Ah, the good old modification butterfly affect. You know – when you tweak one aspect of your truck over here, and all of a sudden, it has a roll-on affect over there. Like fitting larger tyres and the way they throw out your speedo and zap much needed power.
Fitting larger tyres is one of the first mods most of us do to our 4WDs. Larger off-road tyres give you greater traction, ground clearance and puncture resistance in the bush. But how do you get around the backwards step in vehicle performance? This issue, we’re going to take a closer look at the inner workings on your diffs, how they affect vehicle operation and performance, the importance of running the right gear ratios and how to bush-proof your diffs once and for all.
HOW YOUR DIFFERENTIALS WORK
Your diffs have a very important job. They transfer power from the prop shaft through 90° to the axles, which in turn drives the wheels. They also enable the two wheels on that axle to rotate at differing speeds (hence the name) while you travel around a corner. As your tailshaft spins, it rotates the pinion shaft and pinion gear, which turns the crown wheel. Attached to the crown wheel is a housing called the hemisphere. Inside this are the gears that transmit the power to the axles. Inside the hemisphere of a typical open diff, you’ll find a smaller pair of axle gears (which are splined to suit the axles), and a smaller pair of spider gears (which enable the difference in axle speed as you round a corner) 90° to the axle gears. As your truck moves forward, the crown wheel turns the hemisphere, which in turn drives the axle gears, and thus the wheels. When you are travelling straight ahead with both tyres on a solid surface, the axle gears are moving at the same speed, so the spider gears remain stationary, simply following the hemisphere as it rotates. When you start to turn a corner, the planetary gears start to rotate around the axle gears within the hemisphere, allowing both wheels to turn at different speeds. Lockers and LSDs essentially replace this hemisphere with a different set of inner workings again. LSDs use clutch packs to enable some slip while limiting how much, and lockers essentially lock the axles together allowing no slip, when they are engaged.
WHAT HAPPENS WHEN YOU FIT LARGER TYRES
Firstly, the larger rolling diameter throws your speedo out of whack because your speedo is calibrated from the factory taking into account every gear ratio in your vehicle – from your gearbox to your transfer, diffs and tyres. Changing any one of these will affect the others. A larger set of rubber makes your speedo read slower than your actual speed because you are travelling a further distance per full tyre rotation. This means that, the greater the increase in tyre size, the more your speedo will be out. It also plays havoc with your gearing. Each gear is now taller, you’ll find your 4WD doesn’t have as much pull as it did beforehand, it’s slower to get up to speed and up hills you may find you need to drop an extra gear or two to keep your speed up. Off-road it gets worse. Controlled crawling of slow technical tracks becomes a bit hairier because the taller gearing sees you hit it at a bit more speed.The good news is, all of this can be corrected simply by changing your diff gear ratios.
WHAT ARE DIFF RATIOS?
Your diff ratio is the number of turns of the pinion compared to the number of turns of the crown wheel. Or more simply, the number of tailshaft rotates it takes to complete one full tyre rotation. For example, if you have diff ratios of 4.3:1 – your pinion will turn 4.3 times for every 1 full crown wheel rotation. This difference is achieved by changing the number of teeth (and ultimately the diameter) of the crown wheel and pinion gear set, just like changing sprockets on your old push bike. Most 4WDs will have a model number or identification plate on the firewall or on the housing. It will read diff model then have a code. Patrols have model numbers like HG41 or HH43. The number refers to the diff model and the number refers to the ratio, so the 43 will be a ratio of 4.3:1 and so on.
CALCULATING WHICH RATIOS YOU NEED
Before changing diff ratios, you need to know what ratio you have and then you can figure out what you need to change to, to restore your factory final drive ratios. Here’s how to work it out:
- ‘Current Gear Ratio x New Tyre Diameter (mm) ÷ Factory Tyre Diameter (mm)’.
- Using a GQ Patrol with a new set of 35s as an example, that’s 4.1 x 884mm ÷ 804mm = 4.5.
- So a 4.5:1 diff ratio would bring the gearing back to standard spec with 35in tyres. The closest option for the Patrol is 4.6:1 gears, which come factory fitted to the RD28T GQ and early GUs, but are also sold brand new through aftermarket suppliers.
- Some owners opt for somewhere in between, often opting for a 4.3:1 ratio.
HOW HARD IS IT TO CHANGE YOUR DIFF RATIOS?
Diffs are one of those things many people assume are easy to work on because they are small and only have a small number of parts. Unfortunately, this couldn’t be further from the truth. A differential must be setup correctly for it to function properly and reliably. For instance, the bearing caps are not interchangeable, the bearing preload must be spot on and the gear mesh must be setup correctly or you’ll quickly break teeth off your new gears. If you don’t know what we’re talking about at this point, it’s best you leave it up to a professional. If you’ve got a workshop manual and work your way through it, it is possible to set your diff up correctly at home, providing you have a workshop manual, a quality dial gauge, torque wrench and meshing paint.
DIFF RATIO QUICK REFERENCE GUIDE
- GQ 2.8 Diesel = 4.625:1
- GQ 4.2 Petrol = 4.11:1 or 3.9:1
- GQ 4.2 Diesel = 4.11:1
- GU 2.8 Diesel = 4.625:1
- GU 3.0 Turbo diesel = (auto) 4.375:1 (manual) 4.11:1
- GU 4.2 Turbo diesel = (wagon) 3.9:1 (ute) 4.11:1
- GU 4.8 Petrol = (auto) 3.54:1 (manual) 4.375:1
- LandCruiser 40/60/70 = 4.11
- LandCruiser 78 R151F = 4.3
- LandCruiser 78 H150F = 4.10
- LandCruiser 80 = 4.10
- LandCruiser 100 = (auto) 3.9 (manual) 4.3
5 WAYS TO BUSH-PROOF YOUR DIFF
- Never perform a snatch recovery in reverse. This loads up the back of edge of the crown wheel and pinion gear teeth, sheering them off.
- Upgrade the crushable pinion spacer to a solid space and shims to prevent the gears walking apart under load.
- Replace the carrier bearings to heavy-duty units.
- Brace the diff housing with a weld-on diff brace kit to prevent housing and axle warps, particularly near the steering knuckle.
- Install a selectable diff locker to strengthen the centre and increase your 4WD’s off-road capability.