I do not think that Karna was ever born in the family of a charioteer. Rather I think that he is the son of a god, or he is come of the race of the Kshatriyas. My opinion is that he was abandoned (in his infancy), lest his race be ascertained.
नापि सूतकुले जातं कर्ण पन्ये कथंचन॥ देवपुत्रमहं मन्ये क्षत्रियाणां कुलोद्भवम्। विसृष्टमवबोधार्थं कुलस्येति मतिर्मम॥