Dive into Design: Engineering the GRIT Freedom Chair Footrest

When we created the Leveraged Freedom Chair (LFC)- specifically designed for disabled individuals in developing countries - we used a footrest inspired by the Whirlwind Roughrider. We designed it with a toe-guard in front, to keep the rider's feet from hitting the front wheel or obstacles in their path. It's adjustable up and down, but the process requires hand tools and is usually only done once per rider. When we redesigned the Freedom Chair for the US market, we wanted to use the LFC footrest as a starting point, but to make something even better.

Design Requirements

For the new and improved foot plate on the US-focused Freedom Chair, we wanted to address the following:

  • We wanted to make the footrest more comfortable. We commissioned a seating and postural support evaluation by Shonaquip, world-renowned wheelchair fitting experts based in Cape Town. They suggested we design the footplate with a heel plate and a foot strap to better hold the feet in place.
  • We wanted make the footrest easier to manufacture. We're committed to manufacturing the Freedom Chair in the US. Here in the US, however, welding is really expensive. Our developing country version of the footrest requires a lot of welding, so the design needed an update.
  • We wanted to make the footrest more adjustable. Riders come in all shapes and sizes! And American riders are shaped differently than our riders in India and elsewhere in the developing world. We wanted a footrest that was easier to adjust, easier to add straps to, and potentially removable to help with shipping and transportation.
  • The footrest should be strong enough to support a rider's weight. Over 25% of our riders can walk short distances. We wanted our footplate to be able to support their weight without breaking.

Sketching

We started with sketching and brainstorming to see what ideas we could come up with. We explored different strategies: tubular designs similar to what we already had in developing countries, plastic designs, flat plates like on ultralight chairs, and others.

Prototyping

We really liked the possibilities of a sheet metal design. It would be easy (read: less expensive) to make in small volumes, it could be made stronger by changing the geometry (our specialty), and it could meet all of our requirements. We decided to test this first and see how it worked out. We bent together a super-simple version out of some scrap sheet metal (from old seat prototypes) and attached it to a chair chassis. It wasn't as strong as we needed, but it was close! We were ready to start refining.

 
 

 

Modeling

From our testing, it seemed like the concept could work. We developed an intuition for what would make the design stronger, and how to make it easier to build. The sheet metal concept was a slam-dunk if it worked out, so we put the other options on the back burner. If the sheet metal didn't work out we could always go back to them. 

We started off by looking at the big picture: deciding whether we wanted to do a thin piece of sheet metal folded into a complicated shape (each bend adds stiffness to the part, making it stronger), or if we wanted to do a thicker piece of sheet metal instead. 


Thin sheet metal, folded into a box

Thick sheet metal, as a plate


We ran the numbers, which told us that the thinner version would be lighter and easier to make, but wouldn't be strong enough where it attached to the chair. The thicker version also had some strength issues at the attachment point.

In order to make the best footplate we possibly could, we decided to combine the two options! We made a welded base frame and put a sheet metal surface on top. That way we had a strong attachment point, with a lightweight surface.


Concepts for sheet metal over a metal frame


We considered a few different ways to do this, but we liked the concept on the [above] right the most, because it was the simplest. If didn't work out, we could add features from the other concepts until we got it to work. 

Prototyping

At this point, we were pretty happy with the design. We reached out to some local sheet metal shops to get quotes on the part. We picked the best shop and ordered a few copies of the new footrest. We assembled it on the chair and went riding on the gnarliest mountain bike trails we could find. We wanted to beat the crap out of the footplate. It survived!

We also started to show the footplate to wheelchair riders that we were working with in the US.  We learned about the importance of having different strapping options (now we have strap slots), about how US foot sizes are bigger than we thought (we ended up making the plate longer), and why we should put friction tape on the footplate to hold the feet better (we ultimately did). Our rider focus group also suggested that we reverse the footplate from our original concept. This put the support above the footplate, which gave riders more ground clearance, room to put the front support bend down, and more room for the toes. The "2nd footplate" [below] is before the user feedback, and the "3rd footplate" is after the user feedback.

 

Final version

We're really proud of the footrest we designed. It met all of our functional requirements, works great, and most importantly, our riders love it. Our design is:

  • Comfortable: Freedom Chair riders have been raving about the footplate. The slots along the sides allow them to be really creative with straps and the friction tape on the surface holds their feet in place. The upward bend supports the heel and keeps feet from falling off the plate. The upward slope of the overall plate keeps feet from shifting forward into the front wheel.
  • Manufacturable: The footplate can be made by our sheet metal bender and tube welder, both within an hour of our office in Boston. Both components are aluminum, cutting down on weight and guaranteeing that even if they get scratched up on the trail, they'll never rust.
  • Adjustable: We used a bike seat-post clamp for the height adjustment. This off-the-shelf part is really easy to use and lets riders put the footrest at exactly the right height for them. As a bonus, the same clamp allows riders to totally remove the footrest, helping the Freedom Chair pack light when storing in a car.
  • Strong: By combining the best features of tubular and sheet metal construction, we created a footrest that was both lightweight and really strong. The front and back flaps provide support in bending. The box tubing along the middle of the footrest helps to distribute the load along the length of the footrest. We've tested the footrest with over 200lbs on it and it passed with flying colors. The footrest also passed all the ISO-standard and RESNA-standard tests for wheelchair footrests, with rider weights up to 300lbs.
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