I've done several posts on them:
I'm probably not done with posts on internal combustion engines. That's because they are cool, and there are so many interesting ways to configure them.
Today though, I'm delving into a subject that I'm not quite as familiar with, and that is the drive-train of a 4x4 pickup truck - mine specifically. Last plowing season, I noticed that the truck would spin one tire relentlessly. It never did transfer power to any other wheel, which I always thought was the purpose of having a 4x4 truck in the first place.
Recently I spent a little time online, trying to figure out whether there was a problem, or whether I needed to modify something to get better traction. Looks like the answer is a little bit of both.
Below is a really cool video describing how the open differential on your car works to allow one tire to lose traction to allow for your vehicle to make a turn. The first two minutes are prehistoric motorcycle stunters. Skip to the 2:00 mark to learn about open differentials. Note to modern motorcycle stunters: Dress up like cops so the cops won't go after you - even if you don't wear a helmet! (that was sarcasm, in case it wasn't obvious)
Back to open differentials: The above video explains why cars have them, and how they work. For a rear wheel drive passenger car, each rear wheel must be able to move the car forward during a turn, while the inside wheel idles. Meanwhile up front where the steering happens, the wheels are not linked together at all, and so they can independently rotate at different speeds. No problem there.
But what if traction is more important to you than tire wear? Suppose you want both rear tires to be able to grab instead of allowing one wheel to stop? Drag-racing, off-roading, and snow-plowing would be specific cases where you might prefer maximum traction instead of tire longevity.
How can we stop that one tire from spinning when it hits an icy or muddy patch?
There are a few methods that can keep one rear tire from spinning. One way is to connect each axle half, effectively making it into a solid rear axle. The device connecting each axle half is called a "spool". Below is a spool. The splined ends of each half axle fit into it. The ring gear (which is fitted to the driver's side half axle) drives the other half axle at exactly the same speed through the spool.
A spool is not very desirable for a street vehicle however, because the rear wheels are forced to turn the same speed at all times. So if your rear tires are even slightly different diameter or inflation, you are scuffing them. If you ever need to turn, you are scuffing the tires. They will wear out pretty quickly in normal driving.
Are there any alternatives? Yes. Yes there are!
You can install one of two slightly more complex devices: A locker, or a limited slip clutch.
A locker keeps the rear axle locked by default, to provide maximum traction. It will unlock when steering requires one wheel or the other to rotate faster, at which point it unlocks the rear axle. This means that neither rear wheel is allowed to rotate slower than the differential carrier. A locker can either replace the entire differential assembly or just the gears (a "lunchbox locker").
Below, an Eaton locker that would be found on a GM truck. This replaces the gear carrier of a normal differential. (the item labeled "differential case" in the drawing up above)
There are other options. You can buy a selectable remote-operated locker, one that locks with the push of a button. In this case, the axle is normally unlocked, and the driver decides when locking it is desirable. These are expensive though, and there are electrical (or air fitting) connections that might fail when needed.
Lockers also have some disadvantages, compared to open differentials and spools. They make a lot of ratcheting noise as you go around corners, and handling becomes interesting as they lock the axle after completing a turn.
Lastly, there are limited-slip differentials. These are the coolest (and more expensive) route, but probably offer the best compromise between an always locked and always unlocked rear axle. These devices use gears, clutches, viscous coupling, or electronics to limit the differential speed of each half-axle. The Posi-Traction limited slip is possibly the most famous of these, due to the Beach Boys and the muscle-car era.
Below is a limited-slip clutch pack for a BMW.
So that's the story on why I have unlimited slip on the rear of my truck. It's equipped with an open differential, and I'm going to install an inexpensive locker in place of the gear carrier.
The front end of the truck (it's a four wheel drive, remember) has a different issue. The hubs are very likely not engaging. Link: How locking front hubs work
Instead of troubleshooting and repairing something that will probably fail again, I'm probably just going to install manual hub lockers.
EDIT: I did replace the front hubs, and the link to that project is here.
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