rear derailleur front difference question

[CNTO]

I'm building up a touring bike and I'm planning on using a Sugino triple (48-36-24).  I've noticed that most mountain rear derailleurs state a maximum front difference of 22.  With this crankset, the front difference is 24.  Will that be problematic?  Thanks!

[RussellSeaton]

No problem.  You can always exceed the stated limits by a few teeth and sometimes a lot.  Also depends on which shifter you are using for the front derailleur.  Bar ends and Ergo and down tube shifters will accomodate any difference since they are friction.  STI is more sensitive since its indexed.  Put a chain watcher on the seattube for extra insurance against dropped chains onto the bottom bracket shell.

[biker_james]

I use a road FD, but my crankset is now 22/38/48, and the Ultegra STI's shift it just fine. I could probably do something stupid if I tried, and screw it up, and that is why they make those limits. I don't use a chainwatcher, but its probably a pretty good idea.

[CNTO]

I'm using friction bar end shifters.  For my front derailleur, I will be using an IRD Alpina (for "compact" triples).  

I spoke to someone from Shimano customer service, and he said that the front difference (# teeth on big ring - # teeth on granny ring) doesn't matter for REAR derailleurs, as long as everything is within the specified maximum capacity.  If that's the case, then why do they even include a max front difference value for their rear derailleurs?

[RussellSeaton]

The rear derailleur capacity is made up of the difference in the chain rings (big-small) plus the difference in the cassette cogs (big-small).  Assume 52-42-30 crank rings and 11-34 cassette cogs.  52-30=22 + 34-11=23  = 45 total.  A mountain bike rear derailleur will be able to wrap up the amount of chain needed to go from big ring to big cog in all of the cogs using any of the rings.  The reason front ring difference is sort of important is because the cassette difference is sort of fixed at a maximum of 23, 11-34 cassette.  Can't get any bigger spread than that.  But for chainrings, you can substitute those easily and get any spread you want.  Shimano probably assumes the biggest spread possible in the cassette, 34-11=23, then figures what the maximum spread in rings is to get to the max their rear derailleur will handle.

With bar end shifters you can shift any difference in rings without any problems at all.

[bicyclerider]

your using a 22/38/48. Is the rear a 9 speed? If so what is the gearing, 13 to what?
Are you pulling a trailer with that setup?
Are you using it for steep climbs ( asphalt ) over a long distance?
Do you know what the cadence,and average speed was going uphill

Jean Andre Vallery
www.bicyclejournals.net
Sarasota Florida

[biker_james]

It is 9 speed, with an 11-32 cassette in the back. While I try to keep the cadence up, I can't say that I really monitor what it is any more. I used to have a computer with cadence, but got tired of the extra wires. I travel with front and rear panniers, and I have to say that I'm not sure if I've ever used my lowest gear while riding. We are touring on pavement, and the hills are whatever size we come across-nothing like Mont Ventoux though (yet). Its not that often I even use the small chainring, but it is good to know that you have lower gears left if you need them. I toured for a few years with the stock 30/42/52 and 11/32 cassette, but just felt like I would be more assured with lower gears-I hate being halfway up a hill, and knowing that you can't shift down if you decide you want to.