You can basically wear anything when biking, although some clothing makes more sense than others. For example, wearing shorts and a thin t-shirt won't help protect you from the wind or from road rash if you have an accident. However, over the years, most bikers have gravitated towards a particular style. Sometimes, this style fits with the stereotypical look people expect bikers to have, while other times it's only somewhat similar to others. You can put your own special spin on the biker style, but there are some basics you might want to start with.
Most bikers have a favorite jacket that they wear when riding. You'll want this jacket to be warm and durable since it has to protect you from the elements and from the road if you crash. You also want it to be fairly stylish. Many bikers like leather because it's very tough and won't rip easily, but there are other materials out there that also work very well. Once you've selected a jacket, you might want to cover it with embroidered patches. Many bikers love adding biker patches that reflect their beliefs, likes, and style. If you went with a leather jacket, you'll have a bit more work to do since most patches have to be sewn to leather instead of ironing them on.
Bikers also like to wear specific jewelry. There are a lot of different items made for bikers, but there are some common themes. Many biker items are silver, and many incorporate bike design into them. This is especially true for pieces made for men. A bracelet might be made out of chain, for example, or there might be an engine block somehow worked in to the design of a pendant.
