Science is amazing. It turns out that a group of Korean researchers has been inspired by the surface tension of a drop of liquid to create a wheel that changes shape in real time, overcoming uneven surfaces and obstacles with ease. We’ve all imagined a real all-terrain vehicle with something like this… but it was originally intended to help wheelchairs or robots better cope with difficult terrain.
We see wheels and tyres so often, on cars or bicycles, that we take their capabilities for granted. But when it comes to wheelchairs, their tyre’s poor performance when traversing uneven surfaces or small obstacles means their occupant is confined to staying “on the road”.
This is not the first time that the wheel has been reinvented. NASA created indestructible tyres for its vehicles made of a nickel and titanium alloy that could be deformed up to the axle and return to their original shape. There is also the Michelin Uptis, a prototype of an airless tyre (a non-pneumatic tyre) that does not suffer from punctures, leaks or blowouts… but due to their rigidity they are not good for overcoming obstacles, as they are less deformable to adapt to the terrain. And a track system? They are limited to relatively low speeds and consume more energy due to greater friction (there is greater contact with the ground).
The solution lay in the surface tension of a drop. The surface tension of a liquid results from an imbalance in the attractive or cohesive forces between molecules. While a molecule in the liquid experiences cohesive forces with other molecules in all directions, one at the surface of a liquid only experiences net inward cohesive forces. In a drop, as the cohesive force of the surface molecules increases, so does the net force pulling the liquid molecules inward. That’s why a droplet returns to a circular shape.
With this underlying idea, scientists created a wheel of variable stiffness that can overcome rough terrain and obstacles while maintaining the advantages of a normal wheel traveling on flat terrain.
A key aspect of that deformable wheel is its “smart chain structure.” It’s a chain of blocks around the outside of the wheel connected by wire spokes to opposite sides of a central axle. By changing the distance between the two sides of the axle, the researchers were able to vary the length of the spokes, and thus the shape of the outer chain of blocks:
- To the increase the wheelbasethe spokes are shortened, forcing the chain block to move inward and creating a circular wheel to move fast.
- To the reduce the wheelbasethe wire spokes are lengthened and the chain block is loosened, allowing it to deform and pass through obstacles.
The researchers tested their adaptive wheel on a two-wheeled wheelchair and a four-wheeled vehicle. Each was able to adapt and navigate large steps and irregularly shaped rocks 1.2 times higher than the wheel radius. The wheelchair, which weighs 120kg, negotiates uneven and grassy terrain, although the chair does tilt worryingly forward.
Tests have shown that dust and particles get between the blocks of the smart chain and cause damage to the wheel. That’s why they are now working on adding a cover in future iterations. There is great potential in this adaptable wheel.
