Why We Don't Currently Offer a Single Rear Wheel Chassis
In an ideal world, we would offer a chassis with single rear wheels and a 78" track front and rear. Benefits to single rear wheels off-road are numerous, and it is very common in the rest-of-world (non-USA) market.
Unfortunately, legally solving the single rear wheel request on a medium duty truck in the USA is no small matter. Most current US-spec medium and heavy truck axles which are FMVSS-tested and certified for on-highway use are built for dual real wheels. Some of these rear axles are now being tested and certified for "super-singles" that replace dual wheels, but the tire and wheel combination that works safely on the back of the truck will not work well on a driven front axle. (A problem if you want to carry one spare that can work at front or rear positions.)
Not only are there steer-clearance issues, but the reduced rim inset puts the tire position outside of the scrub-radius parameters of the front axle. If you were to install such a large tire at the front position, even if you had the clearance and it "looked good", your tire wear would increase exponentially.
Sure, there are trucks running around on just the outer tire of the duals, but that dramatic increase in track puts much more stress on the axle housing and hub/bearing assemblies. This generally unsafe technique can lead to premature bearing and/or housing failure, and without custom-offset wheels, results in the rear track being wider than the front track. (And why not use custom-offset wheels? Try and find a wheel manufacturer willing to certify FMVSS-compliance on a custom wheel assembly!)
Truck chassis and original equipment manufacturers spend hundreds of millions of dollars on engineering, testing, and certification. We at RUF, Inc. feel it prudent to listen to what the engineers say. We recently inquired with one chassis manufacture, after we received a request to build something with single rears on a chassis with factory dual rear wheels. Our inquiry resulted in the following statement from the manufacturer:
"The chassis is engineered to meet these [FMVSS] standards with the tire & wheel arrangement provided. To change these - especially to such a radically different arrangement as super singles - may adversely effect the completed vehicle's handling, braking and stability, thus taking it out of [FMVSS] compliance.
"Any entity that undertakes any modifications or additions to our chassis is taking on a degree of liability. When those changes/additions fall outside the realm of accepted engineering practices for that chassis, the degree of liability increases exponentially. It is irresponsible for us, or anyone acting as our agent, to suggest, discuss, encourage or provide assistance in anyway to make changes or modifications that may result in noncompliance to applicable FMVSS [laws]."
Yes, there are companies willing to sell vehicles with single rear wheels installed. Yes, there is anecdotal evidence that some folks have run singles with no problems, but the question remains, "Is it really safe?"
You'll even find that some companies will sell you a vehicle capable of speeds in excess of the DOT-legal speed rating of the tires mounted on the chassis! Recent catastrophic tire failures with current US-market Freightliner Unimogs may have dealers paying more attention. If the tire says "55 MPH", it does NOT mean 70 MPH is acceptable.
If you suffer a catastrophic failure while under way, from a tire or other component failure, you may find yourself responsible for the property damage, serious personal injury, or death that results. In court, you may be able to pass that burden of liability on to the manufacturer or modifier that made the unsafe changes, but isn't it better to avoid the whole situation? It is absolutely imperative that the components installed underneath your vehicle are used as the manufacturer intends. Just because it bolts on and looks good DOES NOT MEAN it's safe!
When purchasing a medium duty truck with single rear wheels, check with the CHASSIS MANUFACTURER to see if that configuration is acceptable. DO NOT accept the words of the body builders or final-stage manufacturers, or even (especially) the chassis DEALER. We have personally witnessed chassis dealers (for both foreign and domestic platforms) misinforming us and others about what is acceptable, and what is legal. Many salesman unfortunately have no clue, and just want to make the sale.
Any vehicle sold by RUF, Inc. will be as safe as we can build it. We accomplish this goal by always listening to the manufacturers of the parts we employ. Here are a few more words from the same manufacturer as quoted above:
"One of the main reasons our respective companies have the relationship that we do is your willingness to listen and act upon our advice, and I want to thank you for protecting out interests as well as your own. Too often we hear, "So-and-so has done it for years with no problem..." or other such anecdotal rationalizations. It only takes a split second for a situation to go from all normal to tragically wrong. By adhering to sound engineering practices we lessen that window of liability."
Sincerely,
Darrin Fink - President, RUF, Inc.