For as many years as I can remember, Ford Super Duty pickups have ruled the roost when it came to heavy-duty towing and hauling. Off-road performance, on the other hand, has been left to the aftermarket. Sure, there’s such a thing as an FX4 Off-Road package, but that’s pretty weak sauce. Ram has rightly been able to point at its 2500 Power Wagon and underline how much more seriously Ram takes off-road performance in the heavy-duty pickup segment.
That’s now debatable. The 2020 Ford Super Duty pickup order guide has a new line item called 17Y, the Tremor off-road package. Unlike the Ram 2500-only Power Wagon, the Tremor is available in F-250 or F-350 form, and the engine lineup goes beyond gasoline to also include diesel. Any variety of Tremor can tow and haul more than the Power Wagon, too. Quite a bit more, in some cases.
We can’t say which is better off-road until we get them together in some suitably gnarly location. But nothing is stopping me from getting a head start on that question by taking the wheels off and having a look-see in my own driveway for the latest suspension deep dive. For the record, this is an F-250 Tremor with the optional 6.7-liter PowerStroke diesel V8 engine, a distinction I'll discover actually makes a significant difference to the suspension and what extreme tasks it's best suited to tackle.
The F-250 Super Duty pickup is a no-nonsense heavy-duty machine that rides on a solid front axle that we can’t yet see. But we can see a clue that gives away its existence: a massive radius arm (green arrow) that controls its position in space.
The nose of a Tremor stands 2 inches higher than a regular F-250. A half inch of that comes from these Tremor-specific springs that sit directly atop the axle. The remaining 1.5 inch comes from the larger radius of its huge 35-inch tires — 285/75R18 in new money. Non-Tremor XLT, Lariat or King Ranch F-250s ride on 275/65R18 tires — 32 inches tall in old money.
The Tremor’s unique shock absorbers mount directly to its front axle housing (green) via this welded-on bracket. But the magic is hidden inside. These have a larger diameter that accommodates a 45-mm piston instead of the usual 35-mm one. The bigger piston makes them more sensitive to small motions, and it gives them the capacity to generate the larger damping forces necessary to control heavy off-road impacts. But the most significant benefit may be the extra oil volume that allows them to better stand up to extended running on relentless washboard roads.
Here you can see both radius arms in action. I like to draw a distinction between arms and links. Links attach at one point on each end, but arms have two mounting points (green) on one end that allows them to triangulate the axle’s position. These two arms do the work of four links.
The upside of this approach is it’s beefy and durable. The downside is the suspension’s caster angle will change as the suspension moves. This tendency becomes less significant as you make the arms longer. Also, precise caster control is less important here. This is a 4x4 truck, not a sports car.
These are twin-tube shock absorbers, but they look odd. Most shocks of this type neck down toward the mounting eye, but these bulge out. The reason is crucial to the potential off-road success of the Tremor. Inside there are hydraulic compression and rebound end stops that give them position-sensitive damping. We’re looking at evidence of the compression zone.
The end stops begin to engage during the last 20% of compression travel, which makes these shocks better able to absorb a big impact or, say, land a jump without bottoming out heavily. The ones at the rebound end that we can’t see aren’t as pronounced because full extension events must only contend with the weight of the hanging suspension, not the entire truck.
A Tremor’s stabilizer bar (green) is smaller than it would be on a regular F-250. This move improves articulation, but it also shows that the Tremor is less committed to pure boulder-hopping than the Power Wagon, which has a fully disconnectable stabilizer bar.
The Tremor’s end-link (yellow) connects to the front axle much like the shock absorber does, but the upper end of the link hides something (red) of importance.
That something is the bump stop and its landing pad (red) on the axle housing. That doesn’t look like a lot of space, but it is. I supported the truck by the axle on this side so the suspension is compressed slightly more than usual, not hanging free.
This panhard rod (red) is the fifth link that keeps the axle from moving side to side. We’re looking at the fixed end where it bolts to the frame.
The fixed end is on this end because that’s also where the Pitman arm (yellow) comes down from the steering gearbox. You don’t want these arms criss-crossing each other. The Pitman arm moves the drag link (also yellow) left and right when you turn the steering, which in turn compresses or extends the steering stabilizer (green), a shock absorber that smooths out the steering.
The drag link (yellow) attaches to the right front hub, which then transmits the signal to the left front hub through a long tie rod (green). The cool part here is how the drag link and tie rod share a common axis (white line) and pivot point. You might think this would be universal, but it’s often not the case.
The drag link is quite a bit longer than the panhard bar (red), a difference that means both will swing through different arcs. A driver might have to input a steering correction during extreme travel events.
Alignment adjustments are easy here. Loosen the drag link clamps (blue) to center the steering wheel. One side has left-hand threads and the other has right-hand threads, so you simply turn the middle part to make changes. Likewise, toe-in adjustments are made by loosening the clamps on the tie rod (black), then rotate the center section to lengthen or shorten it as necessary.
No other alignment adjustments are possible because it’s all built into the axle housing, which is set up so the center of the u-joint is precisely aligned with the steering axis. Why u-joints instead of constant-velocity (CV) joints? This is a rear-drive machine until four-wheel drive is engaged, and that’s only supposed to be done on dirt, sand or snow. The slight drive velocity oscillation that comes with u-joints isn’t an issue on loose surfaces.
OK, fine, but what’s up with that hose (green)?
This is a vacuum line that supplies the automatic locking front hub. Hubs that can be locked and unlocked used to be commonplace, and they’re still important in a truck with a massive solid front axle like this. In two-wheel drive you want your hubs to be unlocked or “free” so the u-joints we saw in the last picture (and the axles they’re attached to, as well as the front ring and pinion and the front driveshaft) aren’t being backdriven by the wheels all the time. All that represents a lot of unnecessary wear and fuel waste. Manual locking hubs were once the norm, but getting in and out of the vehicle and moving them from “free” to “locked” every time you shifted the transfer case into four-wheel drive was thought by many to be a real pain.
Auto-locking front hubs grew out of that, but the pure mechanical ones used drive torque to ratchet themselves into the locked position when 4x4 was engaged inside the cab. This was great for convenience sake, but the approach meant that they could unlock going downhill in engine braking situations. Durability wasn’t always so great, either.
These vacuum-operated auto-locking hubs get around that because engine vacuum does the job, not drive torque. When engaged, they can transmit drive torque when powering uphill or when engine braking downhill. Ford has used three variations on this theme, and this is the Constant Vacuum Hublock (CVH) design. Vacuum is automatically applied and sustained to lock the front hubs when you select four-wheel in the cab and is removed to unlock the hubs when you switch back to two-wheel drive.
I know what you’re thinking. What if the vacuum system leaks? Simple. You can get out and manually lock them like grandpa used to do. You don’t have to, but you can if needs must. Just remember to get out and manually set them back to Auto when you shift back into two-wheel drive.
Most newer 4x4s Like Tacomas and 4Runners and countless others with independent front suspension have permanent locked hubs with no dial or mechanism whatsoever at this end, so they can transmit front-drive torque in both directions just like these do. Instead, they connect and disconnect the front axle inside the front differential housing. Their lighter drive axles will still turn in two-wheel drive, but their front driveshaft and ring and pinion gearset won’t.
But this permanent locked hub solution has never been a part of the Super Duty’s beefy front solid axle design, so that’s why it has these vacuum-operated auto locking front hubs with a manual override.
The F-250’s big two-piston sliding caliper brakes are unchanged in the Tremor application. Same size, same hefty rotors. Woof.
At the rear we can see a massive leaf spring pack (yellow) and eight bolts (green) that signify this as a full-floating axle. Full-floaters are found on all F-250 and F-350 rear axles, and the cool thing about this type is that you can replace a broken axle shaft with the truck still standing on its tires by removing these eight bolts.
Remember how I said that gasoline-powered Tremor F-250s differed from diesel-powered Tremor F-250s? Both the spring pack and the axle we see here play a role in that.
Diesel-powered F-250 Tremors are required to be fitted with option 535, the F-250 High Capacity Trailer Tow Package. The package includes a beefier axle and this F-350-esque spring package. A gasoline-powered F-250 Tremor would have a less beefy (but still beefy) rear axle and a very different spring pack. Both would have a lift block (white) but they wouldn’t measure the same thickness.
This diesel truck’s high-capacity suspension has three main leaves (yellow), the gasoline F-250 would have two. Both would have a helper spring (green) that gives the pack a progressive rate, but the dimensions of each are different. Finally, and most obviously, the diesel F-250 Tremor has an added overload spring (red) that is absent on the gasoline F-250 Tremor.
What does this mean for the gasoline F-250 Tremor? With less focus on trailer towing than the F-250 Tremor diesel (or any F-350 Tremor), it might mean better off-road articulation and will almost certainly deliver a smoother unladen ride.
Thing is, a gasoline-powered F-250 Tremor that lacks this high capacity stuff will still tow the same 15,000 through its receiver hitch as the others because that’s the mechanical limit of the receiver hitch. This amped-up suspension supports higher tow ratings, but you have to use a fifth-wheel or gooseneck hitch to get there. A more likely advantage may be the ability to haul more firewood in the bed while towing your 15,000-pound conventional trailer.
For the record, this F-250 diesel Tremor and its high-capacity suspension has a payload of 2,923 pounds. An F-250 gasoline Tremor without this suspension might not be much worse off because it’s not burdened with the extra 700 pounds that comes with the diesel engine. The marquee Tremor payload of 4,210 pounds is an F-350 with the lighter gasoline engine and a beefier version of the high-capacity suspension that makes an F-350 an F-350.
The bump stop (green) is unchanged on a Tremor, but the spring block that raises the suspension has a hidden landing platform that makes it work out right.
All trucks with leaf springs will have staggered shocks that go in opposite directions (green) to lessen the tendency for axle wrap and axle hop. The coil-sprung axle of a Ram Power Wagon doesn’t look like this because it controls its axle position with four links and a panhard bar.
The F-250’s shocks are mounted inside the frame, which puts them very close (yellow) to the middle of the axle. As such, they are best at taming vertical motions. This extreme inset position means they aren’t much good for damping roll in corners. But, again, this is a heavy-duty pickup, not a sports car.
Here’s a nerdy but important point that might not be easy to see without a whiteboard. But I’ll try. The front half of a leaf spring is also a control arm that defines the fore-aft position of the axle. For this reason, the shape of the forward eye of the main leaf matters more than you might think.
This is a Berlin eye, in which the wrap persists until the leaf lines up with the pivot bolt. A standard eye would wrap straight off the bottom. A standard eye unrolls and lengthens the main leaf when the spring is compressed, and that moves the axle back. In corners this pushes the outside tire back, which in technical terms is A BAD THING because it means roll-induced oversteer. A Berlin eye moves in a more upward direction in the same circumstances and therefore does not change the axle’s fore-aft position nearly as much. Less roll oversteer is A GOOD THING.
Here’s a more zoomed out view of the diesel’s high-capacity spring package. The main pack (yellow) has three leaves that move together, with a helper (green) that’s shaped to come in progressively. Above there’s an overload spring (red) and its landing pad that come together later in heavily loaded conditions.
A gasoline F-250 Tremor will have one fewer leaf in the main pack, no overload spring, and four empty holes instead of that landing pad.
The rear shackle (green) allows the spring to grow as it flattens, and that’s why the rear end doesn’t play a role in fore-aft axle location. Side-to-side and up-down, definitely, but not fore-aft.
The main spring leaves are separated by nylon pads (yellow) to reduce the friction that would result if they were allowed to touch. There are central spacers near the middle, so the goal is no touching along their entire length.
Like we saw up front, the rear brakes employ two-piston sliding calipers. The rotors have the same diameter and appear to have the same thickness as the fronts, which says a lot about the heavy loads associated with heavy-duty towing and hauling.
It’s the tires that give the Tremor its stance and bro-dozer personality. These are Goodyear Duratrac LT285/75R18 all-terrain mud-and-snow rated tires that stand 35-inches tall, and they’re mounted on black 18x8-inch aluminum alloy rims. Each one weighs 89 pounds. The spare matches the other four as far as tires go, but it’s mounted on a steel wheel.
What does all of this say about the Tremor’s chances against the Power Wagon in an off-road contest? The lack of a stabilizer bar disconnect won’t help it when it comes to articulation, but it’s got bigger tires and better clearance. The shocks have the potential to be more effective and possibly more durable than the Bilsteins found on the Ram, too.
But after seeing the rear suspension, I’m less convinced about the diesel being the best pick for outright off-road performance because it seems more focused on towing. The F-250 gasoline Tremor might be the jam for bros that won’t tow up near the max. However it turns out, I have to applaud the Tremor for giving buyers the choice. One size has never fit all in the heavy-duty truck world.
Contributing writer Dan Edmunds is a veteran automotive engineer and journalist. He worked as a vehicle development engineer for Toyota and Hyundai with an emphasis on chassis tuning, and was the director of vehicle testing at Edmunds.com (no relation) for 14 years.
- Ford F-Series Super Duty Tremor Off-Road package
- The Super Duty gets some serious off-road chops.
- Image Credit: Autoblog