Hi again..
You can not understand if you have a good rotation just by looking at the amount of track flare. Sure it has an effect but core and surface determines it mostly. Place of bowtie and the shape of track flare are more determinant factors.
Axis Rotation is known as side roll. (Most people confuse it with axis tilt, but that's totally another issue). And it is the horizontal measure of the angle of the ball's revolutions.
Closer your release angle to the direction of the ball, closer your ball to 0 degrees axis rotation. More angle you have in release to the direction of the ball, more axis rotation you generate.
If you release the ball at 6 o'clock position with no finger rotation, ball will roll down the lane directly with relatively no change of direction. Because there is no side roll, 0 degrees angle between the direction of the ball and axis rotation. Please keep in mind that balls direction is down the lane. On the other side if you release the ball with rotating your fingers at 3 o'clock position (9 o'clock for lefties) your side roll will be maximum and ball tends to rotate exactly parallel to the foul line, but perpendicular to the ball's direction. Which means you will have 90 degrees axis rotation. Get a video cam and see it for your self
Another issue is; I don't believe that there is "good rotation" or "bad rotation". There are different types of rotations. Unless you are doing something very stupid, there is always a way to find a ball-surface-line combination for that particular rotation. But ofcourse, again, a solid bowler must have couple of different games which include different releases and rotations
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For your calluses, you may try different finger pitches. Or may be it is just the type your skin. You need to consult with a good driller about it.
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Probably Pin-nut will answer this but "How well does it hit the pins" means:
Is it loosing energy and hits like a marsmallow? Is it rolling out? Skidding too much to create the drive through 1-3-5-9 without deflecting? etc..
In other words, how well is the reaction
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Practicing with spare ball/plastic ball is a solid idea. I always offer it to the beginners for their first ball. They are cheaper and never wear out when you are practicing to develop a technique in the first couple of months
However, spare balls may be 1 pound lighter than your strike balls. That is something advised by many coaches. So if your plastic ball is lighter and you practice with it to simulate a sport condition, your body will adapt to that weight. Must be careful I guess..If your spare ball's weight is same, no problem..
Just my 2 cents..
Thanks
Burak