Surface alternations of English strong verbs suggest that they are highly irregular, and form at best a large number of small unrelated classes. But theoretical innovations, utilizing a markedness approach to characterize morphological irregularity, lead to the conclusion that these verbs are much more regular than they seem, and can be described in terms of seven conjugation classes. Six of the seven conjugation classes share, with minor variations, three ablaut rules. Class VII and one item of Class I require an umlaut rule. The ablaut and umlaut rules are not unique to verbs but are also needed to describe many noun and adjective alternations.
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Hoard et al. (1973) studied this question.