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Old 08-12-10, 09:56 PM   #1
Cachequatch
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7900 crank with 105 drivetrain

Hi all, I was given a huge "hand me down" from a friend today. I am now the proud owner of a DA 7900 crankset and I was wondering about it's compatibility with my current setup. I have an entire 105 drivetrain (5600) I have a FSA vero compact on right now. Besides a new BB, Which im going to assume I have to go DA on the BB as well, what other changes am I going to need to make to make this upgrade happen? Thank you in advance
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Old 08-13-10, 12:31 AM   #2
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Whether the front derailleur is compatible with the new crankset depends on how similar the new crankset is to the old. Double, triple, number of teeth on chainrings, etc. If you describe both cranksets better, you'll get a better answer.
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Old 08-13-10, 06:50 AM   #3
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I have Tiagra shifters, a 105 FD, and a DA RD, and Ultegra brakes with a nashbar double crankset. So no, you shouldn't have any issues, however, what you will need to consider are the following-

If you currently have a double, your FD and shifters should be fine, so long as your new one is a double as well, if your new one is a triple, and your current a double, you'll have to replace your FD and your left shifter, if your current system is a triple, and your upgrade is a double, you'll have to eventually replace your FD and left shifter, but you'll be able to get by riding it the way it is by setting your outer and inner limit screws to allow for there no longer being an extra ring.

The ideal situation is to go double to double, or triple to triple, as all you'll really have to do is tweak the shifting using the barrel adjustor for your FD.

Now, on to other stuff you might want to know-

8,9, and 10 speed derailers are for the most part backwards compatable, there has been some discussion that a 10 speed der can't be used with a 9 speed chain, and I wouldn't try a 10 speed fd with an 8 speed chain, clearances would be very tight I would think, but I'm currently running a 10 speed FD with a 9 speed chain on a double, and i love it. No issues so far, but I'll admit I've only had it setup this way for a few rides so far, but it's been fine. I'll be posting here if that changes.

Dura ace shifters don't care if they are shifting ultegra, tiagra, 105, or sora parts, so yes, you can upgrade a part of your system to a higher level, as long as you stay consistant with the speeds, the shifters in the front should match the gears in the back for the most part, there are some variations to this (9 speed shifters can be used on 8 speed cogs by using the limit screw on the RD to make sure you don't drop off the bottom of the cog) etc, but for the most part, if you have a 9 speed rear cog, try to find a 9 speed shifter for it. Same goes for FD, if you have a double, try to get a double shifter and der, you'll save yourself a lot of time cursing out the chain for dropping off one end or the other of your crank (This from my own experience, if you're running a triple shifter and fd on a double and not having issues, I'm happy for you, I had no such luck).

OK, that should get you started down the DIY upgrade path, feel free to stop back and let us know how it's going.

Joe
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Old 08-13-10, 07:03 AM   #4
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A lot of the info above is interesting but isn't germaine since all you are doing is changing the crank. Assuming the new and old cranks are both doubles, there should be no problems with the swap, and if anything, the DA should shift better than the FSA.

If the DA has conventional chainrings (53/39) and is not a compact, you will have to raise the front derailleur to clear the big chainring and readjust the cable. Otherwise it may be just a drop in. In either case you may have to tweak the fd's limit screws a bit.

You do not have to use the DA bottom bracket as any Shimano Hollowtech II bb will work including those for the Ultegra and 105 groups.
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Old 08-13-10, 08:31 AM   #5
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Just a thing to note... the 7900 crank has approximately a 1mm wider spacing between rings to facilitate cross-chaining with the 7900 setup. The 7900 derailleur moves a bit more than the old stuff to facilitate this extra movement. That means you may get a tad more rub depending on how you set it up and your shifts may not be quite as smooth as they would be with the 7900 shifter and FD.

I've seen 7900 cranks used with everything from Red to Tiagra though and they seemed to work ok. It's just not ideal. You can also use any Shimano BB with that crank so a 105 or Ultegra one will work fine. Unless you are getting a little crazy, you won't see the benefits of the 7900 BB so an Ultegra one on ebay for $20-30 would be perfect.
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Old 08-13-10, 08:59 AM   #6
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Quote:
Originally Posted by HillRider View Post
A lot of the info above is interesting but isn't germaine since all you are doing is changing the crank. Assuming the new and old cranks are both doubles, there should be no problems with the swap, and if anything, the DA should shift better than the FSA.

If the DA has conventional chainrings (53/39) and is not a compact, you will have to raise the front derailleur to clear the big chainring and readjust the cable. Otherwise it may be just a drop in. In either case you may have to tweak the fd's limit screws a bit.

You do not have to use the DA bottom bracket as any Shimano Hollowtech II bb will work including those for the Ultegra and 105 groups.
I know I gave extra info, but from experience once you start upgrading your own bike, it never quite ends, so knowing what you can put on with what helps. Extra info for those who might need it.

Joe
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Old 08-13-10, 11:24 AM   #7
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once you start upgrading your own bike, it never quite ends,

Joe
Now that's sure true.
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Old 08-13-10, 11:38 AM   #8
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Originally Posted by josephjhaney View Post
I have Tiagra shifters, a 105 FD, and a DA RD, and Ultegra brakes with a nashbar double crankset. So no, you shouldn't have any issues, however, what you will need to consider are the following-

If you currently have a double, your FD and shifters should be fine, so long as your new one is a double as well, if your new one is a triple, and your current a double, you'll have to replace your FD and your left shifter, if your current system is a triple, and your upgrade is a double, you'll have to eventually replace your FD and left shifter, but you'll be able to get by riding it the way it is by setting your outer and inner limit screws to allow for there no longer being an extra ring.

The ideal situation is to go double to double, or triple to triple, as all you'll really have to do is tweak the shifting using the barrel adjustor for your FD.

Now, on to other stuff you might want to know-

8,9, and 10 speed derailers are for the most part backwards compatable, there has been some discussion that a 10 speed der can't be used with a 9 speed chain, and I wouldn't try a 10 speed fd with an 8 speed chain, clearances would be very tight I would think, but I'm currently running a 10 speed FD with a 9 speed chain on a double, and i love it. No issues so far, but I'll admit I've only had it setup this way for a few rides so far, but it's been fine. I'll be posting here if that changes.

Dura ace shifters don't care if they are shifting ultegra, tiagra, 105, or sora parts, so yes, you can upgrade a part of your system to a higher level, as long as you stay consistant with the speeds, the shifters in the front should match the gears in the back for the most part, there are some variations to this (9 speed shifters can be used on 8 speed cogs by using the limit screw on the RD to make sure you don't drop off the bottom of the cog) etc, but for the most part, if you have a 9 speed rear cog, try to find a 9 speed shifter for it. Same goes for FD, if you have a double, try to get a double shifter and der, you'll save yourself a lot of time cursing out the chain for dropping off one end or the other of your crank (This from my own experience, if you're running a triple shifter and fd on a double and not having issues, I'm happy for you, I had no such luck).

OK, that should get you started down the DIY upgrade path, feel free to stop back and let us know how it's going.

Joe
Agree with just about everything you said, but I thought I would chime in that I have had chain rub issues using a 10-speed FD with a 9-speed cassette and chain. If it was one of my road bikes, I probably wouldn't worry about it, but since this is my CX bike and wanted to be able to use all 9 cogs in either chain-ring, it was a problem. I "downgraded" to an old 8-speed RSX derailleur I had laying around. The wider cage allows me to use all the cogs. I did have to be very careful in setting the limit screws and I installed a dogfang, just to be safe.

But otherwise, to the OP: you should be fine. I have a Shimano bike that's DA/Ultegra/105/RSX and a Campy bike that's Record/Chorus/Centaur.

Last edited by caloso; 08-13-10 at 11:42 AM.
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Old 08-14-10, 12:13 AM   #9
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Hi all, an update, swapped the cranks, ultegra BB, and raised the fd up a hair to fit the 53/39 in.. tweaked limits a hair and we're ready to roll! I can see where the 7900 fd could help with the shifting a tad, but I don't really see it as an issue. Thanks for the help! Side note.. swapping out the crankset saved me an entire pound of overall bike weight!
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Old 08-14-10, 08:06 AM   #10
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Originally Posted by josephjhaney View Post
I know I gave extra info, but from experience once you start upgrading your own bike, it never quite ends, so knowing what you can put on with what helps. Extra info for those who might need it.

Joe
+100. Years ago someone posted a hysterically funny article on his experience after being given a "free" set of 8-speed Ultegra brifters. One upgrade/change led to another and another and ~$1500 later...... I wish I could find the original as his descriptions of the project were priceless.
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