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The question of Shimano cassette spacing and, in particular, the Dura Ace 74xx series derailleur comes up frequently…
Thanks to Quattro Assi 07 for asking the most recent question and allowing me to screw off at work…
I’ll start with a quick primer on Shimano cog spacing…the below numbers apply regardless of model/year cassette
The amount listed is the distance between cogs( center to center)
7 speed cassettes- 5.0mm
8 speed cassettes- 4.8mm
9 speed cassettes- 4.34mm
10 speed cassettes- 3.95mm
Based on the above, 7 speed shifters need to move the derailleur 5.0mm per click in order to have smooth shifting. 8 speed shifters need to move the derailleur 4.8mm per click, etc….
Now we get to the rear derailleur.
All Shimano road rear derailleurs (except Dura Ace 7400 series) have the same design and parallelogram. That means that any( except Dura Ace 7400 series) Shimano rear derailleur will shift the above listed spacing if attached to the appropriate speed shifter.. Example: the Sora RD has the same Parallelogram as the Dura Ace 7900 RD, etc..One click on 9 speed shifters will move the rear derailleur 4.34mm regardless of the model derailleur( again, except Dura Ace 7400 series)
The Dura Ace 7400 series rear derailleur was a different design and parallelogram than any other Shimano rear derailleur. Because of this different design, one click on the typical Shimano shifter will move the 74xx RD more than all other Shimano rear derailleurs…In order to accommodate the unique geometry , the 74xx RD needed special shifters to move the rear derailleur the proper amount for the above distances between cogs. In the case of 8 speed, Dura Ace 7402 shifters will move Dura Ace 7400 series RD, 4.8mm. However, if you were to connect other Shimano 8 speed shifters to a 7400 series RD, it will move the derailleur more than 4.8mm causing an over shift every click…This is why the Dura Ace 74xx series rear derailleur will only work with certain shifters..
To confuse matters even more, if you connect Shimano 9 speed shifters to a Dura Ace 74xx series derailleur, it will move the derailleur 4.8mm which happens to be the proper distance for an 8 speed cassette…So if you are using a Dura Ace 74xx series RD, you need either Dura Ace 7402 shifters or any Shimano 9 speed shifters to shift for 8 speed.
All of the above information is confirmed by Shimano in their service instructions.
There are also alternate “B” cable attachments that allow 9 speed shifters to shift 8 speed but we can discuss that at another time…
If anyone has similar info on Campy, I would love to see it
Thanks to Quattro Assi 07 for asking the most recent question and allowing me to screw off at work…
I’ll start with a quick primer on Shimano cog spacing…the below numbers apply regardless of model/year cassette
The amount listed is the distance between cogs( center to center)
7 speed cassettes- 5.0mm
8 speed cassettes- 4.8mm
9 speed cassettes- 4.34mm
10 speed cassettes- 3.95mm
Based on the above, 7 speed shifters need to move the derailleur 5.0mm per click in order to have smooth shifting. 8 speed shifters need to move the derailleur 4.8mm per click, etc….
Now we get to the rear derailleur.
All Shimano road rear derailleurs (except Dura Ace 7400 series) have the same design and parallelogram. That means that any( except Dura Ace 7400 series) Shimano rear derailleur will shift the above listed spacing if attached to the appropriate speed shifter.. Example: the Sora RD has the same Parallelogram as the Dura Ace 7900 RD, etc..One click on 9 speed shifters will move the rear derailleur 4.34mm regardless of the model derailleur( again, except Dura Ace 7400 series)
The Dura Ace 7400 series rear derailleur was a different design and parallelogram than any other Shimano rear derailleur. Because of this different design, one click on the typical Shimano shifter will move the 74xx RD more than all other Shimano rear derailleurs…In order to accommodate the unique geometry , the 74xx RD needed special shifters to move the rear derailleur the proper amount for the above distances between cogs. In the case of 8 speed, Dura Ace 7402 shifters will move Dura Ace 7400 series RD, 4.8mm. However, if you were to connect other Shimano 8 speed shifters to a 7400 series RD, it will move the derailleur more than 4.8mm causing an over shift every click…This is why the Dura Ace 74xx series rear derailleur will only work with certain shifters..
To confuse matters even more, if you connect Shimano 9 speed shifters to a Dura Ace 74xx series derailleur, it will move the derailleur 4.8mm which happens to be the proper distance for an 8 speed cassette…So if you are using a Dura Ace 74xx series RD, you need either Dura Ace 7402 shifters or any Shimano 9 speed shifters to shift for 8 speed.
All of the above information is confirmed by Shimano in their service instructions.
There are also alternate “B” cable attachments that allow 9 speed shifters to shift 8 speed but we can discuss that at another time…
If anyone has similar info on Campy, I would love to see it