Installing a limited slip differential | Project Ford F-250

We spent our last update improving the suspension of our 2001 Ford F-250 tow vehicle, finishing with a truck that drove down the road as well as anything we’ve ever towed with: no drama, no issues, no mental strain after a 10-hour tow. The perfect tow vehicle disappears from the tow, and our suspension finally had.

Too bad the driveline wasn’t holding up its end of the bargain. No, our engine wasn’t the problem: Sure, the gas V10 is no coal-rolling powerhouse, but it’s plenty to drive down the highway at 75 mph.

But every stop light held the opportunity for a one-tire-fire from the open differential, especially in the wet. And the clutch throw-out bearing was howling at every opportunity. Hey, at least the manual transmission’s synchros were ruined, too.

[Transmission teardown and picking out a differential | Project C5 Corvette Z06]

Full disclosure: We knew about the noisy throw-out bearing and the worn-out transmission when we bought the F-250, and that’s one reason it was cheap.

But with little else left on the list to fix, all we could think about every trip was how badly we needed to make some fixes and some upgrades. Time to roll the truck back into the garage and get to work.

We should admit that it wasn’t our garage we rolled the truck into. Thanks to our low ceilings, pulling the transmission on our lift would mean pulling it on our backs, and we weren’t willing to suffer through that ordeal. Instead, we had our friendly general repair shop, the Auto Clinic of Ormond Beach, handle this phase of perfecting our tow vehicle.

 

What’s the Best Differential for a Tow Vehicle?

Our Ford F-250 came from the factory with an open rear differential, meaning power is sent to whichever wheel has the least traction. Open differentials are fine for the average driver, as they’re inexpensive, quiet, and reliable.

But we’re not average: We’ve got a V10 with tons of low-end torque and a heavy right foot, plus we hardly ever drive this truck without hooking up a giant trailer first. This combination meant our truck was constantly spinning one rear wheel, especially when pulling out into traffic in the rain.

This isn’t just annoying: It’s dangerous, and it’s more likely to break something when that wheel inevitably regains traction.

And that’s just on pavement: In a muddy paddock or our sandy backyard, we occasionally needed four-wheel-drive just to move a trailer around. Sure, that’s no big deal, but it did have us wondering what somebody towing with a two-wheel-drive F-250 would do.

Here’s the answer: Install a limited-slip.

[What's the Diff? | Figuring Out Which Differential Setup Is Fastest on Track]

In fact, our late grandfather (a lifelong Ford salesman) had a saying for just the occasion: “Only an asshole buys a truck with an open differential!” Rest in peace, Opa.

And true to form, Ford offered a limited-slip as a factory option for our F-250’s Sterling 10.5-inch rear axle. Branded as Trac-Lok, these OEM limited-slips were clutch-type differentials, which limit the difference in rear-wheel speed by putting clutch discs on either side of the spider gears.

These work fine, but they’re not perfect. Clutch-type limited-slip differentials can be noisy and abrupt when they lock, and they also add wear items to the truck: Those clutch discs wear out just like any other friction material.

Still, the sheer number of Super Duty trucks that left the factory with Trac-Loks means they are inexpensive. We bought a used Trac-Lok center section on Marketplace for $100, figuring the low price was worth the five-hour round trip (and $50 of gas) to pick it up.

Of course, that $100 was just the start of things. We spent another $100 ordering an off-brand set of replacement clutches online to rebuild what we’d just bought, then took a closer look and realized we’d screwed up. We’d driven five hours to buy the differential from an older Ford truck, not a newer Super Duty like ours.

What’s the difference? Not much–and what we’d bought would actually bolt right in–but there’s a major difference: The spider gear arrangement. Helpfully, the internet calls these pinion gears in this context, even though they’re not what you’d commonly refer to as the pinion.

Our truck came with a three-pinion differential, while the differential we bought had two pinion gears. Though these two designs are functionally identical, the later style is much stronger–50% stronger, to be exact–and we figured Ford wouldn’t have spent the time and money redesigning the differential if it wasn’t necessary.

We may have installed the two-pinion differential if all we did was drive around an empty truck, but this F-250 never leaves the driveway without being loaded to capacity. Plus, our manual transmission is more likely to shock load, and break, a spider gear than an automatic would. We put the used center section on the shelf and went back to the drawing board.

Add it up, and our cheap differential wasn’t actually that cheap: We’d spent $250 to end up with a used part in unknown condition that was weaker than stock and ended up sitting on the shelf. And it’s no cheaper or easier to install than the right parts. This wasn’t a job we wanted to do twice.

Time to talk to some experts. And if you’re asking about a differential for your tow vehicle, there are few sources better than Eaton.

This massive company spends most of their time making parts for OEMs in a wide range of industries. Need a differential for your Dodge or an actuator for your Airbus? They’ve got you covered. Here’s the cool part: They also sell aftermarket parts to car dorks like us.

One of those aftermarket brands is Eaton Performance. You can buy an Eaton Performnce differential for almost any common rear end, including the full-floating Sterling 10.5-inch found under our F-250 project.

That’s how we found ourselves telling their experts about our adventures buying a used, incorrect limited-slip for our truck, which had them scratching their heads: On something as important as the rear differential, in something as important as a tow vehicle, why cut corners and risk doing the job twice?

It’s one thing to grenade the differential in your race car on track, but it’s a whole new level of misery to grenade the differential in your truck while towing through the middle of nowhere, at night, in the rain. Their catalog lists three options that would fit our truck, each priced right around $800 from Summit Racing.

 

Eaton Detroit Locker Differential:

Detroit Lockers are the most aggressive automatic differentials in the catalog, and lock both wheels together under power by replacing the spider gears with a pair of ratcheting cogs that are either engaged or disengaged. These types of differentials are also called autolockers or lunchbox lockers, and we’ve used them in off-road projects in the past.

That experience is what led us to cross the Detroit Locker off our list immediately: Off-road, nothing compares to a locked differential. But on the street, there are just too many compromises. These are either fully locked or open, with no progressive step in between, so using one on the street means clicking noises, loud bangs, and the occasional chassis disruption during transitions from locked to unlocked.

We’d choose this differential for something like a work truck that drives through muddy pastures every day, or a dedicated off-road build. For our street-driven tow vehicle, though, it just wasn’t the right fit.

 

Eaton Posi Differential:

Eaton Posi is a traditional clutch-type limited slip, similar in design and function to the one Ford offered to install when our truck was on the assembly line. Eaton claims that its modern design and materials mean that a Posi will last the life of your vehicle before wearing out, and these differentials promise much of the off-road ability of a locker with much, much better street manners.

This differential’s clutches allow for progressive engagement and disengagement, meaning you’ll barely notice it on the street, especially if you use the recommended friction modifier in the gear oil to help the clutches slip smoothly. This differential is probably the best compromise in the catalog, which is why its basic design is exceedingly common on OEM truck order forms.

 

Eaton Detroit Truetrac Differential:

Rather than clutches or ratcheting teeth, the Truetrac is a helical limited-slip. It might take an engineering degree to fully understand these types of limited-slip, but here’s the elevator pitch: Helical limited slips use the one-way torque transmission of worm and spur gear combinations to handle the function of a traditional differential’s spider gears.

What’s this mean in English? A helical differential will allow one wheel to be spun faster by the road, but it won’t allow one wheel to be spun faster by the driveshaft.

During tight turns on pavement, these differentials act exactly like open differentials. But as soon as one wheel starts to spin when accelerating from a stop, the differential locks like a clutch-type limited slip.

Helical limited slips are favored by OEMs in passenger car applications because they have exceptional street manners without any wear items. One common brand name of helical limited slip is Torsen, and you’ll find them under some of the fastest Miatas at your local autocross.

If this sounds too good to be true, well, there is a catch, and we’re pretty sure it’s the reason these aren’t more common in heavy-duty pickup trucks like our F-250. Helical limited-slip differentials don’t technically lock: They limit slip by multiplying the available torque of the wheel with less traction across the differential to the wheel with more traction.

On the street or in a sandy parking lot, this is a meaningless technicality: Multiply the torque of a tire on wet pavement, and the result is plenty of holding power for the other tire on wet pavement. But the same isn’t true for one of the common use cases for heavy-duty trucks like ours: When one tire is in the air or in a similar situation where it has zero or almost zero traction, there just isn’t enough torque to multiply, and a helical limited-slip will act just like an open differential.

 

Choosing a Differential:

We’d eliminated the Detroit Locker, which meant we were down to two options: The clutch-type or the helical limited-slip. Both had their advantages and disadvantages on paper, but our decision ultimately came down to a reality check on how we use our truck: We wanted a quiet, seamless experience towing a trailer on pavement, and a small dose of improved off-road ability for muddy paddocks.

Add in a slight price difference and a healthy dose of editorial curiosity, and we ordered an Eaton Detroit Truetrac in the mail for our F-250. Summit Racing’s catalog shows this differential in stock for $713.88 with free shipping, but we’ve noticed prices seem to fluctuate depending on the application.

It seems like if you’re axle is smaller, or more common, or both, then you’ll probably spend less on a Truetrac for your own application.  We didn’t find any meaningfully cheaper off-brand helical limited-slips for our truck, but we did find quite a few lockers and clutch-type limited-slips for sale from other companies.

At the end of the day, the decision will come down to your tolerance for risk, and our tolerance is extremely low when it comes to the truck we rely on to get us safely to and from every race. We’ll happily throw a $300 differential in a project car and see if it explodes, but this F-250 is something we rely on, and we wouldn’t have an issue paying a bit extra for the Eaton name, build quality, and support if we had to order another differential tomorrow.

What about installation? We’ll cover that–and the transmission and clutch replacement–in the next installment. Spoiler: After a few thousand miles of driving with our new Truetrac, we’re thrilled with our decision and could never go back to towing with an open differential. It’s completely invisible until we remember we haven’t had a rear wheel spin in months.

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