why the rear der. difference with double or triple?



upstateSC-rider

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Jul 27, 2004
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I'm in the process of looking for a road bike (really love the Roubaix Comp) and noticed that the rear derailleur for the double chain-ring is a short cage and it's long cage for the triple chain-ring. Just wondering why the front rings would affect the rear der. :confused:
I'm crossing over from mtb'ing but still learning the new hardware.

Thanks.
Lou
 
upstateSC-rider said:
I'm in the process of looking for a road bike (really love the Roubaix Comp) and noticed that the rear derailleur for the double chain-ring is a short cage and it's long cage for the triple chain-ring. Just wondering why the front rings would affect the rear der. :confused:
I'm crossing over from mtb'ing but still learning the new hardware.

Thanks.
Lou
Third ring in front requires longer cage to wrap more chain. Same reason mtb triples use long cages.
 
boudreaux said:
Third ring in front requires longer cage to wrap more chain. Same reason mtb triples use long cages.
By the third ring in the front, you mean the smallest ring, right? Its usually called the first ring.
 
The range that a long cage derailleur has is greater than a short cage derailleur. Touring bikes sometimes use road cranks and MTB cassettes and rear derailleurs because of this wider range.
 
keanu122 said:
By the third ring in the front, you mean the smallest ring, right? Its usually called the first ring.
Actually, it's called the inner or granny ring; don't know anyone who calls it the first ring. Maybe a service manual calls it ring #1, but that's confusing when the terms inner, middle and outer are obvious to all.

In the context of the question, boudreaux's use of the term "third ring" was perfectly clear to me. After all, the difference between a double and a triple is that a triple has a "third" ring; they both have "first" rings.

Doesn't matter which one the "third" ring is for this discussion anyway. The only important thing is the total difference that the RD has to wrap....eg 53-30 = 23 teeth.

Welcome to the Forum.
 
dhk said:
Doesn't matter which one the "third" ring is for this discussion anyway. The only important thing is the total difference that the RD has to wrap....eg 53-30 = 23 teeth.
It does matter. The outer rings on both triple and double cranks have aprox. the same number of teeth. And as we all know, the chainring that can cause the trouble is the granny gear. I was trying to clairify which chainring would actually cause trouble if you were to use a double rear derailleur with a triple crank.
 
keanu122 said:
It does matter. The outer rings on both triple and double cranks have aprox. the same number of teeth. And as we all know, the chainring that can cause the trouble is the granny gear. I was trying to clairify which chainring would actually cause trouble if you were to use a double rear derailleur with a triple crank.
Well, you actually don't even have to use a triple or LC RD with a triple in many cases. Shimanos wrap specs tend to be pretty conservative,and even if the short cage won't wrap all the chain, one can judge it by just staying out of the small cogs when in the GRANNY or SMALL ring. I have a short cage 8 speed DA RD on a triple with 52/39/30 rings and 13x23 cassette and it wraps it all.
 
keanu122 said:
It does matter. The outer rings on both triple and double cranks have aprox. the same number of teeth. And as we all know, the chainring that can cause the trouble is the granny gear. I was trying to clairify which chainring would actually cause trouble if you were to use a double rear derailleur with a triple crank.

Agree the granny ring would cause trouble if the chain wasn't shortened.
But, wouldn't someone installing a triple shorten the chain to accomodate the new low gear they've added? After taking out the links, the problem then moves to the big ring-big cogs where the chain is now too short. Of course, most of us can live without those combinations anyway.