What is the difference between” limited slip” and “locking” differentials, and do they have a downside…like interfering with a car’s steering or extra tyre wear? Stephanie
Differentials not only “affect” the steering: they are essential to it! And anything that obstructs the free operation of a differential will interfere with the steering and cause the tyres to scrub in some circumstances.
The wheels on a driven axle receive a single source of power – from the engine. That power is then sent (equally) to the driven wheels.
Both wheels on the axle rotate exactly the same number of times at the same speed and therefore travel the same distance. And that is fine…if you want to drive in a straight line (no steerage).
However, when you turn a corner, the outside wheel has to travel a longer distance than the inner wheel.
So, if both are forced to rotate at the same speed, the one on the outside would tend to scuff (sliding as well as rotating to make up the extra distance) and/or the one on the inside would try to spin (because it was being forced to rotate more than the distance it needed to travel).
It the road grip was very good, and neither of those options was easily achieved, steering would be severely restricted.
Free slip
Somehow, though powered by a single source and joined in the same axle, the wheels must be allowed to rotate at different speeds to enable smooth cornering.
That is what the differential does, by sensing the difference in resistance in each wheel, and apportioning different levels of power accordingly…and automatically and instantly.
Standard or “free” differentials do that to whatever degree is necessary, ranging from an even split of power to each wheel to all the power to only one wheel and none to the other.
Side-stepping the technical wizardry that achieves this, what the pinion, crown and planet gears do is send power to where there is less resistance.
Perfect for smooth cornering, but not so clever if one wheel is in a slippery ditch and the other is on dry tarmac. The wheel in the ditch can spin easily on its slippery surface (no resistance, so all the power goes there).
The wheel on the tarmac has lots of grip and is trying to get the car moving. Lots of resistance. So, it gets no power. You get stuck.
Limited slip
To overcome this drawback on uneven and loose surfaces, Rallysport invented the “limited slip” differential (LSD) which acts exactly like a “free slip” diff – up to a point - but before the balance gets to 100-0 it obstructs the process at about 80-20.
That way the wheel in the slippery ditch still spins, but the wheel on dry tarmac still gets some power, rotates on the grippy surface, and gets the vehicle moving. Not stuck.
This “boundary” to free slip also gives better control and traction in the hands of skilled drivers hurtling round corners on loose surfaces. The idea was quickly added to some utility vehicles, especially pick-ups.
In normal use, vehicles with LSD behave (and steer) exactly like vehicles with free slip diffs.
Only in extreme situations does the LSD lock in and keep them going where otherwise they might have come to a standstill.
In circumstances where this intervention is anomalous (making a very tight turn at low speed on a grippy surface, e.g. in a car park), the gears can growl, the tyres can scrub a bit, and the steering can be heavy and kick slightly.
On loose surfaces the tyres can spin slightly to release the wind-up in the transmission.
Diff locks
These effectively switch off the differential. When engaged, the drive wheels get an equal amount of power and rotate at the same speed, be they the two wheels at the back, or at the front, or both.
No matter what. Traction is absolutely maximised. Steering is almost impossible. So, they must have an on-off switch.
