Wheels do come adrift reasonably frequently. I don't think I have seen 1 at every meeting, including race day practice, but enough so that most of us see it happen at least twice a year.
The problem is identifying what the causes are.
The most obvious cause is the use of 'old' nuts, either K-nuts or nylocs that can be turned on or off by hand, relying only on the stretching of the stud to hold them secure.
The second is component failure, either stud or more rarely axle (stub or rear).
The third is neglect. How many of us have worn or damaged locators on our stubs or spiders, how many use wheels with elongated holes.
Fourth is ignorance. Many people simply are not aware that you need at least three threads showing when a nut, locking or not, has been tightened. There is a document on the net (http://www.boltscience.com/pages/Why_nuts_and_bolts_can_self-loosen.pdf) that explains why nuts come loose, along with the finding that devices like spring or lock washers actually make it more likely that the nut will try to undo itself. How many people have read it?
Fifth is chance-taking. You know that something should be done but take the chance that you will be OK this time and you will be 'sure' to correct the issue before the occasion happens again. ( I have such a case, I have a set of wet rims that are slightly thicker than others, needing longer studs or countersinking of the bolt holes. I will get around to it....but last weekend was wet and I needed to use those rimes. Luckily nothing happened but when I checked the kart, one nut had come off).
And it isn't any single one of these causes that creates the accident. It's the chain of little events that causes the accident, and anything that breaks the chain will avoid teh accident. It simply isn't possible for scrutineers to break every chain, though it is possible for drivers / mechanics to break most of them. Yes, it means inspecting your kart carefully between outings and using a magnifying glass on your axles, changing 'good nuts' when they seem just a little slack, keeping your wheels clean before putting them on the kart and doing the nut check before every outing.
(At Clay a novice noticed a tiny crack in a brake pad. When he took the pad out, half the pad had detached from the backing plate, was held on only by a tenth of an inch before the pad broke in half. Would we have simply shrugged at an inexplicable failure if he had not noticed or not investigated this anomaly? Was it the scrutineers fault for not seeing a hairline crack in a brake pad?)
Yet we often encourage Dads, who admit to having no mechanical experience or even a tool box, to take up spannering for their child, without ensuring that he has someone with the skills to aid and mentor them at least for the first year. Indeed in some places / classes /events the competitive spirit is such that people aren't encouraged to offer their help unless the dad is paying for the experience.
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