Its great that the Vacu-grips come in half sizes. I've been using them for years. What I've found out through the years is that both my speed and my rev rate increased with Vacu-grips and thats what I was told about Vacu-grips. If this is true for you also, maybe its a matter of your arm getting use to putting more revs ini the ball and it will take time to build up those muscles.
To go on and make this post too long:
I was practicing at The Grove in San Diego (now closed, The Grove, not San Diego)and Ernie Schigel (sp?) came up to me with my pro shop guy and said to give him one of my bowling balls they wanted me to try out some new inserts and the test was free. They came out a few minutes later with the ball and the Vacu-grips and I put my hand in the ball and said, I'll never get out of this. So they convinced me to try. I wsa really shocked I got out of the ball at all but it actually did increase my rev rate and my ball speed. I can't go back to regular grips now or no grips. My Vacu-grips last a little more then 20 games but they do wear out considerable faster then other grips. I just buy a bag of both size grips and change them out when needed. Its much cheaper then having the pro shop change them out.
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Nicanor (Ten On The Deck)