Offroad Wheelchair Update 1 - To Retrofit or Reinvent?

Recently, we at Freeside Atlanta teamed up with the Alchemical Arts Alliance and My Inventor Club to design and build an Offroad Wheelchair so that our friend Robin can get around their events. Together, we've raised about $2,000 for the project.

The design phase is usually the most difficult part of the projects, and it is often the most expensive place to make a mistake. Committing the resources to a poorly-designed project can cause the entire thing to be wasted, so we designed the Offroad Wheelchair project very carefully.

We started with the constraints – We need ease of maneuverability, an ability to overcome obstacles and take fairly steep inclines, longevity to make it through events up to a week long, and recoverability in case it gets stuck. Also, biggest two constraints – Budget (~$2000) and time to prototype (2 months)

From that, we landed on 3 design options –

1 – Electric motors with onboard generator for periodic battery charge

2 – Modifying an existing lawnmower or outdoor vehicle with hydraulic controls and automation

3 – Modifying a zero-turn lawnmower to suit our purpose

After weighing the options, we chose option one for its low noise and high efficiency. Next step - simulate the design. I used the website Study Physics to figure out the torque requirement to pull a simulated wheelchair + passenger up a given slope. Then I simulated it in a spreadsheet starting with the worst-case scenario so I could play with the numbers. The inputs are in orange and the rest are calculations.

In other words, the force required to take a vehicle up a 15 degree slope is a bit more than half of the force required to lift it all the way off the ground. To find the torque requirement, the force needs to be distributed around the wheel, since the wheel radius affects the leverage exerted by the motor.

Therefore, the minimum torque of the motors is about 200 pound feet. Is that feasible for an electric motor? Let’s look at the best motor example I could find - 

This means that the system required two very powerful electric motors, running near peak torque continuously, and geared down 60x. They exist, but they are about $600 each and don’t usually match each other’s output exactly. Plus, the motor controllers are $200-$400 each! That means that this project is neither feasible nor practical, as the entire budget could be spent on 2 motors with their respective transmissions and controllers. It won’t work. But, this is why we simulate. It’s time to drop back and try again.

The hydraulic modification of an existing vehicle seems interesting, but also time-consuming. Plus, it wouldn’t be able to pivot in place like the other two design options, which is important to delivering the rider to exactly their target. Not to mention that the drive of the wheels will probably be locked together, so there is nothing stopping the thing from sliding down a slope. It also requires the modification of a ~$1000 platform with up to $800 in hydraulics and most options aren’t configured for somebody to easily get into from a wheelchair.

Then, Shane from My Inventor Club sent me an email about a zero-turn mower that was available in our price range. Zero-turn mowers use hydrostatic transmissions to convert the high-rpm, low-torque energy from an engine into low-speed, high-torque power with hydraulics. The mower we chose is a Grasshopper 725k. The actual torque isn’t listed in any of the data sheets, but the horsepower (25) and wheel diameter (22in) are. So now we just need to find the torque and update our simulation to see if it will work.

If we know how to find torque then we just need to find the rpm at the wheels at the top speed, discount it by about 30% for conversion losses, and that will be near our actual torque.

First, get the top speed to feet per minute – 

Then, turn that to rpm using (the diameter of the tire) * (pi) to get the ground it covers per revolution.

Finally, you can do the torque conversion – 

 The zero-turn mower is 800lbs, so it is much heavier. The torque required to haul it up a 15 degree slope is 530 pound-feet, so it might be able to crawl along if it can keep traction. However, rolling over would be a concern at that angle anyway. Therefore, the zero-turn is feasible and reasonably meets the requirements of the project.

We can look into supplementing the power of the motors with a winch if the vehicle gets stuck somewhere that the torque/traction can’t overcome and reduce engine noise, but we're in the ballpark now. So, we’ve settled on a design – a retrofit of a zero-turn mower with a winch. It will be faster and more cost-effective to retrofit a used device that has been engineered to a similar task than to design and build something new from scratch. However, this platform could serve as a prototype for some future design, so we can work out the issues through iterative prototyping.

Until then, we’ll do the most effective and cost-effective Offroad Wheelchair that our constraints allow and document it so others can duplicate and improve the idea.

Check in on the Offroad Wheelchair page for links and info, including the simulation spreadsheet that I used (you get to point out my mistakes!) and the first video of the mower that we selected.