Technical Gubbins... continued
Deraileur Limit Screws
Those screws marked H and L on a deraileur are limit screws. They physically limit the motion of the deraileur, functioning as adjustable hard stops of the outer range of the deraileur’s motion. One prevents it from over shifting the chain between the cassette and into the spokes, the other from pulling the chain past the smallest cog. The L for “low gear” screw limits the motion of the deraileur past the largest cog, the H for “high gear” screw limits the motion past the smallest cog.
Rear Deraileur Capacity
Rear deraileurs come in a few different cage lengths, all with a number listed as capacity to determine what gear range is acceptable for a given model. Or more accurately, what cage length is required for a given gear range.
Capacity = (biggest chainring - smallest chainring) + (biggest sprocket - smallest sprocket)
On a spec sheet, capacity is a maximum number. For an example, consider a road bike with a 52/39 chainring setup and an 11-25 cassette.
(52-39) + (25-11) = 27
This means that a short cage rear deraileur with a listed capacity of 29 would be the wise choice. Add a 30t chainring for a triple up front, and you’ll need a longer cage rear derailleur with a higher listed capacity to make up for the added chain slack between the largest and smallest front chainrings.