For any given ball, when dropping from say 15 to 14, and in the case where (most balls) have a significantly higher Rg in the lower weight, would you say that it does or does not matter? Is it still the same ball?

I would disagree with “be the same ball”.

I just dropped from 15 to 14 and drilled up my beloved Hyroad X and IQ Tour Solid and IQ Emerald. The Hyroad has less punch as the diff is lower and the IQs are completely different as the RGs are much higher.

Further - when Marshall Kent was a Storm staffer he had 16# and 15# Hyroads drilled because the RGs and Diffs were so drastically different. It’s essentially 2 completely different balls that aren’t related yet they are.

So - not all are going to be the same or close. Do some research and consult with your pro shop operator as well.

For me - I also liked the Web Tour Hybrid in 15#. When I dropped to 14 same deal - different reaction. Not bad - but not what I wanted either.

Now for the good part - Obsession Tours are low rg in 14# and are excellent. To me it’s an IQ tour with a Hammer logo. So good I have a couple of them.

19525, i think you may have missed the Rg difference part of the question. I agree that 15 vs 14 weight wise have little difference, but the Rg diff is what I suspect will be an issue.
3835, i am both happy and sad to hear your report. It is what I feared. Happy I may be right, sad my favorites might be so adversely affected.

No, I get it.  But when you are talking about 2.49 vs 2.53 that is going to be barely noticeable.  The differential is where you will get burned potentially.  That is why you really need to look at the specs of what you are thinking you want to try in the lower weights.

Agreed, thanks...

The idea behind the different specs rests with the notion of trying to keep the ball's performance relatively the same, despite the weight variance. 

A heavier ball will necessarily have a lower RG value than its lighter version. This is due to the fact that
more torque needs to be applied to the heavier ball (with the hand) than with its lighter version. It makes it easier to provide "lift" ( or revs) with a 15# ball by lowering its RG . on the other hand, in order to try and replicate the effort to impart the same revs on its 14# version, the RG is raised.  It's a balancing act.
 
To a lesser degree, this principle also applies to the variance in differential as well.  It's easier to impart revs on the lighter weight ball, therefore justifying an increase in its RG, while simultaneously decreasing its differential value. 

So you are saying the difference is intentional?

I've always been under the impression it was due to the density of the core material being the easiest place to change weight in a ball.

I think the main question was, why is the RG increased in a lighter version of the same make of ball.  A higher Rg ball requires more effort to rev than one with a lower RG.

First, RG is defined as the distance from a ball's axis of rotation to a point where the total mass of the ball is actually concentrated. For a sphere of uniform density, the total mass is concentrated right at the geometric center of the sphere. For a bowling ball, its total mass can be shifted away ( and more towards its shell) in several ways.

One way is to just decrease the the mass of the core. This will shift the center of mass towards the shell, increasing RG. In reality, there are a number of ways to increase RG. Decreasing the ball's core mass ( but not necessarily its density) is one way. RG increases, the farther away the ball's mass is shifted from its geometric center. For those of you who are interested in the "math", the GENERAL formula for calculating RG is :

  K (RG) = ( the square root of 2/5) X (the radius)

If a 14# ball had the same RG as its 15 # version, its performance would not be quite the same. In order to TRY and replicate the amount of torque applied by the hand in a 15 # ball  v a 14# ball, it becomes necessary to raise the RG value in the 14-pounder. 

There are some exceptions, such as the Purple Hammer in 16 lbs with a .03 diff compared to the .015 and .013 in 15 and 14 lbs,

But in most cases is the small change in numbers really going to matter? Of all the variables to worry about.