Imagine if it is unnecessary to concern the TOTAL CAPACITY of derailleur along with having a wider range of gear ratio options. Can cycling be more unstrained?
There is a limit to cyclist’s physical power; thus, the ideal situation would be having the widest gear range, and amplify power output to be conducive to gradients and cycle further. The leverage is the range of gear ratios of the drivetrain system. In the existing chain drive system, "TOTAL CAPACITY" is the factor related to gear range and has to be.
A chain drive bike needs a chain, which has the exact circumference to wind through all size of your gears, to shift smoothly. By exact length, it has to be long enough to accommodate the largest chainring and sprocket, and still has extra length for the derailleur to stretch and pull.
When shifting, the chain will move between different sizes of gears. According to each gear ratio, the chain should be correctly tensioned in case it slips due to vibration. The task of rear derailleur is to pick up extra length of the chain between ratios through being stretched or pulled. Total capacity is the maximum tooth difference (in the drivetrain) that the rear derailleur can accommodate, and calculated as “adding the chainring tooth difference to the sprocket tooth difference”.
The specification of rear derailleur would indicate total capacity of it. When selecting a drivetrain, you should take the gear size into account according to the capacity of derailleur. Exceeding the capacity makes the derailleur difficult to provide appropriate tension for the chain to drive efficiently and might cause skipping.
As mentioned previously, what will happen if the gear range is wider? The biggest problem is that if the cage has to absorb more chain slack, the length of the cage must be longer and its actuation angle would be enlarged. Lengthening the cage would lead to gaining weight of the components. Furthermore, the ground clearance will be reduced when fully extending, which would bring about the rear derailleur to work at risk. When the actuation angle is larger, the derailleur jockey wheel would collide with the sprockets, causing destruction to parts.
Therefore, although it would be ideal to have wider gear range, choosing the existing chain-drive bike will have to compromise that the options of gear ratio might be limited.