Science: The Secret Behind the GRIT Freedom Chair

Science: The Secret Behind the GRIT Freedom Chair

Nov 5th 2021

The GRIT co-founders remember when the GRIT Freedom Chair was a brainstorming session on a whiteboard, in a long-neglected building in the northwest corner of the MIT campus, in a room we affectionately called the "MIT Mobility Lab." We were trying to reinvent the wheelchair, to design something that could help people with disabilities travel long distances outdoors, over rough terrain and pavement, and then move around indoors when they got to their destinations. Existing solutions didn't work: handcycles were great for distances but were too big to use indoors, and regular wheelchairs were too hard to push off-road or for long distances on-road.

We started with first-principles: the basic underlying physics and biomechanics of the problem. We asked ourselves: what's the most efficient way to use the upper body for locomotion?


Researching biomechanics

Science the Secret Behind GRIT Freedom Chair, researching biomechanics: anthropometric data to calculate push forces through arm’s reach


We spent the next few weeks reading papers. We learned that regular wheelchair efficiency is really low, typically under 10%. This means that 90% of the energy a wheelchair rider uses is wasted! This is due to regular wheelchair propulsion using a lot of small muscles (rather than a few large ones) and the push-rim motion requiring a lot of effort that doesn't necessarily push the chair forward, such as the energy required to maintain a grip on the push-rim.

We wanted to think outside of the box, so we looked at anthropometric data that shows how much force people can exert with their arms in all sorts of positions. Using this data, we could map out ideal movements that would maximize the power we could get out of the rider.

After exploring all the options, we opted for a lever-drive system. It was more efficient than regular push-rims, easy to fit within the confines of a wheelchair frame, and offered the opportunity to change the mechanical advantage by shifting hand position on the levers.


Leveraging physics

Science the Secret Behind GRIT Freedom Chair, leveraging physics: person in off-road wheelchair holding tops of levers


We wanted a way for GRIT Freedom Chair riders to "change gears," just like in a manual transmission car or a multi-speed bicycle. Some lever wheelchairs on the market use complicated gear systems to accomplish this, but we wanted something simpler, easier to use, and less likely to break.

Our insight was that by sliding one's hands up and down the levers, one can have the same benefits as changing gears.

Grabbing at the top of the lever is like grabbing at the end of a long wrench or sitting at the end of a see-saw. It gives the rider a lot of leverage. Push force on the lever is amplified by nearly a factor of 10 being transmitted through the chain and to the wheels, making it really easy to roll over grass and to climb hills.

Science the Secret Behind GRIT Freedom Chair, leveraging physics: person in off-road wheelchair holding bottoms of levers

On the other hand, grabbing at the bottom of the lever is like grabbing near the bolt when you're using a wrench or sitting near the middle of the see-saw. You don't have a lot of leverage. However, you can push the lever through a longer stroke, which means that for each movement of your arm you can make the wheel rotate more: you roll faster!

Optimizing

Science the Secret Behind GRIT Freedom Chair, optimizing: graph showing required lever lengths for varying terrains at peak efficiency

Required lever lengths for varying terrains at peak efficiency.


As engineers, we didn't just want to design a lever system. We wanted to design the best possible lever system. So we started a process of optimization. We thought about a lot of factors: the variety of different terrains riders would roll over (sand, grass, gravel, pavement, snow, etc.) and what kind of friction they would present. We thought about different slopes that were common in day-to-day life outside of the ADA-compliant world. We thought about what gear sizes were commonly available for bikes. We looked at how much force people could exert at each point of the lever stroke and we interviewed dozens of wheelchair riders to find out what worked for them.

We ran the numbers and tried to figure out a few things: what's the best configuration that would let a rider exert their maximum possible force to climb over an obstacle? And what's the best configuration that would let a rider cruise most efficiently on flat ground?

We sized the GRIT Freedom Chair's levers and gear ratios based on these findings, ensuring our riders have the best possible solution for the widest range of riding conditions.

Science the Secret Behind GRIT Freedom Chair, optimizing: graph showing required lever lengths for varying terrains at peak power output

Required lever lengths for varying terrains at peak power output.

Science the Secret Behind GRIT Freedom Chair, optimizing: graph showing attainable velocity at peak efficiency

Attainable velocity at peak efficiency.

Testing

Science the Secret Behind GRIT Freedom Chair, testing: GRIT engineer testing prototype movement on grass


The last step in our process was to test. Running all sorts of numbers in Microsoft Excel is one thing, but we needed to make sure our hypothesis worked out in the field. We designed and built our own data acquisition system that measured how hard riders were pushing, what slopes they were riding over, where their hands were on the levers, their heart rate, and their oxygen consumption. We had riders travel over their regular commute, in both our lever chair and their regular wheelchair, and compared the data we collected.

We were excited to find that the lever system was far better than their push-rim wheelchairs. It's:

76% faster than a wheelchair over the same terrain. Riders got there faster.

41% more efficient. Riders used a lot less energy.

51% higher peak propulsion force. Riders were able to roll over obstacles easier.