4WD MODIFICATIONS - TRACTION AIDS
In the accompanying story Differentials Explained we’ve looked at the different differentials that are popular in the 4WD world and now we’ll look at their selection and use.
The starting point for diff-use information has to be the centre diff lock, because it causes more confusion that any other 4WD control. Most 4WDs that have full-time 4WD or selectable full-time 4WD drivelines have a centre diff lock. What’s full-time or selectable full-time mean, we hear you ask?
In the early days of 4WD popularity, all vehicles except Range Rovers had part-time 4WD drivelines. In a part-timer, the vehicle works as a rear-wheel-drive machine until 4WD drive is engaged, either by moving the transfer case lever to 4H or pushing a button. Many part-timers have automatic or manual free-wheeling
front hubs, to disengage the front axle when the vehicle is running in two wheel drive.
If you’ve got a part-time 4WD you don’t need to worry your head any more about a centre difflock, ’cause you don’t have one.
Full-time 4WDs run in four-wheel-drive all the time, as can selectable part-time/full-time drivelines that give the driver the option of running in four-wheel-drive or two-wheel-drive mode.
OK, so why do full-time 4WDs need a centre diff lock? Think of one climbing a steep, low-range hill. With its centre diff ‘open’ – unlocked – the front wheels may start to spin, because of weight transfer to the back end. When that happens the vehicle won’t move, because all the torque ‘spins out’ the front end.
As a rule, you should lock the centre diff only when there’s a danger of front-end spin-out, on steep climbs or descents and in loose or slippery low-range conditions.
Limited slip diffs
LSDs are the most common fitment to 4WD rear axles, with most production 4WD utes leaving the factory with one installed. Land Rovers and Range Rovers have always been exceptions, having ‘open’ front and rear differentials. Some LSDs are better performers than others and after-market ones are generally more effective than the standard offerings.
The best factory-fitted ute LSDs are in Nissan Patrols and the worst are in Toyotas, with the rest of the 4WD pack fitting in between.
If your 4WD is fitted with a rear-axle LSD as its only traction aid you need to be aware that both rear wheels must have some grip for the unit to function.
LSDs work quite well on flat, loose or slippery surfaces, such as wet grass, mud patches and on sand. LSDs don’t work so well on undulating surfaces, such as trails with rocky outcrops and large potholes. When trail driving with an LSD, avoid dropping one rear wheel into deep holes or on top of rocks and try to keep both rear wheels in contact with the ground.
If your standard LSD has lost its clutch action it’s time for a rebuild or a replacement. A rebuild may often cost as much as a replacement, so if you want to improve off-road traction a more powerful type may be the better option.
TrueTrac and Torsen types are better performers than stock units, because they have better torque-retaining mechanisms that make the most of the limited grip that the lighter-loaded wheel has.
Apart from loss of clutch function LSDs sometimes suffer from too much ‘bite’. If this happens to your LSD, suspect oil contamination or incorrect oil. Clutch-type LSDs require special LSD oil and will malfunction if they don’t get it, or if water creeps into the mix.
The stronger slip-limiting action of an after-market or rebuilt LSD will improve off-road ability, but all LSDs need some traction on both wheels.
LSD up front
Some high-performance, front-wheel-drive cars have limited-slip diffs, to help control wheel spin if the driver gives the go-pedal too big a push. It’s possible to do the same for a 4WD.
A front-axle LSD can reduce front-end traction loss, but as the rear end does most of the work when climbing it works best when the front end is doing its fair share of the traction business.
Fitted to a part-time or selectable full-time/part-time 4WD driveline a front-end LSD will function only when 4WD is engaged. In a full-timer the front-end LSD will limit wheel spin all the time, but unless the clutch pre-load is too strong the ‘loading’ effect on the steering should be noticeable only in marginal traction situations.
LSDs front and rear
LSDs front and rear can be a useful combination for 4WDs that need traction in mainly flat, sandy or muddy terrain. The beauty of this combination is that it fits any 4WD driveline and should have only minor effects on steering accuracy.
However, it’s not the combination of choice for mountain trail driving, because if one front wheel and one rear wheel are off the ground at the same time the 4WD will just sit there, spinning its wheels.
Self-locker in the back
Self-locking diffs such as the Eaton NoSpin Softlocker, Lokka and the Lock-Right are locked unless differential action is needed. The locking action is independent of traction, so a self-locker will drive a vehicle that has traction on only one rear wheel. Another improvement over an LSD is that a self-locker
will prevent one-side wheelspin – both wheels turn at the same speed.
If we consider our trail-climbing scenario once more the self-locker-equipped 4WD will have a definite traction advantage over an LSD-equipped machine. The self-locking rear diff ensures that both rear wheels rotate at the same speed, even if one wheel is off the ground.
Another advantage is that the locking action keeps wheel speed under control. With only a rear-axle LSD as a traction aid one rear wheel is free to spin if it loses traction and that spin occurs at twice the speed of the diff centre.
A rear-axle self-locker contributes to stability on-road; particularly in the case of unladen 4WD utes that are notorious for spinning their inside rear wheels on loose or slippery surfaces, when running in rear-wheel-drive. A self-locker prevents that wheel spin and greatly improves directional stability.
Possible downsides of the self-locker are slight ‘wiggles’ from the back end as the diff locks and unlocks.
Self-locker up front
Self lockers should never – never – be fitted to the front ends of full-time or selectable full-time/part-time 4WDs, but self-lockers can be fitted to the front ends of part-time 4WDs. Why?
Self-locking diffs are locked unless differential action is needed. In the case of a part-time 4WD the front axle isn’t driving when the vehicle is operating in rear-wheel-drive, so the front self-locker isn’t working and steering is unaffected. In the case of a full-timer the front diff is working and if it stays locked during a turn, because of a loose or slippery surface, the vehicle becomes difficult to steer accurately.
In the front end of a part-timer the self-locker is used only when the vehicle is operating in 4WD mode and so the effect on steering is less critical. Even so, such a vehicle is best driven by an experienced off-roader and it should have a dashboard sticker advising of the locker fitment.
A front-axle self-locker works well in conjunction with a strong rear-axle LSD in a part-time 4WD machine. The LSD is useful in controlling rear axle spin when the vehicle is operating in both rear-wheel-drive and 4WD and the front locker ensures that both front wheels must turn at the same speed. The combination is very effective in reducing ‘diagonal wheelspin’ – one front wheel and the opposite rear wheel spinning simultaneously.
Self-locker in the back and LSD front
This combination works well in part-time and full-time 4WDs. The rear locker controls traction positively and the front LSD limits wheelspin if one wheel loses traction. A side benefit is preventing the CV joint damage that occurs when a spinning wheel suddenly gets traction.
We used this combination very succesfully in our Project 75 Series, ‘Harry’, but in 2016 we swapped the NoSpin rear centre for a Harrop ELocker, to give this more modern locker a bush workout.
Self-lockers front and rear
This combination must be used only in part-time 4WD machines that will preferably be driven by experienced off-roaders. also, it’s a fitment best used in vehicles that have very strong front diffs, because all the front axle torque may pass through only one wheel if the other is off the ground.
The correct operating procedure is to use 4WD mode only when conditions are very loose, steep or slippery – as is the case with any unmodified part-time 4WD. In these conditions the front self-locker will unlock to allow steering if both front wheels have traction, but if front axle wheel spin occurs the diff will lock up and steering may become difficult.
Driver-controlled diff lock rear
The best known driver-controlled diff lock is ARB’s Air Locker, but there are TJM ProLockers, Harrop E-Lockers, and a few remaining Maxi-Drive and McNamara units for Land Rovers and Range Rovers.
Toyota sells factory diff-lock-equipped 70 Series utes and wagons, but these locks cannot be retrofitted, because the axle housings and half shafts are different. Factory rear axle diff locks are also available for an increasing number of wagons and utes.
Interestingly, all driver controlled diff locks except for ARB’s have electric actuation. If you tear off an electric cable in the bush it’s easy enough to reconnect the parted ends. Toyota used to have vacuum control, but changed that to electric after too many complaints about vacuum lines ripping off. ARB’s design is also vulnerable to an air line ripping off.
A diff lock in the rear axle replaces the limited slip centre, so the diff is ‘open’ until the lock is activated. The loss of slip-limiting function may affect the on-road handling of some part-time 4WDs – particularly empty utes. However, most new utes and wagons come with electronic traction and stability control that more than makes up for the loss of a rear-axle LSD.
The slip-limiting function loss is less significant in the case of full-time 4WD machines, because four-wheel traction makes an LSD almost unnecessary on-road.
Off-road a rear axle diff lock gives much better tractive ability than any LSD, but there can be some ‘straight-ahead’ steering effects on hard surfaces.
Diff locks are best operated by experienced off-roaders, because incorrect engagement can lead to trouble – either mechanical or the impact type!
Driver controlled diff lock front
Most driver-controlled diff lock installations are rear only or front and rear together, but it’s possible to combine a front diff lock with a powerful LSD or self-locking rear. The most likely such installation is in a ute that needs rear axle spin control when operating in rear-wheel drive, combined with maximum front-end tractive effort off-road.
Driver controlled diff locks front and rear
This arrangement allows positive control of all four wheels, locking them to a common speed, regardless of relative tyre grip.
In theory, if one wheel has grip the vehicle will move.
Double diff locks will take a 4WD anywhere it can get grip, so it’s a combination best restricted to experienced operators. When you do get bogged – and we all do, sooner or later – you’r really stuck!
The downsides are reluctance to steer when both diffs are locked, but skilled drivers know how to engage and disengage diff locks to allow for steering action.
Electronically actuated traction control
Electronic traction control uses the brake system to put ‘drag’ on a spinning wheel, thus ensuring that torque flows to the opposite side of the axle – virtually a variable LSD in action.
Used in conjunction with a centre diff lock that can be either electronically or mechanically lockable, traction control is an excellent traction aid.
However, traction control has its limitations and the main one is the power reservoir that applies the brakes to control wheel spin. This accumulator retains pressure that’s built up during normal driving.
When a wheel starts to spin, pressure is applied to the brake circuit, under computer control, until the spinning stops. Prolonged traction control braking action exhausts the built-up pressure in the accumulator circuit. Our testing shows that this happens, in very slippery or loose conditions, in about 30 to 45 seconds. At that point, the traction control warning buzzer sounds and there’s no more traction control available: awkward if you’re halfway up a long slope, or mid-stream.
Traction control is designed to provide temporary traction assistance, not to vie with locking diffs for supremacy in the traction contest. If you need prolonged traction control, mechanical self-lockers or driver-controlled diff locks are the way to go.
We’ve tested stronger than standard LSDs, self-lockers and driver-controlled diff locks on vehicles that have electronic traction control. The improved control of wheel spin provided by mechanical diff locking action takes the workload off the traction control system.
When to use diff locks
Driver-controlled axle diff locks are often kept in reserve by off-roaders, who wait until the vehicle is stuck before engaging them. It’s a much better proposition to engage the rear diff lock before attempting a climb or a boggy creek crossing and to bring in the front one if the climb is steep enough or the going slippery enough to make the front end feel ‘light’.
Diff locks often won’t engage or disengage if there’s too much torque in the driveline, so it may be necessary to de-power the vehicle momentarily to allow the diff locks to engage or disengage. Most factory diff locks won’t engage before there’s some slight degree of wheel spin, so you may have to provoke
engagement if you need traction for, say, an upcoming mud hole or a steep sand hill.