I'd generally agree with Bill. The sweet spot of a sirupate will change depending on the options chosen and weight distribution but generally it will be about 2/3 of the way along the blade. I think it's very common for westerners to look at khukuris and think that the sweet spot is where the belly is most prominent/curved as though it's an axe. This is true for some of our modern blades (scourge, scourge anniversary, socket, regent) and close to true for some traditional blades (pensioner, mk1). Generally with traditional khukuris is sits slightly behind this point in the angled section that leads into the belly or the transition between these sections. In the case of a sirupate chopping using the convex portion is significantly too far forward. Sometimes this can be good for light brush clearing as this edge portion is moving faster but it lacks momentum for chopping against more solid targets. An example of this
differing performance can be seen here as shelly raskin is getting used to the longer leaner blade shape.
The term "Sweet Spot" is a difficult one and too much analysis is probably outside of the scope for discussion. Generally it is used to mean the point at which the most impact force is imparted to a target, however different schools of thought propose alternative methods of deducting its location. It is used interchangeably with the term Centre of Percussion.
Many in the sword community suggest that it is determined by harmonics and that we should think of a blade like a guitar string.
As it hits a target it vibrates and resonates just like a guitar string and depending on where we hold that sword there is an area of the blade where it vibrates less. The theory is that a strike at this harmonic node will transfer more force as less is lost to vibration. It is worth noting that this node is on many swords generally 2/3 of the way along the blade as previously stated.
In recent years this theory has come under attack and instead I would say that the predominant theory is that of Pivot Points. This is less easily understood and I would also say not particularly well communicated for the layperson anywhere on the internet.
Figure 2 on this page is probably your best bet. This theory is a lot more unified- it tells us more about all sorts of motions that influence a blade's handling and not just impacts.
This theory doesn't argue that harmonic nodes don't exist on swords, but rather that they are only a minor influence on power transfer. I've used some learnings from this theory to help me better understand existing khukuris and design better blades also. I still don't fully understand it but at some point will set aside the time and get it all absorbed.
Take care,
Andrew and the team at Kailash
