First of all using the tortouse shell to absorb a charge is all theory unless if you can point out documentation of that. No expert here but if you watch time commanders (Watling street) you will notice the historians saying they are in the worst possible situation to absorb a charge and the chariots were a coming. This charge was coming while they were in the tortouse shell. So considering the source I do'nt think its a good option to use. There may have been cases where they were blind to the charge due to being under arrow fire and maintaing the tourtouse shell but if they are blind to the charge they will not be embraced for the shock either.

Another thing the historians said was that cavalry for that time period was only good for the charge and should not go unsupported. Time Commanders is a good example for this as I saw plenty of charges totally dominating the infantry but about 30 seconds later the if the contestants did'nt pull there cavalry out they started getting waisted. Once the horse has stalled the rider has lost all mobility while his enemy can turn 360 degrees he cannot and trying to fight while twisting your upper torso may be difficult, especially if armoured.

Well hopefull it will be this way in the game that if your infantry do'nt break the cavalry may soon find themselves the victem