Did Ambedkar think a good constitution was enough to keep a country free?
No, and he said so at the moment of handing it over, when a founder might have been expected to claim more for his work. However good a constitution may be, he told the Assembly, it is sure to turn out bad if those called to work it are a bad lot; and however bad a constitution may be, it may turn out good if those working it are a good lot. The document, on his own account, does not save a people. The people decide what the document becomes.
He located the risk squarely in Indians themselves, now that the British were leaving. Whatever went wrong from here could no longer be blamed on a foreign ruler; the country would have only its own conduct to answer for. This was the opposite of fatalism — a demand that citizens treat the constitution as a responsibility they carry, not a machine that runs itself.
For anyone hoping to improve how the country is governed, this is the operative sentence in his whole legacy. The reform that matters most is not a better clause but a better practice of the clauses there are — because the finest text in the world is, in his words, only as good as the hands it is placed in.