As it doesn't refer to the 'centre' I am always confused by what is meant by the term centreshot. The definition I'm using here is that centreshot is the amount the front of the arrow shaft is moved away (anti-clockwise looking down for a RH archer) from the vertical plane of the bow. A zero centreshot means that the arrow lies in the plane of the bow, increasing centreshot means moving the front of the shaft away from the bow and decreasing centre shot the reverse.
I have only a vague idea about how changing centreshot affects arrow behaviour - so bear that in mind.
I doubt there is some 'optimimum' centreshot for a specific bow/arrow/archer combination, I think its more a matter of getting a good centreshot/button spring combination. The standard rules for setting centre shot run something like:-
"For an aluminium arrow set the front of the shaft one arrow diameter outside the plane of the bow. For a carbon arrow set the front of the shaft around half to one third a diameter outside the plane of the bow."
Is there a logical basis for these ad hoc rules. Like most aspects of archer little is actually understood about centreshot. The following are a few speculative suggestions about centreshot.
Let's split the arrow into two sections, the front section and the back section. The front section includes the pile and a length of shaft roughly corresponding to the bracing height distance. The back section is the length of shaft from around the bracing height position to the nock. When an arrow is released the string force buckling of the shaft takes place over the arrow back section. The buckling effectively acts against the mass of the arrow front section (which we can vaguely assume is located at the front of the back section). The heavier the mass of the front section the more the arrow will buckle dynamically. The centreshot position will affect the strength of any initial torque/bending moment associated with the shaft buckling under the string force. The result is that the centreshot position will affect the initial buckling amplitude and will affect how the arrow shaft rotates into the pressure button. It is well known that changing the centreshot will change the tuning of the bow (weak/stiff behaviour) and that the centreshot and button spring setting as far as tuning is concerned are interrelated.
Suppose we have an aluminium arrow. An arrow with a larger diameter will probably have more mass in the front section leading to more buckling amplitude but by setting the centreshot at one arrow diameter a larger diameter arrow will have an increased centreshot reducing the bending moment/torque effects on the dynamic buckling amplitude. These two effects largely cancel each other out so by using the one diameter centreshot rule we end up with a similar amount of arrow buckling amplitude whatever the particular arrow diameter/shaft weight is.
Carbon arrows have generally a much lighter shaft weight then aluminium arrows so the mass of the arrow front section is lower leading to less arrow buckling amplitude. To compensate the centreshot has to be reduced to get back again to the 'right' amount of buckling amplitude whatever that is.
With a finger release of a recurve bow there is some movement of the arrow nock in the process of transferring the string load from the fingers to the arrow nock. Kooi (via his arrow model) suggests typical values for nock movement of 2mm sideways and 3mm forwards during this load transfer process. It seems plausible that the initial buckling of the arrow shaft under the string loading will be more consistent and archer forgiving if the initial string loading on the shaft runs down the shaft axis rather than at an angle to the shaft axis (torque and bending moment considerations). The relationship between centreshot position and tuning is well established experimentally and is consistent with this assumed relationship between the string force angle on the shaft and its buckling behaviour concept.
If we start with the arrow shaft in the plane of the bow (zero centreshot) then clearly the consequence of the lateral nock movement of the shaft under the action of the release fingers will result in the string load being applied to the arrow shaft at an angle to the shaft axis. An undesirable result in terms of the above argument. If instead we push the front of the arrow sidways a couple of millimeters (around half the shaft diameter) away from the bow using the button centreshot position then this will cancel out the lateral nock movement from the fingers and the string load will come onto the arrow shaft pretty much along the shaft axis.
The end product is that around a couple of millimetres of centreshot position will cancel the finger related lateral nock movement resulting in the bow system being more forgiving to variations in the archer's release.
While bow tuning is mainly about arrow angular velocity at launch the arrow alignment at launch (the initial air flow angle of attack on the arrow shaft) will also affect the tuning (group size). This is not my suggestion but, as far as I know, a suggestion orginating with Rick McKinney which is that you can use the button centreshot value to adjust the arrow launch alignment. Details of the method are in Rick's book "The Simple Art of Winning"
Both Rick McKinney and Vittorio Frangilli have suggested determining the arrow launch angle by "eyeballing" the arrow as it comes out of the bow. An alternative approach based on shooting arrows with front and rear nodes marked with different coloured chalks through a paper sheet has been suggested on the Archery Interchange UK forum.
Last Revision 3 May 2015