The answer I do not know, but one thing is sure: you have to ride a horse, and than think about it again <img src="/forum/images/icons/wink.gif" alt="" />
I just started riding half a year ago, with the intent to joust, fight on horseback and ride in plate in a few month' time. What a moron I was! Well, now I am happy if I could sit on a horse with plate in a year or two.... <img src="/forum/images/icons/wink.gif" alt="" />
Like, you do almost never let go the horse with both hands. So a weapon, like the longsword, which CAN be used with one hand, but needs both to be fully effective, does not make much sense on horseback. Then, you can't just hack and chomp and stomp with a horse, you have to take care for your mount as well. That nasty neck of the animal always tends to be in the way <img src="/forum/images/icons/wink.gif" alt="" />
So if you do not want to hurt yourself and your horse, you certainly will use a weapon that is good for mounted warfare. I would not like to swing around a longsword with 2 kgs hours long one-handed.
The other is, you certainly see arming swords on the plates of Talhoffer, for example, and not longswords, and even so, they are using the thumb-grip on the pics. Like said, just by sitting on a running horse I finally begin to grasp why... It's anough trouble not to fall out of the saddle.
BTW., I just can't understand how a fullplate gothic knight could ride fullspeed with a lance in his hand and NOT fall down <img src="/forum/images/icons/wink.gif" alt="" />
Szab
