What exactly is BBGM - and why should I care?

BBGM is the Bottom of Bore to Ground Measurement, and it is one of the most interesting specifications on a club head when you are talking about shaft tipping and stiffness.

The BBGM can be thought of as the distance from the bottom of the hosel bore to the place where the shaft would exit the clubhead if the head was a thru bore.

The BBGM for a thru bore is 0".   This means the tip end of the shaft goes right through the head to the ground line.

Driver heads are often classified as a Blind Bore, a Standard Bore, and a Thru Bore.

As in most things golf, there are no standards, so Standard Bore is generally accepted to be a head with a BBGM of between 1" and 1 1/2".

A Blind Bore is a head with a BBGM of less than 1" and a Thru Bore is a head where the shaft goes all the way through, and the BBGM is 0".

Why do I care?

The BBGM can impact shaft flex, and you must take this into consideration when fitting and building clubs.

How does the BBGM impact shaft flex?

Let's use an example of a club built to 45" with two different heads and the same shaft.

The first head is a Bore Thru.  We'll insert the shaft all the way through the head and consequently the total shaft length used will be roughly 45".

The second head is a Standard Bore with a BBGM of 1 1/2", so the total length of the shaft to make a 45" driver will be roughly 43 1/2".

Where did that extra 1 1/2" come from in the Bore Thru installation?  It came from the Shaft Butt. We have 1 1/2" more Shaft Butt in the total length of the club in the Bore Thru head - which will make the club play MORE STIFF than the same shaft in a Standard Bore.

Why more stiff?  Because we have added MORE SHAFT BUTT to the total length of the club, and the BUTT is the stiffest portion of the shaft.

To make the Standard Bore play like the Thru Bore in this situation, you would have to TIP the shaft 1 1/2" before butt trimming to 45",  and then you would have the same installed shaft profile - with that extra 1 1/2" coming from the BUTT on the Standard Bore as well.

Many people incorrectly believe that the depth of the hosel stiffens the shaft. It does not change the shaft's bending profile during the swing - though it will make the club feel more solid at impact. 

Think of a Standard Bore with a 2" insertion and a Thru Bore with a 1" thick head. Which will play stiffer?  Why the Thru Bore of course, as it will have more Shaft Butt in the total length of the club than the Standard Bore.

If you purchase a shaft designed for a Standard Bore and install it in the Thru Bore, the shaft will play stiffer. How much stiffer is the difference between a Standard Bore and a Thru Bore. It will play as if you tipped it that distance (again, between 1" and 1 1/2").

If you install a shaft into a head with a 2" Hosel Depth, and a 1 1/4" BBGM, it will play the same stiffness as a head with a 1 1/2" Hosel Depth a a 1 1/4" BBGM. The deeper bore may result in a more solid feel at impact, but will NOT result in a stiffer playing flex. This is important to understand.

Here's another one.  If you install a shaft in a head with a 1/4" BBGM and a 1 1/2" Hosel Depth, and compare it to a shaft installed in a head with a 3/4" BBGM and a 2 1/2" Hosel Depth - which one plays stiffer?  If you answered the head with the 1/4" BBGM you would be correct!  Remember, the depth of the hosel does NOT stiffen the shaft - the BBGM does.

How do I measure BBGM?

It's easy.  Insert a shaft into the hosel and measure the length of the club in the playing position. If this is 46 1/2", and the shaft is 45" long, the BBGM is 1 1/2". That's all there is to it.

If you purchase a shaft designed for a Thru Bore into a Standard Bore, you will need to trim that shaft the distance of the BBGM in order for it to play to flex.

That's BBGM in a nutshell!

-Tim