A parallel spindle has a uniform cylindrical shape, maintaining the same diameter throughout, and is typically used with ball or cylindrical roller bearings. This design is best suited for lighter loads, such as utility trailers and car haulers.
A tapered spindle, on the other hand, has a conical shape, wider at the base and narrowing towards the end. This design helps distribute weight more evenly, reducing stress on the axle and bearings, making it ideal for heavy load trailers like flatbeds and tankers.