Weight and grip of sarissa and shield in macedonian phalanx

[MeinPanzer]

Then, the grip sistem in the macedoniand shield (phalangite) was the same that the aspis? with porpax?

I tried in a phalanx formation with one of these shields, the shield had the shoulder strap that was mentioned before. The movement of your arm is limited for the lenght of the strap and you can only move it up and down. In the other hand with that strap the shield stands always in a correct position protecting you. The real problem is to hold the sarisa with the left hand, the arm of the shield. The shield had a leather porpax in the center, so if you want you can use it or not. When I used the porpax it was almost impossible for me to hold the sarissa with my left hand (and the shield was not so concave as some that have bee found) I had no space to do it and the border of the shield really damages your hand. Maybe it is a mistake in the design of the shield.

When I used no porpax (just the antilabe as porpax for my wrist) I could hold the sarisa better and with this and the shoulder strap the shield still gets the correct position.

This is exactly a topic I am writing about at the moment, and I have come to much the same conclusion you have (though not based on reenactment, so it is very helpful to have your input!). We have no evidence whatsoever that any of the shield types employed by the phalanx were equipped with the porpax, though this has often been stated in scholarly literature. On the other hand, we have this statement from Plutarch, Life of Cleomenes 9.2:

Filling up the body of citizens with the most promising of the perioikoi he created 4,000 hoplites, teaching them to use the sarisa with both hands and to bear shield with strap (ochane), not with porpax.

This is as explicit as any scholar could ever hope such a statement to be: in order for these men to use the sarissa with both hands, they were not to use the porpax but to use a strap which would pass around the shoulders. So, that means that we have one piece of literary evidence against the use of the porpax, and nothing in its favour. And once we accept that such shields were only suspended by a strap (and perhaps a small secondary strap through which passed the left wrist for added control), we have to realize that there was no need to place any restriction on the size of the shield, its concavity, or whether it had an offset rim or not. Therefore, if we accept that the primary mode of carrying the phalangite's shield was with the strap and not with the porpax, we can also accept, for instance, that Argive shields could be carried by phalangites (as Pausanias tells us the Achaean infantry reformed by Philopoemen did), or that the large, very concave shields often found in Hellenistic art could be too.

I am not saying that phalangites never bore shields equipped with porpakes - they very well may have with smaller shields - but merely that we have no evidence for them doing so, and so we cannot assume that they could not have carried a certain type of shield because the use of the porpax would not have allowed it as so many scholars have in the past.