One of our major challenges was making the code for the bluetooth module functional. Although we knew how the data was sent from the Android to the Arduino board figuring out communication the other way round was tricky.
Another one of our challenges was to figure out how the reading from the accelerometer/gyroscope could be converted into acceleration in terms of 'g' (unit of acceleration of gravity).
Coming up with a design for our final helmet required us to deal with making a wearable that could actually be wrapped around any helmet, so we spent quite some time researching how our prototype could be made such that it could be attached to any helmet.
Figuring out the working of Raspberry Pi in unison with the Pi camera was another challenge that we struggled with.