Picture a rotating Harley crank assembly. Now picture the left (drive) side being snatched to a halt (a worst-case scenario) by what it's connected to via its center journal. What will the right side want to do? It's pretty heavy and it's only pumping oil and operating the valves through the camshafts. Won't its momentum tend to make it want to continue rotating? If it gets to, any at all, won't it spin a little on the crank pin if it can overcome the friction between that pin and either of the wheels? Don't forget that when something like that sudden stop happens, there's going to be a lot of force on the left side journal trying pull that journal out of the common centerline with the other side's journal, and this will ease the right-side wheel's effort to spin upon the crank pin, bringing the two journals into a parallel arrangement. You don't want the journal centers to be parallel. You want just one centerline, not two.
Of course, if both center journals are held sufficiently in position in line with each other and the pin is held sufficiently by both wheels this couldn't happen, but are the two journals held that well by the bearings in their bores? Is the pin held sufficiently by the wheels? Evidently not in every case because cranks like Harley's scissor.
It's easier for me to picture this happening by hard deceleration of the left wheel than by merely operating at a lower engine speed on a regular basis. Hard deceleration of the left wheel is easy to see happening by sudden downshifting without matching engine speed to the new gear, or by somehow locking up the rear wheel. It's not so easy for me to see it happening sufficiently because of motoring down the highway at 2k RPM and adding some throttle.
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Topic: Primary side crank runout (Read 8988 times)