Why The Current Ford F-150 Is Actually Three Trucks In One: Suspension Secrets

Why The Current Ford F-150 Is Actually Three Trucks In One: Suspension Secrets

[ad_1]

My fellow Autopians, welcome to the second edition of Suspension Secrets, where we dive deep into the workings of some of the most iconic suspensions ever produced. This time we are going to look closely at the fascinatingly different rear suspensions available on the most popular truck in America for the past 47 years. (according to Ford): The Ford F-150. How did Ford offer such different designs on the same basic truck platform? Didn’t they have to change the frame? Isn’t that ridiculously expensive for an automaker to do? Let’s dive into it.

The Three Different Ford F-150s

While Ford does not break out sales by model, I think I’m right in stating that the majority of F-series trucks sold are F-150’s (versus F-250s, 350s, etc), and the majority of those are work trucks — contractors’ trucks, fleet sales, etc.

Vidframe Min Top

Vidframe Min Bottom

Simply, these trucks are meant to work. They must be durable, strong, tough, but not necessarily capable of fine ride and handling or even off-roading beyond driving in the muck at a job-site. A simple suspension is all that is needed for these tasks. Yes, it’s true that many F-150’s are sold as personal daily use vehicles and will never see dirt. I’m not a contractor nor do I own a fleet, but I’ve owned an F-150 for the past 21 years now, and while I do often use it for trucky type tasks, the reality is that it’s mainly a daily driver. And as you might expect, its ride suffers as a result of its “work-truck” rear suspension.

Ford F150 Composite

But not all F-150’s are meant to be work trucks. The Raptor, for instance, is meant for off-roading, capable of going through the desert at high speed. It doesn’t need to have the payload capacity of a work truck, nor does it need to tow heavy trailers. As such, its suspension needs to be designed to maximize off-road performance through large amounts of suspension travel and lots of articulation with minimal binding.

Similarly, the Ford F-150 Lightning, while still billed as a work truck, has very different demands put on its suspension because of its unique powertrain. An electric motor with its large battery pack need a very different type of rear suspension than either a basic work truck or a desert truck.

While the front suspension is the same or similar on all three of these versions, the rear is where the differences lie, so let’s take a deep dive into each and see what makes them tick and why they need to be different.

 

Before we do that, click the video above for part II of our Suspension Secrets series — this episode goes deep into the three major Ford F-150 suspensions, and it is gold.

Let’s Look At The Base Suspension

Screen Shot 2024 02 01 At 11.02.12 Am

Since the beginning of transportation devices, the most common type of suspension has been the leaf spring. This goes back to the early days of wagons and was carried through into the age of automobiles. The evolution of leaf springs has gone from fully elliptic:

Screen Shot 2024 02 01 At 10.53.14 Am

Image via: Memorabilia: Leaf Springs – Franschhoek Motor Museum (fmm.co.za)

….to 3/4 elliptic:

Three Quarter Elliptic Leaf Spring

Image via: Rios (Wikimedia commons)

There have even been quarter elliptic versions along the way (the Austin-Healey Bug-Eye (Frog-Eye) Sprite and MG Midget come to mind):

Quarter Elliptic Spring

Image Via: 1959 Austin-Healey Sprite for Sale | ClassicCars.com | CC-1800818

But the most common type used in automobiles and trucks over the past 100+ years is the semi-elliptic version in a suspension design known as the Hotchkiss suspension:

Screen Shot 2024 02 01 At 10.59.50 Am

All F-series trucks, going back to the very first one, the F-1 introduced in 1948, have used this basic suspension design, and that continues today. It consists of a solid axle held by two leaf springs. The springs are attached to the frame at their front end with a rubber bushing and a welded-on bracket, and at the back end via a shackle that allows the spring to change length as it flexes Here you can see how the shackle in the rear has to articulate to allow the spring to bow and straighten as it flexes up and down:

via GIPHY

Here I’m pointing at the base F-150’s front leaf spring mount on the passenger’s side:

Screen Shot 2024 02 01 At 11.02.40 Am

And here’s where the leaf spring mounts to the shackle, which mounts to the frame (note: the shackle is upside down compared to the graphic above, but it works the same way — it swings as the spring “straightens’ and becomes longer in the fore-aft dimension):

Screen Shot 2024 02 01 At 11.03.09 Am

For many years, this was the dominant type of suspension in all vehicles, cars and trucks. But as comfort, ride, and handling became more important in cars, the deficiencies of the design led to its replacement first by coil-sprung multilink live axle designs and finally by independent suspensions. Hotchkiss suspensions are simple and cheap, but by asking the springs to essentially act as all the suspension linkages (i.e. not just to be springs, but also be charged with holding the solid axle fore-aft, side-to-side, and preventing it from rotating), they are always going to be a compromise.

The main problems with leaf spring suspensions are wind-up under braking and acceleration, and twist during roll. Wind-up, or axle “wrap,” is caused when the torque of acceleration or braking tries to deform the spring into an S-shape. Here’s a look from the side of the truck:

Staggered Shocks 2

If the engine has a lot of power, the tires can lose traction. When that happens, the spring snaps back to straight. When the tire regains traction, the spring twists back into an S-shape until the tire loses traction again and the cycle repeats itself. In bad cases, this can happen several times per second and cause a violent shaking in the vehicle. Most leaf spring suspensions have staggered shocks to reduce this effect but it never fully goes away.

The other problem is that when the vehicle rolls, like in a turn, or when one wheel goes over a bump, the spring needs to not only move up or down but also to twist. The next animation doesn’t do a good job of showing it but you can imagine how the middle of the spring where it is attached to the axle needs to rotate with the axle, while the ends where they are attached to the frame want to stay with the frame. The result is a twist along the length of the spring and in the bushings — twist that the spring and bushing will try to resist.

via GIPHY

This means the spring stiffness in vertical motion is just the vertical stiffness, while in roll it is a combination of the vertical stiffness and the twisting stiffness, making it much harder to find the right balance between ride, handling, and comfort. This extra stiffness in roll also means the suspension has additional resistance to articulating over rocks and bumps — fine for a work truck, not so good for a truck that has to move quickly over rough terrain. That brings us to the Raptor.

F-150 Raptor Suspension

Screen Shot 2022 10 31 At 8.47.54 Am

When Ford introduced the Raptor back in 2010, it tried to make a production off-road racing trophy truck. For the most part, it worked, but while the image was certainly there, you could argue the performance, while good, wasn’t quite up to the task. The truck got new front suspension bits to push the wheels out, and a wider rear axle, but the basic layout was carry-over from the base truck and suffered from the same leaf spring related issues we discussed before. This is perfectly normal for what was then an unproven concept, since architecture changes are expensive and you need to keep costs down at first.

But as sales improved and the truck gained a solid following, the limitations of the base platform became a liability, not an asset. So, with the introduction of the third generation in 2021, the Raptor ditched the leaf spring rear suspension in favor of a five-link consisting of four trailing links, a panhard rod (or track rod), and coil springs (you can get a close look at the hardware by reading our review of the 700 horsepower Raptor R). Losing the leaf springs meant no longer having to deal with the twisting of the spring in roll and the associated limitations in articulation. A five-link can articulate un-impeded until the springs and dampers reach their travel limits. There is still a bit of twisting going on in the links themselves, but this is much easier to deal with than in a leaf spring. (Here’s our full article on the advantages of this coil-sprung setup of a leaf spring setup)

Of course, changing from leaf springs and their associated brackets to coil springs and multiple links meant some significant changes were needed to the frame. I can’t say for certain, but to my eye, the main fore/aft rails of the frame were not changed, but every bracket welded to the frame related to the rear suspension has been changed. The brackets that held the front of the leaf springs are replaced with a much larger brackets where the upper and lower fore/aft links attach:

Raptor Link Bracket

The brackets where the rear leaf spring shackles attach on the base truck are completely gone but now we have large brackets that support the tops of the coil springs and connect the frame end of the panhard rod:

Raptor Spring Upper Bracket

Similarly, there are now new brackets on the axle itself to connect the fore/aft suspension links and the panhard rod, as well as the bottom ends of the coil springs:

Raptor Spring Lower Bracket

Lastly, the dampers are now both located on the front side of the axle because the fore/aft links take care of the twisting during braking and acceleration. Staggered shocks are not needed with this design.

That’s a LOT of change to the frame, and a lot of expense to spend on new, unique tools to build what is essentially a specialty model. Having said that, as we noted before, F-150s sell in very large numbers. That means a lot of parts need to be made to support that kind of production volume. It is unlikely that a single set of tools could do that for all F-150s, anyway, given how many are sold. Production tools have a limit on how quickly they can produce parts, and if your production requirements are higher than that, you need multiple sets of tools. If Raptor production numbers are expected to be high enough, they might push the production capacity of the existing tools over the edge, requiring another set of tools, anyway.

When a company finds itself in that situation, it really isn’t that much more expensive to make those new tools unique. It then starts to make sense to make a new design that improves the product and gives you a competitive edge. I have no insider knowledge to know if this was the case when Ford decided to make the Raptor rear suspension unique, but I do know that tooling capacity concerns are part of those decisions.

F-150 Lightning

Screen Shot 2024 02 01 At 11.10.19 Am

Being an all-electric truck, the F-150 Lightning (read our ridiculously in-depth technical review here) faces a very different set of challenges than the Raptor. In the base gasoline truck, the engine sits in the front, and a driveshaft runs from the back of the transmission to the rear axle. Since the axle is a live setup, the driveshaft needs to be able to move up and down with the suspension. Unfortunately, the driveshaft and the space it needs for this motion sits right in the middle of the truck where we need to package the large battery an EV requires. We need to get the driveshaft out of there completely so we can use the space for batteries, which means we can’t simply replace the ICE motor with an electric motor and call it good. We need to move the electric motor to the rear, close to the rear wheels, so all of that stuff is out of the way of the battery.

We could just bolt the motor directly to the solid axle, which is what some have proposed as a way to cheaply convert existing trucks to EV power. But this means the motor is moving up and down along with the axle over road bumps. That’s a LOT of mass moving around. Not only that, but the high voltage/high current wires that connect the motor to the battery would need to flex up and down with the motor/axle as well. That’s a recipe for early failure of the wires, and considering the power they are carrying, not a good scenario in general.

We’re left with bolting the motor directly to the frame, and once you’ve gotten to this point, you know you will need a set of halfshafts to connect the motor to the wheels. An independent suspension then becomes the only architecture that really makes sense. And that is exactly what Ford did, and what every EV truck manufacturer so far has done and is expected to do (both the Ram and Silverado EV’s are expected to have independent rear suspensions).

Lightning Rear Suspension

Putting an independent rear suspension in the F150 requires a whole new level of change beyond anything done for the Raptor. Ford chose a semi-trailing arm architecture for the Lightning that consists of a single large aluminum cast arm per side, connected to the frame with a pair of bushings. The springs and dampers have been replaced by coil-over damper modules connected between the rear edge of the trailing arms and the frame.

Lightninr Trailing Arm Attachment

Of course, these new parts require new brackets on the frame to attach to. New cross members have been added to attach both the trailing arms as well as the motor:

New Cross Members

New brackets have been made to attach the tops of the coil spring modules:

New Coil Module Bracket

But there is one change that is not obvious, but is no less significant. The height of the rear frame rail in the Lightning has been lowered, and is now directly in line with the rear bumper. This is no accident, and the reason this is possible is that there isn’t an axle housing moving up and down underneath the frame. This graphic shows how the movement of the Raptor and base truck’s solid axle means the frame rail has to be fairly high to allow it to reach full travel (ignore the spring interfering with the frame, that’s just bad modeling on my part):

via GIPHY

In an independent suspension, there is no axle tube moving up and down so the frame rail can be placed much lower. Both Ford and Chevrolet made use of this fact when they converted their large SUV’s from live axles to independent – Ford in the case of the 2003 Expedition and Chevrolet with the latest Tahoe. In both cases, the lower rear frame height meant vastly improved third row seating package with a much lower floor.

With the Lightning, the lower frame rail has not translated into better room since Ford still uses a cargo box common with the rest of the F-series lineup, but in the future, it could lead to a deeper box. You can see the difference here between the base frame rail:

Base Frame Rail

And the Lightning frame rail:

Lightning Frame Rail

Here’s a clip showing the difference:

But why bother doing this? Why not just keep the same rails and weld different brackets to them? The reason, I think, is that having the frame rail inline with the bumper makes for a much more efficient crash structure in the event of a rear end collision. In the case of the base truck, the forces coming from the bumper during an impact have to jog from the bumper, up to the frame rail and then down again once they get past the suspension. When forces have to “jog” one way or another, they tend to cause the rails to bend and fold, rather than crush, and bending is a very bad way to absorb crash forces. If you can direct the crash forces in a straight line, the rails can be designed to crush nicely and efficiently absorb the crash forces.

Another way to deal with the problem is to make the “jog” area very stiff and strong so it doesn’t fold. You can see how Ford used various brackets in the base frame to add a lot of metal to the rail and stiffen that area. The Lightning rail is much simpler without all those reinforcements.

Another clue can be seen in the Lightning rail. Notice the two square cut-outs on the edges of the rail and the two round holes near them:

Crush Initiators

There are actually two more on the other side of the rail and these most likely form what are known as “crush initiators”. What they do is weaken that area of the frame and cause it to buckle there first. Doing that causes the rest of the rail to crush in an accordion fashion. Here is what that looks like:

Crush Structure

Image via: Aluminum Extrusions Prove Superior in Bumper Systems – Light Metal Age Magazine

The dimples at the top of the left undeformed rail play the role of the square cutouts in the Lightning rail and start the accordion deformation which then continues down the rest of the rail. Notice that the base F-150 rail doesn’t have any cut-outs or other initiators like this. That suggests to me that the rail is not meant to deform but to stay as rigid as possible. In the case of the lightning, this initial upward jog is eliminated and the rail can be designed to crush and efficiently absorb crash energy. A much better design all around.

The F-150 Has Three Very Different Suspensions All In One Basic Truck Architecture

Three very different rear suspension designs — a solid axle held by leaf springs that do everything, a solid axle held by five links and sprung by coil springs, and a fully independent suspension — for three different purposes. Horses for courses, as they say. Ford is fortunate to have the sales volumes in the F-series trucks to be able to afford so many different architectures, consumers are fortunate to get the best setup for the F-150 variant they chose, and we here at The Autopian were fortunate to have some cool stuff to look at and dig into.

 

[ad_2]

Source link

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top