Cycling is a popular activity that has been extensively studied. Unfortunately, within the research, there are sparse insights regarding pitch-over accidents. A pitch-over accident occurs when a bicycle is introduced to an abrupt deceleration, often from a front wheel impact or excessive front wheel braking. Pitch-overs due to front braking are avoided if deceleration levels do not exceed the longitudinal stability of the rider/bicycle system, which is defined by the location of the combined center of mass (COM) relative to the front tire contact point. The purpose of this study was to determine how bicycle designs and geometries, plus rider postures, effect rider/bicycle stability, and pitch-over propensity. This study began by presenting and validating the use of a new force plate method (FPM) for locating rider and bicycle COM locations, as an alternative to a traditional anthropometric method (AM) used by Winter et. al. (2009). COM location estimates developed from the FPM were then compared to estimates derived using the AM. Finally, the FPM was used to evaluate the effects of bicycle types and rider postures on COM locations, and thus deceleration thresholds at pitch-over. The FPM was found to be much faster and more accurate than the AM. Resultant error with the FPM was less than 1cm (7.8mm). Errors in the AM approached 135 mm. The FPM then exposed how both bicycle geometry and rider posture play large roles in effecting the deceleration thresholds of bicycles. Deceleration threshold differences between bicycles with similar rider positions were as large as .20 Gs, and these thresholds approached .21 Gs across different rider positions of the same rider on the same bicycle. The results derived from this study expose the effects of bicycle types and rider postures on pitch-over propensity, and provide an accurate method for examining these effects.