When it comes to differentials or "diffs" in your 4x4, there's a lot of mystery and as to how they are setup and whether the job should be left up to a professional. In this article we intend to dispel some mystery behind your differentials and show you how you can install a locker or regear your diff in your home garage. This guide is based on a Jeep JK Dana 30 front axle but a lot of what is discussed will apply to most 4x4 vehicle diffs out there. Working on an axle is not rocket science, with the right tools and a lot of research, anyone with some mechanical ability can install gearing.
Every 4x4 owner strives to obtain more ground clearance under their vehicle. We can do this by modifying the suspension to lift the vehicle higher, but this doesn't solve all the problem. In most cases (with solid axles) the vehicle differential clearance from the ground has not changed.
Here's where bigger tyres can play a huge part in your vehicle setup. By changing the diameter of your tyre you not only increase your overall clearance, but also the diff clearance as well. There're many benefits to changing tyre size including increasing your traction along with the ability to air down to a lower pressure.
An often unthought of consequence of a larger rolling diameter is the vehicle's ability to accept our tyre choice. The larger tyre you put on, the poorer the vehicle will perform both on and off road. Performance can be regained by increasing engine power output with exhausts and increasing boost etc. however increasing output doesn't deal with the real problem.
Re-gearing your differentials are the most beneficial way to return the performance lost in the larger rotating diameter of the tyres. Everyone who has fitted bigger tyres to a vehicle has noticed the most benefit in re-gearing the diffs to suit the larger tyres.
Re-gearing is essentially changing the gear ratio between the driveline and the axles. The smaller the number (higher ratio) the faster the car will go (with the same number of RPM), the bigger the number (lower ratio) the faster the vehicle will accelerate (at the expense of some highway cruising speed).
Note: You must always change the front and rear ratios at the same time. Never attempt to drive a vehicle with 2 different diff ratios as this will result in axle wind up and will damage a lot of vital components.
Researching diff ratios for your vehicle is an excellent idea. Look into diff gear ratio charts and read about others experiences with different ratios before you jump in and order a re-gear kit. Be sure to order a diff rebuild kit along with the gears so you can replace all of the bearings and seals while it's apart.
For the purposes of this article the Jeep JK Wrangler Sport came fitted with a 3.21:1 ratio, commonly just called 3.21 gears (a relatively high ratio). The new gears being installed are 4.11:1 or just 4.11's. This gear set was specifically chosen for the CRD model Wrangler due to the desired set of 35x12.5R17 tyres that will be fitted.
Differentials allow two wheels on the same axle to spin at different speeds, this comes in handy when you’re doing stuff like going round corners. If it’s not obvious why that’s a good idea just remember that the inside of a curve is a shorter distance than the outside. When you steer round it the wheel on the inside doesn’t have as far to go, so if it’s turning as fast as the outside one it’s going to slip against the surface.
Obviously none of this is a good idea, so your differential is a big help. It works through a complex system of gears that shifts more of your engine’s torque to the wheel that’s turning faster. That’s great stuff on roads, but it’s not always what you want when you're off-road.
When you’re off road you’re going to get in a lot of situations where one wheel’s turning faster than the others. Your unlocked differential
is going to want to shift engine power to that wheel, because that’s what differentials do, but that can be bad news. If one wheel can spin
faster it’s probably because it doesn’t have as much traction for example it might be in the air or slipping in mud or sand. The
opposite wheel will then be at a stand still with no drive to the ground.
With a locked diff, the transmission will keep all four wheels turning at the same speed, no matter how much grip they have. This results in a lot more traction in slippery situatons.
For the purposes of this article we are going to be installing Harrop E-Lockers to the front and rear of the Jeep JK Wrangler. These are an electronic selectable diff locker that replaces the entire carrier of the diff.
Prior to starting, it is very important to research how your diffs are assembled. Obtain a factory workshop manual with torque specifications and bearing preload information before you dive in and start taking apart your differentials.
There is lots of (mis)information out there regarding this particular JK Wrangler front diff and after having ours disassembled I put
together a correct diagram showing all the parts in their correct order.
I suggest while you are disassembling your differential you make a similar diagram on paper outlining where all the parts go. It is also equally important to mark parts along the way with a coloured paint pen or similar to identify which side of the diff they come from. This is very important for the bearing caps (which should be identified with a punch) and shim positions as they will only go back correctly in one position.
This article will not endeavour to describe every detail of disassembly of the diff and will rely on you to utilise your workshop manual to strip the axle housing bare. We suggest you remove all the old seals and bearing races in order to replace them before installing the new gears and locker.
It is also important to note that some OEM carriers will not be able to handle the lower ratio diff gears due to their size. This may
require full replacement of the carrier or installation of a locker of the correct size. It is generally recommended in this situation
to install a locker as we are in this article.
Your differential may require additional specialist tools other than what is listed here. These are the tools required to rebuild
most diffs and are suitable for the JK Wrangler Dana 30 diff. Before you start, consider if purchasing the tools and spending the
time to set up the diff correctly is for you. Look over this list of tools, some of which you may already own.
The stand-out tool in this list is the inch/lbs beam or dial style torque wrench. This wrench is required in order to properly set your pinion baring preload. It cannot be done with a clicker style wrench.
The calipers are required to measure shim thicknesses.
The Dial Indicator is required to measure diff backlash. Another tool that is required for this job.
Differentials use a conical roller bearing in its assembly. These bearings reuire a certain amount of load or pressure to be excerted on them. Essentially the cone needs to be squeezed into it's respective race just the right amount in order to have a reasonable lifespan. If the preload is too little or too much, the bearing life will be significantly reduced. This is why it needs to be measured in the set up of the diff.
Pinion preload is measured by it's rotational torque. To do this the pinion is installed into the bare diff and the pinion nut tightened enough so that there is no play back and forth. The pinion is then turned at a steady rate whilst watching the dial or needle on the inch/lbs torque wrench.
The manufacturer workshop manual will specify the correct torque range for your pinion bearings. In the case of the JK Wrangler D30 it is specified to be 0.79 - 1.24 Nm (7 - 11 in. lbs.).
Backlash is a measure of play between the ring gear and the pinion gear teeth as they mesh together. It's a measurement of the amount of space between the teeth contacting one another. This is measured with a dial indicator in thousandths of an inch. Your workshop manual will specify the range of backlash allowed. In the case of the JK Wrangler D30 it is between 0.005" to 0.008".
Backlash is adjusted by adding and removing carrier shims. If the backlash is reduced too much the gears will mesh together too tightly resulting in overheating and possible failure of the gears. If it is set too wide, it is possible to chip the gears and cause failure.
This installation begins from a bare axle housing with the axle seals, pinion seal and bearing races removed. Removal of these components should be done in accordance with your workshop manual. Bearings and seals should be carefully tapped out or pulled with a seal puller. Be sure to clean the housing of your diff thoroughly with brake clean and wipe out with a lint-free rag.
This is what the diff should look like once you've got it empty and clean.
Bearing Race Installation
The new bearing races should be installed using a seal/bearing driver. Lubricate the bearing race with gear oil prior to installing. A neat trick is to put the bearing races in the freezer the day before, this will shrink them slightly allowing an easier install.
Align the bearing race with the housing and using the correct size driver, pound the driver with a small sledge hammer so that the bearing race presses into place as straight as possible. Be sure to listen to the tone of the hammer as you hit. The sound will change once the bearing race is completely seated.
Install the inner pinion bearing race first, then proceed to the outer. Make sure any components in between the races are also
installed in the correct order. In our case the Pinion Oil Baffle was pressed in behind the inner bearing race.
Axle Seal Installation
It is tricky to get the axle seals in straight. I carefully tapped the seals in with a seal driver working my way around the seal evenly driving it into place. Make sure you lubricate the seal before installation with gear oil. The freezer tip will also work here.