Tires deliver the most performance per dollar than any other upgrade. The right tires can enable four-wheeling success on various off road conditions. On the streets, you expect controlled and safe braking and cornering from your tires. Unfortunately, what works best on the streets does not necessarily work best when off-roading. Installing tires that are best suited for off-roading when 95% of your driving is on streets and highways may not be your best option.
Tread Design - Use the chart to get an idea of how tire choice may affect the daily drive and off-roading capabilities of your vehicle. The better a tire is on the streets the less capable it will be in the mud. Without two sets of tires, you won’t be able to have your mud pie and eat it too. Make your choice wisely so the tradeoffs are expected and acceptable.
Tire Size - Everyone wants to go big, but recognize there may be compromises or accommodations that will have to be made. Bigger tires may require a lift kit, advanced gearing, or wheel and brake upgrades. When upgrading your vehicle, tire size should be the first building block and then other upgrades are chosen based upon it.
Fitting Size - Actual tire size may be different than advertised. Two tires from two different manufacturers with the same advertised sizes, such as 35x12.50-18LT, may be so dimensionally different that one brand rubs when the other does not. It is best to use the measurements from the tire manufacturer’s specification sheets. Here you will also find the recommended wheel widths and the measured wheel width used.
Picking Lift Kit - Lift kit manufacturers often list tires sizes that are compatible for their lift kits. Depending on the manufacturer’s thoroughness, they may also list the wheel sizes. As excellent as this information is, it is still recommended to contact the manufacturer’s technical experts and discuss your planned tire size and upgrades. Keep in mind that using the same lift kit on your vehicle as your buddy may not produce the same results. One truck could be lean and mean and the other loaded with aftermarket parts such as bumpers, winches, skid plates, train horns, etc.
Aspect Ratio - A tire’s aspect ratio is the distance from the ground to the wheel in relation to the width of the tire. Extreme off-road tires generally have a very high aspect ratio, while street tires have a much lower ratio. In the off-roading world, a higher ratio means more sidewall surface and flexibility, which allows the tire to better conform to the various and unusual surface shapes for improved traction. This flexibility is exactly what you would not want in a street tire. In recent years, larger wheel s have become commonplace in off-roading. When the wheel size is increased and the tire’s diameter stays the same, the tire’s aspect ratio is decreased. If a 37” diameter tire is the desired size for your vehicle and you use a 15” wheel, upgrading to an 18” wheel will reduce the aspect ratio. With lower aspect ratio tires, you will have a more grounded vehicle that is a lot less squirmy. The tradeoff is a little less flexibility when off-roading, but improvements in street performance and fuel economy.
Tread Depth - Bigger tread blocks typically make better off-road tires whether you’re in the mud or on the rocks. In the mud, deep block tread acts like paddles and freely releases accumulated mud. On the rocks, those same deep tread blocks conform to various surface angles and grip the smallest terrain features. This conforming grip, which is great off-road, is not what we want on the streets as it reduces braking and cornering performance. You can find a tire’s tread depth on the manufacturer’s specification sheet. Choosing a tire with a little less tread depth would make a better street tire while still having some good off-road capabilities.
Wheels most commonly are offered in steel, cast aluminum alloy, and forged aluminum alloy, and each has positive and negative aspects. The type and fit should take priority. However, the design and style choices you make are up to personal aesthetic preference.
Steel - Steel wheels are very strong, fairly easy to keep looking nice, relatively cheap, and the easiest to repair. What is not so great is that they are heavy. This weight can contribute to a decrease in drivetrain durability, as well as decreased mileage, acceleration, and braking performance. In certain climates, steel can easily corrode. If not maintained properly, this corrosion will significantly weaken the wheel.
One-Piece Cast Alloy - One-piece alloy wheels are the most common. These wheels are strong, lightweight, reasonably priced, and corrosion resistant. What is not so great is alloy wheels are not easily repairable and if the finish is not taken care of, they can become dull. Refinishing or re-polishing the wheels can be very time consuming and costly.
One-Piece Forged - Forged wheels are the lightest and strongest. What is not so great is that they are the most expensive. Forged refers to the process of making an alloy wheel by forcing a raw block of metal into shapes with extreme pressure. Forging is an expensive, multi-step process. As a result, forged wheels are up to 300 times stronger than cast wheels and can take more damage.
Beadlocks - For wheelin’ hard at the lowest tire pressure, beadlocks are what you need to get you there and back and hit all those extra credit sections. But if you don’t plan on doing hardcore off roading, they may be more trouble and expense than you need. Over the years, beadlocks have improved. They were known for having intense maintenance, inconsistent quality, and difficulty balancing. These days, there are enough choices on the market to pick some winners, though they are still expensive and a little more maintenance-heavy. If you just want the beadlock look there are many on the market that mimic the style.
When changing your tire size, make sure your wheels are within the range specified by the tire manufacturer. This information is typically included in the tire’s specification sheet. Also on this sheet you will find the Measuring Rim Width (MRW). This is an ideal wheel width to use. This width is where the tire manufacturer feels their tire will offer the best performance and wear. Tire manufacturers also give a range of wheel widths to use with their tires. It is extremely important to stay within this range. Stay safe, stay in the range.
Backspace, Offset, and Scrub Radius
Backspace is the distance from the wheel mounting face to the inside edge of the bead (not the edge of the wheel). Offset is the distance of the mounting face from the centerline of the wheel. Positve offset is when the wheel is moved inward, thus increasing backspace. Negative offset is the opposite. You can get offset from wheel width and backspace by subtracting backspace distance from wheel width. Mounting wheels and tires that are wider and taller changes with every vehicle because no particular combination of parts and vehicle is the same. Changing offset /backspace changes the way the wheel bearings are loaded, sometimes shortening their working lifespan. Also, more negative offset (less backspace) changes the scrub radius (the point where the tire pivots on the ground when turned). This can increase steering effort and tire wear, while decreasing directional stability. More offset by itself moves the scrub radius outward (positive scrub). Scrub radius changes not only from backspace but from changes in diameter too. More diameter, without changes in backspace, will move the scrub radius in the negative direction (and vice versa). With a buildup, more offset (less backspace) and greater diameter occur at the same time and the scrub radius often stays in, more or less, the same spot. There are complicated ways to determine the scrub radius with any particular tire size and wheel offset but we will not get into that here. One guide that seems to work fairly well in many situations is for every inch of increased height to decrease backspace approximately 1/4-inch. This is mainly applicable to solid axles and is somewhat variable. That will not do anything to help the extra load on the wheel bearings but will keep the scrub radius close to the factory setting. To avoid tire rubbing you may need more backspace than that too. It is a somewhat common belief that changes in scrub radius of 1/2-inch in either direction are not a big deal, getting closer to a one inch change has a worsening effect on stability. Generally, positive scrub is more of a problem than negative.
By increasing the diameter of the tire, the tire grip (with more rubber on the ground), and the weight of the tire/wheel you have also increased the work for the brake system and reduced the OE safety factor. Manufacturers overbuild the brake systems enough so that you can go up in tire size by a reasonable amount without causing noticeable adverse effects. What are these adverse effects? The first is a shorter lifespan for the brake linings, discs, and drums. The next is having to press harder on the pedal to get the same response. In some extreme cases, which could be from a combination of big tires and a lot of weight on little brakes, you just might not be able to press hard enough on the pedal to get the job done. The brakes might also quickly fade and overheat. As far as a guide to upgrade brakes after installing larger tires, there are not many totally universal ones. The best we’ve found is quite general: improve braking efficiency by the same percentage as the tire diameter is increased over stock. Going from a 30 to a 33 is a 10 percent increase in diameter. The question then becomes, how do you improve braking power by 10 percent? We cannot easily come up with numbers in that regard, but we can offer brake upgrades in stages.
Stage 1 - A change to high performance ceramic linings (along with resurfaced or replaced rotors and/or drums) offers significantly higher resistance to heat and a greater coefficient of friction over OE organic linings. If your truck already has ceramic linings, you might want to try performance linings and/or go right to stage 2.
Stage 2 - In addition to stage 1, add ventilated rotors (if your existing is solid) and drilled or slotted ventilated rotors if yours are already ventilated. Also add high performance braided brake hoses. These expand much less so, rather than expanding the hoses, more of that hydraulic force can be placed into applying the brake. Generally, most wheelers will never have to go beyond stage 2.
Stage 3 - In addition to stage 1 and 2, add improved calipers. Most often, this upgrade would take the form of two or four piston calipers to replace the factory one or two piston sliding calipers. This may or may not require an upgraded master cylinder. With this upgrade, you cannot really just slap on the parts. By upgrading the calipers, you are potentially changing the brake bias (the front to rear calibration of braking force) so this upgrade should be done in consultation with an experienced company that knows what parts should go on which vehicle. In other words, buy a kit for your exact vehicle.
Stage 4 - In addition to stages 1, 2, and 3, replace rotors/drums with larger parts, a master cylinder, and a power brake upgrade as needed. Essentially, this is a complete brake upgrade. This radical upgrade would be likely required on a vehicle getting a huge tire upgrade. When upgrading to a tire size increase of 50 percent or more this upgrade would certainly be suggested.
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