The GRIT Freedom Chair has unique parking brakes that are unlike any on regular wheelchairs.
Regular wheelchair parking brakes work well in perfect environments but can have some real challenges in tough environments. This is because many parking brakes rely on linkage mechanisms to maintain wheel compression. Linkages are a great solution to this problem, but they introduce many points of failure: every hinge in the linkage can loosen, tighten, wear out, or require maintenance.
When we started designing our off-road wheelchair at MIT, we knew that any design that required lots of long term maintenance was a non-starter. On top of that, it seems that every wheelchair company in the world patented their own unique parking brake designs. So unless we wanted to pay hefty license fees or deal with a lawsuit, we had to design our own.
Our functional requirements were:
- Reliably lock the wheel in place.
- Be easy to apply for a wide range of physical disabilities.
- Be safe: make sure there aren't any pinch points.
- Extreme durability. Our riders in developing countries don't have the resources to repair their parking brakes, so ours have to be built to last.
We studied linkage-style parking brakes (almost every parking brake on the market uses a linkage) to try to understand what made them work. Activating the parking brakes moved their linkages into, or past, a singularity, making it impossible for the wheel to push back and release the brake. The only way to release the brake would be to push or pull on the brake lever to push the linkage back.
The legs under a folding table perfectly illustrate this principle: when the legs are folded out, the linkage under the table is perfectly aligned, and no movement of the leg will fold it. As engineers, we refer to this point as a singularity. However, pushing in on the linkage under the table pushes it out of singularity, and allows the leg to retract.
While studying other parking brakes, we realized that they, by design, had to compress the wheel in order to prevent it from moving. If we could come up with a way to use wheel compression to lock the parking brake in place, we could avoid using linkages all together.
To keep the brake from releasing on its own, we needed to design it so that in order to deactivate it, the wheel had to be compressed more than in its braking position. The only way to compress the wheel more would be to add energy to the system, meaning that the rider would have to push on the brake lever. Without energy input, the brake will remain locked in place.
With this realization, we quickly designed a cam-style parking brake that compressed the wheel more when you were pulling it into place than when it was locked in place holding the wheel.
After some prototyping, we settled on our tubular parking brake design. It is easy to manufacture, has no linkages or parts that can loosen, and is robust to many different kinds of wheels. It can also be adjusted to make it easier to apply for riders without a lot of strength.
The design process behind the parking brakes is typical of how we designed the entire GRIT Freedom Chair: define functional requirements, understand the underlying physics, innovate new solutions, prototype and test with riders to get the details right. We're proud of how they came out and we're excited to see what you think of them!
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The GRIT Freedom Chair is the most versatile chair on the market, designed from the ground up to handle any terrain. From trails to grass to snow, the Freedom Chair is built for you to push yourself. Born out of research at MIT, the Freedom Chair's patented easy-push levers reduce shoulder strain and put you in control of your mobility. Ready to hit the trails. Learn more about the GRIT Freedom Chair at www.gogrit.us