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Techline - Answers To All Your 4x4 Tech Questions

Posted in How To: Tech Qa on June 7, 2016
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Two-Speed Axles
I was intrigued by Tim Esterdahl's question and your answer in Techline (Mar. ’16). It was spot on. But that issue brought to mind another solution, which used to exist exactly because of this conundrum. Namely, back in the ’30s some manufacturers offered a two-speed differential. I think a two-speed differential would allow a more powerful V-8 to provide both the needed speed and low gearing. Do these two-speed differentials still exist? Are they still made? Would they be usable for off-roading? They seem like a natural fit and could be switched simultaneously in both front and rear axles by a pushbutton on the dash.
Bernie Kressner
Appleton, WI

In the early years of commercial hauling, the trucks had engines with very narrow powerbands and four-speed transmissions. Because the transmissions had so few gears, there were large ratio gaps. As you can imagine, this kind of a combo didn't lend itself well to heavy hauling, especially up or down steep grades. The granny-low First gear common in these transmissions was hardly used except for during technical maneuvers or to get rolling from a stop with a heavy load. First gear is nearly worthless when the truck is empty. This essentially turned a four-speed transmission into a three-speed. A two-speed rearend turned the four-speed truck transmission into an eight-speed by providing the ability to split gears. There are a few different two-speed axle designs out there, one of the more common designs uses a planetary gearset housed in the ring gear. A 1/3 reduction is fairly common. A heavy-load driver might start in First gear, then shift from First to Second, and engage the planetary in the two-speed axle. For the next shift, the rear axle planetary would be disengaged and the transmission left in Second. For the shift to Third, the planetary would be reengaged, and so on. This made for some busy shifting that you had to keep track of. The two-speed rearend is generally shifted via a push-pull air or electric actuated knob near the top of the transmission shift lever. Over time, transmissions with more gears and tighter ratios were developed, eliminating the need for two-speed axles in all but extreme heavy-haul trucks. Most modern big rigs hauling heavy loads have 10, 13, 15, 18, and even 20-speed transmissions available to help keep the powerful diesel engines spinning in their preferred rpm.

Nowadays, most trucks that come with two-speed rear axles are far beyond a 1-ton rating. I have never seen a factory-offered two-speed solid front axle. Because these two-speed axles are fairly large, they really don’t lend themselves to off-roading unless you are using massive tires with a lot of power. There are far too many useable five- and six-speed transmissions as well as aftermarket add-on overdrives and underdrives available. Two-speed axles aren’t really a practical or cost-effective option.

Now, having said all that, the currently offered Jeep Cherokee actually has two-speed differentials, but they don’t work the same as a traditional two-speed axle. Rather than using that second speed to split the transmission gears, the Jeep axles use the additional gearing to provide a low range for technical off-road driving. It basically replaces the low range in the transfer case and is shifted electronically.

Wrangler Tire Sizing
I have been sifting through a few articles that you have authored. They popped up after running a Google search concerning tires for the Jeep Wrangler Rubicon. I wanted to bounce some things off of you. I have a ’08 Wrangler Rubicon with an automatic transmission. It has the stock BFG KM Mud-Terrain tires that just hit 50,000 miles. A few of them are cupping and another has a bad camber effect going on. Anyway, it’s time for new tires. What are the tire sizes and types that I could use for it? I am not an off-roader, but I would like to get out of these mud-terrain tires and either get an all-terrain or an all-season tire that could also work off-road. I just read your article about running 35-inch tires versus 37-inch. The 37s are too much for me, but I gleaned some information from these articles. Are there no issues going to a 33, 34, or 35-inch-tall tire? How big of a metric-sized tire can I fit? Thanks for any thoughts.
Kevin Bulman
Via email

As you have found, selecting the best tires for your 4x4 goes well beyond which tire looks the coolest. You need to seriously consider the on- and off-road conditions that you steer your 4x4 into throughout an entire year, and where you plan to go during the life of the tires. Regular harsh snow and icy conditions will generally dictate the use of a completely different tire than what you might use in mud. In most cases, mud tires will not wear as well as an all-terrain or an all-season tire. However, the fact that you were able to get 50,000 miles from a mud tire is impressive. To get that kind of mileage from a mud tire, you usually have to stay on top of the tire rotations and maintain proper inflation pressures. Solid axle vehicles like your Jeep usually keep alignment better than a 4x4 with an independent front suspension. An improper toe setting on any 4x4 will wear tires more quickly. Cupping can be caused by worn shocks. However on mud tires, cupping is common because the tread blocks usually vary in size and flex differently as they hit the ground. This is one reason why it’s especially important to rotate mud tires about every 3,000-5,000 miles.

Unless the only off-roading you do consists of graded gravel dirt roads, I’d stay away from all-season tires. They are designed to work acceptable in all seasons and they won’t really shine in any on- or off-road condition. Also, tires with higher treadwear warranties are generally made from a harder rubber compound. They may last longer, but they provide less traction on all surfaces. However, if money is more important than on- and off-road performance, then by all means go with the all-season tires.

In most cases you can upfit a 4x4 with a tire size that is one to two sizes bigger. However, I have seen people stuff 38-inch tires under a stock JK. It all depends on how much trimming you are willing to do and how much rubbing you are willing to live with. Your Jeep Rubicon comes stock with 255/75R17 tires. These are about 32 inches tall and 10 inches wide. The stock wheels will limit your tire size options because they have a lot of backspacing, which pulls the tires inward toward the control arms and other areas where the larger tires will rub. You can fit a 265/70R17 very easily and you should be able to squeeze 285/70R17 tires on your Jeep with the stock wheels, but there will be some rubbing. To fit 33-inch tires, you’ll need new wheels with less backspacing or a set of wheel spacers for your factory wheels. You may also want to add a 1-inch lift for added clearance. For tires bigger than 33 inches you will need to make more modifications.

Rubicon Gearing Clarity
I am confused. I have seen you, and some other writers, criticize the Rubicon for being geared too low in the transfer case. Is this concern directed toward automatic-equipped Jeeps only? With the manual it seems that we can choose from 12 forward ratios, 10 if it’s a five-speed, which seems to be plenty for finding the right torque/speed combination. I’m interested in your thoughts.
Jon Kann
Via email

There is no arguing that the Jeep Wrangler Rubicon is the most capable factory 4x4 ever offered. However, as with anything, it’s not perfect. It does have some flaws. The Rubicon capability is focused on rocks and technical crawling. One of my issues stems from the 4:1 low range gearing in the Rubicon Rock-Trac transfer case behind both manual and automatic transmissions. Generally, you can venture around off-road in 4x4 high range just fine. When the terrain gets more technical, you can shift into 4x4 low range. While it is nice to have these 10 or 12 forward gears at your disposal, I think the gap between the Rubicon high and low range is too great, at least for the kind of off-roading I like to do. On loose steep hillclimbs, sand dunes, deep mud, and deep snow, you need to have more wheel speed. In many situations, the 1:1 high range is too high. The engine doesn’t make enough torque to maintain a suitable rpm under load. I find myself toggling from First to Second and back again. The Rubicon 4:1 low range only offers a top speed of about 25 mph before the engine hits redline. For me, 25 mph doesn’t cut it, especially in tall dunes and snow. Now, let’s say you can produce enough wheel speed for your application in Fourth, Fifth, or even Sixth gear in low range, Reverse is still far too slow in 4:1 low. In order to have any wheel speed at all in Reverse, you have to shift out of low range and into high range. This negates the ability to rock the vehicle back and forth to get unstuck if you get mired down. With the Rubicon models, I’ve found that I’m often shifting the transfer case more than I should to meet the wheel speed requirements of the terrain I’m in. The simple answer is that the 2.72:1 low range found in the standard Wrangler models better suits my driving style and the terrain I frequent. Don’t get me wrong, I still think the 4:1 low range in the Rubicon transfer case is an awesome feature that fits the slow technical terrain and driving style of most Rubicon owners. Personally, I’d love to see Jeep offer a three-speed transfer case. Imagine if you had the standard 1:1 high range, a 2:1 middle range, and a 4:1 compounded low range. This would provide great gearing for nearly every scenario. Of course there are aftermarket products out there that accomplish this or something similar, but I’d like to see it as a factory option. Ultimately, if the Rubicon low range gearing works for you, stick with it. But my preferred JK Wrangler would have a 2.73:1 low range in the transfer case.

Rock Rattle
How do you get a small rock out of a ’02 Chevy Silverado 1500 torsion bar crossmember without removing the crossmember? I have a small quarter-sized rock stuck in my torsion bar crossmember, and it rattles and clanks when I hit the brakes.
Elias Corey
Via facebook.com/JohnCappa4x4

It’s not uncommon to get rocks, mud, and sand inside the frame of a 4x4. Some frames are designed to keep debris out better than others, and some frames have plenty of large strategically placed drain holes to let water and other elements out. Stones and pebbles in a frame can be an annoyance, but mud can be destructive, especially when combined with salt residue. To remove annoying stones that rattle in your frame, you can usually blow them out with a pressure washer or air gun. At the very least, you can get them situated so that you can smash them with a punch and hammer through an access hole in the frame. Caked on mud in a frame can be more difficult to remove, however it can be done with a pressure washer. It’s a very time consuming and messy process that will be worth your efforts if you want your 4x4 frame to last a long time. Consider enlarging some of the drain holes to help the washed out debris escape. In extreme cases you may need to cut open a large drain in the side of the frame, wash it out, and then repair the area. Once you get the frame clean, you can spray the inside of it with a rust inhibitor like Eastwood (eastwood.com) Internal Frame Coating. The coating comes in a spray can with a special hose and tip that lets you coat the inside of the frame via boltholes. Once you have the frame protected, you might consider inspecting the frame to see where the majority of the debris is coming from. There are often access holes near tires that will collect flinging dirt, stones, and muck. Plug any offending access holes with rubber plugs or even duct tape, but make sure your frame still has the ability to properly drain.

Titan Trouble
I am really sorry to ask to such a stupid question that even Discount Tire can’t seem to answer. I have a ’11 Nissan Titan Pro-4X that came with P-rated 275/70R18 tires from the factory. The psi was 35, and now I’ve put the same size tire on but it is an LT tire with an E load range. Should I or should I not bump up the psi for the LT tire? Whatever you come up with will really help.
Greg
Camp Verde, AZ

Installing aftermarket tires is becoming more and more difficult as new technologies infiltrate our 4x4s. Tire pressure monitoring systems can be a big hurdle to get over. I find it a little disturbing that the Nissan Titan Pro-4X (an off-road trim package) has passenger car tires from the factory. But I digress. I’ve found that most people run too much air pressure in aftermarket and larger off-road tires. They often simply comply with the pressure molded into the tire sidewall. This can result in poor traction, uneven tire wear, a harsh ride and decreased tire life. I would recommend running the correct pressure in the tires, regardless of what the door sticker, tire pressure sensors, or tire sidewall dictates. The max psi number on the tire sidewall is generally far too much for most 4x4s. Too much air pressure causes the center of the tread to be the only portion in contact with the road. You can check to see if you are using the correct pressure for the weight of your 4x4 by running a chalk line across the tread. Drive straight forward on a smooth flat road for 50-100 feet. Inspect the chalk mark to see where it has worn away. If it’s worn away in the middle only, you have too much air pressure. If it’s worn away on the sides only, you have too little. Adjust the air pressure until the chalk mark wears off uniformly across the entire tread. You’ll likely find that the front and rear of your Titan pickup will require two different air pressures for proper inflation.

The new load range E tires will likely support much more weight than the factory passenger car tires at a given pressure. Hopefully the pressures that your new tires require will fall within the specs of the truck’s tire pressure monitoring system. If not, you’ll end up with an annoying tire pressure warning light in the dash. Some people ignore the system and simply put a small piece of black tape over the light in the dash. However, you may be able to work with your dealership to have the system recalibrated to the new pressures. Many of the more popular and commonly modified vehicles such as Jeeps and Ram trucks have aftermarket programmers available to reset the tire pressure parameters. Unfortunately, I don’t know of an aftermarket programmer that does this for the Nissan Titan.

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