by KCStudly » Wed Apr 26, 2017 3:17 pm
I'm not sure what the stock size U-bolt cross section is, or what grade/alloy they are, but if we are conservative and assume that they are at least 3/8 nominal and at least mild steel, then we can do a little math and see what loads they can be expected to take.
The tensile cross sectional area of a 3/8-16 bolt thru the threaded area is .077 in^2 (a little more for fine thread bolts, which axle U-bolts usually are... so conservative).
If we take the tensile yield strength of mild steel, 36kps, and derate that by 2/3 to get a 1.5 safety factor we get 24kps to yield.
Since any single U-bolt is loaded symmetrically it must fail both legs to fail; similar to a double shear joint except in tension. So we get 2 x .077 in^2 x 24kps = 3696 lbs each U-bolt, or 7392 lbs for the pair.
Some cheesy little axles just use one U-bolt per side, kind of straddling the axle on a diagonal, but I am going to assume the axle in question uses two U-bolts per perch. So we can double this again to look at the big picture scenario of the whole trailer, say hitting a speed bump at speed (as apposed to a single wheel dropping into a pothole... doing this just because I think most people will relate to the total camper's weight rather than just one half/side). So now we have four U-bolts that might have to contend with a jolt from a portly TD at max rating... what are they, something like 1200-1400 lbs? Since some of the trailer weight is borne by the TV hitch, this is again conservative.
So (2 x 7392 / 1400) = 10.56
Which is about what I would look for, anything greater than 10 for a dynamic/shock load scenario is reasonably conservative. Kind of explains why we don't hear all kinds of horror stories about axle U-bolts failing in this scenario.
If the U-bolts have a larger cross section or a better alloy, so much the better.
As to the locating pins, those should not be overlooked, nor should maintaining proper camber orientation, if so equipped, nor should being aware of axle to frame clearance issues (wouldn't want to snap an axle stub due to bottoming out the available suspension travel and spiking those loads much higher).
Disclaimer: yes I am educated in engineering; no I do not guarantee anything or assume any liability for sharing this insight.
KC
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