heavy cavalry engaging heavy cavalry

[Urselius]

Urselius wrote:

A slow attack could be delivered in a tight and regular formation, a charge at speed, a la outrance, unavoidably led to an opening of ranks as individual horses differed in their top speed.

While I agree with you about ranks opening up at speed, in regard to cataphracts, I think that their strength lay in maintaining a tight & disciplined formation when attacking lighter cavalry with maybe lots of wheeling & turning to isolate & destroy enemy cavalry & as Peter pointed out this comes through training, both horse & rider. Because of their heavier armour & bigger horses, the light cavalry of the enemy with their swifter & lighter horses could more than likely evade an individual cataphract's charge & attack him from another angle or in groups (unless they had the lighter cavalry pinned against a natural obstacle like a river, or caught them totally unawares.)
However, as someone pointed out in an earlier post things can go wrong with a tight formation attacking at speed when the lead horses lose momentum or face an obstacle. In 484AD the Hephthalites using feigned retreat tactics lured Sassanid Savaran heavy cavalry to a prepared ditch measuring (from book Shadows in the Desert by Dr Kaveh Farrokh) 30 feet wide & 60 feet long camouflaged with wood, soil, shrubs & foliage and probably in a narrow valley, placing their horse archers in front of ditch to lure Savaran to attack & then crossed the ditch through a safe pre designated passage over the hidden trap with Savaran in full pursuit. Peroz & most of Savaran plunged into the ditch as well as a lot of support cavalry but momentum had stopped & the riders & horses who managed to avoid the ditch were easy targets for the Hephthalite horse archers. The size of the ditch indicates that Savaran were pursuing in a reasonably tight or wedge formation & the carnage inflicted on the Savaran by the trap seems to indicate that they were attacking at speed.

Regards
Michael Kerr

The only really detailed description of the formation taken up by very heavily armoured men on armoured horses dates to the 10th century, describing Nikephoros Phokas' klibanophoroi, but I think that we can be reasonably certain that similar contraints operated at other times. The basic parameters of the Nikephorian formation was that it was deep and compact. I think that such formations would have been difficult to change direction with and even at a trot would have been hard to halt in front of an obstacle. Given that horses wearing barding would have had difficulty cooling themselves by sweating, and in hot climates would have easily suffered from heat exhaustion, I think that they were essentially one-shot weapons.