Pressure points

by James Menzies

MARKHAM, Ont. – The good news is, there’s an easy way to improve your fleet fuel mileage. The bad news is, very few are doing it.

Proper attention to tires, and specifically tire inflation pressures and vehicle alignment, can enhance a fleet or owner/operator’s bottom line by improving fuel economy and extending tire life. It’s no small matter, since tires now account for the third largest expense in running a truck after drive wages and fuel. And tire prices aren’t going anywhere but up as the cost of raw materials continues to surge.

A recent Canadian Fleet Maintenance Seminar panel ‘Tire Maintenance: Keeping the Rubber on the Road,’ highlighted some of the simplest, yet most overlooked ways to get the most of a set of truck tires.

It begins with meticulously maintaining proper tire inflation pressures, which makes perfect sense when you consider the air in a properly inflated tire carries 95% of the load with the casing shouldering just 5% of the weight.

“As the pressure goes down, the load is transferred to the casing,” noted John Overing, commercial business unit director with Yokohama Tire Canada. Not only does that negatively impact the retreadability of the casing, it also hinders fuel economy and performance.

Francois Beauchamp, field engineer with Michelin, said there are six critical factors that can kill your tires – and not surprisingly, most have to do with inflation pressures. He listed the six critical factors as: low air pressure; high air pressure; missing valve caps; irregular wear; dual mismatch air pressure; and dual mismatch tire height.

Of those, low air pressure is the most common concern, Beauchamp noted. Yokohama’s Overing added underinflated tires can cause: shoulder tread wear; reduced braking performance; reduced casing life; poor fuel consumption; and in the worst case scenario, a blow-out.

“The reason inflation is important is that as the tire travels down the road, each section rolls through the contact patch and compresses,” he explained. “What you’re trying to do is match the deflection of the tire to the load, so it is not above or below the designed vertical deflection of the tire.”

An over- or underinflated tire will shift the flex point to the shoulder and closer to the bead, which are typically thicker parts of the tire than the intended flex point, causing heat to build up and increasing the risk of a tire failure.

“A properly inflated tire will flex through the thinnest part of the sidewall, generating the least amount of heat and the least amount of fatigue,” Overing explained.

An overinflated tire is nearly as dangerous. Beauchamp said overinflated tires run harder and are more vulnerable to impacts, increasing the risk of a blow-out.

Overing pointed out an overinflated tire reduces the contact patch and increases the torque-per-square-inch of that contact patch, causing the tire to want to spin.

“You want to maximize the contact patch and reduce the torque-per-square-inch so you don’t spin and that’ll give you better traction,” he said.

Panelists agreed maintaining proper tire inflation pressures is the easiest way for a fleet or owner/operator to improve their fuel efficiency and extend tire life.

Tire pressures should be checked regularly using a tire gauge and Beachamp said missing valve tem caps should be replaced immediately since valve stems are “your main source of low air pressure.”

“It is your seal for dirt and salt and everything that can get in there and affect the valve core,” Beauchamp said. He endorsed the use of alligator-style valve stem caps that provide double seal protection and allow operators to add air without removing the cap.

When inflating drive tires, Overing warned against adding the maximum pressure indicated on the tire sidewall, unless you’re doing off-road heavy hauling.

“The maximum pressure is designed for the maximum load and the laws in Canada in terms of load-per-axle will never require you to put the maximum pressure in your drive tires,” he said.

Beauchamp estimated 95% of irregular wear can also be attributed to improper tire inflation pressures. That’s a big deal when you consider the costs that come with irregular wear.

Understanding how costly irregular wear – and in turn improper tire inflation pressures – can be is not easy if you’re mathematically challenged. Tire professionals, however, can rattle off measurements as though they’re ordering their morning coffee. Take this real-world example from Yokohama’s Overing: A tire removed after 400,000 kms shows signs of irregular wear with a shoulder tread depth of 10/32nds and a centre tread depth of 4/32nds, indicating a loss of 6/32nds of tread. That $400 tire provided 15,385 kms per 32nd. However, if the tread on that same tire wore evenly down to 4/32nds, the tire would have run 525,000 kms before it needed to be pulled, providing its owner with roughly 20,000 kms per 32nd or an extra 125,000 kms.

You don’t have to be a mathematician to understand the potential savings.

Improper tire inflation pressures aren’t the only cause of irregular wear. A truck or trailer that’s out of alignment can also be a culprit, pointed out Leighton Watkins, regional fleet sales manager with Bridgestone Bandag Tire Solutions. He referred to a major Hunter Engineering Company study that performed random inspections on trucks parked at truck stops and found 70-80% were out of alignment.

“That’s a big problem,” Watkins understated. “Fuel is a huge cost and between air pressure and alignment, those are the two biggest causes of loss of fuel that can be easily addressed in a fleet.”

Just as tire experts can rhyme off air pressures and tread depths in their sleep, they’re also good at detecting alignment issues by simply running their hands over the tires and feeling for the telltale defects.

“When our sales guys come to see you, you’ll see them feeling the tires with their hands,” Watkins said. “We don’t have rubber fetishes. We’re just trying to figure out what’s going on with the truck.”

Slight vehicle misalignments can have major implications on tire wear. For example, a 1/8″ toe error creates 7.5 feet of tire scrub for every kilometre the truck travels, Watkins said. If you drive 150,000 kms in a year, a truck with a seemingly subtle 1/8″ toe error will have caused the tire to drag sideways for 343 kms.

“If you drag that tire sideways for 340 kms, what’s it going to look like? And that’s just 1/8″,” Watkins said.

A vehicle that’s out of alignment can cause the following issues: toe-in or toe-out (toe-in is characterized by tires that are closer together at the front while toe-out will involve tires being closer together at the rear); camber (positive camber will be evidenced by the top of the tires leaning outwards while negative camber will cause the top of the tires to lean in); and caster (a forward or rearward tilt of the steering axis, which can cause shimmying).

The drive axles are most commonly found to be out of alignment, followed by trailer axles and finally steer axles, “because we actually pay attention to them,” Watkins noted.

So how often should a truck and trailer be aligned? As a general guide, Watkins suggested having a vehicle aligned: any time there has been suspension work done; when steer tires have been replaced; whenever the truck is in for a safety inspection; and when the driver is complaining of poor handling.

“The majority of OEMs also recommend alignments be rechecked after a new vehicle travels 1,000 kms and the TMC recommends after the first 90 days of service,” he added.

Each of the tire pros on the CFMS panel suggested maintenance managers and truck owners learn how to identify issues by the feel of the tire.

“The best thing to do is feel the tire,” said Overing. “Your hand is your best friend after your pressure gauge in terms of tire maintenance. Any bump in the sidewall could be an impact break. Smaller bumps could be an inner liner spli
ce. If you feel a significant bulge, take that tire out of service; the casing cords may have broken and it compromises the integrity of the casing.”

“The tire is the messenger that is going to tell you what is wrong with your truck,” Watkins added. “If you don’t know how to feel tires, get your reps to tell you.”

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