This is because the engines on these motorcycles usually generate high-amplitude vibrations that may be transmitted through their chassis and handlebars. Such vibrations, within certain frequency ranges, could be too intense to the optical image stabilisation (OIS) and closed-loop autofocus on an iPhone even though both features were designed to be fairly durable. OIS was first implemented on iPhone 6 Plus while the closed-loop AF system is a standard feature on iPhone XS and later, including second-generation iPhone SE. Apple noted that the risk is much lower on motorcycles with small-volume or electric engines but the company still recommends users to utilise vibration dampening mounts to protect their iPhone, as well as its OIS and AF features on those motorcycles. Meanwhile, iPhone users may also want to avoid exposing their devices to strong magnets. In case you missed it, Apple had said back in January that strong magnets could disrupt the magnetic sensors on the OIS and closed-loop AF systems on the iPhone, which would then affect their accuracy as well as movement ranges. (Source: Apple. Image: iFixit.)
