For clubmakers, there is a term that you need a full understanding of and that is the bottom of bore to ground line measurement (or BBGM for short). For example, let’s say you pull a shaft out of one 3 wood and place it into another 3 wood. What could very well happen is the length, swingweight and even the flex of the club can change. If you are asking “aren’t all 3 woods created equal?” then you better listen up.
To explain how the length discrepancy occurs, let us take a look at the anatomy of the hosel area of a club. The dimension labeled HL is the hosel length along the axis of the shaft. This is measured using the lie of the club from the center of the bore to the point it intersects the ground line.
The dimension labeled ID is the insertion depth of the shaft into the head. There is no industry standard for this dimension, but often it is slightly greater than 1” at the minimum and could range to the same dimension as the HL in clubs that are true thru bores or where the shaft exits the bottom of the sole.
The last dimension is the BBGM which is simply the difference between the hosel length and insertion depth. For example, the HL in this diagram might be 2.375” and the insertion depth 1.125”. This means the BBGM is 1.25” or how far the tip of the shaft, once fully seated in the hosel, rests above the ground line.
Now let’s go back to our first statement that we pulled one shaft from a 3 wood. We will use this example for simplicity. If we put that same shaft into another 3-wood that had a BBGM of 1.5”, then we would have a club that now measures ¼” longer. If we had the same head weight, there is a good chance the swingweight will increase and that will have a slight effect on the shaft flex and lie. This becomes an easy fix as the grip can be removed and the extra material be taken from the butt end of the shaft. In this case no harm, no foul other than the clubmaker’s time and cost of another grip
However, now let’s use the opposite scenario where the BBGM is only ¾”. This means the shaft is closer to the ground by 0.5” or the same as if it was tip trimmed ½” more. The length will now shorten by ½”, the swingweight is reduced (with same head weight and CG position), shaft becomes stiffer and the lie flatter. You can fix the length by removing the grip and extending the shaft. The will have an effect of increasing the overall weight of the club as you have to account for the additional weight of the extender. Even though the shaft length and club lie are now resolved, the flex cannot be fixed.
While the BBGM can vary from one model to the next even within the same company’s product line, the good news is the BBGM does not vary within the set of like clubs. That is the BBGM of the 3 iron is the same as the 4 iron, the 5 iron…and so forth. This is one of the reasons why you might need to alter tip trimming for different models or understand you just can’t one shaft pulled from one club and place it in another without some consequences.