Transmissions for a 4×4
Apart from proven reliability, an absolute requirement for any 4×4 transmission is a low range gear set, without which it is less than worthless for any form of serious off-road driving. The types of 4×4 transmission found on luxury SUV’s and/or crossover vehicles, whether full- or part time 4WD, do not have this capability and are therefore excluded from the group of vehicles that can be used for overland expeditions through for instance, Africa, the Australian Outback, or the jungles of South America.
While engine torque remains the same, the extra torque to the driving wheels provided by the gear reductions in the low range transmission, is invaluable if not indispensable to successfully negotiate obstacles such as deep mud, thick sand or very steep climbs and descends. So, how do low range gears work, and how do they assist in overcoming obstacles?
• Gear ratios:
A gear ratio is the difference between the rotational speed of the input shaft and that of the drive shaft at the point where it leaves the transmission, but is further reduced inside the final drive, or differential, by the difference between the input pinion gear and the crown wheel. This reduction also increases torque to the wheels in direct proportion to the diametric differences of the two gears.
All gear ratios are determined by design factors such as the engine torque and power, size of the wheels, overall weight of the vehicle, the purpose of the vehicle and the speeds a particular vehicle is most likely to be used at. This is why a tiny, but hot, hatchback is so much faster than a 4×4 but useless at towing heavy loads, since the purpose of a vehicle determines the 4×4 transmission gearing.
• High range:
Normal high range gearing usually provides sufficient torque to the driving wheels to enable an off-road road vehicle to negotiate “normal” off-road conditions, such as hard, compacted surfaces even if these surfaces are covered with a thin layer of mud, sand, or gravel. These conditions do not place undue demands on a vehicle, and can generally speaking, be negotiated either in 2WD or 4WD with the transmission in high range, albeit in the lower gears, such as second or even third gear.
• Low range:
Some conditions however, demand torque that can only be delivered by the low range, which typically doubles the available torque in any gear. The low range gear set is typically located in the transfer case, where the engine power is split, with one drive shaft leading to the rear diff and another to the front.
Because of the reduction that doubles the available torque, speed is also reduced, with a typical speed of under 10 km’s p/h in first gear. The use of the low range does however, not increase traction but it does significantly reduce the chances of wheels spinning and losing traction in for instance deep mud, or thick, loose sand where constant motion is required not to sink into the ooze. The vastly increased torque to the driving wheels obviates the need to manipulate the accelerator or the clutch, actions that could lead to the instant loss of traction.
• Gear ratios and engine braking:
While the brakes on all off-road vehicles should be in excellent working order, use of the brakes can in some circumstances be positively dangerous, and this is where gear ratios in the 4×4 transmission play a critical role, by providing sufficient engine braking.
Descending a steep sand dune or gravel coated mountain pass requires some skill, if not naked courage. With older off-road vehicles without Hill Climb/Descend systems, Rollover Warning systems, or even just ABS brakes, the only safe way of getting to the bottom of these obstacles is to select a lower gear, preferably first gear, let go of the brake and accelerator pedals, and let the gear ratios work in reverse to prevent wheel lockup, loss of traction and directional control. However, while some sand dunes may be too loose on the surface to use the low range where excessive engine braking (double the normal affect) may have the same result as applying the brakes, in conditions where the surface is more solid, the use of the low range may be the only safe way of getting to the bottom in one piece.
Automatic 4×4 Transmission
While the automatic transmission do have a place in the 4×4 firmament, they have two serious drawbacks: insufficient engine braking capability and very often no low range. While it is possible in many, if not most cases, to retrofit a low range gear set to an automatic 4×4 transmission, there is nothing to be done about the fact that a torque converter does not provide a near solid mechanical connection between engine and transmission.
To effectively use the transmission as a braking mechanism, a solid connection between engine and transmission is required, which a torque converter, being a type of viscous coupling, just cannot provide. In fact, attempting to use engine braking on a long, steep descent with an automatic transmission could potentially ruin a perfectly good transmission: transmission fluid could overheat and boil, or internal clutches could lose their friction linings through excessive heat build-up.
The worthwhile advantage of an automatic 4×4 transmission over a manual transmission is the fact that during deep-water crossings there is no clutch that could lose friction through water getting between the clutch and pressure plates. However, experienced 4×4 drivers are wise to the fact that the clutch should never be used in this type of obstacle since the loss of clutch friction could mean stalling in the middle of a raging torrent, or worse, getting flooded, or worse still, getting swept away.
Another advantage of the automatic Transmission over the manual transmission is throttle/accelerator control. Large obstacles as large rocks or a tree stump can be approached at a very slow speed and speed can be increased without the use of the clutch as in a manual transmission. In a manual you run the risk of burning the clutch with the above manner.
Continuously Variable Transmission (CVT)
Currently only fitted to some sedans, most SUV’s and almost all supercars, this type of transmission, which is a sort of hybrid between manual and automatic transmissions where the best of both systems had gotten lost in the translation, have serious drawbacks even in vehicles with already limited off-road capabilities. Until improvements have been made and perfected, this type of transmission plays no role in serious off-road driving.
The transfer case is an integral part of the transmission and is the mechanism through which power is delivered to both axles simultaneously, and has to be able to withstand the backlash arising from the differences in suspension setups between the front and rear driving axles.
In part time 4WD systems that employ wheel locks to engage the front axle, the solution is simple: building a large amount of play into the drive shafts and differentials makes it possible for much of this backlash not eliminated by tire slippage to be absorbed there.
Electronically controlled 4WD engagement relies on electro-magnetic viscous clutches in the transfer case. Viscous couplings, not being solid, are perfectly able to withstand and absorb slight, momentary differences in the rotational speeds of the two drive shafts without suffering any sort of damage. Electronically controlled Stability and Traction control systems also regulate the amount of power fed to both the front and rear axles through viscous clutches.
Which 4×4 Transmission?
In the off-road context, it is always better to rely on what has been proven to work. However, while an automatic transmission can relieve the tedium of driving across the vast, flat expanses of say Africa, the reality is that Africa is not always flat. There are many rivers to cross, dangerous mountain passes and sand dunes to get down from, not to mention deep mud, thick sand, extreme heat, and gullies by the thousands and even the best automatic transmission may not always be able to deliver the sort of performance required to negotiate the many obstacles that Africa offers serious off-road drivers and vehicles.
Of course, manual transmissions and clutches are not perfect but they have for the most part proven themselves to be able to handle the worst off-road conditions but more importantly, they have proven to be reliable and dependable. While many automatic transmission fitted vehicles have survived murderous off-road conditions, the main reason for choosing an automatic over a manual should never be being able to demonstrate how good a bush mechanic one is; there should be valid reasons and grounds for the choice.
Use your best judgement when choosing a transmission for your 4×4 but always bear in mind that what works best, is what has been proven to work best.