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Old 12-22-08, 01:28 PM   #1
Nota
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Max size rear cog for Dura Ace 7800 RD?

I just got a new ride, one with a Shimano 10sp Dura Ace 7800 x compact series groupo, and I'm wondering what is the max size rear cog I can mount on it.

It came with a 26T, but given that I ride extensively in a steep hilly area, I really want (need!) something like a 28 rear cog. I told the guy at the LBS that I wanted to swap out to the 28T cassette before I picked the bike up, but he came back and said the 7800 series RD has a max rear cog of 27T, and that he'd put that on there for me. He didn't. Now I'm struggling to get up prolonged steep grades in my compact's lowest, 34CR x 26T gearing; which seems way more difficult than my old steed's, 36CR x 28T setup. I'm not much of a math whiz, so I have no idea how much different the gear ratio or "inches" are between the two. Anyone know?

Question: is there any way to get the 7800 RD to accept a 28T cog; perhaps by going with tooth or two larger cog on the small end (like a 12 instead of 11), in order to keep the total chain "stretch/slack" in same "ratio", and maybe even install an extra link in the chain, if necessary?

Last edited by Nota; 12-22-08 at 01:32 PM.
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Old 12-22-08, 04:17 PM   #2
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Most people are of the opinion that the maximum tooth cog spec of Shimano derailleurs are very conservative and can usually be exceeded by several teeth.
It seems odd that you have more trouble climbing on the new bike. 34x26 is pretty close in ratio to 36x28.
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Old 12-22-08, 04:49 PM   #3
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I don't think Shimano road cassette's have ended with a 26 since 8-speed era. Are you sure you don't hae a 23 or a 25 on there as your biggest cog? Or maybe it's an SRAM 10sp cassette as they have one that ends in 26.

Shimano 10sp cassettes usually have a 12-27 option. I think Dura-Ace has an 11-27. And I think I've seen that the Dura-ace 7900 cassette has an 11-28 option. I have no idea on the availability of these. I know I had a hard time finding a 12-27 9-speed a few years ago.

IRD also makes Shimano compatible 10sp cassettes in 11-28 and 12-28 http://www.interlocracing.com/cassettes_steel.html

And the rear derailer will likely run fine on a 28. I've run my Dura-Ace RD on a 30T without any problems.

And 34/28 is about 8% lower than 34/26 which is a noticable difference if you're at your limit on a climb.

Last edited by Gonzo Bob; 12-22-08 at 05:04 PM.
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Old 12-22-08, 05:11 PM   #4
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Most people are of the opinion that the maximum tooth cog spec of Shimano derailleurs are very conservative and can usually be exceeded by several teeth.
Hmm....WWSD? (What would Sheldon do?)

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It seems odd that you have more trouble climbing on the new bike. 34x26 is pretty close in ratio to 36x28.
I agree; particularly in light of the fact that the old bike has 170mm cranks, and the new, 172.5. I just checked Sheldon Brown's gear ratio calculator, and it confirms that the ratio's are supposed to be the same: "2.5" for both.

I can't understand why I'm struggling so much with the new bike; not unless my 2-wk biking hiatus (because of company Xmas parties and other, equally worthless but otherwise "obligatory", training ride preempting functions) has totally atrophied my, once strong and viral, leg muscles?

It seems odd that I could have gone down hill that bad - in just a couple weeks.

If the weather will get up into the 40's this week, I just may take the old steed out and see if it's really me - or the new bike (gearing), that's the culprit.

Regardless, I still want to try to slap a 28T on the back.
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Old 12-22-08, 05:24 PM   #5
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I don't think Shimano road cassette's have ended with a 26 since 8-speed era. Are you sure you don't hae a 23 or a 25 on there as your biggest cog? Or maybe it's an SRAM 10sp cassette as they have one that ends in 26.

Shimano 10sp cassettes usually have a 12-27 option. I think Dura-Ace has an 11-27. And I think I've seen that the Dura-ace 7900 cassette has an 11-28 option. I have no idea on the availability of these. I know I had a hard time finding a 12-27 9-speed a few years ago.

IRD also makes Shimano compatible 10sp cassettes in 11-28 and 12-28 http://www.interlocracing.com/cassettes_steel.html

And the rear derailer will likely run fine on a 28. I've run my Dura-Ace RD on a 30T without any problems.

And 34/28 is about 8% lower than 34/26 which is a noticable difference if you're at your limit on a climb.
You're correct; it has Dura Ace Derailers with an SRAM (26T) casette.

I'll check into the IRD 10sp casettes.

Thanx muchly for mucho!

Last edited by Nota; 08-22-09 at 08:52 PM.
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Old 12-22-08, 05:24 PM   #6
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And 34/28 is about 8% lower than 34/26 which is a noticable difference if you're at your limit on a climb.
Maybe I'm doing this wrong, but I came up with about 1.7% difference, going from a 1.285:1 ratio to 1.307:1 ratio, assuming constant wheel/tire size.
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Old 12-22-08, 05:32 PM   #7
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You're right. I didn't see that the old setup has a 36.

Last edited by Gonzo Bob; 12-22-08 at 05:35 PM.
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Old 12-22-08, 06:01 PM   #8
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Did you transfer all of your measurements from your previous bike to your new bike? Is your knee at the same point relative to the pedal spindle as your old bike?

I guess I am trying to say you may have some fit issues that are not letting you utilize your developed muscles.
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Old 12-22-08, 06:15 PM   #9
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My old wrestling coach/weight trainer gave me a statistic that 20% of strength can be lost in two weeks without exercise. I'm not sure how accurate this is when applied to leg muscles though, seeing as that they are used frequently.
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Old 12-22-08, 07:46 PM   #10
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Did you transfer all of your measurements from your previous bike to your new bike? Is your knee at the same point relative to the pedal spindle as your old bike?

I guess I am trying to say you may have some fit issues that are not letting you utilize your developed muscles.
No. The "fitment", if you can call it that, of my old bike, was wherever the saddle height "landed" - after adjusting it (by myslelf while sitting on it) so as not to feel like I'm either riding a tricycle or about to pull a hamstring. The new bike came with a "professional" fitment. I'd like to think it's reasonably optimal.

I never had issue with the saddle height of my old bike; though I'm (now) fairly certain, in light of the lower backaches I frequently experienced riding the old one, and in comparing it to the fitment of my new ride, that I was stretched out way too far, as its stem is too long and too low.

I just had the same LBS to special order me a new stem (one set up for the old threaded headset x narrower bar diameter) for my old bike, so I can continue to put it to use as a: backup, knock-around, crossbike, trainer ride.
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Old 12-22-08, 07:57 PM   #11
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No. The "fitment", if you can call it that, of my old bike, was wherever the saddle height "landed" - after adjusting it (by myslelf while sitting on it) so as not to feel like I'm either riding a tricycle or about to pull a hamstring. The new bike came with a "professional" fitment. I'd like to think it's reasonably optimal.

I never had issue with the saddle height of my old bike; though I'm (now) fairly certain, in light of the lower backaches I frequently experienced riding the old one, and in comparing it to the fitment of my new ride, that I was stretched out way too far, as its stem is too long and too low.

I just had the same LBS to special order me a new stem (one set up for the old threaded headset x narrower bar diameter) for my old bike, so I can continue to put it to use as a: backup, knock-around, crossbike, trainer ride.
You probably just need to get used to the fitting then. If there were major changes done to your position then you can go back to your old position and then make incremental changes to get to the new fit, but I would not recommend it as this is the time of the year to fiddle with positions and what not so I would just ride what you have until you get used to it. With the caveat that you are not experiencing significant pain.

Anecdotally, I used to ride with a really high saddle and therefore used my quads a lot, when I got a proper fit and instruction on correct pedaling it took me a while to adjust, but it eventually made me a much better rider.
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Old 12-22-08, 08:35 PM   #12
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You probably just need to get used to the fitting then. If there were major changes done to your position then you can go back to your old position and then make incremental changes to get to the new fit, but I would not recommend it as this is the time of the year to fiddle with positions and what not so I would just ride what you have until you get used to it. With the caveat that you are not experiencing significant pain.

Anecdotally, I used to ride with a really high saddle and therefore used my quads a lot, when I got a proper fit and instruction on correct pedaling it took me a while to adjust, but it eventually made me a much better rider.
You may be right, about (not) making such drastic changes in riding position, unless done "incrementally", though, what with the stem being the most drastic change, unlike the seat height & fore/aft positioning, it's not exactly "adjustable" or practical.

here's the difference between the old bike and the new:

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