help?
It’s not our objective to explain how ABS systems work. That’s a job for
your equipment supplier. But if you’ll look at our “Peak & Slide”
chart again, we can tell you that what ABS does is use a fairly sophisticated electronic
and mechanical system to try to keep braking power near the “peak” level and
out of the “slide” area.
Electronic sensors monitor wheel speed, and when lock up is about
to happen, devices called modulators back off on the air pressure to prevent lock up, then
gradually increase the pressure again. The cycles are very fast, up to 3 to 5 times per
second. The idea, as the name implies, is never to let the wheels lock.
And wheel lock up is what causes tire flat
spots, right?
Yes. If the wheels never lock, you shouldn’t get flat spots. Stopping distances
should be shorter and
control much better. |
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How common is flatspotting?
It depends on where and how you drive. One ABS manufacturer suggests that about 1 out of
20 stopping situations (without ABS) results in locked wheels and the potential for tire
flat spots.
Another suggests that wheel lockups might only occur about every
30,000 miles.
If you run a significant number of empty miles, as often happens
for tank truck fleets or fleets who handle diminishing loads, you’re much more likely
to experience brake lock, simply because trailer traction is partly a function of load. So
brake lock is more likely to occur when you’re empty than when loaded.
And, though it can happen almost anywhere, wheel lock up is much
more likely on wet roads or on ice and snow than on dry pavement.If flat spots aren’t common, why worry about
them?
Because of what they can lead to. Since most flat spots happen on trailer tires, even a
pretty severe one may not cause a noticeable ride disturbance.
And that’s part of the problem. The driver may lock up the
brakes and flat spot a tire without even realizing it. The real problem may come along
later.
What’s that?
According to ABS manufacturers, once a tire has a flat spot, there’s a possibility
its wheel may lock up again in the same spot simply because that portion of the tire
is not going to roll through its footprint as easily as it did before.
So, a flat spot may get worse and worse.
And, since there’s less tread in a flat spot, even if lock
up doesn’t happen again, tread depth in that area may fall below your removal depth
or the legal limit long before the rest of the tread is equally worn. And that could cause
premature removal or failure in a roadside inspection, resulting in a delayed shipment
and an emergency tire purchase.
What if none of those things happen?
You can still have a problem with irregular wear. Each time the tire goes around,
especially as it enters or exits the footprint, the forces and stresses created by the
flat spot can cause additional wear, vibration and other problems.
What started out as a small flat spot may grow into a severe
irregular wear problem. This kind of localized damage can become a “seed” of
irregular wear, often of the type called “diagonal” wear.
And, the flat spot can even cause irregular wear to start in a
totally different place or even on a tire beside it in a dual assembly. |
What can we do about flat spots if we already have them?
If you have a flat spot on any tire, you’d better look at all of them. Because
chances are, the tire beside it has one too.
Do what you can to prevent vibration that can lead to irregular
wear. On a dual assembly, you can reduce the potential for vibration by
“clocking” the two flat spots 180 degrees apart.
Another thing that may help is to check dynamic balance of the
tire and wheel assembly, and rebalance as necessary.
If a dual assembly has flat spots, you may be able to
reduce vibration and slow down irregular wear by “clocking” the flat spots 180
degrees apart.
If ABS makes wheel lockup a thing of the
past, will it change the way tires are made?
Probably not. For one thing, ABS is only mandated on new vehicles right now. There are
lots of tractors and trailers out there whose wheels can still lock in a panic stop
situation.
Secondly, there are lots of different ABS systems. Some
sense potential wheel lockup on every wheel end, while others do not. For example, a
system for a tractor might have sensors on the steer positions, but monitor only the
forward drive axle. In that case, the rear drive axle brakes could lock and still cause
flat spots.
And, most ABS systems are designed so that if they malfunction,
they function as non-ABS systems. So wheel lockup is likely to stay with us for a while.
Will ABS save on the life of components
other than tires?
The experts we consulted doubt there will be any effect on the life of brake shoes, drums
or other components. As one put it, if your brakes locked every time you applied
them, you’d wear out tires like crazy, but brake shoes would last practically
forever. So brake components might actually wear out faster with ABS.
Would it make sense to retrofit our non-ABS
vehicles?
From the standpoint of maintenance economics, you’ll have to consult with your ABS
supplier, considering the cost of the retrofit, age of vehicles, resale value, incidence
of tire flatspotting problems in your fleet, and so on.
How will ABS affect my tire costs?
One ABS manufacturer says that because of dramatically reduced flatspotting, some of its
customers saved enough money in the first year of operation to offset the cost of ABS
systems for their new trailers.
And one government study suggests the possibility that ABS may
extend average tire life, perhaps by reducing slippage and minor, near-lock up situations.
Time will tell, but if either is the case, ABS may please not
just Safety Directors, but Maintenance Directors as well. |
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© 2006-2010 Bridgestone Americas Tire Operations, LLC l legal notice |
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