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Plus-Sizing SUVs and Braking Performance

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Plus-Sizing SUVs and Braking Performance

Plus-size upgrades are popular among a wide range of enthusiasts today, so much so that they are more the rule rather than the exception. They are commonplace among not only sports cars and sport compacts, but also touring sedans, pony cars, SUVs, light trucks and even minivans.

Aside from a Radio Flyer kiddie wagon, you name the vehicle, and it´s a candidate for larger-diameter wheels and lower-profile tires.

Plus-sizing -- the act of increasing wheel diameter while maintaining near-original overall tire diameter -- can result in faster steering response and increased corner/curve maneuvering. Let´s face it, however: The vast majority of those who plus-size perform this modification for the sake of appearance. After all, big wheels and short sidewalls look cool (in most cases, depending on the vehicle).

However, plus-sizing has reached the point of the ridiculous, with some customers opting for an all-wheel look with only a hint of rubber cushion between the rim and the pavement.

Good taste (and potentially tooth-clattering ride) issues aside, of major concern is over-the-top plus-sizing on large SUVs, from a standpoint of braking performance. That´s right, brakes.

SUVs, by nature of their design, feature more mass/weight than found in a passenger car. Since many experts feel that the braking performance on many SUVs is minimal at best in original form, braking performance can be greatly diminished by installation of an exaggerated plus package.

Why? Larger-diameter wheels and matched high performance tires that accommodate these larger wheels typically weigh more than an OE tire/wheel package. This increase in unsprung weight (and rotating weight) decreases the brake system´s ability to stop the already-heavy vehicle mass. As a result, the brakes are now trying to slow a heavier and larger-diameter package than before, and some SUV brake systems simply are not up to the task.

There are two approaches to this problem. Either avoid the issue by limiting the upgrade (wheel and tire size/weight package), or upgrade the braking system to better accommodate the increased mass of the new wheel/tire combo. Naturally, any of your real enthusiast customers will opt for the latter.

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LOAD CAPACITY CONCERNS

The typical SUV is a heavier vehicle compared to typical passenger cars. In addition, SUVs, because of their increased cargo area and likely use, may carry greater loads both ahead of the rear axle (passengers and interior cargo) and behind the rear axle (towing, rear-stored interior cargo).

As a result of customers selecting tires strictly based on appearance, passenger car tires are often chosen for an SUV in order to achieve a performance or touring look. They often provide a more comfortable ride as well.

However, a selected passenger car tire may not offer the necessary load rating for this heavier application. In addition, when an SUV customer decides to make a radical plus move, the resulting shorter sidewall places the wheel rim in closer proximity to the road surface. This provides less cushion area for rim protection on pothole-ridden roads.

Using passenger tires on a light truck or SUV should be approached with proper load carrying capacity knowledge. If a passenger tire is considered for use on an SUV or light truck, it´s vital to find a tire with the correct load rating that will safely accommodate the anticipated loaded vehicle weight.

As a rule of thumb when considering a passenger car tire not specifically designed for SUV use, artificially decrease the load rating of the tire to about 91% of the load rating number that appears on the sidewall. This takes into consideration the potentially varied use of this vehicle when lightly or heavily loaded.

For example, if the load rating on the P-metric tire says 1,300 pounds -- perhaps more than acceptable on a passenger car -- you should now view that tire as being load rated for only 1,183 pounds.

Regardless of the tires selected, the load rating for all four tires must always exceed the anticipated gross weight of the loaded vehicle. If under-load-rated tires are run on any vehicle, there´s the very real risk of overloading them and experiencing a tire failure.

WHEEL LOAD CAPACITY

Wheels are load rated as well, so don´t assume that any wheel that "fits" will be correct for the application. If its maker lists an aftermarket wheel as intended for a specific vehicle, the wheel´s load capacity should be factored in and should be fine -- but check anyway.

However, in the case of a wheel that features dimensions (bolt pattern, diameter, width, backspace, etc.) that might allow physical mounting to a variety of vehicles, pay attention to the wheel´s load rating!

As with the tire, you need to consider the wheel´s load rating to verify that it exceeds the vehicle´s gross, loaded weight. The wheel load rating should appear on the front or rear of the wheel. The designation may be listed in kilograms instead of pounds. To convert kilograms to pounds, multiply Kg by 2.2046 (1 Kg = 2.2046 pounds). If a wheel is marked with a load rating of 690 Kg, its load rating is 1521.174 pounds.

Without the benefit of knowing what each corner of the vehicle weighs, we can loosely generalize that four wheels rated at 1,521 pounds each could be used only on a vehicle that weighs less than 6,084 pounds soaking wet with passengers and its fullest potential load, including trailer tongue weight.

We realize that variables exist, including actual weight distribution among the four corners of the vehicle. Other variables might include vehicle use; for instance, off-roading, where sudden impacts are commonplace, would require higher load ratings. The point is to never exceed the wheel´s rated load capacity, and to be safe, you shouldn´t even be close to that limit. An overloaded wheel can distort and/or break. This is an issue that the majority of customers never consider, so it´s your job to educate them. Not all wheels can be used on all vehicles, regardless of whether or not they physically fit the vehicle.

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BRAKE SYSTEM UPGRADE

Where the capabilities of the braking system are deemed inadequate for the vehicle at hand, customers should seriously consider upgrading to performance aftermarket components in the form of a matched system. This can include rotors, calipers, pads and possibly higher-burst-strength reinforced flexible brake hoses.

While several high performance brake system manufacturers offer upgrade components, we´ll cite Brembo´s new Gran Turismo brake system as an example. The GT system addresses the specific needs of heavy SUVs that have been fitted with larger wheel/tire masses.

These packaged systems include high performance rotors (one- or two-piece, depending on application) that feature either drilled or slotted discs. Drilled discs provide increased surface area for improved rotor cooling (in addition to reduced weight). Slotted rotors help to evacuate brake dust more efficiently. Both drilled and slotted rotors are also able to more efficiently clear pad gasses that build up between the pad and rotor during braking.

Braking torque is, in simple terms, the ability of the brakes to reduce wheel motion. It relies on a combination of brake system factors: caliper piston area, effective radius (length of the pad and related disc diameter), brake line pressure and the brake pads´ coefficient of friction.

Nearly all original equipment calipers are of the sliding design, with pistons located on the inboard side of the disc only. This type of caliper is generally heavy (adding to unsprung weight) and flexible, meaning that some braking energy is lost due to the movement of the caliper in its sliding mount.

An aftermarket performance caliper, citing Brembo as our example, uses lightweight aluminum calipers on a fixed style caliper mount, and features opposing pistons (a piston at each side of the disc). Yet another advantage is their larger size, which allows the use of larger pads for greater effective braking radius at the disc. Multi-piston alloy calipers also provide much greater resistance to heat-soak, which can cause brake fluid to boil prematurely.

Brembo recently introduced its Gran Turismo performance brake systems, ideal for SUV applications, especially when these vehicles have been upgraded with larger-mass wheel/tire packages. The four-piston calipers feature differential piston bores (with the piston nearest to the leading edge of the pad smaller in diameter), allowing the pads to wear more evenly along its length. Eliminating uneven, or tapered, pad wear reduces the chance of piston cocking or seizing.

The Gran Turismo discs have a two-piece design, with full-floating construction. This involves an aluminum center section (for reduced unsprung and rotating weight).

The advantage of the floating disc assembly is that it greatly reduces the chance of disc warping. Braking temperatures can exceed 1,100 degrees Fahrenheit (say, for example, when a driver is trying to stop a heavy SUV while descending a steep hill, following a bunch of brake-heating sharp curves), and as the brake disc heats, it naturally expands. In the case of a non-floating disc assembly, the rotor´s center section may remain cooler than the disc area, resulting in a temperature difference that can cause disc warping and cupping.

With the full-floating system, the disc is free to grow (under thermal expansion) freely in relation to the center section (or "bell"). This allows the disc to remain straight and true. The floating disc is equipped with anti-rattle springs that apply a slight preload to eliminate noise.

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SLOTTED DISC BENEFITS

Much confusion exists regarding slotted or drilled discs. Many people believe that cross-drilling or slotting is purely an appearance modification. Even within the braking industry, debate exists over whether or not these features offer benefits. Some believe that either cross-drilling or slotting is of no benefit, while others believe that these features enhance braking performance. Those who truly understand the science of braking know that either treatment offers distinct advantages as compared to a "solid" disc.

As we noted earlier, either treatment improves braking performance by continually cleaning the pad surface, which improves pad grip. In addition, Brembo notes that cross-drilled rotor discs lower operating temperature, as a result of increased surface area. We should caution that disc cross-drilling never should be performed by someone inexperienced, since haphazard drilling procedures can lead to disc cracking or failure.

In other words, if your customer wants drilled rotors, buy them already drilled from the manufacturer, who utilizes exacting tolerances in terms of hole quantity, geometry and procedures when drilling discs.

In short, if a customer has a big SUV, laden with larger-mass wheels, he or she is a perfect candidate for this brake upgrade. The benefits include shorter braking distance, braking repeatability and appearance, since the combination of slotted or drilled rotors, coupled with black, red or silver aftermarket calipers, as seen behind an open custom wheel, simply creates an outstanding visual statement.

Also, because there is an overall reduction in unsprung weight (lighter calipers) and rotating weight (lighter disc assemblies), handling and steering response will improve.

Even if a heavier-than-OE wheel/tire package is not in the cards, your SUV customers seriously should consider this type of performance brake upgrade anyway. A quality brake system upgrade can be sold as an option as part of a tire and wheel package or on its own. Upgrading the brakes on just about any SUV on the road today would not be a waste of money.

While addressing the concern for diminished braking performance with over-sized wheel/plus packages, we do not want to infer that massive plus sizing is a bad idea. Kumho Tire U.S.A. Inc.´s Frank Secondary, senior project engineer -- passenger tires, says that the company ran performance handling tests with a range of wheel sizes from 17 inches to 24 inches on a Cadillac Escalade. "We were amazed how well this massive three-ton vehicle handled.... Handling was much more responsive with the big 24-inch package, as was transient stability." Secondary also says, "the big SUV handled like a car... the handling improvement simply blew us away."

Again, our focus in this article is not to talk customers out of moving to large plus sizes, but rather to consider the effects plus-sizing has on tire and wheel load-carrying capacity and brake system capabilities. A brake upgrade may be necessary to accommodate a larger mass of unsprung/rotating weight.

YOU DO THE MATH

Here´s a handy formula to use when considering the use of a P-metric tire for a light truck or SUV application:

Rated load divided by 1.1 = Reduced load rating for use on an SUV

For example, let´s say a passenger tire is load rated at 1,200 pounds. For SUV use, we would now consider the tire´s load rating as 1,090 pounds (1,200 divided by 1.1). If the total potential weight (including cargo and/or towing tongue weight) placed on the tire set is, say, 5,000 pounds, a set of four of these tires would only be rated to handle a gross weight of 4,360 pounds, which would be too light for the application.

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