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FollowTheRulesOfTheRoad!
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Discussion Starter · #1 ·
Short question: Is a Shimano Dura Ace RD-7401 rear derailer compatible with a freewheel whose largest cog has 32 teeth?

Long story: When the Suntour freewheel on my Centurion Iron Man recently broke, the repair shop did not have a Suntour freewheel so they installed a Shimano freewheel (with a largest cog of 32 teeth), which left me without the option of index shifting. I bought a pair of Shimano Dura Ace SL-7401 downtube shifters and a Shimano Dura Ace RD-7401 rear derailer, and I'm having two problems getting the Dura Ace derailer to work:

1) There is a crack in the derailer (see the picture below) where the cable is mounted and I have not been able to get the cable to hold unless I route it on the other side of the bolt. When I use that routing I have not been able to get all the gear combinations to work smoothly.

2) Even with the B tension screw all the way in the top pulley bumps into the largest cog when the chain is on that cog.

The person who sold me the derailer said he was able to use it with the crack with the cable routed the normal way, but I wonder whether the derailleur is compatible with a freewheel whose largest cog has 32 teeth.

Thanks,
John Link
 

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johnlink said:
The link at velobase says the following:

Max Chain Wrap 28T
Max Cog Size 26

...which suggests that the derailer should not work with 28 teeth, but what is chain wrap?

This is what I was going to say

26
 

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I knew the info would be there, but I didn't read the page fully... Anyway:

http://www.sheldonbrown.com/harris/derailers-rear.html
A note about capacity:

Manufacturers have to assume that their customers are clueless, and will expect the chain to have some tension on it even in the bad gears where the chain is using small chainrings with small rear sprockets. Thus, the rated chain-wrap capacity is very conservative. A competent cyclist who uses the gears properly can generally exceed this by several teeth with no problem.

Most derailers also specify a maximum size rear sprocket. This is often a pessimistic value, based on the largest sprocket that is normally provided as part of that group.
 

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Have two bikes with DA 74xx and 12-28 rear, 53-39 front. One is a 1992 Gazelle in 753, the other an early eighties Nishiki. Shifts fine. The Gazelle have a lot of miles on that drive train.

Tightening the B adjuster screw on the RD all the way will help.

Shown with bad chainline (big ring - second biggest cog) to give a an idea of chain wrap
 

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The hole through which the cable fixing bolt goes looks like it has been spread open, probably by the tension on the derailleur from the cable itself. I don't know what could have caused the break initially, but aluminum is not something to be trusted for resistance to further breakage; it can fail suddenly.

You probably shouldn't have bought it. I can't imagine it's something that can be repaired, especially because the part is going to be under tension, but I'd replace the derailleur, especially if the capacity is exceeded by the cogs on the freewheel or freehub. If the cable attachment goes while you're riding, and it gets into the spokes, you might be in trouble.
 

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I have a 7400 Dura Ace RD that I've run a Shimano 14-26 freewheel without any problems. But it seemd like this was near the upper limit. This was with 53-39 Chainrings. These 7400 (01) derailleurs were never designed for large capacity freewheels.

The broken bolt mount is not easily fixed. You'd be better off starting over with a touring (long cage) RD that was designed for big jumps in teeth to get to the 32T easily.
 

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FollowTheRulesOfTheRoad!
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Discussion Starter · #11 ·
JML said:
The hole through which the cable fixing bolt goes looks like it has been spread open, probably by the tension on the derailleur from the cable itself.
I hadn't thought about that, but I think you're right. I've included another picture looking into the hole, without the bolt.

I don't know what could have caused the break initially, but aluminum is not something to be trusted for resistance to further breakage; it can fail suddenly.

You probably shouldn't have bought it. I can't imagine it's something that can be repaired, especially because the part is going to be under tension, but I'd replace the derailleur, especially if the capacity is exceeded by the cogs on the freewheel or freehub. If the cable attachment goes while you're riding, and it gets into the spokes, you might be in trouble.
Even if I can get the cable to hold for a while I do wonder about a sudden failure.
 

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FollowTheRulesOfTheRoad!
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Discussion Starter · #12 ·
sewup dude said:
The broken bolt mount is not easily fixed. You'd be better off starting over with a touring (long cage) RD that was designed for big jumps in teeth to get to the 32T easily.
Actually my max cog on the Shimano freewheel has 28 teeth, not 32. When I wrote that I needed 32, I had miscounted my max cog, mistaking the 28 it actually is for 32 (I had counted 8 teeth in 90 degrees when in fact there are 7).
 

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There's nothing worse than having something fall between spokes and the frame while you're riding. That cable could release and fly around, snagging a spoke, resulting in your taking a long flight.
 

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good move ditching the broken unit. someone should be ashamed of selling a broken part that is crucial(my opinion). those were race specific parts. ment for tight and high gearing. you can only cheat them so far, til you get premature wear and or failure.
 
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