Selecting bike racks for cars is a tedious job and need lot of research as there are hundreds of manufacturers out there in the market with various designs and materials. To mount more than one bike of capacity approximately 100 kgs you need to select a bike rack made of heavy duty material such as iron. Some of the racks are made of steel and light weight aluminum materials that can be used ruggedly to mount two or three bikes. So basically the weight of the bikes also plays an important role in selecting the material of the rack. But most of the car owners prefer aluminum alloy or carbon steel material bike racks as they are light weight and easy to install.
While installing the roof mounted racks you need to consider the height of the rack so that it fits into your garage or where ever you are parking the vehicle. It depends on your choice that you can remove the bike racks for cars after the usage or keep them mounted as it is. But the material used should be rust free and should have more durability for long time durations. Generally the hitch mounted bike racks are stable and convenient to transport with capacity up to five bikes.