This report covers the research and testing of the rumbling phenomenon found in the drivetrains of high-pivot rear suspension mountain bikes, specifically those made by Norco Bicycles. This includes the project definition and objectives, a theoretical background of the problem, the development and testing of an analytical model, the design and development of a physical test bench, discussion of test results, the applicable findings, and the final conclusion. The problem being addressed in this project is the drivetrain rumbling found in high-pivot mountain bikes. High-pivot mountain bikes allow for less momentum losses when rolling over square-edge bumps than a low-pivot bike, though an idler sprocket needs to be added near the pivot point to address the excessive chain growth. This idler is thought to be the cause of this rumbling; it is the objective of this project to research this phenomenon and try to discover a drivetrain design that minimizes the effect. It was hypothesized that the polygonal nature of the idler, through the pitch polygonal effect, is the main cause for the rumbling. Due to the idler being a polygon, the radius varies as the chain engages and disengages with the sprocket, thus causing slight changes in the gear ratio. These changes can be felt by the rider as a rumbling. By changing the tooth count and position, therefore the wrap angle of the idler, the changes in gear ratio at both ends can be brought either into phase or out of phase by 180°. The project team hypothesized that the ideal setup would be at one of these two extreme cases. To discover the optimal design, the testing was broken up into two groups: first, simulations were run through an analytical model, and second, the theoretical results were validated through testing on a physical test bench. MATLAB was used to create the analytical model. The model was a 2-dimensional representation of the chainring, idler, and cassette, each made up of discrete points. Different parameters such as tooth count, and relative position could be specified. It was chosen that the simulations would be performed at 25% bike sag, the position riders would be pedaling. The final output of the simulations was maximum change in gear ratio for a specific tooth count, as well as optimal relative position for the idler for a specific tooth count. It was found that the best case, given a number of assumptions, was using a 14-tooth sprocket, and the worst case was using an 11-tooth socket. It was decided that these to tooth counts, as well as the Norco standard 16-tooth and second best 18-tooth would be experimentally tested. To allow for data validation, a test bench was designed and manufactured. It was designed to transmit a constant force from a hanging weight through the drivetrain to a scale on the other end, where the tension could be read. The relative positions of both the cassette and the idler could be adjusted, to allow for various sag positions and wrap angles. Through additional pulleys at the weight and the scale, the reduction would be amplified to allow the scale to read the changes in chain tension. The pulley sizes were chosen by calculating the change in chain tension for a set weight and comparing that to the scale’s resolution. To discover an optimal solution, various variables were tested. These include idler tooth count, idler position, sprocket material, tooth profile, and the chain-line. Five individual tests were run for each to allow for more consistent results. The results of each were compared to see how they influence the drivetrain performance. After testing was completed, that both the tooth count and the idler position had the greatest effect on the change in gear ratio. For tooth count, a 14-tooth idler resulted in significant reductions in gear ratio change compared to an 11-tooth idler, almost a 94% reduction. By moving a 16-tooth idler to its theoretically optimal position, reductions in gear ratio change of 38% were observed. However, it was found that both the tooth profile of the idler, as well as is material made little difference. Through this, it was found that a possible optimal bike frame design could exist, and through more thorough research using the above methods, drivetrain rumbling could be reduced to negligible levels during the design of a high pivot mountain bike. The project was completed as of May 5, 2019. The project will be showcased at the BCIT Engineering Expo on May 10, 2019.