weather you want a single-impact or multi-impact resistant helmet. Multi-impact helmets can rebound in to their natural shape after a small to medium sized hit to them, but they don't absorb a large impact as well (read: as safely) as single-impact helmets, which have to be replaced after a significant impact. The reality is that single-impact helmets are a) less expensive, b) better protection from large and potentially serious impacts, and c) almost never have to be replaced if you don't get in a serious crash.