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November 2009 Willie's Workbench

Posted in Features on November 1, 2009
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We often receive letters asking about converting a two-wheel-drive vehicle to four-wheel drive. The situations range from using standard parts and converting, say, a 4x2 Jeep Cherokee or a '70s-to-'80s two-wheel-drive Chevy truck-both of which are pretty much bolts ups-to radical conversions of a vehicle that was never offered in four-wheel drive.

First off, let's get it straight right up front. You'll be a lot happier both financially and mentally if you sell your present ride and buy a factory-built 4x4. Honestly, most people who ask how to convert their vehicles to four wheel drive shouldn't be doing so in the first place. This is especially true when the two-wheel-drive vehicle in question never was offered as a 4x4 or originally had an independent front suspension. Welding, cutting, fabricating, backyard engineering, bodywork, electrical wiring, construction of motor mounts, spring mounts, crossmembers, an understanding of steering geometry (such as caster, camber and steering axis inclination), drivetrain angularity and its related geometry-these are just some of the things in which you must be skilled in to perform a proper conversion. This month, I'd like to clear up a few misconceptions about the problems involved in such conversions.

For starters, you have to decide why you want to convert and what the vehicle will be used for. Will it be car shows, boulevard cruising, mud bogging, hauling heavy loads, truck pulls, serious off-roading, or as a daily driver? From this point, you can then decide how big an engine you'll need and what your drivetrain requirements are.

I will give you an imaginary scenario. Okay, so we've pretty much made up our minds that we want to convert Grandma's 1978 subcompact "marketmobile" we just inherited into a 4x4. It will be used to go back and forth to work and to replace our present CJ-5 for recreational four wheeling. What do we want to use for tires-33x12.50s? Can we open up the fenderwells enough, or will the body have to sit high to obtain the necessary clearances? What are we going to use for a drivetrain? Can we adapt the marketmobile's engine and transmission to a transfer case? Will it be necessary to use an entirely different engine/transmission combination? Changing from an automatic to a manual tranny will require building and adapting all the clutch linkage and related pieces. What about the radiator? Will the new engine need more cooling capacity? Do we have room to go thicker, wider, or taller?

Now that we have all that easy stuff out of the way, what are we going to use for axles and suspension? The marketmobile doesn't have a true frame because of its unibody construction, so before we can install the front axle assembly, we need to build some kind of a subframe up front. Maybe it would be easier to use a complete frame from a 4x4 vehicle. What about new body mounts? How do we shorten the frame to the body's length? Do we want leaf or coil springs, or maybe coilovers? Then there's the steering, which is entirely different on the marketmobile. What changes and modifications are necessary to make it work? Also, most 4x4 axles are going to be wider than the marketmobile's body-can we build fender flares and let the tires hang out, or should we have the axles narrowed? New driveshafts are necessary: Where do we put the slip yoke, do we need a CV joint at one or both ends, and how much angle can we get away with? The front axle has disc brakes, and the marketmobile had standard drums, so what size and type of master cylinder is necessary? The marketmobile also used metric fittings, so how do we convert these to SAE fittings?

Hey, we're doing great. All that's left is the exhaust system and the electrical wiring. Not enough room for the exhaust manifolds? We're going to have to build headers. And look, the oil pan will hit the differential on a hard bump, and the driveshaft's going to hit the starter motor. Extended bumpstops will take care of that problem-so what if there's no compression wheel travel? Oh, gee, we put the shock-absorber mounts in the wrong place. But if we change them, there won't be room for our new custom headers.

Now, if we hook up a few wires, we'll be on the road. If we hook the green wire to the alternator and the red wire to the temperature...but wait a minute. The new engine uses a computer-controlled ignition system, so how do we hook that up? Boy, are we ever glad that we decided not to go with the engine that used electronic fuel injection.

We're on the road now, and what a great feeling. "Yes, officer, I was going a bit fast. What do you mean the bumper is too high? The tires hang out too far? Really, I need an emissions certificate? What headlight-height law?"

I'm sure you get the point I'm trying to make. And I only covered a few of the problems that could come up during a 4x4 conversion. Obviously, not all conversions are, or even could be, this difficult. It's possible that, in some cases, everything is a direct bolt-up. For instance, Chevy two- and four-wheel drive frames are basically the same. Most of the time, all the proper holes are already there. Doing a 4x4 conversion involves nothing more than purchasing the proper pieces and bolting them in place. On some model years, the body mounts are lower on the 4x2 frame, so a body lift kit is necessary to raise the body high enough to clear the transfer case. Fords present a problem in that they use an entirely different front crossmember with different frame rails, so this makes bolting on the 4x4 pieces an impossibility without building a new crossmember.

While you're thinking about a 4x4 conversion, do your thinking with a tape measure in hand. Measure things like wheel track and wheelbase, fender size openings, and engine height, width and depth. Measure spring lengths and their mounting point locations, steering box location, driveshaft length, and anything else that may be pertinent to the conversion. Arm yourself with all this information, then start looking for a donor vehicle. Again, it's much easier to find a complete 4x4 vehicle than to scour numerous junkyards for a bunch of individual parts. We assume that you're not terribly interested in the body, so it doesn't matter if the vehicle is rusted or wrecked, so long as the pieces you need are in proper usable condition. You'll be saving money in the long run by purchasing a complete vehicle. This way, you'll know how everything mounts up, and you'll have all those pieces like small brackets and proper-length bolts that are almost impossible to find at the auto parts store or dealer. Try to stay with vehicles of the same manufacture: use a Ford 4x4 donor on a Ford conversion, for instance. It sometimes makes things a bit easier.

The most important thing about any conversion is to take your time and think before you act. Don't get in a hurry. Sit back, have a soda, and enjoy what you're doing.

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