i do medieval reenactment and there is very little truth in cavalry charges as such. and depends on what era your going for, now, medieval battles in europe tend to be based from 1100AD to the mid to late 1400s [to the end of the war of the roses in england], so, lets base this around the center countries at war, the hundred years war, france and england fighting eachother, there were many cavalry charges but all became quite futile by the french, such as crecy, and agincourt. ill look at 2 battles within that era, and then hastings and the battle of barnet, each giving a point in the use of cavalry
at crecy [26.8.1346], edward brought archers and had them on high ground, and dictated the battle, it would of took over 40 seconds for the french to get to the english archers, who had other foot soldiers around them, ready to defend the archers when the french cavalry decended on them, the problem is, the horses were vulnerable, while arrows were more effective against the armour at 20m, the arrows were still effective on horses at over 80m, and after doing private tests, of which health and safety would have a fit about, i can garuntee, horses could of been felled on the charge, an archer could shoot ~12 arrows in the time it would of took for the french to of got to their target, times that by ~7500 archers, even with them shooting 3 arrows each, the effect upon the french cavalry would be enormous, excuse my bad maths but that would be around ~22,500 arrows, with the estimates from mike loads, it would be ~90,000 arrows in that time. the king of france was estimated to of had 2 horses killed beneath him, and the dead would pile up, specialy in the sorties by the english men at arms, when you think there is an estimate of ~12,000 mounted men at arms, would create an emense pile up, making ground hard for the horses to cross, specialy when some say the french charged ~16 times, 1 cronicaler said it when well into the early hours of the morning afterwards.. but already the bows fall was on the same field, edwards cannon. which comes in with the battle of barnet.
at another battle, the battle of verneuil [17.8.1424] is very good example also, the english outnumbered with around ~+8000 men, while the french with around ~14,000 men, 2000 of them milanesian mercinaries, most of them with armour that was like no armour seen before, tempered steel, it was practicaly arrow proof. arrows [at killing range] would ether bounce off, fall against the steel or stick inside, with no damage done to the knight underneath. the ground was dry, very hard to get stakes in, and upon seeing the english archers start hammering the stakes in, the knights began their charge, the horses would of been wearing the same treated armour aswell, meaning, the archers would of found it very difficult to break the charge. the archers were broken, the cavalry then moved to the main english force and pillaged the baggage train, the forces were scattered. sir john of bedford chose otherwise, he gathered his men, and spurred his men for victory, the battle turned for honour, into a battle of bitter hand to hand combat. the archers had been destroyed, but the armour also made the appearance of percussion weapons increase. eg the poleaxe, favoured by bedford. which could unhorse a knight, which was effective against the milanese knights who chose to stay. and personal tip, it hurts!!!
but while that, the french were forced back into verneuil, pushing the french back into the steep ditch, the rest of the milanesian knights came back to find defeat. the army had been broken.
but also, at the battle of hastings [14.10.1066], cavalry were still being used as they would in the roman era by the romans in western europe, they were skirmish troops, they would charge forward with a javeline or axe, and throw it at the enemy, then come back, grab another from a squire, and ride back, throwing over the shield, or would be used to chase down fleeing enemy, like when the line broke. the lance would be held out, not couched, the next major battle to use cavalry charges was the battle of lewes [14.5.1264] in sussex in the civil war. where impact battle had began in western europe. and in part with crecy, when the english was going over the somme river, it is shown to have the mounted archers [paid 6pence a day to the 4pence a day of the archers on foot] loosing their bows upon the french as they charge accross, repelling the french from nipping the baggage train. cavalry were still using skirmish tactics, and probably still have done after the medieval era.
at the battle of barnet [14.4.1471], during the wars of the roses, the knights dismounted and fought on foot, it not common for english knights to engage battle on horseback during the middle ages at all unlike the french, many choosing to ride to a safe location and then dismounting to fight on foot. to a major loss, the kingmaker chose to have his horse taken to the back to his loss. i chose barnet for this example, for it is a major clue as to how using cavalry could of changed the battle.
there are many ideas on how the kingmaker died, but i tend to find the idea louis [in the links below] to be more likely to of happened: that he was on his horse, his brother had ordered/advised to retreat [sir john neville], he mounted his horse and rode straight into the path of the yorkists, and they managed to unhorse him, got him on the ground and did the cou de gras, he would of been wearing a sallet, probably with his visor down, and they drove a rondel straight through the eyeslit, and into one of his his eyes, a quick death. a dagger with no edges just a fine point, often nicknamed 'angels of mercy', if a soldier saw any wounded knight on the ground, he would put it through the eyeslit and put him out of his misery.
also, it takes allot of recources to train a horse, to feed a horse, to train a man to be able to fight on horseback, you would not waste so much recources to send him to his doom, and also, a horse will not go through a line more then 2 lines thick, and also, even if you manage, you could be pulled down and killed. the greeks had tactics to use against chariots. the english archers had tactics against cavalry, and coming from a person who does mounted combat, your going to chase down or skirmish! and dont stop! you lose all advantages when you stop! not to say these never happened, as explained in a video im going to link, but, only an idiot or mad person would stop..
http://www.youtube.com/watch?v=AAmf4RTBoYk
http://www.youtube.com/watch?v=B-TCIamyYCo
http://www.youtube.com/watch?v=As1rGnx9vng part of series, watch them all...
http://www.youtube.com/watch?v=s9O37Tn9bsI