Recall the popular SUVs like TOYOTA FORTUNER or HONDA CRV. Do you find a spare wheel on their tailgate? The answer is no, the spare wheel is mounted under the body and the reason is very simple as it may increase the effort of lifting/closing of tailgate if mounted on the tailgate. You may say that we can design a more powerful gas spring/strut which will take care of the spare wheel load. But the real trouble arises when you take out the spare wheel probably for puncture repair or so. Now you have to put much more effort to close the tailgate as there is no spare wheel weight on top to assist closing, and the powerful spring/strut what you designed for the tailgate with spare wheel load will no more be realistic to use. Now here I have a very simple and basic solution.
Figure1 shows the tailgate mounted with spare wheel on tailgate. The gas strut/spring is hinged at point A on tailgate and point O on D-pillar. Here gas strut will hold the weight of both tailgate and spare wheel.
Now when the spare wheel is removed from top of tailgate, the same strut will be holding only the weight of tailgate. Thus you need to put an extra effort to pull down the tailgate. To overcome this difficulty, a separate guide mechanism is used as shown in fig.2(red colored).
Here the guide allows the strut to slide from point A to point B as soon as the spare wheel is taken off.
This decreases the resisting torque applied by gas strut to hold the tailgate.
Calculation and analysis has shown that there is 30%- 40% reduction in effort with this mechanism and we know that the weight of spare wheel contributes to about 35% of the total weight of tailgate. so the user have not felt any difference in applied force to close the tailgate irrespective of spare wheel presence.
DOUBT: How gas strut can be shifted from A TO B without compressing the gas strut as length OA seems greater than OB?
ANSWER: The tailgate profile is never linear so as the arc AB, so with intelligent packaging such that point A & B lie on the same circle with radius equal to the length of strut we can shift gas strut from point A to B without compressing as shown in fig.3.
BENEFIT: Vehicle will have more head room or boot space at the rear end as the rear floor can be lowered.
With this mechanism you will surely see more SUVs with spare wheel mounted on tailgate instead of underbody.
NOTE: All the SUVs that you see with spare wheel on tailgate has side opening tailgate not uplifting type.