I agree with the above, No real idea but if I had to guess the fatigue limit is orders of magnitude higher then 500 and not a real concern.
Is it a common failure to experience with aluminum bumpers? I am sure 4runners are not the 1st market for them.
the big aluminum bumper manufacturers are a handful of Jeep outfitters like Savvy Offroad and Genright and general manufacturers like Aluminess.
they all provide a steel crossmember/winchplate that links the frame horns together and use the aluminum as a shell.
the ONLY ONE who doesnt do that is Savvy with their front bumpers, and thats because the wranglers have a huge tubular crossmember at the leading edge of the frame horn. on the JK Wrangler versions of their front bumper, the crossmember is cut off and removed, but the aluminum bumper is also built as a monocoque shell around the frame horns out of 1/4" 6065-T6 that has been heat treated post-welding and it serves as the crossmember. it however has been engineered as disposable in its cost if it suffers a fracture or damage and there is a crossmember in the frame only a foot behind it cradleing the engine.
Savvy and Genright's rear bumpers all use steel tabs for recovery that pass through the aluminum shell. as well, savvy uses an entire crossmember that integrates a steel hitch that passes through the aluminum shell.
6065 series aluminum that these bumpers are built from just can't be reliably trusted for continual fatigue stressors, and at their cost it seems pretty foolhardy to be investing in them when failure is a potential issue you have to deal with.
im not discounting pelfrybilt or hefty's work here - i admire their drive to build it and sell it, but they haven't done the necessary leg work to make this a lasting indestructible product to set the market standard.
i point to the Jeep Cherokee (XJ) as a good example of metal cycles in an every day world. the Cherokee uses a monocoque unibody constructed of steel. its a solid platform that uses the entire body and doors as a structural component. even the windshield glass is used to an extent. the newest cherokee rolled off the assembly line in the summer of 2001, so you can see how old these are starting to get, but thats not the point. there is a big market from offroad manufacturers tailored for the cherokee like TNT Customs and others that provide clamshell 3/16ths and 1/4" steel gussets to plate over the cherokee 'frame rails' to add rigidity. they're manufacturing these, and their common install practice because once you get the cherokee off 4 wheels frequently the incredible torsional stress placed on the unibody exceeds is design parameters. continued torsional stress placed on the unibody through suspension articulation and hauling of gear eventually develops creeks and squeaks and misalignment of door pins. realigning doors is a common practice in abused Cherokees and squeaks are prevalent in every one that has been taken offroad on more than a gravel road.
the cherokees unibody simply wasn't designed to that level of stress, but its incredibly light weight - AND were talking about steel. think of the level of engineering Ford put into the new F150s that still use a steel frame with an aluminum body. the new Aluminum Super Duty's are being released on the 24th at the Texas State fair. the big question there is if ford is building a new boxed frame for the F250+'s or if they're going to continue using 50,000PSI 1/4" C channel frames that are designed to articulate on their own.
its not cut and dry.
