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Top Jeep Tech Questions - Common Queries

Posted in How To: Tech Qa on February 19, 2014
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Believe it or not, the editorial staff of Jp possesses superhero-like superpowers— but they are not as useful as being able to stop a speeding train, deflect bullets, fly, or even prevent crime (above possibly screaming “Stop thief!” and firing off a few well-placed rounds). Our secret powers consist of an extreme knowledge of Jeeps, Jeep problems, and Jeep fixes—all this is the result of being obsessed with Jeeps for as long as we can remember. Sounds fancy, huh? Yeah, it is, and the ladies love it. Um, not really. Also these powers have limits, and we occasionally hit them when we get questions from readers, but luckily for you, we also like learning and sharing what we learn. Having said all that, we love getting questions about Jeeps. Some questions are more of a challenge than others. Some we get over and over again, but that does not meant that they are not good questions. That’s the focus of this article, answering all those common questions that we frequently get and that need answers. Please keep the questions coming!

To Regear or Not To Regear
Hey, ya’ll at Jp, I just bought a Jeep and added a lift kit and big ol’ tars (that’s tires to you and me). Now my Jeep is a pig and I can’t go up hills on the highway without rowin’ through the gears like a raccoon washin’ crawdaddys. I’ve heard I should regear my Jeep. What’s that mean, and do I have to?

Adding larger tires to your Jeep changes the overall drive ratio of the vehicle. The easiest way to rectify this is by regearing the axles. Unfortunately, regearing properly requires lots of know-how and specialized tools. Your money is well spent regearing, and you are better off not trying to make this upgrade a DIY affair.

We get this question all the time, and heck, we all used to be on the other side of this question at one point or another back in our respective “first Jeep” fledgling times. The fact is adding larger tires to your Jeep makes a mechanical change in how it works. Why? Well, the larger tire takes a greater distance to make one full revolution because the outside diameter of the taller tire is also larger than its stock counterpart. The easiest thing is to change the axle ratios in your Jeeps axles back to close to the factory ratio with these larger tires. Here is the math for figuring out your new gear ratio: (new tire size / old tire size) x original axle ratio = new ratio. For example, if you bought a new JK and wanted to add some 35-inch tires, you will want to regear. Here is the math: (35/29) x 3.21= 3.874. This means we would be good with about 4.10s. If the Jeep is a crawler or used for towing, you may want to go a little lower than that, such as 4.56.

Do you have to regear? Nope, but it’s the easiest way to improve fuel economy, regain that lost pep, and your sanity. Also, if the change on-road is not enough to convince you, then the benefits off-road should do it. Off-road you will enjoy better crawling, more usable power, and less wear on parts like the clutch or auto transmission.

Thank Pete
I say good day, Jp editorial staff. What is the largest tire I can fit on my Jeep with X inches of lift?

We’ve been lifting Jeeps here at Jp for years and we have a pretty good knowledge base regarding what tires will fit with what amount of lift, proper bumpstopage, and maybe a little (or a lot) of fender trimming. Use Google to find a story called “What Hits, What Fits” for the answer to your tire size lift question.

Well, thanks to Pete and the experience of the rest of the Jp staff over the years, the answer is pretty easy to find. Either take the analog route and dig up your March ’11 issue of Jp and look for “What Hits, What Fits,” or, if you have any idea what the interweb is, just get on your local Google machine and search for “Jp magazine what hits, what fits.” Information on what tires will fit with what amount of lift for basically all Jeeps ever produced will come up. Having said all that, if your YJ has 500,000 miles, the springs may have sagged a touch and you may want to err on the side of a smaller tire. If your low-mileage cream puff TJ has a brand new 4-inch lift, then you are probably safe running those 33s.

First Things First
Hi Jp, what do I do first, spend my cash on suspension lift and bigger tires, or get a winch and body armor?

Hear us now and believe us later, you can pretty easily screw up the rockers of a nearly-stock Jeep. Having good rocker armor is definitely one of the first upgrades you should make to your Jeep. Furthermore, rocker guards are way easier to install when your rockers aren’t all beat up.

We get this question all the time, and honestly, it is one of those questions that really depends on what you want to do with your Jeep. Planning on going expeditioning 1,000 miles from nowhere by yourself? First, that’s a bad idea, but if you have to, you’d better get yourself a reliable winch, tree strap, a Pull Pall land anchor, water, food, fuel, and so forth. Are you going out to hit the local mud hole back east with all your pals on the weekend? Add your lift and tires—and towhooks. Honestly, regardless of what you are doing, rocker guards should be near the top of the list. We’ve damaged rockers on stock and near-stock Jeeps more than we care to admit. It’s a vulnerable, difficult-to-repair area on most Jeeps, and a little insurance is worth the price of admission.

Yearning for More Go
Yo, Jp, what up? After bolting all kinds of heavy ghetto armor to my Jeep, it’s acting sluggish and I’ve been hoping I can add 200-300 horsepower. Oh, and I am broke! What can I do to get the revs up without spendin’ all my lettuce homies?

Umm…word up! Your Jeep is probably not going to pick up 200-300 horsepower regardless of how much money you toss at it—unless it has a tired four-cylinder and your upgrades include swapping it for a built 401 V-8. We are generally all for upgrading to a free-flowing air intake system and exhaust, and maybe an electronic tuner if your Jeep can use one, but much beyond that, things get expensive and complicated. If you are coming at Jeeps from the hot rod world and speak the lingo of engine building, cam swaps, and raising compression, then you probably know how to make more horsepower. Good for you. If you don’t, you can easily learn about making internal improvements to your Jeep’s engine, but in the end you may not be helping your Jeep work off-road. Why? Well, the vast majority of Jeeps don’t ever get used at or near their red lines where maximum horsepower is made. What and we all want is more torque and more specifically usable torque. That is the grunt you feel in the seat when your Jeep climbs. Many hot rod parts are built to get maximum horsepower at a higher RPM, not maximum torque at a lower RPM. Generally, Jeepers should be interested in an engine that makes maximum torque at 1,500-3,000 RPM while burning low-octane fuel.

Another option is to force-feed your Jeep’s engine with more air with the help of a turbo or supercharger, and there are a few options for some of the Jeep engines out there. Some Jeep engines are strong enough to handle forced induction, while others are not. If you have an AMC-based I-6, V-8 or four-cylinder, the bottom end can probably handle forced induction. The 3.8L V-6 found in JKs, not so much. Either way, turbo and supercharger systems are not inexpensive and generally require use of high-octane fuel. Nitrous? Well, yeah, that will boost horsepower for a brief time at full throttle and high RPMs, and thus is probably not really appropriate for a trail rig or crawler.

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Dubious Axles
All my friends are making fun of me and saying that my Jeep’s rear axle is crap. Why are they doing this? Are they correct when they say my axle is weak and a liability? What should I do?

Pretty much all Jeep axles are up to normal use. Add big tires, lockers, and a heavy right foot, and several of these components start to show their week sides. Luckily, axle swaps are common and aftermarket parts are easily available.

The fact is that some Jeep axles have a bad reputation. Some of that is hype, some is based on maintenance issues, and some of the reputation is based on smallish parts being asked to do way more than they should. The AMC Model 20 in CJs, Dana 35 and Dana 30 with central axle disconnect all do the job that they were designed for when properly maintained. If the two-piece shafts in an AMC 20 or Dana 44 are worn and/or the retaining nut has loosened, something is gonna give. If the vacuum lines on your center axle disconnect are dry rotted and leaky, you can bet it won’t work when you need it. If you are running a locked Dana 35 with 31-inch tires or larger on heavy rock trails, you can expect it to break, and when it does, the C-clip design is gonna leave you stranded if you don’t have a spare shaft. We’ve heard of guys successfully running locked Dana 35s and large tires off-road second hand, but then again, all of us on staff at Jp have heard the snap of a Dana 35-shaft out on the trail firsthand. Luckily, you are not alone. If you have an Model 20 or Dana 44 with two-piece shafts, make sure the retaining nut is tight and then start saving for a set of one-piece shafts. Look for forged shafts with rolled splines as opposed to those with cut splines. Have a Dana 30 with a center axle disconnect? Maintain it and replace any questionable old vacuum lines. Have a Dana 35 in your Jeep? Start saving cash and scouring Craigslist for a Dana 44 to swap in its place or start researching other axles to swap in its place. Tossing money at a turd is a waste, in our collective opinion.

Bendy Bendy!
I have a Jeep built after the mid-’80s, and I think my axle is bent. What do I do, and is there any way I could have avoided this problem?

Bending a front or rear axle is not a new problem, but it does seem to be a growing one. It turns out that if you beat your Jeep off-road while jamming some Hall and Oats down whooped out trails or seek air time like you’re riding your motorcross bike things are gonna fail. Jeeps are amazing off-road and they do many things well, but there are limits and most Jeeps are not set up as Baja-level race cars. Also Jeeps seem to be getting bigger and bigger. Heavy Jeeps with big tires are gonna cause failures at the weak points and a Dana 30 hefting a 7,000-pound JK around is gonna fail.

The weak link in most modern Jeeps is the weakish thin-walled axletubes and relatively tall cast inner knuckles. Does your axle look like it’s smiling? It may, and there are a couple of ways to check. First, your smart phone probably has a handy level already on it. If it doesn’t you can probably download one. Activate the level and place it on the flat surface of the axletubes as close to the knuckles as you can. Are you getting the same reading on each? If not, your axle may be bent. Finding a bent knuckle is a bit more complex, but should be very apparent with a simple alignment or when tires start to wear oddly (like the inside tread wearing more quickly than the outside). If your axle is bent, the easiest remedy is to get a replacement housing. There are plenty in junkyards around the country, and a few are available in the aftermarket. A good example of that is the TeraFlex Tera30 Heavy Duty Front Replacement axlehousing (PN 3503000 for 0-3 inch lift JKs and PN 3503004 for 4-inch-and-up JKs, both at $1,799). If your housing is not yet bent or can be straightened, you can add a sleeve and gusset kit like the D30/D44 Front Axle Assurance Kit from Synergy Manufacturing (PN PPM-8012-50-30 for JK Dana 30s, PN PPM-8012-50-44 for JK Dana 44s, $212 plus installation and soon available for TJ, XJ, ZJ Dana 30s, and TJ Dana 44s).

At the end of the day, a bent axle is not the end of the world unless it is causing your expensive off-road tires to wear funny or if it is eating axle seals. Luckily for us, several off-road companies have come up with solutions to modern Jeeps’ more bend-prone axles.

PhotosView Slideshow

Old dudes who work at Jp, I just added a 3.5-inch lift to my ’98 TJ and it vibrates like crazy on the highway. I added the T-case drop spacers and that helped, but now my center skidplate is hanging down too low. One of my BFFs from the Jeep Internet forum said I need an SYE and all my problems will be solved. What is an SYE, and do I need one? Thx.

Slip-yoke eliminators have been around for a while, and prices for them and a custom double-cardan driveshaft are pretty reasonable. If you are a true cheapskate with an XJ, know that depending on your rear axle you can use the front double-cardan driveshaft from a four-door in the rear after installing a slip-yoke eliminator.

Oh good…now there’s emailing with text talk and abbreviated spelling. Damn kids. A slip-yoke eliminator (SYE) is a part that bolts to your Jeep Wrangler’s T-case. It allows the use of a slightly longer driveshaft than your factory rear driveshaft, which BTW, is crazy short. It also means that you can add a double-cardan joint to your rear driveshaft. This allows you to rotate the pinion of the rear axle up, which further reduces the extreme angles that cause your vibrations. As a basic rule of thumb, if you have a short Jeep Wrangler sold after ’86 and before ’07 (i.e., non-JK Wranglers), you probably need a slip-yoke eliminator if you plan any lift at all. With longer Jeeps like XJs, ZJs, WJs, LJs, and so forth, you’ll probably want a slip-yoke eliminator and a new driveshaft for any lift over 3.5 inches over stock.

The Jeep Shake
Howdy, Jp folks. Round here my ol’ Jeep “Bessy” has a started shimmying and a shakin’ like a ornery bronc’ with a Greenhorn rider. This here shakin’ happens just as I reach my old cruisin’ speed of 40 mph on yonder two-lane interstate. What’s a causin’ this, and can I fixer? Or is it off to Dead Jeep Hill for the old girl?

Death wobble is scary. Also it’s usually caused by a combination of things, not just one worn part. The main contributors are usually worn-out tie-rod ends, control arm and track bar bushings, worn ball joints, alignment out of specification, and so on. Also, tires out of balance or mud in the wheels can contribute to death wobble. Fighting it can be frustrating and expensive, but at some point you’ll probably figure it out if you follow our advice.

Don’t give up on Bessy just yet. What you describe above is what we in the Jeep world like to call death wobble. It happens, especially to newer Jeeps with link-type front suspension. The problem is that death wobble can be caused by many different things or by a combination of problems. The best thing to do is to start looking for worn components and replace them even if they are only a little bit suspect. Check tie-rod ends, control arm bushings, track bar bushings, ball joints, and unit bearings. The best way to test all these parts and possibly find the one that has failed is to park the Jeep and get a friend to saw the steering wheel back and forth while you get under the front end and have a look. Hear a pop or see something moving a fair amount? That’s probably your culprit, or at least a co-conspirator in the death wobble. Then jack one front tire off the ground at a time and push/pull at 12 o’clock and 6 o’clock. Any play here means your ball joints are bad. Then push/pull at 9 o’clock and 3 o’clock. Play here could be a bad tie-rod end. Also, death wobble can be caused or at least exacerbated by out-of-balance tires, mud in the wheels/tires, and improper alignment toe-in, camber, or caster. You can try having the front tires re-balanced or having an alignment done. Heck, a bent wheel or caliper-retaining clip behind an aftermarket wheel could cause death wobble. Let’s not forget poorly-worn tires from lack of rotation. Hopefully after all this, Bessy will run down the road as straight as an arrow.

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