Gaining leverage: Designing a wheelchair for rugged terrain

Wheelchair users generally want to be able to go out and enjoy the countryside. However, activities as simple as family trips in the park, attending music festivals or ambling along country paths can be difficult or even impossible in regular wheelchairs due to uneven ground, steep terrain and mud. While some traditional wheelchair designs are available with chunky, 'off-road' tyres, they still require the user to push the wheels to move the chair, resulting in muddy hands and difficult to move wheels. It was this frustration that saw inventor Tim Morgan come up with the Mountain Trike, a wheelchair specifically designed for off-road use in difficult conditions that was able to deliver the performance being demanded by active people who are wheelchair bound. The Mountain Trike is designed for crossing ground that would be uncomfortable or even impossible in traditional wheelchairs such as woodland, sand, snow, mud and cobbles. The large front wheels mean users can easily ride up and over kerbs or other obstacles, yet the chair is narrow enough to fit through doorways. The chair is also able to fold flat, with the rear wheel folding neatly underneath the frame. The chair uses an aluminium space frame to keep weight under control, and also features a smaller rear wheel, and two small, anti-tip front wheels to prevent the chair from rolling over if braking quickly or when going up or downhill. In addition, independent suspension on each wheel not only gives a smooth and comfortable ride, but also means users can turn or travel across slopes safely and with confidence. One notable difference is the use of two drive levers, pulled by users to transmit power to the wheels. This overcomes a number of shortcomings. First, on off-road terrain it stops the user's hands from getting covered in mud and dirt. Secondly, they are able to transmit more power to the wheels, using optimised gearing to give good speed on flat ground, but making hills less of a challenge to climb. However, as mountain bike designers will know, operating in these conditions puts a significant amount of additional strain, debris and wear on components. From the outset it was clear that the Mountain Trike needed to be both lightweight and reliable in these rugged conditions. The clutch was a key component, which would enable the gears and lever system to be effective. The lever drive was essential to the design, as it not only allowed users the ability to propel themselves forward, but also tosteer and brake. The levers are connected to the wheels using a similar system found on mountain bikes. However, rather than a standard bike freewheel, Morgan chose to specify an overrunning clutch to transmit the kinetic energy from the levers to the wheels. "When the wheels are spinning freely on a mountain bike, without drive from the pedals, the freewheel clicks during the rotation," says Morgan. "The Mountain Trike is intended for use during family walks and other lifestyle activities so I wanted to make sure it would run silently. I also found that bike freewheel has a small amount of free travel before it engages. Because the levers are quite long, this translated to several inches of free travel, which was quite jarring for the user and resulted in less efficiency overall. Using an overrunning clutch eliminated both these issues." However, while the initial overrunning clutches specified on test models performed as desired, it became apparent that longer term use brought questions of durability, which was likely to only be exacerbated when used off-road and on challenging terrain. The reliability and durability issue was traced back to poor seals on the overrunning clutch unit. It meant that mud and contaminants could creep inside, causing seizing and undue wear, while the poorly machined internal components led to slipping and eventual failure, due to the shock loading caused by disengaging and engaging the drivetrain with every push. Morgan, aware that he needed to source an alternative component that could survive in the most extreme terrain, approached Stieber Clutch with the problem, hoping for a solution. He found that a similar configuration of overrunning clutch was available off the shelf and, persuaded by its promise of superior quality, decided to trial the component to see how it compared. "I was supplied with Stieber's CSK PP overrunning clutch for testing and the superiority of the component was immediately apparent," says Morgan. "The seals were clearly far better protected and the movement was so much smoother." The Stieber clutch offers a number of unique design features that help ensure it will last, even over dirty and difficult environments and terrains. The sprags of the clutch component are manufactured using the company's 'Formchrome' process, which diffuses a chromium carbide into the working surface of the sprag. This substantially increases the surface hardness and results in far superior wear protection. The sprags are also precision machined to ensure they are identical, to prevent any uneven rolling movement. The sprag retainer assembly also features a free-action design that permits each sprag to have free and independent action. This independence allows the sprags to adapt to variations in annular space and eccentricities, so when the clutch is engaged the load is proportionally shared throughout. This is an important part of the component as it eliminates shock loads, jarring motions and the possibility of clutch damage from the entire load being absorbed by just a few sprags. In operation, springs energise the sprags into position for instantaneous engagement with virtually no backlash. "During the testing process it was clear that the Stieber clutch offered far more in terms of reliable performance," says Morgan. "Since they have been installed, we have had no reported failures, even from some of our more adventurous customers. Overall, it gives riders an amazing feeling of freedom and independence."