The Tire and Wheel Combo

Every tire has a range of rim sizes that will offer the best performance and wear. The rim diameter is a given. A 15 inch tire has to go on a 15 inch rim. Period. With regards to width, the tire manufacturer list a range for a given tire. A 12.50-inch wide LT tire, for example, might show a range of approved rim widths from 8.5-11 inches. Some will show a measuring rim width (MRW), which is the rim size used to obtain the section width specification. A narrower rim will make the tire narrower and a wider one will fatten it up a bit. A good rule-of-thumb is that for every half-inch of rim width larger or smaller than the MRW, the section with will change approximately a quarter inch. This can be useful in fitting a tire that almost fits but rubs a little.

The ideal is to make the rim width equal to the MRW, where the manufacturer designed the tire to run. At least stay within the recommended width. If you don’t, squirrelly handling and increased wear could result on the highway. A narrow rim tends to pull the edges of the tread up and away from contact with the road. Too wide tends to make the tread pull up in the center. On the trail, either of these extremes could make the tire more vulnerable to rock damage or being rolled off the rim at low trail pressures.

Some Jeepers like to run at the minimum end of the width spec to help protect the rim on the trail. Others think the wider end of the spec is better as it lets the tire flex better. This is one of those controversies where both sides are right… as long as the rim is within the recommended width range.

Larger Diameter Rims

A recent trend… and a welcome one… is larger diameter  rims. You can now upgrade from 15 or 16 inch diameter rims to 17, 18 or 20 inch rims. The selection of tires to fit these wheels is growing rapidly.

The particular advantage from such a tire/wheel combo comes mostly in the street area. A 37 inch tire on a 15 inch rim has a lot of tire profile (10-11 inches) between the contact patch and the rim. Even when aired up, that makes for a squirrelly tire because the tires tends to roll and flex when braking or cornering. Day to day, with a careful, clued-in driver, that may be livable… except in unexpected emergency situations. The larger rims reduce that profile while keeping the same overall tire diameter and the result is better and more predictable street handling. Many people think the profile of a 33 inch tire on a 15 inch rim, approximately 8-9 inches, is the maximum profile for a tire used day to day on the street. If you run a tire bigger than 35 inches in diameter, you are most in need of a 17-20 inch rim.

The downsides are relatively minor and come on the trail side of things. With the reduced air volume of the lower profile tire, you may not be able to air down quite as much for the trail. Most Jeepers with dual-purpose street/trail machines find it an easy compromise. The profile of a 33 offers plenty of room for airing down to get the flex needed for hard ‘wheeling. Only the most hardcore Jeeper needs more, and the odds are his rig isn’t a daily driver.

Tread Design

This is one of your most critical choices. It will dictate street and trail performance, influence street fuel economy and alter the tire wear equation for the better or worse. The main choice involves evaluating your trail traction needs in the places you most often frequent and balancing them against your street driving needs.

Tread design can be broken up into three very basic categories, the street tire, the all-terrain and the mudder. Four if you want to include the rock crawling variations of the mudder. We won’t talk about street tires at all, other than to say they do the best on the street if that’s what you need. In this case, your 4x4 is likely an all-weather vehicle and you’d want to choose an all season street tire or even a snow rated tire. Mudders and Rock Tires

The more aggressive the tread design, generally, the better the trail performance. The open tread design of a mud or rock tire divides the tread into small blocks, leaving many void areas. In mud this makes for a tire with lots of edges that work more or less like paddles. The void areas are open enough that the mud that collects in them can be thrown out via centrifugal force, leaving the tread blocks open to supply grip as they rotate back down into the goo.

On rocks, the blocks of tread are more flexible to conform to irregular surfaces and have many biting edges with which to grip the surface. On top of that, the ground pressure on these blocks of tread is high. In some cases, the weight of the Jeep is concentrated on just a few of these tread blocks and are practically “glued” to the rock.

What’s the difference between a mud and rock tire? Often very little if we are talking about radials. Most radial mudders perform very well in rocks. Not so much the bias ply. The specialty rock tires differ from a true mudder in way the tread blocks are staggered and aligned and many times are directional. Often, the biggest difference is in the composition of the rubber. Dedicated rock tires often use a softer rubber compound to offer better grip. Grip will be very much better than a standard composition but tread wear will be very much reduced when used on the street. Some rock crawling tires also use extra plies in the carcass to resist sidewall damage.

Aggressive tires have downsides that appear mostly on the street. In the fuel economy department, you may notice a small decrease because they create more rolling resistance.

Tread wear will also be reduced according to how aggressive the tread pattern and the rubber compound. They are often noisier as well, though modern tread design has greatly reduced that trend. Finally, street handling is reduced because of the deep, flexible tread designs. Choose a mudder or rack tire if trail performance is your most important consideration.

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All Terrains

“All-Terrain” is a trademarked name used by BFG since the ‘70s but one which has become a generic term for an all-around tire that does “OK” in all venues. There are more and less aggressive ATs but if you find yourself spending most of your time on the street, or in relatively mild trail situations, the all terrain will deliver better MPG and tread wear than a mudder, while offering better handling to boot. Ice and snow performance (on the street) is better as well. With less void area, the AT puts a bit more rubber to the ground than the MT but the tread is more stable and offers more predictable handling. ATs are also quieter than mudders.

On the trail, the ATs are at their worst in mud but are good to very good everywhere else. The more open tread AT designs tend to perform better in the dirt and vice versa. The opposite is generally true on the street. Due to improvements in technology, the more open AT tread designs can do much better on the trail than ever before. Tread depth is still usually less than a mudder, even on the more aggressive ATs, so they aren’t quite as flexible in conforming to irregular surfaces. Most times, an AT will outperform an MT in sand. Few ATs will equal an MT in rocks, however. Choose an all terrain if your Jeep gets a substantial amount of street miles or is used in winter climates that have snow.

Speaking of snow, there are two tire ratings of interest to those in the snow belt. The first is the old “Mud and Snow” rating (M&S), which was basically the tire manufacturer saying, “Yeah, it’s OK in the snow. Well, obviously, that’s open to some interpretation. The more recent, and useful, rating is ’99 up Winter Tire Traction rating. These tires have to meet an actual winter tire performance standard and those that do are allowed to wear a “snowflake on the mountain” symbol. Many all-season and some all-terrains carry this rating.

The Forgotten Tire

Any tire can be punctured or cut so a good spare is a vital tool. “Anything that will hold air,” is the common cheapskate’s credo. That’s probably a workable philosophy on the street, where a few miles will carry you to a tire shop. On the trail, replacing a grippy-but-deflated tire with a bald spare reduces your traction potential by as much as 25 percent. You then may struggle on the spots you would normally ace. If the diameter is different than the other tires, you could run into diff and locker issues. An automatic locker goes positively nuts with two different diameter tires and may go bye-bye as a result. Ouch! The best tip… have five identical tires and rims. Include the spare into the rotation process and you will be able to significantly extend the life of any set of tires.

Wheel Choices

Your two basic wheel choices are aluminum alloy or steel. Steel wheels are tough, malleable and relatively inexpensive, but heavy. Cast or forged alloy wheels are lighter, usually stronger than steel (especially forged) but not as malleable. The malleability aspect come into play when repairing a bent wheel. Alloys are more problematic to repair, though forged units tend to be more forgiving in that regard than cast.

The weight of the tire and wheel is the, “Grizzly in the outhouse,” when it comes to tire choices. The increased rotational inertia of bigger tires and wheels puts a larger load on your brakes on the street and reduces handling qualities. On the trail, that inertia adds to drivetrain loads and can help break axle parts.

An “average” 33 inch tire weighs about 55 pounds. A stock 225/75-15 tire weighs 28 pounds. Aftermarket steel wheels are usually heavier and stronger than the stock wheels. In averaging a few aftermarket 15x10 inch steel wheels, we’ll use 26 pounds as a number versus about 16 for the stock wheel. So, where the stock wheel and tire combo weighed 44 pounds, a 33 on a 15x10 steel is 81 pounds. An “average” cast alloy wheel (forged are a bit lighter) come in at 16 pounds, dropping the upgraded tire and wheel back down to 71 pounds.

There are two basic types of alloy wheels to consider, forged and cast. There are also modular wheels, which usually consist of a cast center section with a spun aluminum rim. These wheels are way-weak for the trail, so avoid them. In the cast realm, there are low-pressure cast wheels and counter pressure, or “vacuum cast,” wheels. The latter system results in a stronger wheel because fewer impurities are introduced into the casting. Forged wheels are the strongest and lightest because the wheel are made from billet material that is rolled or pressed into shape by millions of pounds of pressure. This double forging makes the alloy stronger by tightening it’s grain structure.

Beadlocks  … Or Not

Hardcore Jeepers that run extremely low trail tire pressures often find beadlocks necessary to avoid rolling the tire off the rim. Most beadlocks use a clamped on ring on the outer bead to grip the tire. Others use an inflatable tube inside the tire (with a second valve stem) and some use a spacer that fits between the beads.

You don’t see many daily drivers with beadlocks. Part of that is the extra expense and part is the potential problems an owner may encounter. For a trail rig seldom driven on the street, these problems aren’t an issue, but for a daily driven, dual purpose machine, they may be.

The first problem to note with most beadlocks is that they are heavier and much harder to get balanced than a standard wheel. Some owners report the clamping ring type beadlocks fail at high speed, causing sudden deflation and parts flying around the highway. There are a still a few do-it-yourself beadlock kits out there that depend greatly on the skill of the welder/fabricator. Many, if not most, beadlock problems can be traced back to human error, either honest mistakes or inexcusable stupidity.

Careful owners report good service from most styles of beadlocks but, expense aside, the first choice is to evaluate your needs. If you are seldom in extreme terrain, seldom aired down to extreme levels and drive your Jeep regularly on the street, why deal with the potential downsides? The inflatable beadlocks and the spacer styles, however, seem the most trouble free in a daily driver situation.

Backspace, Offset

These are two commonly mixed up terms. Backspace is the measurement between the wheel mounting flange and the inside edge of the rim. Offset is the distance from the mounting flange to the exact center of the wheel rim, bead to bead. If the wheel centerline is offset to the outside (away from the axle), the mounting flange moves to the inside and the wheel has negative offset. If the centerline is to the inside, the wheel flange moves outboard and it’s called positive offset.

You can determine offset by measuring backspace, subtracting that dimension from the rim width and then dividing by two. An 8 inch rim with 5.5-inch backspacing has a 1.25-inch offset (8.0-5.5=2.5/2=1.25-inches). Remember that rim width is technically from bead to bead, not between the outer edges of the rim.

A bigger tire almost always comes with an upgrade to a wider rim. Most times the extra width will be added equally to the inside and outside. That keeps the load on the wheel bearings in the place where the factory intended. A stock 7 inch rim with 3-1/2 inch backspace might be replaced by an 8-inch rim with 3-/3/4 inch backspace. One-half an inch is added to the inside and the other half-inch to the outside. How much you can add to the inside can become a problem on the front, which has to steer, so in some cases, more of the extra width is added to the outside edge, for example, an 8-1/2 inch rim with 3-3/4 inch backspace that has an inch added to the outside. That keeps the backspace the same but changes the offset. Changing the offset a great deal in either direction can cause early wheel bearing failure and tire wear issues.