Tech Corner: A Beginner’s Guide to Building a Rock Crawler, Part 1

There comes a point in time when looking the part simply doesn’t cut it anymore. Whatever your reason for wanting to step up to the big leagues, it’s important to understand that building a rock crawler —a real rock crawler—requires more than just some gadgets and gizmos that make a rig look trick. Upgrading a 4×4 to tackle true rock crawling requires thoughtful planning, smart shopping, and a lot of research.

Today, we’re going to give you a starting point for building a true rock crawler out of your favorite toy.

Axles and Differential

So, where to start? Well, the first thing to ask yourself would be whether or not your existing axles can handle the abuse. Rock crawling is like kryptonite to weak axles. All it takes is a good amount of momentum behind a wheel and a sudden gain of traction for everything to go to hell. “Upgraded axles should be on the short list of any rock crawler,” says Dan Guyer, Category Manager of Wheels and Tires at Keystone Automotive. “Pull your stock axles and keep them for trail spares. You never want to be that guy on the trail with a broken axle. Nothing ruins a weekend of fun like broken drivetrain parts.”

What upgrades will you need to make when building a rock crawler? Should you even keep the factory axle? Well, this really depends on what’s under your rig. Though, there are two things to consider. First, you should at least set yourself up with chromoly axles and upgraded u-joints, regardless of the axle type (because, again, good solid axles are your best friend here.) And, second, if you’re working with IFS, you might want to at least consider the idea of swapping to a solid axle. It’s quite expensive to do, but ultimately it will give you more durability and suspension travel than an IFS can offer. If this is something you’re interested in learning more about, check out our piece on Solid Axle Swaps.

So, axles aside, let’s talk about the gearing and carrier.

Low gearing goes hand-in-hand with rock crawlers for many reasons—one of which is providing precise throttle control when you need to put a tire in just the right spot to clear an obstacle. Slipping your clutch is definitely not the way to control your vehicle. “This is why the Rubicon has an advantage over nearly any other stock platform,” explains Guyer. “It has a 4:1 transfer case stock. You can upgrade a D300 or NP231, but the cost is going to be in the $1K range. Or, go straight to an Atlas at close to $3K until all is said and done (new driveshafts, etc.),” he adds.

Low gearing also helps combat the weight of larger tires. “An aggressive set of oversized treads is a great way to help your 4×4 get farther down the trail. The added ground clearance, along with a larger footprint, can do wonders in the dirt. But don’t over-tire your drivetrain,” explains Driving Line Magazine. “There isn’t a steadfast rule for this, as there are dozens of factors to consider (vehicle, suspension, and axle type being among some of the most important).”

This is where that research element comes in. As Driving Line points out, just because you can run huge, gnarly tires when building a rock crawler, that doesn’t mean you should. Your axle might not be able to handle it. “You’ll have more fun and go farther on a smaller and more aggressive tire, over one that’s simply too much for your drivetrain to handle,” says Driving Line.

“Tires are insanely important,” echoes Guyer. “In Moab, a good all-terrain tire will suffice because the rock is dry. But in the slop of the East and Pacific Northwest? Big tread voids are a requirement. Consider checking out the new Mickey Thompson Baja Boss or Yokohama X-MT.”

So, let’s talk carriers.

Right now, locker-type carriers are the most popular type of carrier being used. Why is this? Well, limited slips work fine and so do spooled differentials, but the gap between the two creates some issues. Limited slips don’t positively lock the wheels together to make sure you have the most possible traction at all times, while spools don’t offer much flexibility between spots and can be a handful on the road. Being able to lock up the differential on command or leave it open as needed, brings a level of versatility to the table which is virtually unmatched by the others.

“You have many choices when it comes to your carrier, and it’s up to the owner to decide which is best,” advises Guyer. “An open carrier gives the best highway ride, but traction is very limited off-road, while a limited slip works well on road, but cannot put all the traction to the ground. A locker is very popular because it provides acceptable highway manners, but it is nearly 100% traction off-road. And a spool is the ultimate in traction, but they’re a handful on the highway. The best of all worlds is the selectable locker, but with that comes a higher price and higher level of complexity,” finishes Guyer.

Suspension

One of the biggest challenges when building a rock crawler is working with a production vehicle. Suspensions can be limiting and, when building from stock, you’re forced to work with what the dealer gave you. To further complicate things, we have to come to terms with the fact that very few of us can afford to build an Ultra4 car. However, this doesn’t mean we can’t look at what they have, weigh it against what we’ve got, and then mimic what we can. This doesn’t need to translate to the actual equipment used, but more so how it functions.

Travel is a big concern in the rock-crawling world. If your tire isn’t in contact with terra firma, you go nowhere. This may lead you down some difficult roads in terms of building—especially if you decide to move from an independent suspension setup to a solid axle. Even if you don’t, this will mean that the lift kit you should use for this build will require extensive involvement to install and you will likely want to spring for professional help.

Ultimately, each vehicle owner is going to be facing different challenges.

Some of you own JKs with 4 links, Toyotas with independent front axles, old Fords with leaf springs all around, or something else entirely. It’s nearly impossible to speak to all suspension styles, but it is important to remember that money will be spent here because you don’t want to lean on a cheap kit.

In any off-roader, but especially when building a rock crawler, you need your suspension to be strong and reliable so it can perform well. Additionally, in many cases, you will also be going fairly high so you can squeeze large tires underneath to combat breakover and traction issues.

If you’re a newcomer, there’s absolutely no reason not to work your way up, unless you have a specific recipe you are following. “The great thing about starting small is that you can typically avoid additional modifications like extending the driveline or installing a slip yoke eliminator,” points out off-road accessory manufacturer Bushwacker. “The trade-off is that you’ll most likely need a transfer case drop down bracket, which will lower your skid plate clearance by about an inch. You’ll want to do this to keep your driveline as close to its original angle as possible. Otherwise, you’ll feel driveline vibration and potentially break u-joints.”

The Building Blocks of Building a Rock Crawler

Axles, differentials, and suspensions are your big three here. These are what will give you the biggest edge in terms of being able to theoretically crawl over anything. However, there’s more to it than just sitting high and having a strong setup. While this will get the wheels turning for right now, we’ll be back to further discuss other modifications—like brakes, tires, and exterior accessories—for building a true rock crawler.