Dually Travel Analysis

In developing the new Ahrens Austin dual-suspension design, I set out to achieve a few broad goals for the suspension system.

GOALS:

1. Minimum rear wheel travel ~5.5" [140mm]
2. Shortish chainstay length for good handling and climbing ~16.9" [429mm]
3. Neutral BB height that's just tall enough to avoid cranks/pedals hitting the ground under full load ~13.5" [343mm]
   
4. Rear axle path traveling upwards/rearwards for optimal traction
5. Leverage ratio ~3:1 or less to minimize shock overloading
   

In analyzing and refining the design, it became apparent very quickly that achieving these goals would not be easy! But with some diligence and great 3D modeling software, pivot placement was continually tweaked until all of the original goals were met. Below is a graph which details only the swingarm motion (from zero to full-travel). It does not take into account front fork motion at this time.

FINDINGS:

1. The rear axle path moves in an upward/rearward fashion without any abrupt direction changes. Smooth travel upwards, smooth travel downwards with progressive damping in both directions - simple and effective.

2. As rear wheel travel increases, so does BB height and crankarm/pedal clearance. This trait is unique in that most frame designs exhibit the opposite; more travel typically yields lower BB height and less crankarm/pedal clearance. And since the front fork will ultimately be part of the overall system, the BB height should hover near 13.5" which has been my sweet spot for many years.

3. The leverage ratio on the rear shock is 2.97:1 which is acceptable for this frame style. The frame will accept shock lengths of 7.875" x 2.0" stroke and my plan is to offer both Fox RP23 or DHX 5.0 options depending on the application.

4. Chainstay length is fixed at 16.9" and with a concentric rear axle pivot, no chain growth can occur which means pedaling efficiency should be excellent. This setup also minimizes 'brake jack' to some extent although the ideal solution is a fully-floating setup which hasn't been designed yet!

5. Overall travel tops out almost 6" (151mm) which is a nice bonus since it's more than originally planned.

In summary, the main objectives listed above have all been met and in some cases exceeded. When I envision riding this bike, I want the suspension to do it's job in a clean, quiet and effective manner. This design will inspire confidence with its stout construction, lateral rigidity and frame geometry that's proven to handle well in a variety of terrains.