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Thread: Tire & Wheel Info
06-07-2008, 09:55 AM #1
Tire & Wheel Info
I found this article interesting and thought I would post it here...I do a lot of reading of different forums and I think I will start copying/pasting interesting or useful pieces over to the Jeepz.om forums, since I like it here and figure that instead of being a fly on the wall, I will add some goodness to the community. I stumbled on this one when trying to google to figure out the optimal tire pressure for my rig on the highway. I probably won't go seeking out a scale to weigh my Jeep and get as detailed as he gets here - but for those that like to get technical - this is probably some good discussion.
Original link is from 4wd.com/advisors/tire-wheel-advisor dot aspx (I don't have 5 posts here yet so it won't let me link it for you. hehe)
Tire and Wheel Advisor
by Jim Allen
For most Jeepers, a tire upgrade is the biggest trail performance bump for the buildup dollars. It offers improvements in both vital trail performance parameters, traction and clearance. To start, please refer to the Suspension Advisor for more tips on what tire sizes will fit on your Jeep. While you can get an improvement in performance from even a small size upgrade, the benefits multiply as you go bigger… within limits. There comes a place, and it varies from Jeep to Jeep, where the cost, complications and compromises have to be very seriously considered and balanced against your end use of the Jeep.
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 4x4 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.
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 rockcrawling 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 treadwear will be very much reduced when used on the street. Some rockcrawling 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.
Treadwear 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.
“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 treadwear 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.
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.
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.
Many Jeeps use a small 5 on 4.5-inch pattern. This is fine until you get into large diameter tires. Big and tall tires will focus a lot of stress and strain over that fairly small area and trouble can result. It can appear in the form of broken wheel studs, bent wheel flanges, damage to the wheel at the mounting points, among other things. Many experienced hardcore Jeepers think the 35 inch tire is the point where some consideration should be given towards an upgrade to a 5 on 5.5 inch pattern. Tires over 35 inches, especially on a hard worked Jeep, make an upgrade even more necessary. Since hub conversions are generally an aftermarket product that’s of higher quality than the OE piece, you end up stronger in more ways than just the wheel flange. Eliminating the unit bearing hub found in XJs, YJs and TJs offers one less weak link to offer up as tribute to the Trail Gods. In some cases, you can include a full-float conversion to the rear axle.
A spacer is sometimes necessary to cure minor fitment issues where the alternative is prohibitively expensive. A common example is to correct the offset or backspace of a wheel and tire that’s touching where it shouldn’t. When kept within an inch, that’s an OK option. Keep in mind that the bolts holding the adapter to the hub, which lie unseen under the wheel, can work loose if not properly torqued and regularly checked! Check them a few times after mounting the adapter. Use spacers as a minor corrective device, not so you can use totally unsuitable wheels. A quality spacer or adapter is usually made of billet aluminum and pretty strong but spacing a wheel out some ridiculous amount is potentially dangerous.
Tech Tip: Finding Street Pressure For Larger Tires
When you increase the tire size, those factory pressure recommendations go right out the window. If your stock 225/75R-15 tire had a 29 psi street rating, the 31x10.50 you replaced it with might need only 22psi to carry the same load. You need a certain volume of air to support a given amount of weight, regardless of the tire size. When you have to carry that weight on a small tire with a small internal volume, you have to really pack it in and this increases pressure. Putting the same volume of air into a larger tire, which has more internal volume, creates less pressure but it carries the same weight.
A typical 35 x 12.50-15 might show a maximum load rating of 2550 pounds at 35 psi. That means at 35 psi, those four tires can carry 10,200 pounds, or 5,100 pounds a pair. Your Jeep weighs 4,500 pounds. How much pressure do you need again? There are many ways of finding the new street pressure for a new set of big tires. All involve some trial and error. Here’s one way.
Step one is to get your Jeep weighed so you get the front and rear weight separately. Load the Jeep as it’s most often driven, including a full tank of fuel and you. If you want to factor in some different scenarios, weigh it light and then with a capacity load. You can get a vehicle weighed in a variety of places, including the local landfill, grain elevators, scrapyards, and truck scales. Once you have front and rear weights, you need to see the manufacturer’s load vs inflation charts for the exact tire you are using. The tire shop will probably have this information, or can get it. You may also find it on the manufacturer’s website or by calling them.
First, divide the weight at each end by two to get the individual weight on each tire. Take the per-tire weight and match it to the chart the get the pressure you need for that weight. If the pounds don’t match exactly, move up to the next higher pressure. These load vs inflation charts will look like the one in the sidebar, which is a generic version for some popular tire sizes from the Tire & Rim Association.
You likely won’t weigh side to side but if you did, you might find the tire on one side is loaded more than the other. You can compensate by measuring the tire profiles and adding pressure to the low side until it equals the high one. Take the measurements on flat ground and on smooth concrete. Front and rear weight, and therefore pressure, will also be different because on end of the vehicle is almost always heavier than the other.
The final check is the heat test. All tires generate a certain amount of heat running down the highway and this heat will raise the tire pressure somewhat. This is normal. Pressure too low will generate more heat than normal. Check for this by measuring tire pressure after a short, high speed run.
You already have your cold pressures. Take a fifteen mile run at highway speeds (55mph and up, but within the speed limit), stop and measure the tire pressure. If you live in a particularly hot area, do this in the morning before the pavement is blistering hot from the sun. If you live in a cold area, do the test in the warmer part of the day or wait for warm weather. Pull over in a safe spot and immediately measure the hot tire pressures. They should be about 10 percent higher than the cold pressures. If they are higher, add a few pounds of air to each pair (front or rear), let the tires cool and repeat until you get the correct results.
Tire Load vs Inflation Charts*
Each tire is rated differently by manufacturer for maximum load at a given maximum inflation but this chart will give you a general idea of how much weight a given size tire will carry at a lower inflation.
Radial Tire Load (pounds) vs Inflation (psi) Charts
Tire Size 25psi 30psi 35psi 40psi 45psi 50psi
30/9.50-15LT 1240 1410 1570 1715 1855 1990
33/9.50-15LT 1565 1780 1980 2170 2345 2510
31/10.50-15LT 1400 1595 1775 1945 2100 2250
32/11.50-15LT 1575 1795 1995 2185 2360 2530
33/12.50-15LT 1765 2000 2225
35/12.50-15LT 2015 2295 2555
*This chart was compiled from Tire & Wheel Association Charts and doesn't represent the recommendations of any particular manufacturer. Use it as a general reference.97 TJ Sport-4.0L-Green: 33X12.5's (BFG AT)-Amrcn Rcng Chmbr Tfln whls; 4" Tera lift; JBA Header; 4.56 Grs; Body Armor rear bmpr w/Swing-Away tire carrier, 2 jerry cans, & hi-lift jack; Body Armor frnt Triple Hoop bmpr w/ Smittybilt XRC-8 Winch; Tuffy expand-a-trunk, cntr cnsl, & glv bx; Wet Okoles; Cobra 75WX; Viair onboard air-450C/2.5; K&N Air; Optima Yllw Top; 4 Hi-Liters; Bestop Fndrs; Yakima Rk; Ravelco ...and more coming!
06-07-2008, 03:32 PM #2
Re: Tire & Wheel InfoLarger Diameter Rims
A recent trend… and a welcome one… is larger diameter 4x4 rims. You can now upgrade from 15 or 16 inch diameter rims to 17, 18 or 20 inch rims.
08-20-2008, 08:03 AM #3
Re: Tire & Wheel Info
TEEJ - If you feel like pasting a correct version here, I'll go back and edit the original post.
08-31-2012, 12:02 AM #4
Re: Tire & Wheel Info
There is a much easier way to find the correct street pressure for your tires. First load the vehicle as it is normally driven, fill the tires to the maximum rated pressure and find a purely level parking lot that will allow you to drive about 100yds without turning, next take some cheap soft sidewalk chalk and draw a solid straight line squarely across the tread of each tire at 90 degrees to the sidewall, mark the side wall so you can find your chalk line incase you wear it completely off. Now just drive in a completely straight line for a SHORT distance (yards or feet), stop and check your line across the tread. If you started at maximum inflation the line should be wearing off more in the center of the tread than at the edges. drain off a couple pounds of air and rechalk the lines and repeat. What you are trying to do is find the inflation point where the chalk line across the tread wears off completely even without turning your steering wheel. when you reach that point you have 100% contact of tread patch with the road, and this is the optimum inflation pressure for THAT TIRE in THAT Position with THAT load in the vehicle. Every other method is just an estimation and guess. This method is based off of actual tire contact and pressure of the the tread against the road surface. it will ensure the longest lifespan of your tires and the mose even wear of the tread. You will note thar each position will often have differing pressures. this is due to vehicle weight distribution and individual tire variations. I was taught this back in 80 by a Cepek tire dealer...
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