Curtiss One Design, Pt. 2: Suspension

Curtiss motorcycles are battery electric. The lack of an internal combustion engine creates a noise-free environment for a pure riding experience. Lack of vibration and unnecessary dedicated cognitive space to modulate an ICE means that road feel will be unmasked and unfiltered, compelling us to re-examine the status quo.

Motorcycle suspension has been a process of refinement post-WWII with telescopic fluid-filled front fork tubes and coil-over rear shock arrangement. The modern motorcycle with its highly refined version of this system has proven the validity of this formula, but we submit that it has done little to address its underlying weaknesses.

Those being:

Stiction – The reliance on bushings within the telescoping elements to provide free play between the two causes minute pauses in motion, especially in directional changes from rebound to compression.

Torsional Rigidity – Fork tubes are prone to twisting forces, and rely entirely on the front axle attachment points to counteract twisting around the steering axis.

Wheel Trajectory Control – The critical relationship between steering neck angle (rake) and front wheel contact patch yields the trail value. With a linear front fork (deflection not withstanding), trail values will always have a non-alterable trail curve to suspension travel ratio when graphed.

Dive – Applying force to front wheel brake discs results in a high degree of compression of the front fork tubes as the entire mass of the motorcycle and rider now pivots around the abruptly introduced pole of moments around the front axle. Weight transfer vectors greatly reduce the effectiveness of the front suspension by asking the springs to suspend, while concurrently resist deceleration forces.

Mass Centralization – Two front fork tubes filled with fluid and springs, positioned far from the steering axis resist quick directional changes.

Un-sprung Weight – All fluid within the front fork tubes is considered un-sprung, as is a large percentage of the heavy steel coil spring and valving that travels with the wheels.

Longevity – As any motorcycle restoration expert will attest, the first thing that typically fails on a motorcycle are the fork tube seals. The underlying premise of a large diameter linear seal is inherently flawed.

Our exploration begins with a look into the past. Currently regarded as an anachronism, the girder-style front fork does have advantages, especially with the application of modern materials, bearing surfaces, and fully understood geometries. Pushing further into possibility, the unification of the two halves of the girder via a cross brace into a single piece both increases rigidity and reduces complexity. Narrowing the frontal area of a motorcycle is germane to the fundamentals of motorcycle aerodynamics and efficiencies at speed.

Stiction can be greatly reduced with the use of a multi-link front fork that does not rely on isolating bushings for the tubes to telescope smoothly, or rake angles to counter deflection of those tubes as they vector impact forces from irregularities in the road surface to the chassis. Multi-link front suspension systems have inherent torsional rigidity, and when properly tuned, are inherently anti dive as the loads applied during braking are not transferred directly to the chassis through the spring.

The true benefit of a multi-link type front suspension lies in front wheel trajectories that can be altered by varying length and placement of the connecting links with eccentrics. This trail value fine tuning has the advantage of altering trail values for any given suspension or rake angle setting.

Trail adjustment on the Curtiss One can be carried out easily by the employment of the tuning eccentrics to satisfy varying conditions, and rider preferences. These gear driven units alter the effective length of the upper control arms.

Our motorcycle is the first series production motorcycle to feature a truly adjustable rake angle. The steering neck itself bolts onto the chassis side plates with two positions available and eccentric adjusters made with safety block-outs for either rake position. Both of these systems (adjustable rake/trail) working together give us something totally unique in all of the motorcycling universe.

Available trail values are 3.5 to 4.5 inches in either the 27 or 31 degree rake position.

Multi-link suspension has two deficiencies. The mass centralization problem is still present and un-sprung weight is an issue. By replacing the traditional steering stem (and wasted space around it) with a hydro- pneumatic cylinder, the mass of the suspension component is therefore concentric with the steering axis, and effectively neutralized. The cylinder that now does double duty as structural steering stem and front suspension component, can be further re-purposed to act as the rear suspension component as well. Our design initially utilizes traditional coil springs. With the hydro-pneumatic system being offered on future production models and as an upgrade to existing clients.

With unsprung weight as the final hurdle to overcome, we turn to composites. The technology to reliably manufacture structural composite pieces is well understood, the only drawback being the cost of labor invested in production of the parts.

Optimized swingarm angles, vectoring thrust into a motorcycle chassis, are a critical piece of the geometric puzzle that yields a proper handling motorcycle. The angle of the swingarm dictates the action of the rear suspension under acceleration, and in a delicate balancing act either causes compression or rebound. By attaching the swingarm pivot directly to the driving motor shaft and utilizing an adjustable suspension push rod, alternate swingarm angles can be selected by adjustment of the push rod length. This adjustment makes precision tuning possible for a variety of wheel sizes, rake angles, and rider weights while maintaining appropriate ground clearances. Formula for every conceivable iteration can therefore be understood and offered to the client. Post-purchase personalization becomes more than merely an exercise in cosmetic enhancements.

The added value of the longer life expectancy of the smaller seals used in coil over shocks, and the inherent modularity of these units means a motorcycle that is more immune to age related obsolescence. Unlike previous efforts to graft a battery electric system onto a chassis designed for internal combustion engine with short lifespans, the Curtiss One is a new kind of motorcycle.

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