Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Differential terminology?

My 70’s Ford truck has a “limited slip” differential. I’ve taken it apart before, and there’s slew of clutch discs inside, that’s apparently what’s doing the slipping.

I’ve hear the term “locker”, and “air locker” applied to differentials now. I don’t understand this terminology. What’s the difference between “limited slip”, “locker”, and “air locker” differentials?

A locking differential allows no slip whatsoever and both wheels are locked together and turn at the same rate. The air locker is engaged with compressed air powering the actuator. Some others are engaged with and electric powered actuator. A limited slip over time can slip as they wear as they are in use more often automatically and can not supply equal torque to both wheels as a locker will always do…if you can get it engaged. The normal open differential is quite problem free where variations have more moving parts and require more attention over time. Because they are used in conditions that are often severe, the differential and accompanying actuator takes lots of external abuse, both in the way it’s used and the conditions it faces…mud, water, bouncing off rocks etc. If you don’t have a locker, you tend not to go in these “places” as much. I have one in my truck in the rear and on my tractor,both front and rear…they can make a world of difference and nothing works better…when they work.

Ok, I think I see what you mean. If you’re doing a really tough hill climb say, in a 4x4 Jeep or something, like the photos you see in those magazines where the wheels are going every which way over big rocks, the Jeep is covered head to toe with roll bars and big thick plates of steel underneath to protect the drive-train, and you want the very maximum amount of traction, you’d rather have “lockers” than a “limited slips” installed on your Jeep, right? But you’d never engage (“lock”) them except when you intend to go straight, or at least you are on a slippery enough surface. Otherwise you’d damage the tires.

Am I on the right track? “Lockers” are for the really advanced 4WD enthusiast?

"Lockers" are for the really advanced 4WD enthusiast?

I would think so, and probably drag racers.

Actually, it’s an open differential that delivers equal torque to both axles, and it’s the tire with the least traction that determines how high that torque is.
A locked rear end provides equal speed to both axles, even if one of the tires is not even on the ground.

What do you mean by “open”? Is that the regular kind found in rear drive cars, where the ring gear (driven by the pinion) is bolted to the third-member housing, and the housing has a rod that goes through it, and the rod has two gears on it, which engage with each of the gears that the axel shafts go into? So the pinion turns the ring gear which turns the housing which turns the rod which, via the two gears on the rod and the gears attached to the axels, turns the axels?

"Lockers are for really advanced 4wd enthusiasts "
Well…yes, and no. Anyone who gets stuck while in part time 4 wd…and you can get stuck, either in deep mud or snow or getting hung up, can benefit more from a locker then from a limited ship including those operated by your ABS. It’s just that non enthusiasts seldom even use their 4wd except on slippery roads where lockers should not be used over about 5 mph. So most manufacturers don’t give you one. But, it is interesting that both the Honda Ridgeline and Honda Pilot have a quasi locker. Because their differential for power going front to back is in the rear differential and they don’t have a center one, locking it also locks the two rear wheels. It works quite well in moderate conditions under 10 mph to get you going.

Plus, it should be use to keep it working. Because it isn’t something that is commonly automatic, most drivers today can’t be bothered learning how to use it.

dagosa: Most drivers today can’t be bothered with actually driving their vehicles period. They can’t wait for the self driving vehicle so they can waste more time on their phones. Google maps has only sent me to a dead end three times.


What do you mean by "open"? Is that the regular kind

Yes, just a regular differential or “unlimited slip”. Slip is another misnomer because nothing slips in a normal differential except maybe the tire with the least traction.

Also, a locked rear end tends to fight your effort to steer the car especially if the front end is light during acceleration, in case you are wondering why so many dragsters get disqualified from crossing the center line on the drag strip.

Trucks mostly use locker differentials. GM had a fetish for them on trucks because clutch type diffs tend to make towing a little strange if there is any small variation in rear tire size from side to side. Also with unloaded beds, the trucks tend to push like a cheap lawn tractor on wet grass on slippery surfaces.

Lockers are harsh heck when they engage. Slip a tire on wet pavement and you’ll get a BANG from the rear as it engages. An open diff will just spin. GM has adopted an even worse diff on trucks called a rev-loc which prevents the locker from engaging above about 25 mph.

Lockers are not used much in drag cars, they usually lock the axles together with “spools”. Spools are not acceptable for street use!

The best differential is the gear type or Torsen or Tru-Trac. The torque is vectored to the wheel that can accept the greatest amount of traction. No bang, no trailer drift, no push. They won’t work for rock crawlers because if one tire is off the ground completely, they spin like an open diff. Hence the need for an air-locker.

I’ve never really had a feel for how the Torsen works:

Worm gears and spur gears add up to a lot of friction and wear…It looks expensive to manufacture…

Good explanations, thanks.

Most drivers today can’t be bothered with actually driving their vehicles period. They can’t wait for the self driving vehicle so they can waste more time on their phones.

It’s not really about driving anymore. It’s about talking while in motion.

Google maps has only sent me to a dead end three times.

My GPS certainly makes occasional errors. Not many dead ends so far, but plenty of not-so-near misses, ranging from a block or two to just over a mile.

Another faux name used by manufacturers is calling a differential a “limited slip” when in reality, the differential is an ordinary open differential. It can be made to behave like one by simply using the traction control with the throttle limiter cut off in the traction control system. (Usually by a special switch on the dash) Normally when a drive wheel then slips on an axle, instead of cutting power as well, the ABs system will only apply brake pressure to force some of the torque over to the other wheel. You can then spinn both wheels which is good in deep snow and mud. When that wheel Slips because it is on ice, some drive power is then transferred to the other drive wheel on a solid surface. It works quite well but it should not be called “a limited slip differential”. Because the differential is the same.

The disadvantage is, it’s really hard on the brakes if it’s used continuously like climbing a long slippery hill. A mechanical limited slip or a locking rear differential would be better. The advantage of the ABs activated one is…it never wears out as long as the brakes function and eventually, as the clutches on a mechanical limited slip decreases effectiveness over time, these traction aids remain consistently good. The other advantage is, it’s easier to incorporate them on all open differentials, front or rear and even combine it on front with a mechanical limited slip or real locker on the rear.


I did that manually on farm tractors. If I was pulling real hard and one wheel started spinning, I would brake that wheel to force the other wheel to turn.
Tractors have separate brake pedals for each rear wheel for the benefit of those who never used one.
Those brakes were also used to aid turning when pulling a plow.

Somehow, I suspect that anyone who accelerates aggressively enough to activate this feature routinely during normal road conditions is going to experience short brake life anyway.

This all reminds me I had an embarrassing event as a teenager involving differentials. While on a first date I somehow managed to get the passenger side rear wheel of the car positioned over a deep rain gutter in downtown Denver, enough so the wheel was in mid-air. I step on the gas and hear nothing but Whirrrrrrrr … lol … I had to round up some passers-by to push the car away from the curb. As you might expect, there was no second date.

Here’s another thing I don’t really understand about differentials, in this case the plain Jane open type. The whole case rotates, and there’s a rod, called the pinion shaft I think, that inserts into two holes in the case. On each end of the pinion shaft are pinion gears, which mate with the drive gears (what the axel shafts spline into).

My first question is: Does the pinion rod rotate w/respect to the two holes of the case, or is it fixed w/respect to the case?

And second: Do the pinion gears rotate on the pinion shaft? If so, do they rotate on bearings, or just bushings?

@‌ GeorgeSanJose
I believe those are called “spider gears”, the pinion is what drives the ring gear. The spider gears only spin on their shafts if one rear wheel is turning faster than the other.