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It was over a year ago, in Volume 1, Issue 2
of Real Answers magazine, that we first examined the effects of higher speeds on tires and
their performance. Since speed has a great effect on so many characteristics of tires, we
wanted to share with you the latest update on the subject. |
Have
your recommendations changed on using tires at 75 mph?
In some ways yes, in some others, no. As we’ve said many times, nothing good happens
to your tires when you use them at 75 mph.
Wear rates increase, irregular wear is more likely, and both
retreadability and fuel efficiency tend to decrease.
Do we need to change load rating and inflation pressures
when we run at 75?
Previously, we reported guidelines from the Tire & Rim Association (TRA) that
specified a need to increase inflation pressures and decrease loads on tires used at
speeds higher than 65 mph.
Since that time, we have determined that most current
Bridgestone medium truck and bus radial tires are suitable for operation at 75 mph without
load reductions or inflation pressure adjustments.
That simplifies things, but how do we know which tires are
affected?
If you have a copy of the Bridgestone Medium and Light Truck Tire Price and Data Book for
1997, you’ll notice that each of the technical data charts has a new column, labeled
“MAX SPD (MPH).” It’s the last column on the far right of each chart, and
lists the maximum speed for each tire model and size.
(If you’d like a copy of the Data Book, just check the box
on the business reply card we’ve packaged with this issue.) |
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What
if the chart lists a maximum speed of only 65 mph?
If the tire is rated at 65 mph, then you must use the load reductions and inflation
pressure increases specified in the TRA charts.
The chart shows load and inflation changes for speeds at or
below 50 mph. How do those work?
If you know that you are going to be operating a tire at a lower than normal speed, you
may be able to increase the load you put on it.
What the chart tells you is that if the vehicle is stationary
(like a stationary crane, for example), you can increase the load up to as much as 185
percent of normal (65 mph) ratings by adding 40 psi to the inflation pressure.
As we know, the slower the speed, the less heat is generated.
That is why loads can be increased. If you change speed however, be sure to adjust the
load and inflation pressure accordingly.
And, never exceed the load and inflation capacity of the rim or
wheel. |
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Are
there Bridgestone tires that should not be operated at sustained speeds above 65 mph?
Those tires are listed in the chart above. In order to operate these tires at sustained
speeds above 65 mph, load must be reduced and inflation pressure increased per TRA
guidelines.
Have all tire manufacturers re-rated their tires this
way?
No. Each has taken a different approach. Some still adhere to the original TRA guidelines
(that are based on 65 mph) for all their truck tires, some require only inflation pressure
increases. You’ll need to consult your tire manufacturer before operating their tires
at speeds from 65 to 75 mph.
For Bridgestone tires, consult the Bridgestone Medium and Light
Truck Tire Price and Data Book, or call the Bridgestone Technical Hotline at
1-800-847-3272. |
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