Complex sentence: In the early 1970s the only proven method to secure one's bicycle was by the use of case hardened security chain with hexagonal links, but some cyclists were making the mistake of using inexpensive chains or cables that could be breached by thieves using commonly available tools.
Split into simpler sentences: In the early 1970s the only proven method to secure one's bike was by the use of case hardened security chain with hexagonal links. But some cyclists were making the mistake of using inexpensive chains or cables that could be breached by thieves using commonly available tools.