Bottom of Bore to Ground Line Measurement

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.

22 comments

  1. Cleve Porter says:

    Knew this but I like the way you explained it.
    Hope to see you at the show.
    Cleve

  2. Thanks for laying this out so clearly. This issue often come into play in flex matching and MOI matching differing models of fairway woods.

  3. Bob Citrak says:

    In the first case I would have tip trimmed it 1/4 inch to maintain the same shaft flex remembering working with steel shafts to keep the heel to first step dimension the same for both clubs or am I thinking wrong and just trimming the butt end would have the same effect on the flex of the shaft? Bob

  4. Jeff Summitt says:

    Bob:

    In a situation where you are working with a parallel tip steel shafted iron I would have removed the extra material from the tip like you stated. Using a taper tip steel is not an option if you want the shaft to seat to the bottom of the bore or you didn’t want to re-bore the hosel. However in our example with graphite (like 99% of 3-woods today) you may have been able to tip trim the shaft. But unlike steel, tip trimming a graphite you don’t necessarily get the predictable change in stiffness as you do with steel. Granted the 1/4″ wouldn’t be that far off, but if the dimension was say 1″, then I think the safer bet would be to remove the extra material from the butt end.

  5. Bob Citrak says:

    Jeff, Thanks for your response. Had always used the same rules for steel and graphite. Will try it out.

  6. Norman says:

    Jeff,

    Is there any reason why the BBGM is not included with the online and catalog specifications information supplied for every head you sell? I use that information when building clubs, and it would be really nice to not have to measure it myself.

  7. Jeff Summitt says:

    Norman:

    We don’t publish it on-line due to lack of space, but the catalog does include that information (just below the specification box). The on-line master trimming guide also includes it.
    http://support.hirekogolf.com/wp-content/uploads/2012/01/2012-Hireko-Trimming-and-Length-Guide.pdf

  8. Thomas Ray says:

    My driver has a BBGM of 1″ and the matching fairway woods have a BBGM of 3/4″. I want to reshaft them with the Aldila NV 65 shaft. Aldila’s trimming instructions are 0″ for the driver and 1″ for all the fairway woods. Should I trim the shafts for the fairway woods 1/4″ less than what is recommended since there is a 1/4″ difference between the BBGM of the driver and fairway woods?

  9. Jeff Summitt says:

    Thomas:

    There are a number of options. The instructions tell you that you can take up to 1″ off of the fairway woods. For instance, if you had a 3 and 5 wood and wanted to have a progressive frequency slope, you may elect to tip trim the 3 wood 1/2″ and the 5 wood 1″. If it were a a 3, 5 and 7 wood combo, then you may want to trim 1/2″ off of the 3, 3/4″ off of the 5 and 1″ off of the 7 wood. If you want to adjust for the bottom of bore difference, then you can take 1/4″ less off of each. If not, the 1/4″ would amount to @ 2 cpm or 1/5th of a flex stiffer.

  10. Thomas Ray says:

    Jeff. Thanks for your comments. Since the BBGM for my 3 wood is 1/2″ less than my driver then isn’t that the same as being tipped 1/2″? With that in mind I was thinking of using the following tipping sequence. 3W – 0″, 4W – 1/4″, 5W – 1/2″,
    7W – 3/4″ and 9W – 1″. Would this be a good option?

  11. Jeff Summitt says:

    Thomas:

    Assuming your BBGM is the same for all the fairway woods,that would be a perfectly acceptable option to create a nice gradual progression in stiffness..

  12. Thomas Ray says:

    Aldila’s website says to tip all the fairway woods that same at 1″. Does it really make sense to tip all the fairway woods the same since the heads obviously get heavier as you go from 3w to 5w, to 7w.

  13. Jeff Summitt says:

    Thomas:

    The trimming instructions by manufacturers are often suggestions rather than absolutes. Shaft manufacturers make the flex of their wood shafts based on a driver which most manufacturers have similar head weights then offer (in most cases) alternative trimming for the fairways. The only problem is not all fairway woods weigh the same from one company to another and sometimes model to model in their line. This is especially true with a 7-wood.

    By tip trimming 1″ off of all fairway woods, butt trimming to length and making sure the swingweights are the same, you end up with a flat line frequency slope. Where as progressive tip trimming creates a sloped frequency (higher frequency the shorter the club). There are golfers who can hit flat line sloped woods (or for that matter hybrids and irons too) well and other that don’t.

  14. Thomas Ray says:

    The BBGM in a set of woods I have is 1″ for the driver and 1/2″ for the fairway woods. The Proforce 65 Gold shaft calls for 0″ tipping in the driver, 1″ tipping in the 3 wood, 2″ tipping in the 5 wood and 2″ tipping in the 7 wood. I have a set of woods consisting of driver, 3W, 4W, 5W, 7W and 9W. If I install the Proforce 65 gold shafts what is your tipping recommendations?

  15. Jeff Summitt says:

    Thomas:

    Most clubmakers would follow the recommended as suggested. However, if you want a recommendation to make a nice even flow then it would be as follows:

    Driver 0″
    3 wood 0″
    4 wood 0.5″
    5 wood 1″
    7 wood 1.5″
    9 wood 2″

    This is based on our head weights.

  16. Thomas Ray says:

    Does anyone know anything about the regular flex John Daly Signature shaft made by Penley? Would this shaft be a good replacement for a player who swings at 85 mph and hits the ball on a medium to high trajectory with their current driver?

  17. Thomas Ray says:

    I have another question about Penley shafts. I am trying to get some specs on an older Penley golf shaft. The shaft is black and the graphics are white but all the graphics say on the shaft is “Penley made in USA San Diego, CA.” The shaft is new but was made at a length of 45 inches instead of the customary 46 inches. One person told me it preceded the Penley Tour Light shaft which has the 4 yellow rings and has the specs written on the shaft. Can anyone help? Thanks.

  18. Jeff Summitt says:

    Thomas:

    The John Daly Signature shaft was a model designed with a lower launch angle. If you have a shorter or more abbreviated swing or have a quick tempo, then the shaft may be fine for your 85 mph swing. If you have more of a longer or smoother swing, it could be perhaps too stiff.

    I don’t have my old files anymore on the Penley shafts to identify what shaft you may have as that has to go back close to 10 years ago or even longer. The Graphite Light was one of their oldest shaft so there there is a chance it could have been an early version of it, but there is no way of know without being able to measure the weight and torque.

  19. Thomas Ray says:

    I am going to have some UST shafts installed and the tipping instructions on the Golfworks website call for tipping:

    driver 0″
    2W 1/2″
    3W 1″
    4W 1 1/2″
    5W 2″
    7W 2″
    9W 2″

    The lofts of my fairway woods are strong. 3W-14*, 4W-16*, 5W-18*, 7W-20* and 9W-22*.

    I was wondering if it would be okay to tip each one of my woods 1/2″ less than what is suggested. The reason I am asking is because my 3W is is only 1 degree less than a 2W loft.

    Best regards
    Thomas Ray

  20. Jeff Summitt says:

    Thomas:

    It is the weight of the head that dictates how much to tip trim and not the loft as weight affect the flex. You certainly can tip trim less as the 1/2″ would amount to 2-3 cpm which is not that noticeable, but I would check your head weights first.

  21. Thomas Ray says:

    Hi Jeff
    I have my Srixon XXIO metal heads and my UST 65 Gold shafts in regular flex and .350 tip ready to be assembled. I weighed the heads and here are the results in grams.
    Driver 193, 3W 204, 4W 210, 5W 215, and 7W 218. I didn’t have a very sophisticated scale so the weights probably have a margin of error of 1 or 2 grams. Now before I spend a lot of money on assembly I want to make sure the tipping is done correctly. UST says tip Driver 0″, 3W 1″, 4W 1.5″ and all other fairway woods 2″. Would you follow those recommendations or make adjustments?

  22. Jeff Summitt says:

    Thomas,

    Based on the head weights, I would follow the recommended trimming.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>