Why Chinese crossbow were the strongest

The following essay is by HackneyedScribe of Historum, one of the most knowlegeable if not the most, scholarly poster on any history forum. 
The full text can be seen on the original thread here : http://historum.com/asian-history/69...-crossbow.html


-Power of Han 6 stone crossbow (medium sized, most common) = 3676.5 inch pounds 

-Power of Han 8 stone crossbow (large sized) = 4902 inch lbs

-Power of largest medieval handheld crossbow Payne-Gallwey, huge in size probably used with prod, 20 pounds heavy = 4200 inch lbs

Explanation below :

Han Dynasty Crossbow Technology

Range and Draw Weight


Just how powerful a crossbow could be is glimpsed in the excavated Chu-yen slips, from which records of crossbow maintenance was kept. From the slips already excavated, we have available a set of records showing six crossbows shooting 168 to 280+ meters. Each of these crossbows had only draw weights of 3-5 stone, as compared to typical Han era crossbows of 6 stone. Of these crossbows, two was tested for their penetration ability, both puncturing a wooden wall (most likely a plank or fence) at 252 meters.


What must be noted is that crossbows were typically weaker than bows pound for pound due to their shorter powerstroke. Powerstroke is the length required for a string at rest to be drawn back to the trigger. This means crossbows must have heavier draw weights than bows in order to shoot the same projectile at the same velocity. Typical crossbows of Medieval Europe generally need to have three times the draw weight of a bow in order to give a similarly powerful shot, but made up for this by utilizing winches to maximize the draw weight.


However, ancient Chinese crossbows thanks to their trigger design allow the nut of the trigger to be placed near the back of the stock. This results in relatively long power strokes rivaling that of a bow, which greatly reduces the crossbow’s usual weakness of energy transference inefficiency. Han dynasty crossbows would have a draw length of 24 inches, implying a powerstroke of 18-20 inches, or rivaling that of a bow.



Generally speaking, the power of a bow or crossbow measured in inch lbs = 0.5 x powerstroke length x draw weight. The most commonly mentioned crossbow in excavated accounting records were of the 6 stone (387 lb) type. In comparison most bows would only have draw weights of 40 to 100 lbs.


Heaviest standard 8 stone Han crossbow power = 516 lbs draw weight * ~19 inches powerstroke/2 = 4902 inch lbs 
Heaviest Medieval crossbow found from Gallway = 1200 lbs draw weight * 7 inches powerstroke/2 = 4200 inch lbs
Typical 6 stone strength Han crossbow's power = 387 lbs draw weight * ~19 inches powerstroke/2 = 3676.5 inch pounds
Heavy Song dynasty bow made to pierce armor = 160 lbs draw weight * ~20 inches powerstroke/2 = 1600 inch pounds



Advantages and Disadvantages


The primary strength of the crossbow is without doubt its strong armor penetration capability. With a median draw weight of 387 lbs, and a powerstroke rivaling that of a bow, Han crossbows could shoot a projectile farther and faster than that of a bow. 


The Han crossbow does come with shortcomings. Han crossbow prods were composite, including sinew and horn materials. Compared to metal prods of winched crossbows in Medieval Europe, composite prods need to be longer in order to have the same draw weight. Thus each crossbowman requires more horizontal space for shooting, although this is partially offset by the fact that composite prods tend to shoot projectiles with higher velocity as compared to steel prods of the same draw weight. 



For those too lazy to read, here's the TLDR :

-Crossbow were the most common ranged weapon in Han and Song times

-Most important for the power of a crossbow : Draw Weight x Power stroke (draw length)

-Chinese trigger design were the best in the world as it allowed crossbow to have long draw length compared to european trigger design with very short draw length (power stroke) : 19 inch vs 5-7 inches

-Average draw weight of typical chinese crossbow 400 lbs

-Power of Han 6 stone crossbow (medium sized, most common) = 3676.5 inch pounds 

-Power of Han 8 stone crossbow (large sized) = 4902 inch lbs

-Power of largest medieval handheld crossbow Payne-Gallwey, huge in size probably used with prod, 20 pounds heavy = 4200 inch lbs