Double or Triple rear mech for single chainring?

[vbc]

I'm rebuilding my redundant Kaffenback as a commuting hack with flat bars, 9 speed rear mech and a triple chainset with the outer and inner rings removed, a 39T ring on the middle.

My first question is - do I need a triple (long cage) rear mech - as the single chainset is in effect a triple - or would I be better off with a double (short cage)?

I have a low mileage Shimano 105 long cage rear mech if needed, although I might be building up an Uncle John cross bike with most of an Ultegra/105 9 speed triple group as I upgrade my best bike to Ultegra 10 speed triple - the 105 rear mech would be needed there.

So should I buy a new Sora rear mech to blend in with the low profile, mostly re-used parts on the Kaff and save the 105 mech for the planned Uncle John project?

Second question - can I get away with a fourth bike (the UJ project) without Mrs C noticing?

 

[GrasB]

Generally you want the shortest arm mech that'll give you your required tooth capacity. This means less chain to pull round & also the mech tends to respond more quickly to slack chain.

 

[RecordAceFromNew]

Second question - can I get away with a fourth bike (the UJ project) without Mrs C noticing?

__________________

Old age & treachery will always defeat youth & skill.

You've answered your own question, depending on who is older and more treacherous.

 

[andrew_s]

If you think you will ever put an MTB cassette on (>=30T), you'll need an MTB rear mech.