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Capstone project is an adapter that goes between a bike's pedal and crank

It may come as a surprise, but having limbs of different lengths is actually quite common. Most of the time, however, this discrepancy is slight (think a centimeter or less) and people can easily compensate.

That’s not the case for Andrew Mills. The 9-year-old Provo boy was born with a condition where his right leg grows much faster than his left, causing a gap that has reached nearly 4 inches at times.

On top of regular procedures requiring plates and screws in his leg to slow the growth, the condition — which also affects his leg muscles and range of motion — makes a lot of things harder for Andrew, such as running (“It’s more of a skip-hop,” his mother says), jumping and biking. 

Mechanical engineering students at BYU are helping make one of those things easier for Andrew and the 4.5 millions Americans with a similar condition. They’ve created a special adapter to a bike pedal and crank system that allows him to ride smoothly and painlessly and keep his foot on the pedal.

“He was beaming when he got off the bike after the first time using the adapter,” said his mother, Rachel Smith Mills. “The funny thing was, he was so excited because his knee was tired from him actually being able to use it on the bike.”

The student-created system goes in between the crank and the pedal and provides a drop arm to the existing crank. The device reduces the size of the circle the leg needs to cycle, making it easier to move the pedal around and requiring the knee to bend less. The invention means Andrew’s adapted way of riding, where he swings his right leg off the pedal with each cycle, is no longer needed.

“I can be faster and I am able to go the full cycle,” Andrew said.

The students devised the system as part of the BYU Engineering Capstone program, which takes on around 30 projects each year sponsored by companies and institutions across the country. While the student projects are often for big-name companies like Boeing, John Deere and Union Pacific, occasionally students take on projects like this one for Andrew. (Like this wheelchair and this bike trailer.)

“I’ve loved working on a humanitarian project,” said senior mechanical engineering student Trent Porter. “There’s a lot more of a connection with people. Meeting Andrew and seeing how happy he was to use the system we’ve created carries so much meaning.”

The students are starting with Andrew, but hope their device will help many people with limb-length discrepancies, knee flexibility issues, osteoarthritis or other disabilities. They are creating a website where anyone can enter their dimensions and receive the parts and instructions needed to make a similar bike-pedal adapter.

“Every single person I’ve talked to about this project knows someone this adapter could help,” Smith Mills said. “The work and ingenuity and creativity of these BYU students is going to help so many more people than they can even imagine. It’s so special to be a part of it.”

Added Andrew: "I think it is so cool that I can help people that have the same problem that I have."