What was the real purpose of bulbous arrow nocks?

I think one possible reason is that the added surface helps to keep the nock on the string thanks to the side pressure of the base of the index finger.

It would be nice to see sample pictures of your nocks.

Daniel,
The Turks originally made thicker strings. Most of the surviving arrows are sport arrows for flight or target. The bulbous nock was necessary for large strings but it was rarely of greater diameter than the broadest part of the shaft. You do see greater diameters in reed arrows, but they are often of very small diameter to start with and needed the extra width to fit the large strings. I have reed arrows that are 6.5 mm in diameter with nocks designed for 4 to 5 mm strings.

Having started making bulbous nocks, there were other advantages for archers. It is easy to make sure that the nock is on the string properly by feel alone. If you use the swelling of the nock to seat it with the index finger, you can run into problems with the nock striking the bow. Pressure should be very slight.

Another thing which seems to be missed: most Oriental nocks do not grip the string. This appears to be a Turkish invention used with both solid and composite nocks (bashpare and bakam). The Persians, Moguls, Chinese, Mongols and Koreans have nocks with parallel-sided slots. Even the Turkish nocks do not closely fit the string like modern plastic nocks. They clip on, but will slide up and down easily. I have tested this with real Turkish arrows on an associated Turkish silk string. It appears that the bulbous outside profile convinced people that the slot was the same shape in all these types of arrows.

If this were the case for nocks, then how did the chinese keep their arrows from sliding up the string while on horseback? Is it just by using a nocking point?

The nocks are usually a good fit. I do not use a nocking point and have no problems keeping the arrows in position. Most of the traditional techniques relied on constant practice.

The technique is to hold the arrow in position with your index finger and release most of that side pressure as you draw. Once the string is starting to form an angle, there should be no tendency for it to slide up the string. The very slight pressure from the index finger tells you whether the nock is moving off the string. If the slot of the nock is too big this will happen. If the slot of the nock is too small, a powerful bow will split the arrow. The bulbous style of nock gives you a good tactile feedback.

I have seen people pull the nock off the string when drawing a bow because they are still holding onto the arrow with the bow hand. When the friction of the arrow sliding between the index finger, thumb and bow grip exceeds the friction from the index finger of the drawing hand on the nock, the arrow comes off the string.

Hello :)

I made some bamboo arrows with nocks in "turkish style". actually nocks are massive, but I think it has its charm :) Photos:

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Here is more photos: http://yfrog.com/3pimg5786jx

On your request Jose,
from left to right: baspare and bakkam gez nocks made of hardwoods and horn

Daniel, that's first class work. Thanks.

I imagine the solid nocks are stronger than the composite nocks.

Nice, Nice.

Jose, majority of the arrows from the picture above have been used in field for over 6 months. Haven't had any problems with the nocks. Looks like they past the test. They're getting a facelift now :D