We have examined the two possible JT distortions in allene and spiropentadiene radical cations, which can proceed by torsion and by bond-length alternation. Depending on the substituent pattern, substituted allenes and spiropentadienes relax through pseudo-JT distortion in one of these ways into C 2v or D 2 -like structures. We show that this difference is well-described by a simple frontier MO picture, where distortion by bond-length alternation relates to stabilization of linear π-MOs, while torsion relates to stabilization of helical π-MOs. In the parent allene and spiropentadiene cations, the helical distortion provides more stabilization than the linear one, and both radical cations relax into twisted D 2 symmetry structures with equal bond lengths.