SHAFT TORQUE (and measuring same)
Technically speaking shafts don’t have any torque because torque is a force. When we refer to torque in a shaft we are referring to it’s resistance to twisting when a given force is applied to it. The only thing that seems to be an industry constant is the amount of force we are talking about, a foot pound. To understand what a ft-lb is, just imagine the force that 1lb of weight would cause if it was located 1 foot away from the center axis of the shaft tip. Now if you put 1/2lb 2 feet from the shafts center that would also = 1ft-lb, likewise if you put 2 lbs 6 inches from the shafts center that would also = 1 ft-lb.
This is important because while torque measurements have always been fudged by shaft companies they have not been updated to account for today’s larger, higher MOI heads.
Please keep in mind that the average weight of a driver head has been near 200 grams all along. What’s changed is how much further those 200 grams has moved away from the shafts center axis. As you can see from the diagrams and pictures, the bigger the head, and the more extreme the head design is, the more load you put on the shaft.
So with this given amount of load we are going to measure how much the shaft twists in degrees and that is the torque number assigned to that particular shaft. Okay that’s easy enough, right? Well not exactly, there is still 1 more thing to consider, the length of the beam. Let’s take a typical wood shaft that comes 46", if we put a 5" clamp at the very end of the butt and a 1" clamp at the very end of the tip we would be measuring a beam length of 40" The beam length is the length of the exposed shaft in between the clamps!
So lets say at that 40" beam length 1ft-lb of load caused the shaft to twist 5.6 degrees, well that would be the torque at that beam length. If you moved your clamps closer together, lets try 5" closer together so now our beam length
Well if we keep moving the clamps closer and closer together we keep getting a lower and lower number, in this case 6/100 of a degree per inch. Well if the marketing dept of the Wamma Wamma 2000 shaft co wants to claim 3.5º of torque because that’s what they think you want to hear then for that to be a real number they would have to move those clamps so close together that there would only be a 5" gap between them, but your not playing with 5" of shaft.
We have used any number of beam lengths over the years, but recently were involved as part of the design team for the new Fierce brand of golf shafts and we decided to determine and establish reasonable and proper protocols for this measurement. Here’s what we’ve done and our thinking behind it.
We are using a modern 460 cc driver head and inserting the shaft and then measuring out to 45" of playing length and marking the butt there. Then we are subtracting 11" from the butt to allow for the grip as we feel that the grip will certainly contribute more to twist than the shaft in this section. We then clamp the butt of the shaft at this point. Now on the tip end because the 1st 1 ½ "of the tip of the shaft is epoxied into the head and therefore has no torque we do not account for this area and use a 1" clamp beginning 1 ½ "from the shaft tip.
Now if you have paid real close attention here you might think I’m fudging by an inch by starting the tip clamp at the top of the hosel, but if you consider that many will play their drivers shorter than 45" such as 44, and these wood shafts will also be used in 3, 5, and 7 woods then I think you will agree that a 30" beam is very appropriate for a wood shaft measurement.
This provides us with a beam length of 30" for wood shafts, so if we tell you a shaft has a certain amount of torque that is how we derived that number, and it is a real measured number, not some pipe dream from some shaft companies marketing dept.
Until next time,
David Dugally
