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Tire Tech Tips

Posted in How To: Wheels Tires on January 14, 2013 Comment (0)
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Tires are the shoes for our off-road rigs and traction is what gets us where we want to go. Tires are odd in some ways. They are seemingly simple devices, yet there are many performance subtleties and some characteristics to make note of.

Tires on trailered rigs that see only time in the dirt often present different concerns than tires spinning on daily drivers. Many of us use off-road tires for double duty, hitting dirt and rocks part time and running down paved streets the rest of the time. Hence, tire wear, fuel mileage effects, and highway safety are greater concerns for those in the latter group.

We recently spoke with Matt Hanlon, who's part of the Motorsports Tire Development team for Michelin supporting BFGoodrich off-road tires. We wanted to pick his brain with a few tire questions we had on our minds. Here's what he and the team he works with had to tell us about tire use.

Q: Tire tread blocks wear and leading edges become rounded with use. Is it OK to rotate of-road tires from side to side to reverse rotation?

A: It is usually not a problem to rotate worn of-road tires from side to side or front to rear. Typically, a fresh sharp edge will give an improvement in bite of the tire until it starts to round of. However, as a word of caution, a fresh edge on a treadblock is normally not as well suited for high-severity applications. If you ask too much of a fresh edge before rounding it of, you will start to see tearing just below the leading edge of the tread block. Typically, this will not do any long-term damage, and the tire will eventually acquire the familiar rounded appearance.

Q: Tread blocks tear of tires on occasion and sometimes sidewalls can become scarred or cut. What are the warning signs or wear limits an owner should be concerned with?

A: It is dificult to set a consistent rule on the threshold for severity. In general, if a cord or wire is exposed to the elements by a cut or a tear, it is not recommended to continue use of the tire on public roads. The efects of moisture over time on the internals of a tire should not be ignored. In racing and competitive applications where the tire will only be used for a short period of time, the guidelines may be slightly diferent. If there is a small cut that exposes a cord, we will typically allow a tire to be run as long as there is no breakage or fraying of cords. Patching or vulcanizing any sidewall damages, especially those exhibiting broken cords, is strongly discouraged.

Q: Obviously taller, fatter of-road tires afect overall fuel mileage. Are there further variations in rolling resistance across types of tires that might afect highway fuel mileage?

A: Many aspects of a tire design influence rolling resistance, with the tread area being a significant contributor. Rolling resistance generally increases as tread depth and lug sizes increase. The shoulder area at the edge of the tread surface is particularly influential for rolling resistance— having more tread mass or depth in this area has an exaggerated efect. As a result, your tire's rolling resistance actually improves as your tire wears: It is usually best just as you are ready to replace it! The formulation of the rubber compound also has a large efect on the rolling resistance. In general, compounds formulated for racing tend to have much higher rolling resistance, which is normal since its design is for a particular application rather than general on-road travel. The tire's rolling resistance is just one of many factors that impact overall vehicle fuel economy. The overall dimension of the tire/wheel assembly come into play, as there are erodynamic drag efects that impact the vehicle's rolling resistance. A larger-diameter tire may have the same or lower rolling resistance in comparison to a smaller one, but the extra height it gives the vehicle may yield a lower net fuel economy.

Q: We often air down for of-road runs. What are the dangers of running under-inflated tires on the highway and how do I know if they're under-inflated?

A: When an of-road driver moves back to on-road conditions, it's critically important for the tire pressure to be set back to the vehicle's recommended tire inflation pressure. On-road conditions require diferent performance from the tires, including high-speed stability, handling, and durability. These are all seriously compromised by incorrect inflation pressure. Besides the safety issues, low tire pressure greatly impacts fuel consumption and increases tire wear. Resetting the proper inflation pressures is even more important when the vehicle is heavily loaded or towing a trailer.

Q: Some of us have weekend rigs that may see relatively little tread wear and longer than average tire ownership times. What's a good guideline to tell if tires are getting too old for use, despite having plenty of tread left on them?

A: It is important to keep good records of when you purchased each tire. Due to variations in storage conditions, it is often dificult to judge the age of a tire. Luckily, the week of manufacture is molded into the sidewall of each DOT-approved tire. Look for a four-digit number at the end of the string of characters, following the letters DOT on the lower part of the sidewall. The first two numbers indicate the week and final two numbers denote the year of tire manufacture. Tires that have been in use for five years or more should continue to be inspected by a specialist at least annually. Consumers are strongly encouraged to be aware, not only of their tires' visual condition and inflation pressure, but also of any change in dynamic performance like increased air loss, noise, or vibration, which could indicate the tires need to be removed from service to prevent tire failure. It is impossible to predict when tires should be replaced based on their calendar age alone. However, the older a tire, the greater the chance it will need to be replaced due to the service-related evolution, or other conditions found upon inspection or detected during use. While most tires will need replacement before they achieve 10 years, it is recommended any tires in service 10 years or more from the date of manufacture, including spare tires, be replaced with new tires as a precaution, even if such tires appear serviceable and even if they have not reached the legal wear limit.

BF Goodrich Baja T/A Tires

For our Project Ford Fast-150, we’d decided to run 37-inch tires. We turned to BFGoodrich for a set of their Baja T/A tires in a 37x12.50R17 D-rated size. The tires have proven themselves to be tough, and these are the DOT versions of the legendary race tire.

Tire construction consists of two full-width steel belts and four nylon sidewall plies. The tread blocks are designed to provide maximum dirt performance, but can still provide acceptable on-road behaviors. These tires come in just under 37 inches mounted and weigh only 75.3 pounds—lightweight, but tough.

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