XTNDED Motoclimb Extensions
Motoclimb is a unique sport where participants race modified, off-road motorcycles of various engine displacements uphill through a specifically marked course, with the goal of having the fastest time from bottom to top. The race courses are designed to challenge rider and machine, often riddled with large rocks, ledges, and sharp turns. In the professional classes, all machines are outfitted with swingarm extensions that increase the wheelbase of the bike by adding approximately 1ft of length to the rear swingarm, allowing more aggressive riding while reducing the tendency of the bike to flip over backwards on steep climbs and under heavy acceleration.
A final wall at the Nitro Nationals in Columbus, Montana
Given the inherently harsh environment of racing and the vastly different racing surfaces found at events across the country, a feasible design must satisfy the following requirements:
1. The design must be able to safely handle any and all foreseeable racing scenarios.
2. The design must be as lightweight as possible.
3. Length adjustments must be possible to account for different racing surfaces, as well as minor adjustments at each length for chain tension
4. The design must bolt directly onto the stock motorcycle without requiring any permanent modification or adjustment.
5. The design must be produced for sale at a $500 price point.
The design process began by reverse engineering one of the more popular designs currently available, and running FEA simulations to determine what type of loads it could withstand. This was chosen as the benchmark due to its well known durability, however the design could be improved upon by reducing weight and increasing the range of wheelbase adjustment available. After multiple iterations and input from racers, as well as drawing from my own racing experience, an initial design was created that would save weight by utilizing the stock rear axle rather than require a heavier custom steel one, as well as double the available wheelbase lengths for more precise chassis tuning. FEA simulations were run to ensure the design was on par with the sample unit.
It also proved to be challenging to maintain proper alignment of the rear brake caliper hanger and chain guide, while simultaneously allowing them to move forward and backwards as the rear wheel location is changed during length adjustment. This was achieved by creating a sliding system with a friction clamp for the chain guide, and multiple mounting locations for the rear caliper hanger block.
A FEA simulation assessing stress points
A first article prototype was machined, then tested for a full race season. It was found that after a period of time a small amount of bending had appeared in the test part as well as the swingarm. However, due to the way the part interfaced the swingarm, additional bending was halted before any additional damage could occur. As the sport continues to evolve, so do the speeds at which riders are hitting obstacles and loads seen by chassis components are increasing at an exponential rate. These increases in intensity were not accounted for in the first design, and this proved to be an error.
Exploded view of final extension design
The design was subsequently updated after the failure was assessed, and a new prototype set was machined and tested the following season. Overall height was increased to greatly improve strength at a minimal weight penalty, and an additional mounting feature was added to help alleviate pressure on the flanges that had bent on the previous swingarm. These changes proved to be effective, and the system lasted the entire season with no noticeable degridation of integrity.
Final design mounted to Austin Teyler's Suzuki RMZ-450