Time for a new motorcycle helmet? That question is not so easy to answer. Certainly after an accident or a hard knock, you should get a new one even if there’s no visible external damage. That’s because the internal structure may be damaged. Even barring accidents, there comes a time when your old helmet is due to be replaced. But when is that? The answer depends primarily on individual usage. The general rule of thumb is that modern helmets should be replaced every five to seven years. That’s also the interval recommended by many manufacturers. Dekra, a testing organisation in Germany, recomm…