At the InterBike 2009 Show in September, Cannondale unveiled the design for a new active suspension system quite unlike anything else in the mountain bike world. Though not officially on the market yet, the system, called Simon, is designed to automatically react to impacts and vibrations even before a rider feels them in the handlebars.
A drawback of suspension systems with a fixed damping rate is that they force the rider to balance the need to absorb impacts against the need to control the bike by smoothing out accelerations on the seat and handlebars. But common wisdom in the industry was that a fully active suspension was the Holy Grail a long way off, at best. In fact, in the summer of 2006, a chief designer at a competitor forecasted the possibility of an electronically controlled suspension within 20 years. Meanwhile, Cannondale and its partner Enfield Technologies had secretly made a fully functioning prototype of just such a technology.
What sets Cannondale's real-time damping system apart is that it incorporates a series of sensors to monitor the ride conditions and adjusts specific damping on-the-fly based not only on fork velocity and position, but also impact and rider inputs such as weight. In all, Simon offers more than 10,000 combinations of damping profiles.
Matt Timmerman, a former professional mountain bike racer and now an engineer at Cannondale, has tested Simon and is enthusiastic about its potential. "In downhill racing I've experienced the shortcomings with other suspension systems. My reaction here was 'Wow, this is really exciting.'
When you hit something really fast, you have a tiny displacement, and you basically want the damping to open up and not provide too much resistance," Timmerman explains. "But when you hit something and retain shaft velocity like jumping off a ledge you don't want the valve to open up as you'll go flying through all your available travel. You want some resistance. So even though shaft velocity is the same, damping requirements are not."
Given the thousands of different terrain responses and the fact that even little washboard bumps require lots of processing and valve activity, the design effort wasn't trivial. Moreover, engineers had to figure out how to fit the entire system inside the narrow confines of Cannondale's Lefty Headshok fork tube, with a 36.5 mm outer diameter and 90.0 mm length.
Five years in development actually make that 12 years if you count back to when Cannondale first patented a concept for an active suspension (U.S. Patent 5,971,116) the technology was brought to life by Cannondale Design Engineer Stanley Song and a team of engineers at Enfield Technologies headed up by Principal Engineer Dan Cook.
Song first contacted EnfieldTech in 2006, because of the company's expertise in proportional servo valves and control/drive electronics primarily for industrial markets.
"When we first got the call, Stanley was having some difficulty describing what he was interested in doing without giving away his project," recalls George Haithwaite, VP sales & applications engineering, EnfieldTech. "We had a very broad discussion about whether we could come on board as a technology partner to develop an electro-mechanical suspension system. I am also a mountain biker, so fortunately it was an application that I actually knew quite a bit about."
How Simon Works
In a suspension system, a spring absorbs the energy from a force input (like hitting a pot hole or rock) and a damper controls how the energy is dissipated. As the spring shock of the bike compresses or rebounds on impact, oil is forced through apertures in a valve and piston assembly. Since how quickly the spring shock is allowed to compress or rebound is directly proportional to the viscosity of the fluid, the size of the restriction and the velocity of the piston, Cook and his team integrated a high-precision, electro-hydraulic servo valve inside the fork tube.
The servo valve is actively changing the aperture opening (and therefore fluid flow rates) every 500 µsec with a resolution of 0.01 mm based on an algorithm that calculates the required response using recent rider data, a series of integrated sensors and ride map variables.
Linear Motor Delivers High Force
The precision valve is directly driven by a high-speed, linear-force motor, which is a permanent-magnet motor consisting of a floating coil and set of high-energy, rare-earth magnets. "Overall we needed a low-mass, high-force, extremely fast, linear electro-magnetic actuator that could generate the highest forces in the smallest space," says Cook. "We chose this motor over a typical solenoid because the force-to-mass ratio is high, the linearity of force versus position and current is better, and the moving mass is low to dissuade impacts from affecting valve position, as it's vertically oriented."
The static field motor housing employs radial magnet segments that allow for a more intense field strength while at the same time not increasing the valve OD, which was a major design constraint. The coil was also encased in aluminum to assist in mechanical rigidity while enhancing dynamic performance.
Power was a major concern, as the requirements called for eight hours run time on a 14.8V/2,600 mA-hr lithium-ion-cobalt battery. "A rider isn't going to want his battery to die in an hour," says Cook. "Our design consumes less than 1W continuously, compared to 11W to hold open our off-the-shelf industrial valve. Small solenoids are typically 3 to 4W."
The valve incorporates a light-force spring to force the valve closed upon loss of power. Rather than using a mechanical spring, which would degrade battery life since a constant current is required to maintain valve position, engineers used the existing magnetic components to effect a magnetic spring.
Using as many off-the-shelf components as possible to avoid cost and risk, EnfieldTech engineers incorporated the best features from existing valve designs and combined them with a precision-manufactured, annular sliding gate to manage the size of the aperture. The design provides infinitely variable and linear proportionality with near-zero hysteresis.
Using the magnetic spring as a target, the temperature-compensated Hall Effect sensor independently monitors the valve stem position so its accuracy is not affected by fluid temperature inside the valve. However, since fluid viscosity is dependent on temperature and viscosity affects differential pressure and therefore damping rate, temperature is a critical variable in the valve control algorithm.
The system accommodates temperature-related changes in viscosity both mechanically and electronically by directly measuring temperature and incorporating this into the valve control algorithm.
When the damping fluid expands due to the temperature rise caused by the energy conversion, a pre-charged, spring piston apparatus inside the damping chamber expands to accommodate the additional volume. A temperature-independent encoder on the fork delivers velocity and position information to the master damping control algorithm.
Of considerable concern to engineers was the fact that the valve body is itself a structural member required to withstand forces up to 1,000 lb. Engineers addressed this concern by maximizing the strength-to-weight ratio of the materials. They selected AL7075 and conducted a stress analysis in concert with internal component design to ensure stress points were identified early on and managed. Once the first design freeze was reached, prototype bodies were manufactured and a tensile stress rig designed and used to conduct physical testing to achieve maximum confidence in the body design.
Cost has been a concern all along the way, says Cook, noting that a custom servo valve for a comparable aerospace application can run several thousand dollars, which is close to the entire cost of a high-end bicycle.
Due to time constraints and limited volume, as well as the materials, engineers decided it was beneficial to design components for CNC machining centers. Additional tooling costs were eliminated by working with the vendor of the rare earth magnet to design toward materials and dimensions with existing tooling. The internal design meets all of the design requirements while making the assembly operation a simple, cartridge-type insert thereby reducing assembly time by 90 percent.
Cannondale has been cagey on the topic of when Simon will hit the market, though VP of R&D Peck is positive about the design efforts on the tricky fluid control system thus far. "It's at the heart of Simon, as the fork's performance hinges on the speed and precision of fluid flow, " he says. "EnfieldTech delivered an electro-hydraulic control valve that surpassed our requirements.
Several standard deviations above the mean when it comes to testing in an industry that's obsessed with it, Cannondale is currently doing field evaluations with the elite teams it sponsors. The feedback will help engineers tweak the design and nail the performance, pricing and competitive positioning.
Click here to listen to former mountain bike professional Matt Timmerman talk about Simon.
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