I have been to look at a new roller coaster trailer and they still have the same problem with the keepers/split Pin and roller Axles.
If I am correct then your Axles are Oval and the roller is held on by a penny washer and split pin, and not a crown cap
The problem is that the hole in the washer is round and the axle is oval . therefore the gap between the washer and the flat of the axle is too big and the washer in time overides the split pin and the roller falls off, usually scratching your lovly gellcoat in the process!
The easiest remedy is to replace your normal size spilt pins with much longer ones ( at least an inch) and bend em around a bit in the recess of the roller , this offers a lot more retention to the washer!
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