First of all we have to establish what Good performance or "High performance" really is..
I would say tuning the bow first to find the sweet spot is going to yield the best all "around performance" of that particular bow. It's hard to generalize because each bow is different. But over all "High performance" is a combination of the least amount of noise and vibration which in turn will provide the highest amount of energy transferred to the arrow . this can be measured on the chrono... If you are talking about how much difference in speed, or how much difference in noise or vibration? these numbers and amounts are going to differ a lot between different limb designs.
Here is a good example:
If we look at a long bow limb design with a very low preload at brace, and a lot of limb mass moving forward. The "preload" or the tension on the string at brace is what stops the forward motion of the limbs. At that point the transfer of energy stored in the limbs goes to the arrow shaft.... When you don't have enough preload to stop the limbs clean, a much higher percentage of your stored energy stays in the limbs and goes back into the riser in the form of vibration. This stored energy is lost completely.... and for sake of comparison lets say we end up with 168 fps at 10 gpp measured in the chrono......
With a bow like this i'd bet the brace height could be moved as much as an inch without a measurable difference in speed.
Ok.... Now lets take a recurve bow design, or a hybrid long bow with a much higher preload at brace..... These limbs are designed with a much shorter working portion with much less actual limb travel. The higher preload stops the limbs dead and transfers a much higher percentage of the stored energy in the limbs to the shaft. It also has much less vibration felt in the riser because the limbs are not flopping around. For sake of comparison, lets say this bow shoots 190 fps at 10 gpp through the chrono. The tension on the string at brace or preload is going to change a lot more with a 1" difference in brace height than the long bow with lower preload.... so that 1/2" difference is going to be measurable by as much as 4-5 fps... and the noise level if tuning with a bare string is going to be much more noticeable too.....
Are we splitting hair here? Sure we are...
How much difference does it really make? Well each archer out there has a different idea of getting the most out of there set up. They are all different, and nobody is wrong... But higher performance bows will be more sensitive to brace height adjustments, and they all got there own sweet spots.....
You guys don't need a chrono to find that sweet spot. tune your bow with a bare string using lighter weight shafts for noise, then switch to heavier shafts once you've got the sweet spot in the brace established. If you start getting the brace too high, the bow will still be quiet, but you will notice a drop in trajectory the farther you twist the string.
btw.... there is another way you could measure the optimum brace height on a bow using an in-line cable tensiometer on the string. But they are expensive little buggers... [/QB][/QUOTE]
thanks for that explanation Kirk.