The Definitive Guide to Running 37s on a Jeep JL

Jeep JL 37s

The Definitive Guide to Running 37s on your Jeep JL or JLU – What to Consider Before Throwing 37″ Tires on your Jeep JL

Over the years, larger and larger tire sizes have become the norm. Years ago with the Wrangler TJ, many people were running 33s on their Jeeps on some pretty difficult trails. With JKs and their longer wheelbase, the new normal became 35s. Now with the JL and its stronger factory Dana 44 axles, 37s are what many people want to run for the increased ground clearance, breakover angle, and capability.

If your JL doesn’t see the trail, putting 37’s on doesn’t require too many other upgrades. Driving around on the street with 37” tires doesn’t take all that much. However, if you want to extract maximum performance, articulation and stability on and off-road from your JL or JLU with a large tire, there are quite a few things to take into account. Keep reading to learn the secrets of getting the most out of 37s on your Jeep JL or JLU.

Lift Height

Jeep JL 37s

Search for “best JL lift for 37s” on any forum and you’ll be bombarded with many opinions – 2”, 2.5”, 3”, 4”…the list goes on. The amount of lift height you choose really accomplishes three things: how much up-travel your suspension has, how much droop can be achieved, and how your vehicle looks at ride height. The minimum lift height to run 37” tires is a 2” lift, but this really has more to do with the amount of bump-stop you run which we will dig into in a following section.

The more you lift your Jeep, the higher your bump stops will sit above the axle. This means that when driving over bumps, potholes, etc, your suspension has more room to compress before your bump stops engage. The more up-travel you have, the more comfortable your Jeep can be on and off-road. I recommend aiming for at least 4” of up-travel before your bump stops engage for optimal ride comfort.

Ideally, your shocks or limit straps limit how far your axle can droop so that it is impossible for your springs to move or fall out under full articulation. Higher lift height springs generally have longer uncompressed lengths making it less likely that this situation can happen. Some manufacturers such as Synergy MFG make dual-rate springs that have a “flex-rate” coil section that makes it near-impossible for the springs to become un-seated even with exceptionally long shocks. Sorry XJ guys, but XJs often run short, single rate springs and long shocks which is why you may have seen this situation play out on that platform.

There are some drawbacks to too much lift height. The higher you lift your JL, the higher your COG (center of gravity) will be. Excessive lift height can make your Jeep unstable both on and off-road. A low slung Jeep will be more stable in all conditions.

Don’t waste your time with body lifts – they do not offer any performance benefit and only serve to raise your center of gravity.

Bump Stops

Jeep JL 37s

Bump stops have two purposes. They prevent your shocks from bottoming out (which can cause serious internal damage) and prevent your tires from rubbing your fenders.

When you encounter a large bump in the road or trail that your suspension cannot adequately absorb, your suspension compression will be stopped by your bump stops. Without bump stops, your shock absorbers could be the limiting factor during suspension compression which would cause damage to your shocks over time.

Bump stops also limit wheel up-travel to prevent your tires from rubbing on and damaging your fenders.

You also don’t have to run the factory Jeep bump stops which are very firm and cause a very uncomfortable bottom out experience over large bumps. Many aftermarket companies such as Metalcloak and SumoSpring manufacture softer bump stops that will soften the impact of bottoming out your suspension.

Whichever shocks and springs you choose, take note of the minimum bump stop spacer height they require. Many 2-4” lift kits require anywhere from 2-4” of bump stop extension. You do not want your springs going into coil bind or your shocks to bottom out under full compression. You do, however, want to set your bump stop spacers to use as much up travel of your shocks as possible. Aim for about a centimeter of shock shaft to be showing when your suspension is at full compression.

Shock Absorbers

We will discuss two things to think about when choosing a shock package for your JL: compressed/extended length and shock performance.

Shock absorber length relates directly to how much flex your Jeep has. As I mentioned in the previous section, up-travel is limited by bump stop extensions and shock body length. Your down travel is limited by your shock’s fully extended length. Maximizing these compressed/extended measurements is the key to getting the most flex out of your Jeep.

Shock performance is dictated by a number of factors – damping characteristics, fade resistance, adjustability, and more. Common shocks for the JL platform generally fall into two categories:

2.0 IFP Shock

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2.0” diameter internal floating piston shocks are the most common type of shock on offroad vehicles. They can be budget-friendly and have pretty good performance per dollar. Fox 2.0s are some of the most popular shocks for JLs and provide a comfortable ride and good flex. 2.0s are best for mostly street-driven vehicles that don’t see much high-speed offroad use. Their limited oil and nitrogen capacity means that they can overheat during extended offroad use where they are working hard to control your vehicle’s motion. These shocks are generally considered “throw-away” and will be replaced when they wear out.

2.5 Remote Reservoir Shock

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2.5” diameter remote reservoir shocks are a much more performance-oriented option. They will generally ride stiffer than their 2.0” counterparts, however, they are much more fade-resistant. 2.5” shocks have much more oil capacity making them ideal for prolonged high-speed offroad use. These shocks have a longer life than 2.0s and are also rebuildable so they don’t have to be completely replaced when they eventually wear out. Some 2.5″ shocks are adjustable which is great for tuning your shocks for the terrain or the load you’re carrying.

Because the internal floating piston is moved to the reservoir, these shocks have shorter collapsed lengths than a 2.0 IFP shock. These shocks have more travel, meaning you will get more overall flex with a remote reservoir style shock (if your suspension is set up to take advantage of that travel).

2.0” remote reservoir shocks are also available however they often do not have the adjustability and outright performance of a 2.5” shock.

Wheel Specs

Jeep JL 37s

Wheel backspacing is the single most important thing to consider when shopping for new wheels for your Jeep JL. Backspacing is measured from the wheel’s mounting surface to its inner lip. With 37s too much backspacing will cause your tires to rub on your inner fenders at full suspension articulation. Too little backspacing will cause your tires to rub on your fenders and will increase your scrub radius.

Scrub radius impacts the effort required to steer your vehicle due to the tires being pushed out away from the axis on which they turn (your ball joints). Excessive scrub radius will also shorten the life of your wheel bearings and ball joints.

For JL Rubicons with their wider axles, 4.5” to 4.7” backspacing is ideal for 37” tires. Less backspacing is required for larger tires such as 40s. For Sports and Saharas with their standard width axles, 3.5” backspacing works best.

Wheel width should be in the 8.5-9” range to support both 12.5” and 13.5” wide tires. 17” diameter wheels are the standard and are what you should run. 20” wheels reduce the amount of sidewall your tires will have which negatively impacts off-road performance.

If you’re serious about offroading, beadlocks are an amazing upgrade. They allow you to run super low pressure while offroad without popping the tire bead off of the wheel. I routinely run 5-10psi off-road with my beadlocks with no issues. I would be limited to around 12psi without beadlocks which would not provide as much traction on the rocks.

Tires

Jeep JL 37s

When it comes to tires, there are some strong opinions out there. Many people are loyal to one brand or another. I’m not going to go into specifics around which brand or tire is best. Instead I’ll give you some information on three important topics to consider when choosing a 37” tire – tire width, weight and terrain rating.

Width – most 37” tires come in either 12.5” or 13.5” widths. 12.5” wide tires require less effort to steer due to their slightly narrower contact patch and lighter weight. Off-road performance is similar between both sizes.

Weight – 12.5” wide tires are lighter which will improve acceleration and braking over 13.5” wide options.

Terrain rating – most popular 37” tire options are all-terrains or mud-terrains. ATs will provide a smoother ride on-road, less road noise, and good traction in wet on-road conditions. MTs will provide better loose dirt/rock traction but worse on-road performance and more road noise.

Regardless of which tire you choose, there is one thing that all larger tires have in common – their PSI requirements. When running a 37” tire, you do not want to run the same PSI as you did with your factory 31s or 33s. You’ll want to run less pressure to ensure that the entire width of your tread is contacting the pavement. Start with 30psi cold and perform a chalk test to find your optimal pressure. This will allow you to get the best handling and most tread life out of your tires.

Engines and Gearing

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All of the available engines have plenty of power to turn 37” tires. What will impact your Jeep’s acceleration, ability to maintain speed up a grade, and MPGs is your axle gear ratio.

JL Rubicons (non-diesel) come with 4.10 axle gears. This works ok at sea level with 37” tires, however, you may lose the use of 8th gear on the highway. That will not harm anything and your highway RPMs in 7th gear will be similar to when the Jeep was stock in 8th. If you live at altitude or have a Sport/Sahara with lower factory gearing you will want to regear to 4.88 or 5.13. This will slightly improve your acceleration and will allow you to hold your overdrive gears (7th and 8th in the automatic and 6th in the manual transmission) up steep grades and at higher speeds.

Axle Strength and Upgrades

Jeep JL 37s

JL Rubicons come with Dana 44 axles front and rear. These are capable of supporting 37” tires for mild to moderate wheeling without any real upgrades. If you plan on doing more serious off-road trails, you may want to consider upgrading a few key axle components.

The factory axle Dana 44 shafts have small u-joints that do not have full coverage c-clips. This means that after some abuse off-road (or if you have a heavy foot) they are prone to coming apart. A broken axle-shaft u-joint can leave you stranded and even damage other axle components when it breaks.

Many companies offer upgraded axle shafts with larger u-joints. Most of them also delete the FAD (front axle disconnect) which normally keeps your front driveshaft from spinning when the Jeep is in two wheel drive. Deleting the FAD means replacing the factory two-piece passenger side axle shaft with a one-piece unit. I personally do not like the idea of deleting the FAD because your front driveshaft will always spin. Factory driveshaft or not, this will cause the driveshaft to wear out quickly and can contribute to driveline vibrations at high speeds.

RCV Performance offers a 2 piece (FAD) axle shaft set for the JL D44 with their amazing CV joints. These joints are greaseable, eliminate crow-hop while turning in 4WD and are incredibly strong. I am running these axle shafts and have had zero issues with lots of on and off-road miles.

JL Sport/Sahara models come with Dana 30, 35 and 44 (rear only) axles. These do not have adequate gearing for 37s and the Dana 30/35s are not strong enough to reliably support 37s for anything more than mild-moderate off-roading and street driving. I recommend upgrading to some Rubicon take-off Dana 44 axles if you plan on more serious offroading – it is not very cost effective to add upgrades to a Dana 30.

Some other common upgrades to help your axles handle abuse are a weld-on axle truss to strengthen the housing, upgraded ball joints to get rid of the factory plastic lined ball joints, and thicker steel differential covers that won’t peel back or bend like the thin factory covers.

Best Supporting Upgrades

Jeep JL 37s

Steering components are one of the main upgrades to consider. An upgraded tie rod and drag link will not only take much more abuse than the thin stock links, but will improve your steering feel and precision on-road.

Adjustable control arms and track bars are also very important to maximizing your vehicle’s performance. They are not entirely necessary for a small 2” lift but become more important as the lift height increases. Control arms and track bars allow you to recenter your axles under the vehicle and dial in your driveshaft angles.

At a minimum I would recommend upgrading to some adjustable front lower control arms – this will allow you to set your caster angle to 5.5-6.5 degrees which will dramatically improve your Jeep’s on-road handling and straight-line tracking.

Driveshafts are also something that you will want to consider upgrading. Larger tires mean more stress on your driveline and the stock driveshafts aren’t the strongest out there. I would recommend upgrading to a 1350 front and rear driveshaft. Generally a 29” long (at full extension) front shock is the longest you can run without upgrading the front driveshaft due to the limited range of motion that the stock front driveshaft has.

How my JLUR on 37s Drives

Jeep JL 37s

I have been incredibly happy with the way my JLUR drives on 37s. I just completed a 5000 mile road trip with many days of hard off-road trails in Colorado and Utah. The Jeep drove great on and off the trail throughout the entire trip.

I really love some of the specific components on my Jeep – the Fox 2.5 DSC shocks, the upgraded suspension components, and BFG tires – but when your components are carefully selected to work well together, they come together to create a cohesive vehicle.

Throughout this article I’ve touched on many different areas to consider when upgrading your Jeep and this can be overwhelming when trying to piece a setup together. Luckily, there are many companies on the market who sell fantastic kits that include nearly everything you’ll need. Some of the companies top of mind are Metalcloak, Synergy MFG, Rock Krawler, Dynatrac, and Terraflex. Hopefully after reading this article you’ll be armed with the knowledge to decide which kit and components are best for you!

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