What you want to do can be done, but I wouldn't use adapters. First you need to look at what you're trying to do with dual rear wheels, then there are some other things to consider.
I have a friend with a body shop who specializes in rebuilding less-than-one-year-old smashed Dodge trucks. He has three dually diesels right now. His latest is a 2012 Megacab that he welded two frames together to fit an 8' box. The truck is longer than anything you can get from the Big Three. It also was not a dually when he bought it.
The first thing you have to consider is if you're going to pull a big trailer, you'll want a high axle gear ratio. That is easily changed when you don't have a front axle to match it to. The higher ratio will not necessarily reduce your fuel mileage because even though the engine is running faster, when it's not under load it will need less throttle to maintain speed. You will notice the decrease when pulling a load, and that will get worse the faster you go.
The biggest thing people overlook is braking. The dual-wheel axle is going to add a lot of weight. That is not supported by the springs so those aren't an issue, but there is going to be a lot more weight on the rear of the truck. Manufacturers spend a real lot of time and research into designing a brake system that is perfectly balanced front-to-rear for your exact model with its list of optional equipment and its placement on the truck. You're going to be changing that balance by having more rear weight with no additional braking power to go with it. Big trucks already have a problem with front brakes wearing out much faster than the normal two times per once for the rear, and you'll be asking the front brakes to do even more of the work now. Besides the size and stopping power of the front and rear brakes, there is an issue with rear wheels locking up easily under hard braking when most of the weight shifts to the front. All cars and trucks have a proportioning valve built into the brake's hydraulic system that limits brake fluid pressure to the rear brakes during hard braking. Most minivans and pickup trucks use a special height-sensing proportioning valve in the rear because there can be such a wide variation in loading, either with many passengers or with a big load. Since they can't design a one-size-fits-all braking system for these conditions, the height-sensing proportioning valve was the solution. The problem is that valve responds to changes in the ride height of the box and frame relative to the axle housing. The valve was chosen for a specific "unsprung weight", meaning weight not supported by the springs. The dually axle will add weight to the braking system but it won't be seen by the proportioning valve.
For most of us that just means the rear brakes won't do as much work as they could as far as stopping power. "For most of us" does not include lawyers and insurance investigators. They love to find these kinds of modifications, and especially raised trucks and lowered cars because they will be able to shift part of the blame for a crash caused by their client who ran the red light onto you. They will convince a jury that you were partly at fault because you were less able to avoid the crash, ... And they will be right. Common sense says it's the other guy's fault, but when have you known a lawyer to use common sense? Being a suspension and alignment specialist, I really get annoyed when people alter the ride height of their vehicles without being aware of the consequences, but I also would not be looking out for you if I didn't mention the effect other modifications have like snow plows and things like that.
Another thing to look at is your engine. Ford and GM designed their own engines and they do a fine job for what they were intended for, but a lot of people add "chips" to increase the power. Those are more or less a gimmick that gives the illusion of more power with a gas engine, but they really work with diesels. The problem goes back to what the engineers expected the engine to do, and increasing the power can stress the lower end and the transmission. The axle should be able to take it. Chrysler went a different route. In the '60s they designed the 426 Hemi for NASCAR, then "detuned" it to put in street cars. Since it was developing a lot less power than it was designed for, adding modifications to get some of that power back didn't phase it. GM and Ford took perfectly fine street engines and went the other way and tried to beef them up for use in racing, and had rotten results.
The same is true with diesel truck engines. Chrysler figured out someone else already had a design that was better than anything they could come up with, so they took that, detuned it, and put it in their trucks. Here again, adding a chip just gets back some of the power the engines were designed to handle and they had before. My friend used his truck to drag a 32' goose neck trailer with an 8' high box built around it, stuffed with his sister's stuff to move her from Texas back to Wisconsin. It was so grossly overloaded that he snapped all the wheel studs for one pair of wheels on the trailer. A company that repairs semi trailers on the side of the road couldn't lift that trailer without going back for more equipment. Up to that point he had no trouble keeping up with traffic going 70 - 75 mph, but the fuel mileage went from 24 mpg unloaded to around 15 mpg. An extra 5 mph makes a real big difference in fuel use with diesel engines.
If you plan on pulling a heavy trailer, drag it around first with what you have now to see if you're happy. The dual rear wheels are not going to give you more pulling power. They will give you more stability and more weight-carrying capacity. If you're not happy with the pulling power, a better alternative to a chip is to change to a higher axle ratio. That will not add much stress to the transmission.
To get back to the brakes story, a different axle is likely to have bigger brakes, and may have disc brakes. If you have drum brakes now and switch to discs, you'll need to change the master cylinder too. Drum brakes use a "residual check valve" in the master cylinder's port to keep a little pressure on the fluid all the time. That valve would cause disc brakes to drag and seriously overheat. Bigger brakes that came on an F-350 are going to upset that front-to-rear brake balance too. You have the advantage that you're trying to turn your truck into something that has already been designed and manufactured. That means you can use parts from the appropriate truck in a salvage yard. My friend didn't have that luxury because he built something that didn't exist before. If you have four-wheel anti-lock brakes, that will overcome an increased tendency for rear-wheel lockup, but it won't help with the unmet need for more rear braking power. The way we addressed this on the Megacab was to turn the ABS system off, then readjust the link for the height-sensing proportioning valve to find the most braking power we could get without easy rear-wheel lockup and skidding. That truck came with disc brakes on the back and they were the same size on the dually axle. That kept to a minimum the number of new variables that were added to the truck.
You'll need fender extensions for the box. For a '99 model you should be able to find those in a salvage yard too. On a lot of newer trucks the boxes are built for a dual-wheel axle. You can add the extensions yet, ... I think, but there's nothing you can remove from a dually box to use it with single wheels. Adding fender extensions is not going to be a significant factor for the braking system because they aren't nearly as heavy as some of the loads you normally carry.
Tuesday, October 28th, 2014 AT 2:42 AM