Aftermarket Accessories, Part III: Tired of Your Tires?
Or, One Thing Leads to Another! by Bill Burke

tiresiconIn the last two articles about accessories, I talked about having the proper recovery equipment and winches. There are several different ways to go from here–tires, suspensions, lockers, body lifts, body protection and interior refinements et al. Things like CB radios, first aid kits, fire extinguishers, gas cans, spare tires, tools and spare parts also have their place in the accessory line-up.

The most common improvement seen as an aftermarket accessory is the tire. Right off the showroom floor, we see that the average tire installed on the vehicle by the manufacturer is usually a “P” (passenger) rated tire. Yes they usually put a passenger car rated tire on your 4WD rig. Understand that the most amount of driving for any SUV is on the pavement and the manufacturers aim toward that end with the initial setup of the vehicle, hence the very large aftermarket accessory availability. Not all SUV’s come with “P” rated tires; however, some have the “LT” (light truck) designation.

Changing the tires on your rig means a significant investment, especially if you want to go up to a bigger size. It can open a Pandora’s box! Usually, though, adding more ground clearance is the reason to go to bigger tires, as is gaining better traction. Most tires on stock vehicles are in the 29″ range diameter. On any trail, you will usually see rig tires in the range of 31″ up 35″ in diameter. There are bigger tire shod vehicles out there, but I will stay in the realm of the average 4WD owner. I will not get into tread design or tire make except to say that I like an aggressive tread pattern. I have found them to be good on all terrains, even sand if aired down enough. It also depends where you spend most of the time 4-wheeling. But, that’s another article!

And here is where one thing leads to another! With bigger tires comes suspension and gearing related modifications. These affect steering geometry, power transference from the transmission/transfer case and driveline angles. Depending on the stock ring and pinion ratio and the overall final drive ratio that is already installed, you may be able to get away with going up a size or two from the stock tire, unless it is already an option type tire that is in the 31″ diameter range.

To put those 33’s or 35’s on, you must modify the suspension and maybe do some body lifting or fender trimming. You will also have to change the gear ratio in the axles–the ring and pinion. And while you are in there, you might as well install those air lockers you wanted. You can see that, indeed, changing your tires does open up many possibilities.

Can the wheel bearings, drive train, axle housing, engine and even the brakes handle/stop the added mass of bigger tires and wheels? That poor little 4 cylinder barely gets over Eisenhower tunnel as it is. Add another couple hundred pounds and it’s struggle city. Now you are thinking about a V-8 conversion! If the tires and axles are done right, usually the drive train and power train will handle the extra weight just fine, unless you have an older rig with lots of miles that has been ridden hard and put away wet too many times.

You bought that new SUV and you just got the IRS refund check! It’s time for new tires. You’ve also been talking to your friends who want to spend your money real bad. They say that ARB lockers are the cat’s meow. Of course, since you are going to put bigger tires on, you have to change the ring and pinion and add a suspension lift. And since you are messing with the suspension, you might as well do a shackle reverse kit and a body lift to keep the tires from hitting the fender wells now that you have all that articulation. STOP! Does it never end? Well, not really! What about the highway performance and street ability? Will they be compromised? If you (or the shop you choose) do a clean and proper installation, then the vehicle will handle really well on and off highway! So, you see, it is not as easy as just going out and buying a new set of tires.

You’ve picked out a set of tires and, depending on the size, you must install a suspension, or you just want to improve the handling characteristics. Adding a couple of inches lift to your rig is not out of the realm of possibility. Lifting a rig over 3 or 4 inches gets complicated and must been done with the proper equipment and engineering, whether or not you are doing a leaf, coil, independent or mixture. Driveline angle is the biggest concern and steering geometry the second biggest. Make sure the caster angle is corrected on the front end. This helps the steering tires return to center.

Some lift kits include a transmission/transfer case “drop” kit to lessen the angle on the driveline. Otherwise, you will be going through u-joints prematurely. Some lift kits are not really good suspension systems. They just lift the vehicle up and don’t afford any better articulation. A good suspension will allow the tires to flow over the terrain with lots of flexibility. Downward wheel travel is the key to articulation. It is great that the tires come up into the fenderwells and the axle bump stops work, but the real key to suspension travel is how far down it goes.

Flexibility is another important aspect of the suspension. When one wheel goes down, the other wheel (on the most common solid axle set up) moves up, and the springs must flex and twist to allow for the added drop rate of the other side. Some suspensions, like the independent front end style, will allow one wheel to move independently of the other, hence the name! The steering is affected by all this movement and must be strengthened also with a stabilizer, support brackets as needed, and possibly a steering gear box support.

The shock absorbers or suspension dampers, must be changed as well. Usually a good suspension will come as a kit–springs, shocks (dampers), steering damper (stabilizer), bushings and hardware. A reputable company will have a matched kit for your specific vehicle and the type lift you are looking for. Don’t cut yourself short by just trying to get by with one item of the kit. Save those pennies and get the kit all at once.

There are also protection plates for the spring pads, depending on what type suspension and shock mounts. These plates will keep rocks from shearing off the heads of any bolts, holding the shocks or springs in place. Stronger mounting shackles are available that will hold the springs in place (leaf springs) better and in proper alignment during all that flexing and springing taking place on the trail.

A good set of skid plates or, underbody protection, are a must if you are going to do any rougher 4-wheeling than dirt roads to the trail head or fishing spot! Protection for the gas tank, transmission/transfer case, differentials and or the front end are a consideration. There are aftermarket protective bars that will save the side rocker panels and fender corners. Some 4-wheeling gurus will even cut off the ends of the bumpers to allow the tires to climb easier over obstacles instead of having the bumper hit the rocks.

Tires, suspensions, axle gears, lockers, nerf bars and skid plates each have a book full of information alone. I have tried to inform you about the things needed to upgrade if you really want to go to a much larger size tire than the stock application. As I said earlier, to just go up a size or two from the tire that came with your rig would be alright, as long as that tire was not already an option or add on. You can usually stay with the stock rims if they are between 71’2 and 8 inches wide. They will hold a 31/10.50 X15 size tire.

I have mounted a set of 33/12.50 X15 tires on an 8 inch rim. I like the fact that the sidewalls bulge out and help protect the rims. Usually when the tire fits the rim flush, the rims get scuffed. On some trails, rocks or tree roots will catch the rim and bend it up, releasing air. So, it is alright to have a tire bulge on the rim somewhat. Don’t let the tire store sell you rims when you may not need them. An experienced tire dealer will listen to you and know what you are getting at.

Now that you know that upgrading tires can open a Pandora’s box, I hope you have fun using them. It might not take doing all that modification to the suspension, etc. if you just want to get a new set of tires that are a little more aggressive. These can afford you many miles of off-highway pleasure without all the “extras” that extensive modification requires.