# maximum front difference value for REAR derailleurs



## CNTO (Aug 4, 2006)

I've noticed on the Shimano site that most of their mountain REAR derailleurs state a maximum front difference (# teeth in big ring on crankset - # teeth on small ring on crankset) of 22 teeth. I spoke to a tech person from Shimano, and he gave me the impression that the max front difference doesn't matter on the rear derailleurs (as long as you are within the maximum capacity. If that's the case, then why do they bother adding specs for max front difference for their rear derailleurs?

I'm curious, since I'm planning on using a 48-36-24 triple (24t front difference).


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## orange_julius (Jan 24, 2003)

*It matters*



CNTO said:


> I've noticed on the Shimano site that most of their mountain REAR derailleurs state a maximum front difference (# teeth in big ring on crankset - # teeth on small ring on crankset) of 22 teeth. I spoke to a tech person from Shimano, and he gave me the impression that the max front difference doesn't matter on the rear derailleurs (as long as you are within the maximum capacity. If that's the case, then why do they bother adding specs for max front difference for their rear derailleurs?
> 
> I'm curious, since I'm planning on using a 48-36-24 triple (24t front difference).


It matters. The rear derailleur is the component that adjusts the chain tension as move across your gears, front and back. If the front difference is too large, the rear derailleur may not be able to properly adjust for it, as you shift using your front derailleur.

This is why owners of compact crankset with 50-34 = 16 tooth difference find that they often drop their chain, more often than the standard crankset with 53-39 = 14 tooth difference. The rear derailleur basically yanks the chain clean off the chainrings when they shift from the 50 to the 34. 

The problem is worse then you switch from the 50 to the 34 while your rear derailleur is in a large sprocket, say a 25. Compact owners will do better by judiciously switching their rear gear to a smaller sprocket before switching the front from the 50 to the 34, although when they are panicked, losing momentum, and climbing a steep hill, people often forget .


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## brblue (Jan 28, 2003)

*Measurement issue*



orange_julius said:


> It matters. The rear derailleur is the component that adjusts the chain tension as move across your gears, front and back. If the front difference is too large, the rear derailleur may not be able to properly adjust for it, as you shift using your front derailleur.
> 
> This is why owners of compact crankset with 50-34 = 16 tooth difference find that they often drop their chain, more often than the standard crankset with 53-39 = 14 tooth difference. The rear derailleur basically yanks the chain clean off the chainrings when they shift from the 50 to the 34.
> 
> The problem is worse then you switch from the 50 to the 34 while your rear derailleur is in a large sprocket, say a 25. Compact owners will do better by judiciously switching their rear gear to a smaller sprocket before switching the front from the 50 to the 34, although when they are panicked, losing momentum, and climbing a steep hill, people often forget .


I think you should add the max front cap to the usual max rear cap (32-11 ?) to get a total cap.
I.E if you want to use a front difference of 24 teeth you should use in the back a max difference smaller with 2 teeth (30-11)
That's for the chain not to hang loose in the small/small or be too stretched out in the large/large (although you won't use those combinations)

As an example, this will prevent u to use a short cage der with a triple crank (given the fact that u only know the capacity of the der and no cage length/model info)

Good luck,
br


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## C-40 (Feb 4, 2004)

*no such thing...*

The only thng that matters is the total wrap capacity. You have to add the front difference to the rear difference to get the total. If the total does not exceed the stated wrap capacity, then you'll have no problem. I run a setup that exceeds the 36T wrap capacity of a campy medium cage RD (36T), with my 53/39/28 and 12-25 (a 38T wrap). This never causes a problem, because no one would ever use the 28/13 or 28/12, where the chain will hang loose.

I'll have to diagree with orange_ julius. Shifting from a large cog in the 50T down to the 34T on a compact should never be a problem, because there is plenty of chain tension at the start of this shift. The rear deraileur tension does not "yank the chain clean off the chainrings". That's pure conjecture. It may indeed be wise to shift 2-3 cogs smaller before making the chainring shift, but what this does is improve the chainline, preventing the dreaded chain drop. It has nothing to do with excessive chain tension. The same thing can be seen on a triple, shifting from the 39T to the 30T. It's not a large jump and the tension is not high in the 39T and even the largest cog, but if the chain tends to drop off to the left, despite proper adjustment of the FD, performing the cog shift first can cure the problem. The real culprit in this problem is probably the shifting ramps being a bit too agressive. Short chainstsy also seem to make this problem more pronounced. I never has this problem on a triple until I installed one ona Cervelo R3 with ultra short chainstays. I could not get the chain to quit dropping unless I was in the third from largest cog (or smaller) when the shift to the little ring was made.


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