The fronting of back vowels before the segment represented by the runealgwiz in Old Norse (Runic: kūRON: kýr‘cows’) is problematicbecause there is no consensus on the phonetic value to be reconstructed for theconditioner, a reflex of Proto-Germanic+/z/. This study presents anacoustic experiment testing the fronting effect of codas [z], [ʒ],and [ɹ] on realizations of the representative back vowel /u/, asthese are possible reconstructed values of algwiz. Based onobserved coarticulation, we argue that a voiced alveolar fricative[z]—and not postalveolar [ʒ] or rhotic[ɹ]—is the best reconstruction for the conditioner ofR-umlaut, and that this reflex of PGmc+/z/ in North Germaniccausedumlaut before rhotacism and merger with PGmc+/r/.