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Nuts & Bolts - November 2013

6x6 Truck
Fred Williams
| Brand Manager, Petersen’s 4Wheel & Off Road
Posted September 16, 2013
Photographers: 4-Wheel & Off -Road Staff

Your Tech Questions Answered!

Q Since both of my trucks have bad frames I want to take the rear axle off the 2WD truck, which is a Ford 9inch with posi-traction, and the front and rear axle off of my 4x4, whose axles are a Dana 44 front and a Ford 9-inch rear. What I want to do is make a tandem axle street-legal pickup truck with a tube frame body—the whole nine yards. But I plan on using the axles from both of my trucks. I have been looking on the Internet to try and find a way to connect the two Ford 9-inch axles together to make them both drive axles. I plan on making this into a 6x6 truck.
Cody P.

A Wow, this is a major project, but not impossible. You need to determine if you are going to run two driveshafts from the transfer case to each axle or if you want to run a driveshaft from one Ford 9-inch to the other. No matter what, you have a load of work and expense ahead of you. I believe the Dodge M37 and Jeep M715 transfer case, known as the NP200, can be modified for dual rear outputs. These cases had an offset rear output and a parking brake on the centered rear output. By replacing the parking brake with another driveshaft yoke I think you can make it dual rear output.

Then you need to figure out how to run the driveshafts to the dual axles. You could replace the rearmost differential with a Tru Hi-9 third member ( and have one driveshaft run low to the front rear axle and the other run high to the rear rear axle. To do this I think you would want to have the pinion of one of the other offset so you were not crashing into the other differential at full suspension compression. One problem I also see is the NP200 transfer case has a passenger front output, and I believe from your letter that you have a Ford front or driver-side front axle.

There is another option for running power to the rear rear axle, and that is to custom-build a gearbox that mounts to the front rear axle with the pinion support bolts, and when power comes in it turns a gearset or chain drive that then powers a second driveshaft up and over the differential. This may result in the rear driveshaft turning the opposite direction as the front rear driveshaft depending on how many gears you install in the gearbox. This would be a fairly complex custom machining project.

I have also seen people custom-building an extralong pass-through pinion that ran straight through the axlehousing and had a rear output as well as drove the front rear axle ring gear. There were companies in the past that offered 6x6 conversions, but I do not believe any are still in business, so you would likely need to build your own components.

One concern I have with a 6x6 vehicle is the ability to turn sharp. I think having two axles pushing forward and only one trying to turn may make this vehicle more likely to want to go straight when you’re trying to weave around and over obstacles off-road. Some giant 6x6 vehicles have the rearmost axle that steers to help in maneuverability. 6x6 is usually great for hauling heavy loads, but I’m not sure it makes the most sense for an off-road vehicle. But hey, I think you should build it anyway and let me know how it turns out. Good luck!

Bump What?
Q What is bumpsteer? I hear it mentioned all the time but don’t understand what it is.
Randal C.
St. Louis, MO

A Bumpsteer is when your 4x4’s suspension causes the steering to turn during suspension cycling due to mismatched geometry between the steering and the suspension. Most often bumpsteer is found on solid-axle trucks, but it isn’t impossible with independent front suspension either. It is all based on the geometry of your steering movement and your suspension movement.

When your front suspension compresses, such as when you go over an obstacle or through a ditch at speed, the axle will move upward toward the frame. As this happens it usually moves in some sort of arc based on the style of suspension, and the goal is to have your steering not change direction as the axle goes through its arc of motion. The steering of your vehicle allows the axle to move with a drag link that runs between your frame-mounted steering box and your axle. Because one end of the drag link is attached to the frame and the other to the moving axle, it also moves in an arc. If the arc of the suspension and the arc of the drag link do not closely match, it can cause bumpsteer. Bumpsteer can be a minor twitch of the steering wheel when you cross a rough section of road, or be so bad it tries to change the direction you are traveling.

When building a suspension I recommend making the track bar of a three- or four-link suspension as equal in length, angle, and location as possible with the steering drag link. This is why many suspension companies have track-bar drop brackets to match some sort of drop pitman arm and keep both the drag link and track bar in line when installing a suspension lift.

If you’re building a suspension with leaf springs you may be able to add a crossover steering system to get a longer drag link, thus reducing the arc, or you can run the drag link parallel to the leaf springs to some sort of bellcrank steering on the driver-side knuckle as seen in many suspension lift kits. When running the steering parallel to the leaf springs it often works best to have the drag link equal in length to the front half of the leaf spring as seen in some older Dodge or Toyota trucks.

Crossover steering on a leaf-spring suspension may seem counterproductive, as the axle wants to move in and arc front and back during suspension compression, whereas the crossover drag link arc wants the axle to move side to side. However, with a long drag link a lot of the bumpsteer can be hidden due to the very gradual steering arc. It may still be there but not very noticeable.

I have seen vehicles where the suspension geometry and the steering geometry did not match and the owner didn’t even realize he had bumpsteer. It was so minute it never bothered him. A lot of times your suspension may not be perfect, but if it works good enough to run down the trail or road then it is fine. In fact, you may only have bumpsteer at the very ends of the suspension movement, such as when you are fully articulated or fully compressed. This is a situation you may not see very often during normal street driving, but it can be hard to deal with when you are driving off-road and your vehicle is using every bit of suspension to get over obstacles.

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