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Discussion Starter · #1 ·
Just to show how easy it is these days to get a reall light rear derailleur:

DA 7900 standard: 165g
pictured are all standard parts which later get replaced: 45g
tuning parts: 21g
removed cable adjustement bolt. not needed since you still have barrell adjusters on the shifters.
tuned DA7900: 143g

Note that i still use the standad plates. There's lighter carbon plates available but i still have them on my old DA...those should be another 5g lighter or so. i just thought that close to 140 is low enough. I also didn't install the Titanium spring. I would have needed to modify it too much since it didn't fit perfectly...so i still use the almost 2g heavier standard spring instead.

the DA once again fits perfect also with a 11-32 cassette in the back.
 

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Discussion Starter · #4 ·
Slobberdoggy said:
Nino,

What titanium spring do you speak of?

I didn't think rd-7900 was compatible with xtr shifters etc. . .
it's the main spring around the pivot bolt.

No problem. shifts as good as did the 7700 or 7800 before.it's just 16g lighter right away.
all you need is a DA tuningkit from Torontocycles and some Pulleys like the Extralites i use. The tuning is done in 5 minutes!
 

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Question for Nino

Nino, you say that the DA once again fits perfect also with a 11-32 cog in the back.

I just want to verify that it works well in spite of Shimano's contention that the largest cog should by no larger than 28t, and the Total Capacity is 33t.

What are you largest and smallest chain rings, if you don't mind me asking?

Do you exceed both the Total Capacity and largest cog, and it still shifts as well as an XTR derailleur? If this is so, what reason is there to choose an XTR (besides aesthetics)?

Thank-you for your help.

Frank
 

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Discussion Starter · #7 · (Edited)
my setup..

Frank_Zuccarini said:
Nino, you say that the DA once again fits perfect also with a 11-32 cog in the back.

I just want to verify that it works well in spite of Shimano's contention that the largest cog should by no larger than 28t, and the Total Capacity is 33t.

What are you largest and smallest chain rings, if you don't mind me asking?

Do you exceed both the Total Capacity and largest cog, and it still shifts as well as an XTR derailleur? If this is so, what reason is there to choose an XTR (besides aesthetics)?

Thank-you for your help.

Frank
front chainrings: 27/42
rear 11-32

XTRs asthetics is No1 reason to get another derailleur! XTR these days is just so ugly and box shaped. Swapping XTR derailleurs for DA saved me 100g total !! That's even with using a Speen top-pull adapter on my front derailleur. if i had downpull it would be about 110g lighter.I'm using DA on my bikes since 1999. Durability is absolutely on par with XTR too.

A sidenote:
XTR "invented" shadow derailleurs which should hide the derailleurs better for less chance of damage in crashes or less chance to catch bushes etc...that's the exact same advantage you have using short caged DA deraileurs! besides that DA still allows for faster and easy tire removal/installation while with shadow you need to touch the dirty chain all the time, same with SRAM derailleurs...i hate that!
 

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More Questions, Please.........

Okay, Nino, I'd like to ask you a few more questions if you don't mind.

The Dura-Ace RD-7900 has the following specifications:

Maximum Cog = 28t
Minimum Cog = 11t
Max Front Difference = 16t
Total Capacity = 33t

Now, from what you have told me, you run:

Maximum Cog = 32t
Minimum Cog = 11t
Max Front Difference = 15t
Total Capacity = 36t

My bike, on the other hand, has a 44/32/22 triple crank, an XTR rear derailleur and, at your suggestion, I will shortly be trying a DA 11-27t cassette. With this arrangement, my specs will be:

Maximum Cog = 27t
Minimum Cog = 11t
Max Front Difference = 22t
Total Capacity = 38t

Max and min cogs should be no problem, but my Maximum Front Difference will far exceed yours, and my Total Capacity will be significantly higher.

Question 1: Do you think that the DA RD-7900 will work under these conditions? If not, I will continue to use my XTR long cage.

Also, by my calculation, my current Gain Ratio (Gear Inches) with an 11-34 t cassette runs from 1.2 - 7.6.

If I simply substitute a DA cassette (11x27t), my Gain Ration will be 1.5 - 7.6. I will lose some low end, but I rarely use the small ring now, so this should not be a problem. Just more frequent shifting for me.

Your Gain Ration appears to be 1.6 - 7.2, so if I follow your example, I will lose both low end and high end gearing. This will hurt me on the hills, both up and down, but it leads me to my next two questions:

Question 2: What crank do you use, with 42 and 27t rings? Certainly not Dura Ace.

Question 3: Can you easily shift from the small to the large ring? It looks like it would be difficult.

I'm just trying to decide what I can do to lighten my bike a bit more, without making it un-ride-able for me.

Thanks for you help................Frank
 

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Discussion Starter · #10 ·
10t pulley...

bubba888 said:
Nino - I don't see how this could work without the top pulley whacking the 32, as the 7800 would do this sometimes even on a 12-27 (saw a friend break his 7800 trying to run an 11-32...)
i use 10t Extralite Pulleys which have a slimmer diameter than 11t...anyway - it works!
 

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(I know that you don't like that size but..) Have you tried to use an 11-34 cassette with the 7900RD? Do you know if it's possible(with a custom long cage or a smaller jockey wheel)?
 

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nino said:
A sidenote:
XTR "invented" shadow derailleurs which should hide the derailleurs better for less chance of damage in crashes or less chance to catch bushes etc...that's the exact same advantage you have using short caged DA deraileurs! besides that DA still allows for faster and easy tire removal/installation while with shadow you need to touch the dirty chain all the time, same with SRAM derailleurs...i hate that!
Glad to know I'm not the only person who finds this unacceptable.

On the road, I can pop a wheel off and slap one on in under 15s (I saw a video of it later) when I have to; officials aren't prone to giving 2 free laps, and a crit field going 30mph+ waits for no one. With SRAM, Shimano, and Campy road RDs, it's never been a problem.

With my MTB, not only do I have to get the rotor inside the caliper, I have to wrangle the derailleur out of the way, getting my hands filthy in the process.
 

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nino said:
A sidenote:
XTR "invented" shadow derailleurs which should hide the derailleurs better for less chance of damage in crashes or less chance to catch bushes etc...that's the exact same advantage you have using short caged DA deraileurs! besides that DA still allows for faster and easy tire removal/installation while with shadow you need to touch the dirty chain all the time, same with SRAM derailleurs...i hate that!
Just watch out.. i learned the hard way that 7800' da rd's aren't meant for the rigors of mountain biking. I had catastrophic failure at the derailleur pivot. There is VERY little material holding the derailleur together at the pivot. XTR is the clear winner there.
 

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Discussion Starter · #15 ·
Bs!

superlightracer said:
Just watch out.. i learned the hard way that 7800' da rd's aren't meant for the rigors of mountain biking. I had catastrophic failure at the derailleur pivot. There is VERY little material holding the derailleur together at the pivot. XTR is the clear winner there.
sorry - this is just plain BS.

By the way - i'm using the exact same aluminium pivots on XTR and DA alike...bad luck can strike you with every derailleur.
 

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nino said:
sorry - this is just plain BS.
not quite.

Look at the machining on the XTR vs the Dura Ace rd at said pivot.

Dura-Ace is extremely hollowed out at that point, the actual wall thickness is 2mm max.

The XTR derailleur has a completely different design at this point.

The dura-ace derailleur is not designed to handle the same fatigue loads that the XTR is subject too.

Dont get me wrong, the DA functions flawlessly on the showroom floor, but under the rigors of racing or hard riding the XTR is a better choice. When my DA failed it was not under load or in a situation that conventionally eats up derailleurs.
 

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Discussion Starter · #18 ·
superlightracer said:
not quite.

Look at the machining on the XTR vs the Dura Ace rd at said pivot.

Dura-Ace is extremely hollowed out at that point, the actual wall thickness is 2mm max.

The XTR derailleur has a completely different design at this point.

The dura-ace derailleur is not designed to handle the same fatigue loads that the XTR is subject too.

Dont get me wrong, the DA functions flawlessly on the showroom floor, but under the rigors of racing or hard riding the XTR is a better choice. When my DA failed it was not under load or in a situation that conventionally eats up derailleurs.
still BS

you might have bent it or crashed before so it eventually ripped off. DA gets used on cyclocross-bikes too all over the planet and isn't known for any weakness at all. i am using DA derailleurs for at least 8 years as well and never had any issues.in the same time my friends destroyed several SRAM derailleurs...so i'd say SRAM derailleurs are weak...c'mon!
 

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Mind you, while lots of mtb pro's run DA rear derailleurs... they also don't think twice at having to replace three or four derailleurs per season. They're not exactly paying retail for their parts.
 

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DeeEight said:
Mind you, while lots of mtb pro's run DA rear derailleurs... they also don't think twice at having to replace three or four derailleurs per season. They're not exactly paying retail for their parts.
You're dealing with people who replace the derailleur spring with a Ti one to save 2 grams!
I check the WW forum frequently for a good laugh.
More money than sense. Clearly the global economy can't be that bad, right?

I know Nino will say I am negative, but if the amount of time, energy and resources spent saving grams was redirected, we could cure cancer and end world hunger AND global warming.

go ride your bike!
 
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