Mounted crossbowmen appear in Polish army in the 13th century and become more popular in the 14th and 15th century. These were usually the riders who couldnt afford to buy the horse good enough for a knight and served in the army as lighter than knights cavalry, however they could have somtimes use the lance.

It is assumed that during the reign of Casimir the Great in the half of 14th century, the number of mounted crossbowmen was higher than number of knights in the Polish army.

 

Usually the mounted crossbowmen were servants of the Polish knights, who were recruiting them in their own estates and to large extent participating in the costs of their equipment. Their role was to support heavy cavalry. When they have finished shooting, they were participating in hand to hand combat as well.

 

And affcourse their crossbows were of lighter kind compared to those used by the infantry.

 

As agatharchides    said they were probably pulling the bowstring on standing horses and they had metal tools to do it (i dont know english words for it). They could have ride to shooting range, shoot, pull the bowstring and shoot or withdraw if necessary and come back with crossbow ready to fire.

 

Crossbowmen were considered to be lower than Ant dung by the knights or elite warriors of their day.  An untrained common soldier could do in a well trained well armored and equipped Knight with one well placed bolt.

 

BTW- the Chinese invented a successful version of a repeating crossbow.  It held and could launch a succession of six bolts before reloading.

 

 

 

Aye, capability of english longbow to pierce the armour was proved to be a myth.

 

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Battle of Agincourt- St. Crispin's Day, Oct. 25, 1415,

 

The English had a total of 6,000 - 1,000 men-at-arms in heavy steel armour from  head to toe and 5,000 lightly armoured men with longbows, he said.

The French  had 10,000 troops, each with an attendant servant who could also fight, plus 4,000 men with crossbows and other fighters – totalling more than 24,000.

As French lines broke down under volley after volley of arrows and panic set  in, English archers killed thousands by stabbing them in the neck, eyes,  armpits and groin through gaps in their armor.

 

Iv read the similar description of the battle explaining how the English did win.

 

Oh I would imagine you could shoot a longbow from the saddle, at least once or twice before someone got you.  The average English Longbow was between 6 and 8 feet long.  Arrows averaged 30-36 inches in length.  You try nocking a shaft and putting a full pull on the bowstring[to your cheek]  All while holding the bow horizontal and at attack gallop.  The Japanese made an art of mounted archery, and there recurve bow was less than 4 feet.  I don't think you could get a full pull without the leverage of standing.

Besides,  The longbow's value would be wasted.  It's value is in the effects of massed archers.

 

 

 

 

No doubt written by the French.

 

 

"[My yeoman father] taught me how to draw, how to lay my body in my bow ... not to draw with strength of arms as divers other nations do ... I had my bows bought me according to my age and strength, as I increased in them, so my bows were made bigger and bigger. For men shall never shoot well unless they be brought up to it.

As to why someone would use mounted crossbows versus mounted archers, the qualities of the crossbow themselves make it readily apparent: 

1) crossbows require minimal training to use compared to a longbow;

2) the trajectory of a crossbow is basically flat, while a bow needs to fire in a high arch if the shot is to cover any significant distance.  The crossbow is more or less a point and shoot type of weapon (see also #1 above);

3) the flat trajectory and firepower of a crossbow bolt allows it to be used more effectively in bad weather conditions where bows and arrows would be rendered useless;

4) bolts could be loaded and held at the ready indefinitely.  A nocked arrow can only be held for just so long

Even if a crossbow designed for mounted use has to be small enough to be no stronger than a bow, thus giving no special advantage for range or penetration, the above to qualities could be enough to justify the existence of such a troop.

And because wikipedia is such an awesomely reliable source  it appears the Germans used mounted crossbow troops to fire a single volley at close range before drawing swords and charging.  While a short range volley like that might be pretty effective in terms of dealing damage, since it's a single shot volley, the effect was presumably more psychological than anything else.