The ugly stub of metal you see above is the end of a Jeep Wrangler axle—actually the end of a custom-manufactured, aftermarket chrome-moly Jeep Wrangler axle. It met its fate at the Overland Expo—not on a driving course, or one of the rocky, off-camber obstacles in the Camel Trophy Skills Area, but while the owner attempted to simply drive up on one of the sets of rollers we use to demonstrate the effects of differential action, lockers, and traction control. Normally getting a wheel up on one of these rollers is little more difficult than climbing a curb if your vehicle has either traction control or a diff lock.
So what happened? I’m told the owner hadn’t done anything wrong—his speed was correct and he hadn’t applied any overenthusiastic bursts of throttle. But the sound of the axle self-destructing was clearly audible to bystanders and the instructors.
The most likely explanation is that the axle splines had been partially sheared by previous, more difficult excursions, and were primed to fail at the slightest stress. Another possibility is that the axle was improperly heat-treated and was actually too stiff—an axle shaft with a bit of torsional flexibility can sometimes absorb shocks that a harder axle cannot. Either way, it brings up some points to consider.
I loaned the owner of the Jeep a couple tools he didn't have, and in doing so got a very brief look at the way it was set up. It was clearly modified toward what I’ll call the rock-crawling end of the 4x4 spectrum—a lift higher than two or three inches, and tires that looked to be at least 37 inches or so in diameter (I did not get their exact size). While such modifications are useful for Class 5 trails, in areas where outside assistance is nearby or the vehicle is in a convoy, they are contraindicated for an overland vehicle, where reliability is the number one priority and one might be traveling solo and a long way from help if a breakdown occurrs that one cannot repair on-site with on-hand spares.
Installing larger wheels and tires probably puts more additional stress on a vehicle than any other modification. To gain ground clearance, ledge-climbing ability, and—perhaps—extra traction, you are adding significant rotational mass and inertia, which has measurable deleterious effects on acceleration and braking, and puts extra stress on the suspension, wheel bearings, steering components, and . . . axle shafts. Our Wrangler owner had conscientiously upgraded those axles, but clearly a combination of things, including but perhaps not limited to the larger wheels and tires, had nevertheless overstressed them.
Another modification often done at considerable expense after buying larger-diameter tires—installing lower-ratio (i.e. higher numerical) differential gears to compensate for the increased rolling circumference—can drastically reduce the strength of the differential, since the only way to get a lower ratio is to decrease the size of the pinion gear. This not only weakens it, but can reduce the number of gear teeth in constant contact with each other. The sole alternative is to completely replace the axle with a larger unit.
Notice how much smaller the low-ratio pinion gear on the left is.
These issues are why experienced long-distance overlanders strongly advise sticking as closely as possible to stock tire and wheel sizes. Going up an inch or so in diameter is unlikely to cause any trouble, but jumping up five or six or more is very likely to do so.
Our Jeep owner accepted the mishap with remarkable aplomb. He rolled the vehicle into an unpopulated corner of the training area, removed the axle, and cut off all but the outer stub with a borrowed angle grinder. This allowed him to reinstall it securely enough so the Jeep could be rolled on to a tow truck.