Mechanical (Auto) Lockers

Now I see the topic of mechanical lockers come up time and again on lists. There are descriptions all over the place of these things, and most are right in their outcome, and wrong in their “why”. So this is the “why”.

This mainly relates to the Lokka and Lock-Rite lockers. Detroit has a similar process, however I have never pulled one apart and I believe there are some differences.

First, Mechanical lockers are NEVER locked. Any document that says they are locked is either over simplifying, or doesn’t have a clue.

They have ONE SIMPLE RULE:
One wheel will be coupled to the crown wheel at all times. The other wheel *may* do MORE of whatever the system is doing at the time, but not less. If it is placed under a load that attempts to make it do less, it will become the coupled wheel.

This is best understood from two different perspectives – internal and external.  

Internal and External Perspectives.

External

1. When you turn a corner the outside wheel has to speed up and the inside wheel slow down. The system is driving, so no wheel is allowed to spin slower than the crown wheel. The inside wheel takes all the drive load. This unloads the outside wheel allowing it to do more (go forwards) freely – it uncouples and free-wheels forward.
2. When you come off the throttle and start to engine brake downhill the situation reverses. The crown wheel is trying to slow the car. The load will transfer to the outside wheel. The inside wheel, going slower (more of that the crownwheel is trying to do) will uncouple and freewheel.
3. In a straight line both wheels take the load, but neither is locked. The load actually is applied varied between the wheels, but the flex in the suspension, axles and tyres evens out the uneven power to deliver what feels to be equal torque to each wheel.

Internal

1. The principle of operation is actually simple. A pair of drive rings are forced outward into the side gears (or outer dog clutch in some models) engaging with them and forcing them to rotate. It is not locked here, it is just that both wheels are being driven. A key point to note is why they are forced outward. The cross shaft in the middle of the diff bisects the two drive rings. They have an elongated hole for the cross shaft to sit in. This results in a portion of the rotating torque being transferred into a strong outward force – pushing the drive rings into the side gears and holding them engaged. Think of it as a wedge splitting a log – pushing the rings apart from each other. There is also a very weak set of springs to assist the process, and some dowels to keep the two rings nearly rotationally static in relation to each other.
2. Now lets turn a corner. The inside wheel tries to slow down, and the outside to speed up. The fixed coupling of the side gear stops the inside wheel from slowing, so it starts to take all the power. The outside wheel is speeding up and becoming unladen, as it wants to go faster then the crown wheel. When unladen the outward force on that side drive ring is reduced to near zero. There is another angle manufactured into the dog clutch teeth. This angle is steeper than the one in the cross shaft. It will exert LESS force inwards than the cross shaft exerts outwards, but the principle is similar. Now that there is no outward force from the cross shaft bevel, the side gears slide over the beveled angles on the drive ring forcing it inwards against the very weak springs. This allows the outside wheel to “cam” freely forward. The outside wheel is freewheeling, the inside is driving. I could do this on my Hilux using my little finger, it takes almost no force.

The “clicking” sound from these diff’s is the dog clutch engaging and disengaging. It’s normally only audible in closed spaces like carparks.

The often commented on dog clutch teeth that are not undercut are designed to be that shape. This shape is what provides the force to uncouple the dog clutch. When driven this is overcome by the shallower cross shaft angle, forcing the clutch to be engaged.

The weakness I can see with the system is that the engagement depth with the side gears is very shallow, and on the tips of the teeth – not their strongest part. On the other hand it is engaged the full length of every single tooth – far more than the 2 or 3 teeth under load normally in a diff. I believe that under the right shock loadings it would be possible to catch the diff in a partly engaged scenario and tear the tips off the teeth, although I haven’t seen it done. The best way to do this would be vicious bouncing of the wheels transferring load side to side. Smoothly driven I never had a problem.  

The issue if the inside wheel driving, but the outside wheel braking does result in some rear axle steer under throttle transition. It is evident partly as bush compliance moves the axle, and partly as a slight understeer effect. I noticed it mostly with the large throttle movements from cruise control on winding roads. In a long wheelbase Hilux it was not noticeable under normal conditions. I never had a problem with it towing trailers on steep roads.

In the wet there is an increased propensity to spin up the inside wheel, BUT and this is a big BUT, only to the point where it was going as fast as the outside wheel, then they both drove out of the corner. There was never any significant loss of traction toward oversteer.

I could never see any reason why tyre wear would increase. The unlocking is so gentle I could do it with my finger. The inside wheel would do more work, but the outside less, and as I go around as many left as right corners, this wasn’t a problem.

Documents like these mislead people (since removed) about how mechanical lockers work. After my experience with ARB airlockers, I know I would prefer the mechanical in the rear.

There is long discussion on mechanical lockers here, but you’ll need to read it a few times to catch all of the details.

My personal preference is an AWD car, with a Torsen LSD in the centre and electric locking. A mechanical locker in the rear and a Torsen in the front, preferably with electric locking over the top. Unfortunately I can’t buy this combination.

NOTE: In the diagrams below clearances and angles are exaggerated for clarity. The actual cross shaft hole is eccentric rather than triangular. The side gear teeth slots are a much closer tolerance to reduce lash.