Transfer Case Reduction Gears

By: MICHAEL BORG , Photography by: Matt Fehlberg


In a quest for better engine braking performance, sometimes you’ve got to gear up to slow down

 Go Slow Gear

There’s nothing quite like a near-on vertical downhill descent to remind you of the real importance of low-range gearing in a 4WD, or in my case, engine braking. Yep, when gravity and Mother Nature team up to throw you a super steep curve ball, the last thing you want is to rely explicitly on your vehicles braking system to keep your speed in check. Why? Well, apart from the increased chance you’ll have of braking traction and entering and losing control, you’ll probably have a hard time washing your duds out afterwards too!

Now last year I made the decision to abandon the old 5 speed manual gearbox I’ve been rocking around the countryside for the last 15 odd years, in favour of a fancy 4 speed automatic transmission. While it gets around effortlessly on the black top and offers substantially more control on rough or rocky terrain, there’s a distinct lack of engine braking thanks to an older hydraulic style automatic transmission. With that in mind, it’s time to break out the tools and install a set of brand spanking new reduction gears!

Borgy tests the new reduction gears out the only way he knows how - The hard way!

SLOW AND STEADY WINS THE RACE

Apart from more efficient engine braking, why would a 4WDer want even lower low-range gearing? Well, there are a few reasons and having a heck of a lot more control over your vehicle on tough terrain is a massive one! This is especially true with a camper trailer firmly in tow. Yep, there’s nothing worse than having to hit an obstacle at Mach 2 just to stop the engine from stalling. Chances are the trailer will end up bouncing around like a frog in a sock as it plays a sneaky game of Russian roulette with a nasty rock ledge or two. Sure, we’ve all had that one mate chanting at us to bury the accelerator through the firewall to conquer an obstacle, but the reality is that places a heck of a lot of strain on your entire driveline, and your mate isn’t going to foot the repair bill, is he?

Now an optimally geared vehicle should be able to ascend and descend a variety of technical terrain at a constant, steady speed. In fact, it shouldn’t need much if any assistance from the accelerator or brake pedal. This is actually where the term "crawl" comes from, as in apart from steering it you can let the vehicle crawl over an obstacle pretty much by itself. Having a fully loaded camper hanging off the back inherently places more load/weight on the vehicle, but a set of reduction gears is a good way to counteract this off-road. Plus, the hidden beauty of reduction gears is the extra stump pulling grunt you get, which is hard to knock back, eh?

 Borgy finally decides to give Blake a hand, instead of just drinking all the beer

VEHICLE GEARING - WHAT ARE THE OPTIONS?

There are plenty of ways to alter how a vehicle is geared, but most changes you make will have a consequence. For example, simply fitting a larger set of tyres can enhance your vehicles off-road ability, but the increased rolling diameter of your tyres will diminish things like braking performance and acceleration. It’ll alter the engines rpm, especially at highway cruising speeds and will lower fuel economy in most cases too. Plus, your low-range gearing speed’s up as well. To bring it all back into line, you may need to install different diff ratios depending on how much larger the tyres are from standard.

Replacing internal gear components is best left to the specialist; even mechanical workshops send them away these days

Now the rear differential gears are constantly engaged, so changing your diff ratios will affect the overall gearing of the vehicle whenever it’s in motion (just like your tyres do). That means if you’re happy with how the vehicle performs in high-range on the black top, but are hoping to adjust the low-range gearing side of things, that’s when low-range gears in the vehicles transfer come into play.

To the untrained eye, we have a big blob of gears!

WHAT ARE REDUCTION GEARS?

Every true 4WD vehicle has low-range gearing. They’re basically a separate set of gears in the transfer case that allow your vehicle to drive at a much slower speed compared to high-range at the same engine RPM. It achieves this by meshing two (or a series of) gears of different diameters together. The idea being to reduce the rotational speed found at the input shaft to a lower speed at the output shaft. While the two gears may be different diameters, the teeth on them are exactly the same size and design.

A set of reduction gears gives you so much more control during a technical low-range drive

The only variation is the number of teeth found on the gear is relative to its circumference. So if the larger gear has 10 teeth, and the smaller gear has 5 (half), the difference in size means the smaller one will have to make two full revolutions in order to spin the bigger gear one complete revolution, which is where you get your speed reduction.

When it comes to aftermarket reduction gears in a 4WD, the idea is to replace the factory fitted gear set with another set of gears that offer a larger reduction, which is ideally better suited to your needs.

Reduction gears are generally sold as a two piece set with a pre-determined reduction of standard gearing

REAL WORLD EFFECTS

So what can you expect in the real world after installing a set of reduction gears in your 4WD’s transfer case? Well, quite a lot actually. Having lower low-range gears means you get much more engine braking than in the past, which is thanks to the lower speed compared to the corresponding engine revs. Long story short, it’ll make steep downhill descents a much more pleasant overall experience.

The extra torque available is perfect for those super steep rock climbs

Tackling steep inclines is a little different too as there’s a fair bit more stump pulling torque available, which means less chance of running out of steam. The trade off is it’s a bit harder to build speed or momentum up if you’re trying to get a run up. It’s a bit of a drawback when it comes to tackling large sand dunes too. In fact, I have to select completely different gears for specific obstacles compared to the past. For example, 2nd gear used to provide enough speed to tackle a steep dune climb, but these days I have to be in 3rd gear to achieve the same speed and momentum. Although, having an automatic transmission means it pretty much sorts itself out these days.

Reverse gear is quite slow in low range. I mean it’s fine with my auto transmission, but it would be bordering on impractical with the manual gearbox I would think. I used to whack it low-range to help reverse caravans and camper trailers into tight spots, it would reduce the need to ride the clutch and give you a little more time to react to the trailers direction, but it’s a bit too slow for that now.

Check the integrity of seals and joints

HOW SLOW IS TOO SLOW?

Now the reduction gears I installed offer a mere 25% reduction from factory gears in the transfer case, which is basically the best I could do for my particular vehicle with parts available off the shelf in my budget. In saying that, there are gear sets available for other vehicles that offer much more of a reduction. In fact, it’s not uncommon to see up to 80-85% reductions available. So what am I trying to say? Well, different gear sets will produce a different amount of reduction over standard gearing, which means the pros and cons are quite diverse. So do your research before dropping a wad of cash on the table.

For example, if you do plenty of beach work you may find having super low crawler gears might be too slow. It’s a delicate balance, but one worth looking into in the long run.

QUALITY COUNTS

All driveline components a crucial to your vehicles reliability and performance, and low range gears gears are no exception. Plus, their buried well within the transfer case so it’s pretty hard to access if something goes wrong. With that in mind, you really want to make sure the parts you’re putting in are high quality and made to last. The problem is it’s hard to tell the difference between a quality bit of kit and a cheap knock-off.

Quality Counts

For this build, I chose to buy the kit from Road Runner Offroad. The gears are made from Japanese alloy SCM415, which is known for its great strength and wear resistance. In fact, this particular metal has been tested and proven to outperform other materials used for the same application by most other suppliers. In essence it’s less prone to chipping and cracking and retains its strength under high-temperature use.

For more information visit - www.roadrunneroffroad.com.au

FINAL THOUGHTS

Well, I started this particular venture with the hope that installing a set of reduction gears would give the mighty LandCruiser a bit more engine braking. While it worked to an extent, it’s still not exactly where I need it to be – granted, I’m a bit of a fussy bugger like that.

The problem lies in that fact that a larger gear set won’t physically fit inside the transfer case housing, so a 25% reduction is the best I can do with this particular modification. It’s definitely made a difference, so I’ll chalk it up as a win. But stay tuned – something tells me my bank account is about to cop an absolute flogging soon!