Retail

Extreme plus sizing

Mike Mavrigian
Posted on November 1, 2008
Moving up to a Plus-Seven change can double the wheel/tire package weight. Depending on the vehicle, a brake upgrade may be a good idea.
Moving up to a Plus-Seven change can double the wheel/tire package weight. Depending on the vehicle, a brake upgrade may be a good idea.

OK, so let’s say a customer rolls in with a large SUV that’s OE-equipped with 19-inch wheels. Since he or she feels the need to be a sheep and blindly follow the “bling” crowd, the customer informs you that he or she wants to max-out the wheel/tire package simply for the sake of appearance. The new target: 26-inch chromed alloys mated to massive section-width performance rubber.

Such a severe plus move, in this case, Plus-Seven, may provide the visual statement that the customer desires, but there’s more to the picture that the customer needs to see and consider. Transitioning to such an extreme plus move is going to result in a heavier unsprung weight factor, which, in turn, may adversely affect fuel economy and steering, suspension and brake system performance and durability.

Unsprung weight

Gross vehicle weight can be separated into two broad categories: sprung and unsprung.

Sprung weight refers to the weight of, well, everything that’s supported by the vehicle springs. That would include the frame, body, engine, drivetrain, fuel, passengers, etc.

Unsprung weight refers to the weight of components that are not supported by the springs, which would include tires, wheels, brake rotors, brake pads and calipers, brake drums, hubs, axles and control arms.

Actually, depending on suspension design, a percentage of control arm, half-shaft or CV shaft weight would account for unsprung weight, since a portion of component weight is supported by the vehicle springs. If the weight of the item places load on the springs, it’s sprung weight. If not, it’s unsprung weight.

Keep in mind that even using an alloy wheel can result in greater unsprung weight if the wheel size increases. For instance, moving from a 19-inch wheel to a 26-inch wheel results in a heavier wheel, and because of a wider section width, potentially a heavier tire as well. As an example, a typical 16-inch or 17-inch wheel/tire package will likely weigh around 40 to 50 pounds or more. For the sake of comparison, moving to a 26-inch wheel and tire package can easily more than double that weight.

Braking system

The lighter the rotating mass (wheel/tire package), the more efficient the braking system is likely to become. On the opposite side of the coin, as you increase rotating mass, the brake system must work harder to overcome the heavier load. And there’s the rub. Many original equipment brake systems simply are not designed to accommodate these heavier masses in terms of rotor diameter, rotor design, caliper design, friction pad size and friction pad composition.

Under heavy braking conditions, heat buildup will contribute to loss of braking efficiency as pad grip begins to fade. Keeping the brakes ventilated will help maintain braking efficiency.

While a larger wheel and plus-sized tire package may be heavier than its OE counterpart, a larger wheel diameter conveniently provides a larger cavity that would accept a larger-diameter brake rotor and larger brake caliper. In short, moving to an extreme plus size should be accompanied by at least the consideration of moving to a more robust aftermarket performance braking package.

Tire and wheel weight

Listed here is merely one example of the weight differences, comparing a typical OE setup to a variety of plus changes.

Tire -- Wheel -- Pounds

265/70R17 -- 17-inch/OE -- 68

305/50R20 -- 10x20-inch -- 94

305/45R22 -- 10x22-inch -- 94

305/35R24 -- 10x24-inch -- 97

305/30R26 -- 10x26-inch -- 105

As far as inflation pressures are concerned, when moving to a notably larger wheel diameter and wider tire, you need to take a close look at load index matching, since inflation pressure may require a slight increase beyond the OE specification in order to provide enough support for the “big envelope” of inflation area.

Brake system upgrade

Some large SUVs feature braking systems of marginal performance to begin with. If the tire/wheel package will increase substantially in terms of mass, 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.

These designed systems can include high performance rotors 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, and both drilled and slotted rotors are also able to more efficiently evacuate gasses that build up between the pad and rotor during braking.

Braking torque (in simple terms, the ability of the brakes to reduce wheel motion) 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.

The majority of OE calipers are of the sliding design, with pistons located on the inboard side of the disc only. This type of caliper is generally heavy, as compared to most aftermarket performance alloy-body calipers. Also, the sliding design of the caliper results in a slight loss of braking energy due to the movement of the caliper in its sliding mount.

A typical quality aftermarket performance caliper features lightweight aluminum calipers on a fixed style caliper mount, featuring opposing pistons (a piston at each side of the disc). And, since a larger-diameter wheel offers more clearance room, you’re able to use a larger size caliper (and 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.

Multi-piston calipers may also feature differential piston bores, with the piston nearest to the leading edge of the pad smaller in diameter. They allow the pads to wear more evenly along their length. Eliminating uneven, or tapered, pad wear reduces the chance of piston cocking or seizing.

Brake rotor assemblies are also available in a two-piece design, with full-floating construction. They are designed with an aluminum center section for reduced unsprung and rotating weight. This type of design greatly reduces the chance of disc warping.

Since braking temperatures can easily exceed 1,100 degrees Fahrenheit in severe braking conditions (for example, when trying to stop a heavy SUV during a steep hill descent), the brake disc will expand as a result of the generated heat. In the case of a two-piece rotor, the disc is free to grow under thermal expansion in relation to the center section (or “bell”). This, in turn, allows the disc to remain straight and true, benefiting braking performance and minimizing the chance of rotor warping.

Drilled/slotted discs

Aftermarket performance brake rotors are often offered with either a pattern-drilled feature (holes strategically placed on the rotor surface) or with straight or curved slots in the rotor surface.

Either treatment improves braking performance by continually cleaning the pad surface, which improves pad grip. Cross-drilled rotor discs are designed to lower operating temperature because of their increased surface area. Slots -— in addition to drilled holes -— provide an escape path for brake pad gassing, which normally occurs when the friction material contacts the rotor surface. This out-gassing of the pads can create a hydroplane effect as the gas layer builds under compression, which reduces braking friction.

In order to counteract the strain on the braking system caused by a larger rotating mass (the result of extreme plus sizing, remember), upgrading the brake system with larger diameter rotor and properly matched performance calipers will help improve braking performance.

The benefits include shorter braking distance, and braking repeatability. There also is a benefit in 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.

A quality brake system upgrade can be sold as an extremely value-added option as part of a tire and wheel package, or on its own. In reality, upgrading the brakes on just about any SUV, while possibly not mandatory in each case, would, at the very least, not be a waste of money on the part of the customer.

Suspension/steering components

An extreme plus move will result in a larger contact patch. This is a good thing. However, in terms of traction, dragging this greater wheel/tire mass through turns, from parking lot maneuvers to on-road driving, can place greater strain on steering system components.

The entire suspension and steering system is trying to cope with this larger rotating mass, which, depending on vehicle design, may result in reduced service life for various system components. If the aftermarket wheels of choice also feature an offset that moves the tire centerline further outboard relative to the lower ball joint, this can place even more stress on the components and result in premature wear.

In short, if customers want to “enhance” their vehicles with extreme plus-sizing, they need to know up-front that they will likely need to pay more attention to suspension and steering component conditions. You should advise them to pay regular visits to your shop for inspections, parts replacements and wheel alignments.

The morale: With appearance comes added maintenance responsibility.

Load capacity concerns

The typical SUV is a heavier vehicle as compared to a typical passenger car. 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).

While it’s always critical to select tires that are able to support intended loads, this becomes an even greater issue with regard to SUVs, especially when customers choose to upgrade the vehicle’s appearance by switching to larger-diameter wheels and lower-profile tires.

As a result of customers selecting tires strictly based on appearance, it is entirely possible that a passenger car tire may be chosen in order to achieve a performance or touring look. 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, this creates a situation that involves a shorter sidewall, which places the wheel rim in closer proximity to the road surface. This means less cushion area for rim protection on pothole-ridden roads.

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.

When considering the passenger tire for SUV use, artificially decrease the load rating of the tire to about 91% in order to provide a safety margin (considering the potentially varied use of this vehicle when lightly loaded or heavily loaded).

In other words, if a passenger tire is to be used, you should consider its load rating as only 91% of the load rating number that appears on the sidewall.

If the load rating on the tire says 1,300 pounds, which is common for P-metric tires used on passenger cars, you should now view that tire as being load rated for only 1,181 pounds.

Here’s a handy formula to use when considering a passenger tire for a light truck or SUV application:

Rated load (the load rating that appears on the sidewall) 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.

If the total potential weight placed on the tire set is, say, 4,800 pounds (vehicle weight plus all cargo and/or towing tongue weight), a set of four of these tires would just meet the required load rating. Based on the safety margin formula, however, the tires would be rated to handle a gross weight of only 4,360 pounds, which would be too light for the application. We all know how often light truck and SUV owners overload their vehicles, so play it safe when selling tires to your customers. And use the formula as a sales tool.

Regardless of the tires selected, their individual load rating (times four for a set) must always exceed the anticipated gross weight of the loaded vehicle.

If your customer runs under-load-rated tires on any vehicle, he runs the very real risk of overloading and experiencing a tire failure. Bear in mind that this would not be a fault of the tire; rather, it would be a direct result of selecting the wrong tire for the application.

The load rating may be marked on the tire sidewall in pounds (1,140 pounds, 1,200 pounds, 1,300 pounds, etc.) or may be identified by a load rating index number, which will be either a two-digit or three-digit number that represents specific pounds.

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 an aftermarket wheel is listed by its maker 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 individual load rating is 1,521.174 pounds.

Without the benefit of knowing what each corner of the vehicle weighs, you can loosely generalize by saying that a single wheel rated at 1,521 pounds could only be used 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 include vehicle use. For instance, when owners are off-roading, sudden impacts are commonplace and require higher load ratings.

The point is to never exceed the wheel’s rated load capacity —- to be safe, you shouldn’t even be close to that limit. An overloaded wheel can distort or break.

Cause and effect: Will plus sizing affect braking performance?

If your customer is moving to a substantially heavier wheel and tire package, he might have to change to an aftermarket braking system in order to help negate the heavier rotating mass.

So make sure he understands his options and what they mean to the vehicle. In order to maximize braking performance, he may have to add the following:

* larger diameter rotors.

* larger-capacity calipers.

* pads with greater friction surface area.

However, you can’t blindly assume that braking performance (braking distance) will be lessened as a result of an extreme plus move. Another variable in the equation is the tire itself.

The increased weight of the tire/wheel package may become a moot point if the plus-sized tire has greater traction capabilities than the tire it is replacing. A wider tread area (resulting in a larger contact patch) and tire compounding can increase traction.

Calculating tire aspect ratio: Follow the tiremaker's recommendations for selecting the correct rim width

The size marking on the tire will indicate aspect ratio (245/50R16, for example, has an aspect ratio of 50). However, if you’re measuring in order to determine what aspect ratio will best work on an application, you easily can determine aspect ratio on your own.

In order to calculate a tire’s aspect ratio, simply divide the tire’s section height by its section width.

Example: If the tire’s section height (bead to tread) is 5 inches, and section width is 10.2-inches, 5 divided by 10.2 = 0.4902. Rounding this off, this would mean that the tire has an aspect ratio of 50 (a 50-series tire).

Determining section height is easy. If you know the overall tire diameter, subtract it from the rim diameter. Then divide by 2.

Example: If the overall tire diameter is 26 inches, and it’s intended to mount to a 16-inch diameter rim, 26 minus 16 = 10. This means that the tire section height across the diameter is 10 inches (section height at one spot plus section height at a spot 180 degrees away). Now simply divide this by 2 in order to determine section height of the tire from bead to tread. In this case, 10 divided by 2 = 5-inch section height.

When considering mounting a tire on a wider-than-design rim, the tire section width will increase, so keep this in mind when considering installed clearance on the vehicle. A rule of thumb is that for every 0.5-inch increase in wheel width, the tire section width will increase by about 0.2 inch.

Example: If the tire was designed to mount to a 7-inch-wide rim, but you’re considering mounting the tire to a 9.5-inch-wide rim, the difference in wheel width is 2.5 inches. Then divide the 2.5-inch difference by the 0.5-inch incremental unit of rim width, which shows that the tire section width is increasing, in this case, by 5 increments.

When you multiply the 5 increments by the 0.2-inch section width increase rate (remember, the section width increases 0.2 inch for every 0.5 inch of rim width), you find that the tire section width has increased, in this case by 1.0 inch. This changes our original section width from 10.2 inches to a new section width of 11.2 inches.

Since aspect ratio results from the relationship of section height to section width, you now need to consider the tire’s new section width to realize your new aspect ratio.

In his case, you divide the section height of 5 inches (this has not changed) by the new section width of 11.2 inches to realize a new aspect ratio of 0.446 (approximately a 45-series). As you can see, you can alter final aspect ratio by deviating from the mounted rim width.

We point this out only to provide a better understanding of how mounted tire dimensions can affect aspect ratio.

This, in addition to possible improper mating of the tire bead angle to the rim if the tire is too wide or too narrow for the wheel, serves to illustrate why it’s important to follow the tiremaker’s recommendations for selecting the correct rim width for the tires at hand.

Related Topics: light truck tires, Plus-sizing, SUV tires

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