[Gene Green]

I read somewhere that the European crossbows had short power stroke and high draw force, while the Chinese crossbows had long power stroke and lower draw force, thus resulting in the same energy stored but with half the force required to load (although applied for a longer distance).

It seems that the draw force seems to be a major limitation on how easily or quickly a crossbow can be loaded (or whether loading it would require special assist methods). It seems that the lower draw, longer stroke crossbows make more sense. What was the reason for the European crossbow designs to take the opposite direction?

[Pieter B.]

Imagine if you will a nice Porsche or Mercedes going 60 miles per hour and hitting a speed bump or a pothole.

If everything is okay the suspension, which we can imagine for the moment as a coil spring suspension, will initially compress and then slowly spring back releasing the energy it had absorbed.

Now imagine that you take out the factory provided coil springs and have them replaced by the coil springs typically found in an 18 wheeler truck or some other heavy duty vehicle. In case you don't know these are much stiffer springs.

Now the Porsche or Mercedes going 60 miles per hour again and hitting the exact same speed bump or pothole is going to behave a bit different. Its the same car, same speed and same obstacle so at least theoretically you're looking at the same amount of energy involved.

However because the new set of springs is so much stiffer they will compress much less because they can store the energy in much less 'distance' or compression. The flip side of this is that this energy will also be released over much less distance or expansion of the spring.

I think you can sort of intuitively feel the end result of this. Deceleration and acceleration over a much shorter distance will make your bones rattle, compresses your spine and might shake your fancy car apart. For the same energy a stiffer spring will be able to accelerate a heavier mass.

A weightless, frictionless spring that has a drawweight of 150 lbs at 20 inches is going to store the same amount of energy as a spring that has a drawweight of 460 lbs at 6.5 inches.

Key difference here being that the latter spring is about 10 times as stiff as the former one and thus its going to store and release the energy in a very different matter. Disregarding friction and mass the latter spring should be able to accelerate a heavier projectile than the former spring despite storing the same amount of energy. Now that additional mass may not even impact the kinetic energy of the projectile that much but when you consider momentum and terminal ballistics this can become much more important.

Then again I am not entirely sure how this works. Stiffer springs tend to be heavier and I have heard some explanations regarding energy transfer talk about how a high mass can more effectively transfer its energy to another high mass object.


At any rate springs with a different degree of stiffness behave in different manners even if they store the same amount of energy. Using stiffer springs which can indeed be harder to span must have conferred some advantage to the impact side of the equation that made it worth the trade-off.

[Gene Green]

Thank you for such a wonderfully concise explanation.

[Sean Manning]

"Why" is often an unanswerable question! The old crossbow-makers did not write about that, and they could not go to the Victoria and Abert Museum and pick and choose features from crossbows made made all over the world hundreds of years apart.

We can say that the Franks were very satisfied with their simple nut-and-lever locks, which are nothing like say the Han Dynasty multi-piece cast locks. The Franks had a talent for clockwork and fiddly mechanics, but they were satisfied with a simple durable lock. And we can say that the "two-foot" crossbows shooting long bolts with presumably a long powerstroke, which the Franks used in the 11th, 12th, and 13th centuries and were probably spanned with two feet and a belt hook, seem to fall out of use by the 15th century when we have intact crossbows. They chose to stop making those in favour of bows with a short powerstroke, but why is a hard question to answer!

Further Reading: https://bookandsword.com/resources/fashion-in-the-age-of-datini/crossbows/

[Jonathan Dean]

[Duncan Hill]

I've always figured that the reason for the European short power stroke is related to the length of the prod, its power, and the material from which it is made.

So, one could make a wooden (self) prod with a longish power stroke, say 12 inches. But there would be a limit on how powerful the prod could be because there is a limit on how thick it could be, given how much it may need to bend to achieve the 12 inch power stroke. Thus, the armbruster in question would need to find a sweet spot among the draw weight ( thickness) of the prod, length of the prod, and powerstroke (amount of bend) to get the most bang for the buck (most weight for each inch of powerstroke). Of course, that sweet spot will vary based on the materials used. As we see today, modern materials like carbon fibre and glass, can be very heavy AND have a long power stroke, and they are light weight, so all in all, you can make a short, powerful, lightweight prod. Conversely, a steel prod is heavy, and can't bend a lot without risk of breakage (particularly in the medieval and renaissance periods where the process of hardening and tempering steel was much less refined and therefore less predictable result).

In a nutshell, I believe that in order to get a powerful enough crossbow to do some good on the battlefield (or in the woods), given the available materials, prods had to be very stout, with high draw weights, and thus had to have short powerstrokes so that there was less chance of breaking the prod.

Edited to add: presumably the length of the prod was related to maneuverability as well, for European bows?

[Sean Manning]

I think that the properties of 15th century steel might explain why the Catholic European steel crossbows were what they were, but European wooden crossbows were made of wood just like everyone else's, and European horn crossbows were made from horn, sinew, and wood just like everyone else's. For some reason, Catholic Europeans seem to have lost interest in long-powerstroke designs by the 15th century.

Edit: Also, some of the Arab treatises mention long-powerstroke crossbows. Any Frank with money could pick a few of those up in Alexandria and have them copied, and with enough time and resources they could import crossbowmakers trained in the Islamic world (King John of England's two best paid crossbow-makers had Christian names but were called "the Saracen" and "the Moor"), but by the 15th century they don't seem to have been what Franks / western Europeans / Catholic Christians were interested in.