How tolerant is the range of front derailleurs?

Say you have a front derailleur cited as having an 18T range, with a 42T main ring. According to the specification, the smallest inner ring can be 24T. But how strict is that? Is it possible to get away with 22T? In particular if the expected use of this ring is very rare: you climb that 25 feet of vertical rock face, and then shift back to one of the other rings.

Similarly for 28 versus 26, et cetera. For example, I have a bike here whose derailleur doesn't go lower than 28T according to spec, and that's the size of the inner ring. But when the chain is on that ring, there is a quite a bit of clearance left between the chain and the cage. I can push the chain down down so that it touches the midpoint of the chain-stay. Then it just about touches the bottom of the cage.

• Probably, it's worth a try. With a small ring the chain could rub on the link of the blades. The RD is more sensitive because it has to take up the slack of the chain. – Carel Jul 13 '16 at 8:12
• I think it is not important if the gear is used often or not- on every settings chain must not rub against derailleur. Otherwise it does not fit. You can measure if it fits - measure derailleur's vertical clearance taking into account vertical position change between lowest and highest gear, then measure "distance" between lowest and highest gear, add some surplus and you're done. – krzyski Jul 13 '16 at 8:49
• Hard to know without trying it. There appears to be a bit of wiggle room, but that's there to allow the chain line to go to the big and small rear cogs without rubbing the cage. – Criggie Jul 13 '16 at 9:26
• Figure it out. A 24T ring has a nominal diameter of 7.6". A 22T ring has a nominal diameter of 7.0". So the reduction in radius will be about 0.3", meaning the chain will exit the ring that much lower. – Daniel R Hicks Jul 13 '16 at 11:32
• @DanielRHicks Better calculation: 24T to 22T is a 2T loss. 2T = 1 inch. Since circumference is 2πr, the reduction is 1/2π inches, or only about 0.16" or around 4mm. It seems trivial. – Kaz Jul 13 '16 at 15:00