Sounds like a way to get some really low gears. He might need a longer chain. I like to set my bikes up so the chain is long enough to let me shift on to the large chainring and large cog, in case I mistakenly shift into that combination while oxygen-deprived. A new 9spd chain is what, $20?
It should be obvious that an MTB RD that will handle at least a 22T difference on a triple will handle a 16T difference with a compact. Just don't get an XTR RD, the cable pull is backwards.
I think you'll find the big jumps between the cogs isn't a great way to go for road climbing.
Here's how to set the chain length:
Two simple tests will determine if the chain is the correct length. First, it must not hang loose in the little ring, little cog combination. If there is no tension on the chain in the little ring, little cog combination; remove two links (one inch) at a time, until there is. When the ends of the chain are brought together, some movement of the lower pulley should occur, indicating tension is being applied. Two more links (another inch) may need to be removed, beyond the point of absolute minimum tension, to keep the chain from rubbing on itself or the chain guide tab as it passes under the upper derailleur pulley. If you want to see how much lower pulley movement will occur, without removing the extra inch of chain, shift up four teeth (11 to 15 or 12 to 16). This has the same effect as removing two links. Once this is done, the chain is set to the maximum useable length. Removing additional links will do nothing but reduce the derailleur's capacity.
Second, the chain must be long enough to avoid over-extending the rear derailleur when shifted to the big ring and biggest cog combination. If the chain is set to the maximum length as described, it should always pass this test, unless your setup exceeds the derailleur's stated wrap capacity. If you deliberately exceed the derailleur's capacity and the derailleur is over-extended in the big ring/largest cog combo, then you must either avoid that combo or add another inch and avoid using the little chainring and the smallest 3 or 4 cogs (since the chain will hang loose).
Can be done. Been done before. Don't see any reason why not this time. Pretty sure I could tow a car in a 1:1 gear. That'd be fun.
Oh, he may need to check for chain wear more often. The more tension is put on a chain the faster it wears. Since small chainrings amplify the leverage that the rider has at the crank upon the chain, they amplify chain tension even more.
I'm pretty sure that most high-end shimano mtb rear mechs are available in high-normal or low-normal (rapid rise) formats. Care should be taken to get the desired one.
Considering I run medium cage rear mechs on my mtn bikes without any major problems (11-32s and triples up front with 22 tooth difference from top to bottom) then a medium or long should be able to take up the slack of any gear combo.
"Considering I run medium cage rear mechs on my mtn bikes without any major problems (11-32s and triples up front with 22 tooth difference from top to bottom) then a medium or long should be able to take up the slack of any gear combo."
That was EXACTLY what I was thinking, exactly.
Model is Shimano Deore LX M570 which is now 'called' M580, I believe.
I've used the Deore LX with 11-34 gearing on a road bike. I was using a 39 small chainring though. Might even be able to get away without lengthing the chain if you are careful, and this isn't permanent. Just make sure you stay out of the 50 when using the two or three biggest cogs.
That setup should work fine. Is your friend trying to ride up Mt Washington?
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