The tailgate mounted types are generally the cheapest option.
They strap to the edge of the door and have some contact pads on the metalwork to distribute the loads. As modern tailgates include large plastic roof spoilers, it’s becoming increasingly difficult to find tailgate mount bike carriers that will fit. Reference Autoexpress 2016
Note that the tailgate mounted racks are not offered by the vehicle manufacturers. This is because the chances of damage to the vehicle and the bikes are high. Straps and pads moving on the paint will scratch it.
Also there is not much security for these systems; a thief can cut the straps and walk away with your bike(s) pretty easily.
Probably best to have a small step or sturdy box to hand.
Stay away from the vacuum type. These are in process of being banned by International Standards Organisation (ISO).
Firstly you need a set of cross bars. The original manufacturers ones are best, as they will have tuned the fit. You can easily dent the thin roof panels with a poorly matched part. If you have some rails then the risk is a less, so an after market product is not so big a risk. Secondly you need to fit a cycle carrier or multiple carriers. The most common type clamp on the down tube. It is advisable not to clamp carbon frames.
People complain that you can hear wind noise from the fittings, but the main problem with the roof location is the load can be forgotten, with devastating effects at car park entrances and garages.
You see a lot of racks and bike carriers left on, which illustrates that the loading time and effort is high. Not removing the racks, is however costly at the pump, see RAC Fuel saving tips.
The towbar system is the most accessible, but they are also the most expensive: up to twice the price of roof rack types, when you take into consideration the cost of the towbar and electrics. Please see some explanation and guidance on different towbar options.
There are 2 types of towbar bike rack;
the hanging type are cheaper. The bikes are hung onto 2 prongs. If the bike’s top tube is not horizontal then the bikes will hang at an angle, which can be a problem for ground clearance.
You should fix lights and a number plate to be Legal.
the platform type where the wheels sit horizontally avoids the ground clearance concern. Lights and number plate are also integrated. Most fix to the towball which really limits load to 3 bikes.
The driver is responsible to make sure that any load is ‘safe.’ There have been cases of cycle falling from cars. In order to safeguard against this a secondary cable through the bike frames and carrier, to the vehicle is recommended. If this has a lock attached it may deter theft too.