I hate to disagree (not really!), but the 70% number that you quote is a carryover from the pre-modern era. 70% may be accurate in some cases, but it's a combination of cover material and surface, not just cover material. In terms of the importance of the core, it varies greatly, based on bowler's styles. There are only two factors involved in ball reaction: friction and resistence. Friction comes from the cover, and resistence is a result of the low RG. Bowlers with high ball speeds and rev rates rely primarily on friction. In their case, cover is probably closer to 90% than 70%. Bowlers with lower ball speeds and rev rates, however, need to be aware of the core resistence if they want to get a ball that will be effective for them. Resistence is closer to 40%-50% for them.
If you happened to see the US Open on TV a couple of weeks ago, notice that most of the bowlers on the show were using very aggressive bowling balls with low RG's. The one exception was Pete Weber who was throwing a Storm Rocket (much higher RG) for the simple reason that his lower ball speed made it a better choice for him. Often companies have come out with low RG balls with polished surfaces and promoted them as "going long." The one thing that these balls have in common is that they tend to disappear as quickly as they appeared.
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