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Old 02-03-12, 06:12 PM   #1
masi61
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Can Dura Ace 7401/7402 rear dérailleur wrap chain to run close ratio triple?

So I'm road testing my new Retroshift shifter pods on my 126 mm rear triangle road bike. I'm using Shimano 105SC 7 speed levers with an older Deore LX rear dérailleur. The crank is an Ultegra 6503 triple. The Deore LX rear is not as crisp as I would like. And this is where I started think: it's either the 105SC levers, or it's a set up issue with the Deore LX rear. I'm running a Dura Ace 7400 Uniglide 12-25 7 speed cassette.

Here's an idea: What if I convert the cassette to a 12-21, change the shifter to a Dura Ace 7401, change the rear dérailleur to Dura Ace 7401. Would the rear dérailleur have enough chain wrap to keep the chain taut enough with the 30 tooth chainring on the Ultegra triple crank? Thanks for any opinions you can offer on the advisability of this build, from Bill.
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Old 02-04-12, 06:36 AM   #2
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Can somebody who has read this post render an opinion? I hate to bump my own post. Oh, well, perhaps this belongs in mechanics forum, but with threads on making long cage derailleurs for Campy, why not early Dura Ace?
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Old 02-04-12, 06:41 AM   #3
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Max chain wrap on the DA is 28T, according to velobase. You don't mention what your large chainring is, but assuming it's 50T that would be a 20T difference up front, leaving you an 8T allowance in back. Your proposed 12-21 cassette would have a 9T difference, so you might be able to get away with that.
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Old 02-04-12, 10:49 AM   #4
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Big chainring is a 53. Sounds like its not quite going to work. How do you calculate maximum chain wrap?
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Old 02-04-12, 10:59 AM   #5
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Originally Posted by masi61 View Post
How do you calculate maximum chain wrap?
Those are specs provided by the manufacturer.

If you're asking how to calculate chain wrap it is:

(Largest chain ring teeth minus smallest chainring teeth) + (largest rear cog teeth minus smallest rear cog teeth)

So, in your case, your chain wrap is 32.

53-30=23
21-12=9
23+9=32

For what it's worth, I have seen a long cage mounted to a RD-7400 body. I'd assume this could also be done on a 7401 or 7402.
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Old 02-04-12, 11:16 AM   #6
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Can you describe what you mean by 'not as crisp as I would like'. Do the 105sc have a friction mode, if so will it shift as fast as you would like? If so perhaps it is the indents on the 105sc that are worn or not 'crisp'? Also what chain and cassette are you using? Perhaps if older or worn they chain will not be getting help up onto the smaller cogs like a modern cassette with ramps?...

Just thoughts. Any help?

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Old 02-04-12, 01:07 PM   #7
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Originally Posted by ColonelJLloyd View Post
Those are specs provided by the manufacturer.

If you're asking how to calculate chain wrap it is:

(Largest chain ring teeth minus smallest chainring teeth) + (largest rear cog teeth minus smallest rear cog teeth)

So, in your case, your chain wrap is 32.

53-30=23
21-12=9
23+9=32

For what it's worth, I have seen a long cage mounted to a RD-7400 body. I'd assume this could also be done on a 7401 or 7402.
Thanks for breaking that down. Looks like, since the Dura Ace chain wrap is maxed out at 28, I have a need with at least 32. It would be tempting to disassemble the cage from a Shimano long cage rear derailleur and retrofit it onto the RD-7400 body. I'm almost tempted to try it myself. It would be great to get a little You Tube tutorial on just such a retrofit.
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Old 02-04-12, 01:24 PM   #8
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Originally Posted by retroshift View Post
Can you describe what you mean by 'not as crisp as I would like'. Do the 105sc have a friction mode, if so will it shift as fast as you would like? If so perhaps it is the indents on the 105sc that are worn or not 'crisp'? Also what chain and cassette are you using? Perhaps if older or worn they chain will not be getting help up onto the smaller cogs like a modern cassette with ramps?...

Just thoughts. Any help?

Cheers,

The Goats
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Thanks for your interest RetroShift Goat!

A few answers to your questions on why it is not as crisp as I like:
1) When I say it is not as crisp as I would like I can just tell you what it did when I rode 43 miles on 2 different occasions this last Early February week. I can hear a faint clattering sound in certain gears when riding down the road. Also, the upshifts are sometimes late but for some odd reason the downshifts are also sometimes late. I thought it might be related to the "B tension adjustment screw". This pulls the upper jockey pulley closer to the cogs, but the setting must be just right apparently. I tried the setting with the pulley up tight to the largest cassette cog & found some clattering from that so I let it out some. The lazy up shifts and downshifts were more prominent when in the big chainring. The middle and 30T small chainring both work really great. I find myself using the middle ring more with this set up, forgoing similar ratios on the big chainring.
2) The 105-SC shift levers are brand new. I got them NOS off eBay about a month ago. I just thought of something though. I cut some really nice housing with stainless ferules and dura ace downtube micro-adjusters. I forgot to use my compressionless housing. I have a sneaky suspicion that my Shimano standard housing might have some give to it. If you think so, let me know. I'm going to go ahead and replace the housing for the rear shifter with Shimano compressionless. I'm using SRAM teflon coated derailleur cables. They seem pretty nice, not real thick/very flexible which I guess is a good thing.
3) For a chain, I am using a SRAM PC-890 that is basically new with about 100 miles on it. It feels very smooth and quiet, with no slop whatsoever.
4) The cassette is a Dura Ace Uniglide 7 speed 12-24 that has about 1,000 miles on it. this have a twist tooth design that while not ramped, when set up properly should permit very positive shifting hopefully over the full range of gears.
5) Previously I was using Mavic 8 speed indexing downtube levers and really got used to these. Even though they were for 8 speed, each hard detent somehow had a friction wiggle that would let you pull just a little more cable. I set up a spot-on drivetrain using those levers. I didn't fool with it for 10 years. Now I had to change them out due to this RetroShift project.

Last edited by masi61; 02-04-12 at 02:22 PM.
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Old 02-06-12, 01:05 PM   #9
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Hi Masi61,

I am only guessing but the next step I would take in troubleshooting would be to release the cable attachment bolt on the rear derailleur and pull the cable back and forth with the shift lever or just fingers if you can (take off shifter) and see if there is any binding.

It does sound to me like there is some resistance that might account for this type of behavior. Could be cable, cable ends, cable ferrules etc. Also without cable make sure the shift levers are moving free and properly.

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Old 02-06-12, 02:26 PM   #10
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you could look to replacement big ring (a 50 or 48) to get you within (or close to) the chain wrap capacity of the RD
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Old 02-06-12, 02:39 PM   #11
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Also depends on your fault tolerance. You can use a compromise length chain, then make sure you avoid big/big and small/small combinations, which is a good practice anyway. But now, shifting into these combinations could actually break your derailleur and ruin your ride. I wouldn't do this on a bike I plan to ride in dark, as it may be hard to avoid when you are not sure what gear you're in.
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Old 02-06-12, 08:02 PM   #12
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In a chain wrap-challenged situation, I'd size the chain to avoid breaking the RD in the big-big combination. Regardless of chain wrap, I never select big-big or small-small combos when riding - there's absolutely no reason to with any reasonably well sorted out drivetrain. Of course, accidents can happen.
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Old 02-06-12, 09:22 PM   #13
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Sounds like a worn RD or goof'd cable issue. LX RD's are very good units, there's no reason it shouldnt shift.

I always run the longest chain I possibly can.....
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