The price of complementary goods can affect the demand for a good. In this case cycle helmets are complementary to the cycle. They have a relationship which exists through price. Cycle helmets complement cycles just as cars complement petrol. A rise in the price of one will cause a decrease in the demand for the other.

Say if the price of cycles increased drastically for some reason, this will have an effect on demand for cycle helmets. If cycle prices increase enough to make them too expensive for most people to buy, less will be bought. Cycle helmets are bought to be used with a cycle, so if fewer cycles are bought the demand of cycle helmets will also decrease.

Tastes and preferences will also be a factor, what people like could influence the demand. If they dislike the `look' of cycle helmets or the `feel' of them...