Rating and selecting a vehicle: A guideline

[Jonathan Hanson]

Goodtimes, you're correct about payload and the vehicle frame . . .mostly. Historically, certain vehicles have suffered from serious frame issues even when not loaded to capacity. Mid-1980s General Motors trucks were notorious for frame cracks and broken motor mounts. The Arizona Game and Fish Department had something like 75 percent of their Chevrolet trucks certified as unsafe at one point. One truck was found with only a single mount holding the engine in. This was back in the days when Chevrolet told its customers "Oh, the frames are designed to flex."

So I consider proper frame design a bedrock foundation for an expedition vehicle. Toyota has been boxing the frames on Land Cruisers and trucks for decades, something American truck builders only recently adopted. (And even Toyota had some issues with the rear parts of frames on some Tacomas, I understand.) The Land Rover frame is a work of art in its intricately welded box section design (unfortunately of thin gauge steel and prone to rusting). Modern hydroforming techniques are making frames stiffer at no weight penalty.

Only a very few vehicles, such as the Unimog, are truly designed with frames that flex in a controlled manner.

 

[goodtimes]

Jonathon, you are correct. There are concerns with frames of certain vehicles, particularly for off hiway travel. 73 - 87 GM trucks and blazers (full size) tend to crack around the steering boxes when used heavily (big tires, etc). Most compact and mid size trucks seem to have rigidity issues right above the rear axle. This is where the MFR's tend to cut back on material...most go from a boxed design back to a C-channel, as they assume there wont' be a whole lot of stress on the frame in that last 2 or three feet. Of course, us OHV travellers then install tow hooks, etc., and once we use them, we end up bending the frame. Hard pulls from one side of the frame can also bend things....better to pull strait from a properly designed attachement point centered between the frame rails, IMO.

There really aren't too many non-custom solutions to fixing these areas. DeMello sells a frame stiffining kit (basically some 1/4" steel plate that gets welded to the outside of the frame, right above the rear axle) for the tacoma's...apparently this is a big enough issue to have some aftermarket support. Several companies sell a steering box brace for the GM vehicles...basically a stamped piece of steel that fits over the steering box mounting location and gets welded on. A reciever hitch will help stiffen up the back end of the frame on pretty much any vehicle, and provide a attachment point for pulling. But none of these items will actually increase your payload. That is still governed by the wheel bearings (and brake system, etc).

There are alot of reasons that a really rigid frame is a good thing, but it really doesn't affect your ability to carry a heavy load. My concern with poorly built frames would be with a heavy vehicle that gets used on very technical trails on a regular basis...you will lose the fit of the body panels, get waves in the sheet metal, latch and hinges will get loose, you develope squeaks, rattles, and thumps......and eventually you will start breaking things like motor mounts, suspension mounting points etc. For the type of trails that most of us spend time on, over working a frame probably isn't a huge concern. Now if you like playing on the extreme rock crawling trails, yea, frames do crack, link captures break off, etc. Which is one of the things that is spurring on the huge number of buggies that are being built. You are able to build a chassis that will hold up to the abuse of rock crawling, where you won't find a suitable frame to accomplish the same.