PARALLEL UNIVERSE
Of course, the V configuration isn’t the only way to enjoy a twin. Some of the most important and iconic bikes of all time were parallel twins. Yamaha RDs, LCs, and Power Valves, Triumph Bonnevilles fromway back when to the present day and let’s not forget any number of Honda CB twins that established the company as a serious bike manufacturer. Parallel twins usually have their crank big-end pins offset by either 180º or 360º (the latter being indistinguishable from no offset at all). Although Yamaha played an unusual aural trick with their TDM and TRX twins; they offset the crank pins on these engines to 270º apart. This was merely to make these bikes sound exactly like a 90º V-twin, thanks to exhaust pulses that mimicked a V-engine.
Sadly, the 850 and 900cc Yamaha parallel twin’s odd crank offset didn’t yield the balancing advantages of a 90º V-twin. Therefore, one of the major limiting factors of any parallel twin is engine vibration. In the case of the smaller two-stroke engines, like Yamaha LCs for example, vibration could be mitigated by using rubber engine mounts. Larger four-stroke parallel twins need something more elaborate to quell their vibes though. Additional balance shafts, or crank driven, piston opposing counterweights are needed and the extra cost, weight and space of these balancing systems limit the ultimate performance potential of the parallel twin engine configuration.