| July 01, 2011 |
How to crack the codes for lower cost and longer-life tires
By Tom Jackson
Next to fuel and wages, tires are the biggest operating cost for heavy construction fleets.
Yet a surprising number of contractors care little about their tires as long as they’re round, black and rolling.
Such indifference is financial folly. The experts we talked to say there is an OTR tire shortage looming on the horizon, if not already here (see sidebar on page 54). And the last time this happened, 2005 to 2007, prices for some sizes of earthmover tires doubled and even tripled. Back then new machines shipped barefoot (without tires) to customers and trade shows, and some contractors got so desperate they bought machines at auction just for the tires.
If you want to keep working through this pending shortage and maximize your tire life and minimize costs you need to understand the science and nomenclature behind today’s off-the-road tires. There are a lot of numbers and codes and formulas to decipher, and to make it even more confusing there are industry wide codes and nomenclature and then there are manufacturer-specific designations.
The tires that come on new machines tend to be general purpose and not always the best tire for low cost per mile or hour. But when it comes to the second and subsequent sets of tires you can lower your tire cost per mile as a well-informed tire customer.
Here is what you need to know.
The use of these tires on the designated machines, however, is not necessarily cut and dried.
“You see a lot of L-5 tires on graders in mine sites, because they’re exposed to sharp rocks as they’re grading the haul roads,” says Pete Kearney, ORT sales and marketing in the southeast for Titan International. “The reason most quarries and mines put L-5s on their graders is so they can put them on and forget about them. They’ll last forever unless you impact them and it’s pretty tough to impact an L-5 on a grader.”
Loader tires can be overkill for a grader in other applications though. In dirt, sand and gentler conditions, the extra protection is unnecessary. And loaders by their nature put extreme weight and stress on the front tires when the bucket is full. This demands a robust bead design and thick sidewalls. All this mass makes a tire that’s easy to heat up and slow to cool down. But heat is normally not a problem for L tires on loaders because loaders don’t go very fast or very far.
Earthmover tires, by contrast, run at much higher speeds and aren’t built with the same mass and thickness that the loader tires receive. A beefy loader tire would quickly overheat on a speedy haul truck, plus the extra weight and rolling resistance of a loader tire would rob a truck of some of its carrying capacity. “You can move an E tire to a loader if you don’t over stress it, but you couldn’t move an L tire to an earthmover machine,” Kearney says.
Quarry face work generally needs an L-4 or L-5 tire, says Steve White, market segment manager, Michelin Earthmover Tires. They offer more wear life in high scrub applications and have more protection against impacts or punctures. They are also heavier, he says, so if the job doesn’t require the robust construction or tread depth, an L-2 or L-3 will be lighter and more fuel efficient. L-2 and L-3 tires, because of their more open tread designs will also give you more traction for crowding the pile, he says. Using an L-5 instead of an L-4 or L-3 in a sand and gravel application is a waste of money, White says. The tread on the beefier tires wears slowly and the casing would become fatigued before the tread wears out, especially if the tire goes through more than the normal number of deflection cycles.
In the earthmover category or E-service tires, E2 and E3 rated tires likewise have less tread than E4 and are used in higher speed applications. Their shallower tread depth and quicker wear is offset by their greater Ton Mile Per Hour (see below) rating and ability to move material more quickly.
On the E-service category, you don’t want to use a shallower E3 or E2 if the speed and distance don’t require it, White says. “By using an E4 where high speed is not needed, you will get more hours from the tire. On the other hand, using an E4 when a higher speed E3 or E2 is required will mean having to lower the transport speed to stay within the capability of the tire. “If you don’t you will overheat and cook your valuable asset,” he says.
Examples like these show the importance of picking the correct tire for the application. If the tire is not robust enough it may be damaged before the tread wears out. If the tread is too robust, the casing may give out, leaving you with a lot of used tread – and tread that’s robbed you of some fuel economy as well. White says the key is to understand your conditions and the tire options and tradeoffs and then consult with a manufacturer’s representative or tire dealer to help make the proper choice.
Ton Mile Per Hour rating
A critical formula for selecting and managing tires on large quarry and mining trucks and scrapers is the Tons Mile Per Hour or TMPH. The mathematical formula for figuring TMPH is below, but simply put it is the average tire load times the average speed for the shift.
“TMPH is an expression of the working capacity of a tire and is a function of the maximum allowed internal operating temperature of a tire,” says White. “If you are exceeding this limit you are overheating a tire which could lead to catastrophic or premature failure.”
The relationship of speed to heat is easy enough to understand – the faster you go the hotter your tires get. But load plays a big role in heat generation as well. The sidewalls on an overloaded tire flex more than normal and flexing adds heat – just like when you bend a coat hanger, says Roger Best, senior field engineer Bridgestone Commercial Solutions Group. Bend it once – no difference. Bend it repeatedly and it becomes warm.
Every tire design will have a maximum allowable TMPH. “A bias OTR tire that contains multiple layers of fabric has a tendency to build up more heat,” says Aaron Murphy, vice president, CMA. “The radial OTR runs cooler due to the single layer of steel cord wire.”
Big OTR tires have a TMPH rating manufacturers publish, but it’s up to you the contractor or fleet manager to calculate the TMPH of your operation to make sure the work your trucks are doing doesn’t exceed the TMPH limit of the tires. And more importantly, that number can change, and change frequently.
“As your operation grows, your weights, speeds and distance to the crusher are going to change,” Kearney says. When those change, the TMPH changes also. “Staying on top of that is usually up to the tire dealer and manufacturer, but it can change daily,” he says. But a good fleet manager will keep a close eye on these variables and understand if the haul distance increases, the speed or weight of the loads may have to come down in order to stay under that maximum TMPH number for the tire. And conversely, a shorter haul may allow you to increase loads or speeds.
“It’s not very hard to get in your truck and re-do the cycle times and weights to find the TMPH,” Kearney says. “And if you find that the tires fall outside the TMPH or that something has changed, you can go to the superintendent and say, ‘Look, is it possible to slow the trucks down, because this is what’s going to happen if we don’t.’”
White adds that overly tight curves on haul roads can also affect tire loading and TMPH considerations. “This overloads the outside wheel positions, particularly in the front,” he says. This can be corrected by slowing down in the corners and maintaining proper speeds or by banking the corner more gradually. Steep grades also affect tire loading, overloading the rear tires going uphill and the front tires going downhill. “We prefer to see maximum grades of about 6 percent and no more than 8 percent,” he says.
Likewise pay attention to your truck’s suspension. A suspension problem may result in uneven tire loading even though material in the bed of the truck is distributed uniformly, White says.
There are four service categories of tires for construction machines. These are indicated by the following industry-wide, single-letter designations: E, L, G, and C; followed by a number indicating tread type.
E = Earthmover, but more specifically it means tires for end dumps, scrapers and articulated trucks. These are designed for speeds up to 30 mph and round trip haul distances up to five miles.
L = Loader/dozer tires. Built for a top speed of 5 mph and travel distances no more than 250 feet, one-way load and dump.
G = Grader tires. Designed for unlimited distance and speeds no greater than 25 mph.
C = Compactor tires. Includes both pneumatic and solid tires for use on pavers and compactors.
Tread types are indicated by numbers 1 through 5. Higher numbers indicate more tread area and less void or cavity around the tread and a thicker tread. Lower numbers give you more traction, higher numbers more protection. There is also a 7 designation in the earthmover service category which is a flotation tire (but doesn’t follow the same tread depth criteria), and an S designation in compactor tires.
S = Smooth.
1 = Rib: 100 percent tread depth
2 = Traction: 100 percent tread depth
3 = Rock: 100 percent tread depth
4 = Rock deep tread: 150 percent tread depth
5 = Rock extra deep tread: 250 percent tread depth
7 = Flotation tire
To get the average tire load, multiply the loaded and unloaded weight per tire in tons of the truck and divide by two. To get the average speed, multiply the miles per trip times the number of trips and divide this by total hours in the shift. Or to put it into an equation:
Empty tire weight + loaded tire weight x Average shift speed x number trips per shift
2 Hours per shift
For example you have a truck that puts 10 tons on each tire when it’s unloaded and 17 tons on each tire when it’s loaded and you drive it 9 miles per hour, 15 trips per day over an 8-hour shift. Then the math looks like this:
10 tons + 17 tons = 13.5 tons average weight
9 mph x 15 trips = 16.9 mph average mph
13.5 tons average weight x 16.9 mph average mph = 228 tons mile per hour (TMPH)
To prevent heat problems you should have a TMPH rating of 228 or higher, and this should be the minimum for the highest rated tire on the truck. Also note that the formula doesn’t work for tires loaded more than 20 percent above their rated capacity or for haul distances longer than 20 miles.
What kind of rubber?
Another important consideration is what tire manufacturers call the rubber compounding. In the chemical composition of their rubber manufacturers tweak the formulas to make either a standard compound, a cut-resistant, abrasion-resistant or a heat-resistant compound. And some compounds will be a mixture of these properties.
“OTR tires are application specific, so these compounds are another way the manufacturer pursues having the correct tire in the correct environment to ensure optimum performance and cost per hour, mile or ton,” says CMA’s Aaron Murphy.
There are no industry-wide standards for designating compound types – some use letters, some use numbers and some use both – so you’ll have to ask.
“If you’re in a rough application – slow haul, short distance – the best thing is a cut-resistant tire,” Bridgestone’s Roger Best says. “As long as you don’t fatigue the tire, it should match life for wear. If you’re in a high-speed haul with a sandy surface, a heat-resistant tire would be best because it on a sandy surface you’re not wearing very much.”
What heat does to tires
Heat will destroy any tire for one very simple reason. Rubber is vulcanized or cured, if you will, at approximately 300 degrees Fahrenheit. Once cured rubber starts getting close to that temperature it reverts back to its natural state (depending on the chemical makeup), which Bridgestones’s Roger Best says is a powdery mess. “It will create gasses that will
Running underinflated is often the cause of heat problems, Best says. “It’s basically the same as overloading.”
bulge out the sidewall and the tire will usually split in the shoulder to vent those gasses. If it’s extreme heat it may separate the belts from the tread package. And once there is separation, it’s not going to go away. It’s going to get worse. If you heat seperate a tire, the tread’s going to come off the casing.”
Running underinflated is often the cause of heat problems, Best says. “It’s basically the same as overloading.”
The new tire shortage
All the tire companies we’ve talked to tell us that we’re either in an OTR tire shortage now, or predict that we will be in one soon. Highway haul truck tires are more affected now than OTR tires, but the situation for OTR tires is growing worse.
The last time this happened (2005 to 2007) the shortage was caused by demand – the developing world, the wars in the middle east and the booming U.S. economy. This time, the experts say, the shortage is being driven by a limited supply of natural rubber from Malaysia.
“We’re starting to get phone calls from the OEMs and once that happens you know something’s coming.”
“Things are getting tighter,” Titan’s Pete Kearney says. “We’re starting to get phone calls from the OEMs and once that happens you know something’s coming. You can get 2700/49s right now, but you’re going to have to wait six to eight weeks for them. A month from now you may be waiting 12 to 16 weeks.”
The smart move for equipment managers in the face of this looming shortage is to build inventory and to maximize the life and cost effectiveness of the tires you have now. Partner with your dealer to make sure you’ve got the right tire and the right cost per hour or ton. Dealers can also set you up with tire tracking software that can give you a history of your tire successes and failures and apply that knowledge to make better choices in the future.
And last but not least maintain a rigorous air pressure monitoring system. “If you’re letting your air pressure drop 10 percent, you may be losing 20 percent of your tire life,” Best says. “Don’t buy a premium tire and then neglect the air pressure so that it doesn’t last any longer than a standard tire.”