 Drive
axle radials, like the Bridgestone M726, tend to deliver very high removal mileages. And,
they can often be retreaded, for use either on drive or trailer axles. But before you pull
those drive tires and send them to the retread shop, there may be a way to squeeze even
more miles from them — by NOT using them as drives. |
 |
| How can I make my M726 radials last even longer?
First of all, do all the normal stuff: Make sure the M726 is
the right choice for your equipment and operation, maintain
them and your tractors regularly, including always running the
right inflation pressure, and rotate tires, if necessary, to
equalize wear. But once they’ve done their job on your
drive axles, don’t retread them.
Wait a minute! Bridgestone is telling me NOT to retread my
M726 radials?
Not right away. Here’s the idea: A fleet customer of ours (a tandem drive axle
long distance line hauler with an excellent tire maintenance program) was running M726s,
and getting extraordinarily good results.
With an average of nearly 450,000 miles on their tires, they were averaging nearly
9/32" of tread left when removed. This works out to over 21,000 miles per 32nd of
wear.
That was better performance than they had experienced with any other
drive radial.
Why remove them?
For a variety of reasons. Most fleets pull drives at about 6-8/32" to improve
retreadability, but before they get that low, drivers sometimes complain about the
appearance of their drive tires. When remaining tread depth gets to 12/32" or less,
treads don’t look as aggressive as when new.
Drivers want deep tread on their drive tires. Since the tires started
out with 30/32" when new, with 8/32" to 12/32"
remaining, some drivers feel the tires look worn out.
What happened next?
It was important to try to satisfy the tire appearance preferences
of drivers. But cost per mile is still a very important goal.
So, instead of sending drive tires to the retreader with all
that usable tread still on them, only to have it buffed away,
we moved them back to trailer positions. Then we tracked their
performance, targeting them for removal from the trailer at
2/32" to 5/32" remaining tread depth. |
At
30/32" original tread depth, the M726 drive radial is one
of the deepest in the industry, and over half an inch deeper
than many trailer radials.
|
Drive Axle Mileages |
| Tire Number |
Drive Axle Miles |
32nds @ Removal |
Wear Rate Miles/32nd |
Cost Per Mile |
| 1 |
483,737 |
8 |
21,988 |
$0.00062 |
| 2 |
483,737 |
9 |
23,035 |
$0.00062 |
| 3 |
483,737 |
8 |
21,988 |
$0.00062 |
| 4 |
483,737 |
7 |
21,032 |
$0.00062 |
| 5 |
483,737 |
8 |
21,988 |
$0.00062 |
| 6 |
483,737 |
8 |
21,988 |
$0.00062 |
| 7 |
445,095 |
10 |
22,255 |
$0.00067 |
| 8 |
445,095 |
11 |
23,426 |
$0.00067 |
| 9 |
445,095 |
11 |
23,426 |
$0.00067 |
| 10 |
445,095 |
10 |
22,255 |
$0.00067 |
| 11 |
445,095 |
9 |
21,195 |
$0.00067 |
| 12 |
402,550 |
8 |
18,298 |
$0.00075 |
| 13 |
402,550 |
9 |
19,169 |
$0.00075 |
| 14 |
402,550 |
7 |
17,502 |
$0.00075 |
| 15 |
402,550 |
10 |
22,255 |
$0.00075 |
| 16 |
402,550 |
8 |
18,298 |
$0.00075 |
| Average |
448,950 |
8.8 |
21,256 |
$0.00067 |
|
Based
on estimated initial cost of $300 per tire. |
| Trailer Axle Mileages |
| Tire Number |
Trailer Axle Miles |
32nds @ Removal |
Wear Rate Miles/32nd |
Total Miles @ Removal |
Cost Per Mile |
| 1 |
138,939 |
3 |
27,788 |
622,676 |
$0.00048 |
| 2 |
138,939 |
4 |
27,788 |
622,676 |
$0.00048 |
| 3 |
138,939 |
3 |
27,788 |
622,676 |
$0.00048 |
| 4 |
138,939 |
2 |
27,788 |
622,676 |
$0.00048 |
| 5 |
138,939 |
4 |
34,735 |
622,676 |
$0.00048 |
| 6 |
138,939 |
4 |
34,735 |
622,676 |
$0.00048 |
| 7 |
192,206 |
4 |
32,034 |
637,301 |
$0.00047 |
| 8 |
192,206 |
5 |
32,034 |
637,301 |
$0.00047 |
| 9 |
192,206 |
4 |
27,458 |
637,301 |
$0.00047 |
| 10 |
192,206 |
3 |
27,458 |
637,301 |
$0.00047 |
| 11 |
192,206 |
3 |
32,034 |
637,301 |
$0.00047 |
| 12 |
133,792 |
4 |
33,448 |
536,342 |
$0.00056 |
| 13 |
133,792 |
5 |
33,448 |
536,342 |
$0.00056 |
| 14 |
133,792 |
3 |
33,448 |
536,342 |
$0.00056 |
| 15 |
133,792 |
5 |
38,441 |
637,301 |
$0.00047 |
| 16 |
133,792 |
4 |
33,448 |
536,342 |
$0.00056 |
| Average |
157,627 |
3.8 |
31,492 |
606,577 |
$0.00049 |
|
Based
on initial cost of $300 per tire and total mileage
logged on both drive and trailer positions. |
|
Why not just leave them on drive axles? First of all, by replacing the drive axle tires, drivers were pleased that
they were getting brand-new drive tires.
Secondly, based on projections, had we left the tires on the drive
axles, they probably would have been removed in about 50,000
miles or so, given the fleet’s standard practice of retreading
drives at about 6-8/32" remaining tread depth.
How did moving them to trailers change that?
Trailer positions tend to be the easiest on tires. Trailer tires are free-rolling, so
they’re not subjected to the high torque of drive axles.
And, while many steer axle radials are carrying upwards of 6,000 pounds of load, even
with a fully-loaded trailer, the 8 trailer tires carry a maximum load of 4,250 pounds each
(34,000 lbs total) – often much less.
The result is that especially in line haul operations, trailer tires
often wear much more slowly than any other position. And our
test case was no exception. As you can see, in terms of tread
wear, trailer positions produced over 31,000 miles per 32nd,
half again as many as drive positions.
What effect did this have on cost per mile?
A big one. On average, these M726 radials, when used only on drive axles, cost the
fleet about $0.00067 per mile. Ordinarily, that would have been the end of it, because the
casings would have been sent out for retreading.
But, by moving these tires back to trailer positions, then running
them down to normal removal depths, the average cost per mile
for all the tires in the study fell to $0.00049. That’s
an average savings of about 25 percent in tire cost per mile.
But wasn’t this a lot of extra trouble?
Not really. We found a trailer needing tires and positioned it back to back with a
tractor with drive tires at 8/32" to 12/32" remaining tread depth. We moved the
tractor tires to the trailer, demounted the trailer tires, mounted new drive tires on the
trailer wheels, and installed the new drive tires onto the tractor.
This was made simpler by the fact that both the tractor and trailers
used the same size and style wheels. |
| Are there any drawbacks to this idea? Not many. But there are some important requirements. For one thing, the sizes
of both the tires and wheels you use on drive axles and trailer axles must be identical.
And, you must have a program for checking your drive tires regularly, so you can pull
them at the right time. Based on this study, that would be at about 8-12/32" of
remaining tread depth.
Dual assemblies also need to be well-matched in remaining tread depth
to prevent rapid wear on the shallower tire of the assembly.
As a general rule, tread depths should match within 2/32"
for best results. So be sure to mark the remaining tread depth
on the tread surface for quick dual matching.
How did the M726 perform as a trailer tire?
Very well. The fleet averaged over 150,000 miles on these M726 radials in trailer
positions—miles that normally would have been buffed away in the retread shop.
In addition, irregular wear (a typical concern on trailer tires) was virtually
nonexistent because the wear pattern was already well established when the tires were on
drive positions.
Best of all, it was a "win-win" situation: The fleet got 25 percent longer
tire life, reduced trailer tire irregular wear, had to mount new tires less frequently and
reduced its overall cost per mile.
Drivers got brand-new drive tires earlier than normal, which gave them
added confidence, especially along snowy winter routes.
How would this work for fleets that don’t retread?
It’s ideal for a fleet that doesn’t retread. They get extra
miles, and the casing credit is virtually identical, regardless
of whether the casing has 8/32" or 2-5/32" remaining
tread depth. So, using a partially worn drive radial as a trailer
tire can be a really innovative way to cut your tire cost per
mile. |
Calculating Tire Cost
Per Mile
Buying tires on the basis of price alone—or
on the basis of removal mileage alone—really doesn’t make
sense. You need to know the real value of tires, over their entire
useful life.
The best way to calculate this is to figure cost per mile. Most
of the time, the "mile" part is pretty simple—at least for the tire’s
original tread life. If it has to be removed for a repair, or if you retread, things can
get more complicated.
The same is true of "cost." There’s initial
purchase cost, Federal Excise Tax (F.E.T.), mounting and balancing, repairs, retreading,
etc.
Make these calculations as simple or as complicated as you want,
but essentially, you’ll always divide costs by miles to determine cost per mile. The
more true cost factors you take into account and the more accurate your mileage records,
the more accurate your calculation will be.
You may simplify your calculations, by using
only initial cost and original tread mileage. The thing that is
critical is to make sure you always make your calculations the same
way. |
Calculating
Cost Per Mile |
To
determine total costs,
ADD:
 |
Initial tire cost |
|
F.E.T. |
|
Mounting & Balancing (initial) |
|
Repairs |
|
Mounting & Balancing (repairs) |
|
Retreading |
|
Mounting & Balancing (retreads) |
|
Tire disposal fees |
And SUBTRACT:
|
Casing trade-in values |
|
To determine total miles,
ADD:
|
Original tread mileage |
|
Retread #1 mileage |
|
Retread #2 mileage |
|
Retread #3 mileage |
|
Etc. |
|
|
Cost
per mile = Total Costs/Total Miles
|
For best results, track cost and mileage performance of each tire
individually. You’ll need to brand your tires or use some other way to positively
identify them, and a computer can be a big help.
Which
tire is the best investment?
| |
Tire A |
Tire B |
Tire C |
| Initial Cost |
$200 |
$300 |
$350 |
| Removal Mileage |
100,000 |
175,000 |
190,000 |
If you consider
cost alone, tire A is the clear winner. On the basis of removal
mileage, tire C is best. But when we calculate cost per mile, notice
what happens:
Which
tire is the best investment?
| |
Tire A |
Tire B |
Tire C |
| Initial Cost |
$200 |
$300 |
$350 |
| Removal Mileage |
100,000 |
175,000 |
190,000 |
| Cost Per Mile |
$0.00020 |
$0.00017 |
$0.00018 |
Now it is clear that neither tire A nor tire
C is the best choice. The tire with the lowest cost per mile—and
the most potential to deliver profits for you—is clearly tire
B, a fact not obvious when only initial cost or removal mileage
is considered. |
|
