In 2015 we ordered a Ford Transit T-350 cargo van with the intention of building an off-road capable adventure van for full-time mountain bike travel and exploration. Our van is a combination of DIY and professional build-out. The build includes a robust solar/electrical system, a full galley (sink, 2-burner stove, refrigerator & freezer), Espar on-demand hot water and heat, 25-gallon water capacity, a queen-sized bed, and a garage for bikes, packrafts, and gear storage. All of these systems and design features are for livability and functionality. We plan to do a deeper description of these in later articles. For now, let’s focus on the off-road capability and exploration part of our van build. One of the biggest factors on vehicle mobility; the drivetrain. Or specifically, the guts of the rear axle differential.
The Ford Transit is equipped with a 9.75″ Sterling full float rear axle. This axle is pretty common and as a result, a number of factory and aftermarket gearing and differential options (Locking Diff, Limited Slip, etc.) are available. This translates to options for you to customize the drivetrain to meet your needs.
Spoiler – We ordered our 2015 3.5L EcoBoost Transit T-350 L2/H3 with the factory Limited Slip Differential ($300) and standard 3.31 gears. While the LSD was a great add, we eventually upgraded this to an aftermarket ARB Selectable Locking Rear Differential. Also, since we run AT Tires that are ~2.5” larger diameter than the stockers, we also swapped the 3.31 for lower 3.73 gears.
For a general overview of what we selected for our application, why we selected it, and a demo on how an open vs locked differential performs, check out our video on the MtbNomads Youtube Channel:
OK, you watched the video and want more info, lets dive into these a bit more:
First, factory gearing of 3.31, 3.55, 3.73, and 4.10 are available for the Ford Transit. Not all of these gears are available for all engine options due to Ford’s matching of torque, horsepower, max GVW (Gross Vehicle Weight) and target application to gearing. The 3.31 gears are the most common rear differential gears available for the Transit configurations. The 3.31 gears are the “tallest” or highest gearing option available for the transit. These refer to the ratio and mechanical advantage of the rear differential – the linkage between the engine/driveshaft and the rear axle. As an example, 4.10 represents a gearing ratio of 4.10:1, meaning that the driveshaft turns 4.10 times for each turn of a wheel. The relationship of gear and tire is important, more on this later. As the numerical ratio goes up, mechanical advantage is increased (more wheel torque, towing capacity). This means that a van with 3.73 gears will have greater towing capacity (or lower gearing…) than a van with taller 3.31 gearing. But everything has its trade-offs. The engine of a van with lower gearing (4.10) will have to operate at higher RPM to maintain speed than a higher (3.31) geared van (read: reduced fuel economy).
So…what’s the Diff…??
A differential is a gearing mechanism in the rear axle that takes the engine/driveshaft output and transfers torque to the rear wheels. The rear wheels of a van travel at a different speed and distance during a turn. Long ago, smart people designed the open differential to accommodate this varied wheel path length through a separation (…or “differentiation” …) of the supplied torque, allowing wheels to spin at different speeds. This is important due to the fact that in a turn, the outer wheels spin faster than the inner wheels during a vehicle turn. The open differential transfers most or all torque to the faster rotating outer wheel. In off-road situations, this can be a huge detriment. With an open diff, torque is transferred to the “path or least resistance” or fastest spinning wheel – you know the one with the least amount of traction and ground contact. Your 2WD with an open diff is effectively a 1WD off road. Since this is less than ideal, the the smart people developed additional differential options.
Nope, not lysergic acid diethylamide. A Limited Slip Differential (LSD) reduces torque to a slipping wheel, and prevents torque from being sent to a single wheel, and most importantly for low traction situations, keeps both wheels in powered rotation at all times. Due to this improved functionality, a LSD can be a worthy upgrade for driving in snow, mud, and light duty off-road. A LSD is functioning at all times and does not require driver selection or activation. There are a number of both OEM and aftermarket types (clutch, torsen helial gear, viscous coupler, e-LSD, etc.) and brands of LSD available for the Transit differential. The OEM Ford LSD option ($300) is a no-brainer for those opting for a new Transit.
Pro tip – in some instances, off-road performance of a LSD can be improved with ‘left-foot brake’ driving technique or temporary and partial application of the emergency brake. These actions essentially trick the LSD into sensing traction (or resistance) with both wheels and not limiting torque to either wheel – pretty much creating a temporarily “locked” differential. Pretty rad.
A locking rear differential locks both wheels together as if they are on a common shaft. Both wheels turn at the same speed regardless of traction available to either wheel. A locker must be selected or activated by the driver before its needed. Currently, lockers are engaged by air or electronic systems. So, yeah, additional systems introduce additional “opportunity for failure”. Also, a locked differential is exactly the opposite of an open differential. In tight turns, the inside wheel still turns at the same speed as the outside wheel, resulting in additional tire wear and stress on axle components. This is why a locker should never be engaged in high traction situations like highway or high traction bedrock (sandstone slickrock) driving. Also, a locker is not a good option for snow on pavement. All this said, the locker seems to be the most capable of the off-road differential options. This rings especially true if you plan to encounter difficult to severe terrain and one or more wheels leave the ground, as full power remains on the other wheels keeping you moving.
Check out CRAWLpedia.com if you want a deeper dive into differentials and options. They also have a bunch of additional calculators that you might find useful when considering your drivetrain setup or upgrade process.
Differential Gearing vs Tire Size
Ok, I said we’d revisit this, so here we go. As mentioned above, the gear ratio is the number of driveshaft rotations related to the number of tire rotations. What happens when ya ditch the undersized, puny, poorly paired 235-65-16 (~28” diameter) stock tires on your Transit for a solid e-rated set of 10-ply 245-75-16 (30.5” diameter) All Terrain tires? Here’s an example of what to consider.
Recall that our Transit came with the stock 3.31 gears and that we would want to remain near the stock gear ratio with larger tires. Turns out, CRAWLpedia.com ( ) has a gear ratio calculator related to tires. To run this, all you’ll need is the stock gear ratio and old/new tire diameters.
According to this calculator, we’d need to install ~3.6 gears. So, 3.73 gearing swap would be a good choice. The 3.73 gearing is slightly shorter (lower) and widely available for the Transit Axle.
Why change gearing? Well, the Ford engineers paired engine outputs, intended use (towing, GVW, etc) with specific gearing options. When you bolt on larger tires, you depart from this. With larger tires, you may notice increased engine “lug”, a change in shifting, and reduction in available torque and power. Since the EcoBoost is such a powerful engine, it can take a lot of change, but at the expense of optimal performance. And, add in a few thousand pounds of adventure van build, fuel, water, and gear and you’ll want to ensure that your van is properly set up for performance and longevity. In our view, this is a good investment for the vehicle.
Bringing it all together
So, let’s get back to the MtbNomads Van Build. Remember, we wanted an off-road capable mobile basecamp for our travel, adventure, and exploration. So…how did we bring all this together?
Right after we got our Transit, we quickly bolted on a new set of Ultra Toil 470 Wheels and 245-75-16 Falken WildPeak AT3W tires. These are ~2.5 inches taller than the anemic stockers. While an amazing upgrade in both on and off-highway travel, the larger tires reduced our effective gear ratio to ~3.10 – much taller than stock. We found that the powerful EcoBoost was working way too hard to push our 9,000 pound rig up steep climbs and seemed to lug its way along on big mountain passes. We also wanted to increase off-road capability beyond that of the LSD.
By now, you can probably see where this is going. We contacted Agile Offroad in southern California to see what our options were. They were able to install an ARB Selectable Locker (9.75″ Ford Air Locker RD150, 34 Spline) system and 3.73 gears in our rig for a total (parts & labor) cost of about $3000. While the locker does add a layer of complication (air compressor, switches, air lines, etc) it has proven to be a worthy upgrade and is meeting our purposes pretty dang well.
The gearing and locker are only part of our quest for a capable mobile mountain condo. We’re in process of developing additional content for lifting a Ford Transit, other worthy off-road improvements & upgrades, and all the other components and considerations of MtbNomads van build. Stay Tuned!
What else do we have along for backcountry travel and recovery? We have an assortment of shackles & attachment points, a recovery strap and vehicle recovery boards, upgraded recovery jack, and an air compressor to re-inflate after lowering tire pressure for off-highway travel. Check out our Backcountry Travel and Recovery Gear Amazon Ideas List for other gear to consider throwing in the van.
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