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centre pins break all the time. almost the first thing you look at when diagnosing spring troubles.
rarely see a broken u bolt. I never have.
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Yes, I too have seen a spring pin shear... but it's still not much of an arguement. After all, it does not matter what set up you have at that point... your still in the same boat. I mean, if your pin breaks/shears under throttle, the axle runs along the spring in the direction of travel until something gives... or it hits. At least that's been the case in the examples I've seen. At that point, no matter what your running (blocks vs springs) your still not going anywhere until the pin, and potential resulting mess, is addressed.
I prefer springs, but will use a block as long as it's not over 4". For the front, I would only use springs. But, I'm still suseptable to pin failure either way... like every other wheeler running spring packs.
Just my 2 cents...