The purpose of this study was to investigate the mechanical aetiology of quarter cracks by comparing the strain patterns (i.e. magnitudes and orientations at midstance, peak tension and compression, and maximum strain rates over an 18 ms interval) collected for shod and unshod hoof conditions using 4 rosette gauges attached to the lateral heel of the right forefoot. The working hypothesis of the project was that shoeing may destabilize the wall by altering the pattern of heel strain. The general pattern of heel strain was characterised by high proportions of tensile strain in the posterodistal zone and biaxial compression near to the coronary border. Numerous trends of significance differentiate the shod and unshod responses, namely the absolute and relative magnitudes of tensile strain, the relative timing of peak events, and the strain orientations at the quarter gauge. Thus shoeing distorts heel function. For some hoof shapes imbalanced strains may progressively weaken a zone of wall (e.g. the quarter) such that microscopic flaws evolve to macroscopic proportions and cracking results.