Well, ok I made those two up. But today’s part does have a few names, depending on who you’re talking to. It’s typically called the center section, or third member. Often it’s called the pumpkin due to its similar dimensions, though this term is also used to refer to the entire visible bulge in the axle which contains the differential and gears, including that part of the axle housing.

(As inconsistent as all these names might seem, it’s not even close to the confusing lexicon of computers … ask any computer geek what the word filesystem means, and if you only get one answer, look elsewhere for information in the future.)

In the Ford 9″ design, the center section is a discrete, detachable assembly containing the pinion gear which attaches to the driveshaft, the ring gear which attaches to the differential and meshes with the pinion, and the differential itself, which selectively couples power to the axles. Power is transferred through these three parts in that order. The ratio of ring gear teeth to pinion teeth determines the “rear end ratio” which in my case is 4.11 to 1, usually expressed by car buffs without the decimal or the 1, e.g. “411 gears”. Stock is usually less than 3 to 1.

These parts are held together by a single piece of cast metal often referred to as the diff housing, carrier, or case, which is made variously of cheap grey iron, nodular iron, or in some cases aluminum. I’ve seen one made of titanium, but you won’t catch me sporting one of those until I win the lottery. This one, made by Strange Engineering, is cast out of nodular iron, which is heavy but very strong.

The differential, located in the center of the ring gear, deserves some mention here as well, as its design directly affects the dynamic characteristics of the vehicle. A typical differential is called an “open diff”, meaning that there is no connection between the wheels. They’re allowed to spin at different speeds so they don’t scrub on the pavement when going around curves and corners, but this also means that if one wheel should lose traction, all available power will go to that wheel making it spin uselessly, with no power going to the other one. Not so good for rolling around off-road, and not so good for racing either. In drag racing especially you want both wheels to turn at the same speed, which is why serious racers use what’s called a spool — it’s just a tube of metal that locks both axles and the ring gear together, making it effectively a single axle. The vehicle is really hard to steer with a spool, but it does straight lines with confidence.

Somewhere in between an open diff and a spool are what are commonly known as “limited slip”, “positraction” or “posi” differentials (though there seems to be no consistent agreement on any of this terminology), which employ various methods like viscous couplings or clutch mechanisms to allow the wheels to spin at slightly different speeds, but not extremely different. This is very helpful with off-roading and autocross, as it prevents a loss of traction on one wheel from causing you to lose all power. Finally, somewhere in between a limited-slip and a spool is what’s called a locker. There are automatic lockers like the Detroit Locker, and manually-controlled ones like the ARB Air Locker. Mine is an automatic type made by Strange called the Lenco Billet Locker, similar to the Detroit but supposedly a little stronger. Probably a bit stronger than I’ll need. An automatic locker will “ratchet” to allow the wheels to turn at different speeds when they’re forced to, going around corners at low power. But they will lock the two axles together at the first opportunity, and will hold fast as more torque is applied. It’s about as close as you can get to a spool without wearing away your tires as you turn.

The side of the center section with the visible gear plugs into the opening on the axle housing, and then the axles are inserted through the housing tubes into the holes you see on the sides. Finally the assembly is filled with oil. What’s somewhat unique about the Ohmbre is that the gearing in this center section will probably be the primary source of mechanical noise on the whole truck. In most EV conversions, the transmission ends up being the noisiest part of the vehicle while it’s moving; most of what you hear from inside is gear noise along with some tire and wind noise. With no transmission and with a high-power motor, it may be a contest between the rear-end gearing and the resonating motor coils as the controller pulses power during hard acceleration. It’ll be a happy day when I finally get to hear what it sounds like from inside the truck.

Link to today’s part.

Update: I’ve received confirmation that the Lenco Billet Locker is indeed manufactured by LENCO Equipment Company for Strange Engineering. LENCO’s site doesn’t mention the locker product, but they do sell them direct as well.  I’ve been curious about this for a while, but had assumed in the absence of googleable details that it was a Strange product and that the name was a coincidence. Although Strange is hardly a second-rate manufacturer, I’m actually glad I was wrong.