The weight, and therefore the speed, of all types of shuttlecock can vary a great deal due to the amount of moisture that has been absorbed over time. Feathers, cork and nylon moulding materials all absorb or lose water depending on the relative humidity of the storage conditions. A shuttle stored in damp conditions can easily vary by as much as 0.2g compared to dry air conditions. This is a lot for a shuttlecock and can turn a medium speed into a fast or slow speed until the moisture content has returned to normal and this can take half a day or more. Combined with the affect of air temperature and relative humidity during play, shuttlecock speed can be difficult to account for.
Additionally the hardness of the base material has a significant affect; cork has natural variation which can make a large difference in speed but plastic foamed materials have considerable variation due to process. Whatever the material, they all soften and lose resilience as they are used. Having discovered how much hardness affects the speed we are now testing for base hardness and factoring this into the weight/speed calculation. Feather shuttlecock producers are using launchers to test speed but unless the shuttle is hit the factor for speed off the racket is not accounted for. All in all, using a bit of tree bark can't be the way forward so we are developing a completely new composite material which will replicate the performance of cork without all the inconsistencies; we hope to complete this in 2019.
Hi, I'm the designer of the revolutionary Bird2 shuttlecock. Let's change Badminton for the better, together; all comments and feedback are essential to perfecting our products.