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Old 08-03-10, 03:26 PM   #1
Aero-X
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SRAM PG1070 cassette with 7800 rear der

I need lower gearing on my bike. My current set up is compact double(50/34) + D/A 10spd 7800 12/27 cassette. So I'm thinking about getting a SRAM Rival 1070 12/32 cassette to keep the cadence above 80 on steep hills. Now the question is will I need a different rear der? For the 7800 12/17 cassette, I pretty much maxed out with a short cage rear der. If I go PG-1070 12/32, will I need a 7800 long cage rear der? Of course, a new and longer chain in addition.

Any advice appreciated!
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Old 08-03-10, 04:23 PM   #2
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I believe that you'll need to swap out the 7800 for a MTB RD. The 7800 is designed to go up to a 27 but many have used it on 28 and 29t sprockets but a 32 is really pushing it and the upper pulley will rub on the largest sprockets.
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Old 08-03-10, 04:53 PM   #3
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My plan is to use the long cage 7800 rear der, the one used with triple cranks. I just want to make sure the long cage rear der will work with a 12/32 cassette.

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I believe that you'll need to swap out the 7800 for a MTB RD. The 7800 is designed to go up to a 27 but many have used it on 28 and 29t sprockets but a 32 is really pushing it and the upper pulley will rub on the largest sprockets.
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Old 08-03-10, 05:15 PM   #4
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My plan is to use the long cage 7800 rear der, the one used with triple cranks. I just want to make sure the long cage rear der will work with a 12/32 cassette.
A 7800 triple RD will not work with a 32t sprocket due to the design geometry.

What I am trying to explain to you is that the geometry of road rear derailleurs and mountain bike rear derailleurs is different. Mountain rear derailleurs will clear the 32t sprocket while a road derailleur will not. It has nothing to do with the length of the cage. Sure, the long cage 7800 triple RD will be long enough to take up the slack but it will only be able to go up to the 28 or 30t sprocket before the upper guide pulley starts to grind on the cassette.
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Old 08-03-10, 05:22 PM   #5
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So what about just getting a compatible Sram Rival rear deraileur to go with the PG1070 cassette? It has short and mid cage options. For 12/32 cassette, which one should I get?

http://www.sram.com/sram/road/produc...ear-derailleur

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A 7800 triple RD will not work with a 32t sprocket due to the design geometry.

What I am trying to explain to you is that the geometry of road rear derailleurs and mountain bike rear derailleurs is different. Mountain rear derailleurs will clear the 32t sprocket while a road derailleur will not. It has nothing to do with the length of the cage. Sure, the long cage 7800 triple RD will be long enough to take up the slack but it will only be able to go up to the 28 or 30t sprocket before the upper guide pulley starts to grind on the cassette.
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Old 08-03-10, 05:39 PM   #6
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Quote:
Originally Posted by Aero-X View Post
So what about just getting a compatible Sram Rival rear deraileur to go with the PG1070 cassette? It has short and mid cage options. For 12/32 cassette, which one should I get?

http://www.sram.com/sram/road/produc...ear-derailleur
That would work, if you also swapped out your shifters for SRAM shifters.
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Old 08-04-10, 05:47 PM   #7
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Originally Posted by Ingleside View Post
That would work, if you also swapped out your shifters for SRAM shifters.
What if I use XT CS-M771-10 10spd cassette(11/32) instead of SRM PG1070? That way I only need to replace chain, cassette and rear der. I can keep everything Shimano. I assume XT 10spd is compatible with D/A 7800 STI shifters and drive train.
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Old 08-04-10, 06:10 PM   #8
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SRAM and Shimano cassettes are fully interchangeable,.

There are two factors to consider when choosing the rear derailleur: max cog limit and capacity limit.

Using a cassette with 32 or more teeth will necessitate a MTB RD (this is independent of how long the cage is).

If the capacity of your system [(largest chainring + largest cog) - (smallest chainring + smallest cog)] exceeds the total capacity of a short-cage MTB RD (GS), you'd be best off using a long-cage version (SGS). Shimano XT GS RD has a capsity of 33T. 50/34T crank x 11-32T or 12-32T cassette would have a capacity need of 37 and 36, respectively, so you'd be best off with the SGS version, which has a 46T capacity.

It's a good idea to replace the chain whenever the cassette gets replaced.
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Old 08-04-10, 06:22 PM   #9
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So I just checked on shimano's site. Found a 7800 rear der PDF doc. It says a 7803 GS(long cage) rear der has a capacity of 37 teeth(see img). I guess a D/A 7803 long cage rear der can work with an XT 10spd 12/32 cassette? The reason I'm asking is that I happen to have a 7803 rear der. I will try to make it work without buying another 10spd MTB long cage der.



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SRAM and Shimano cassettes are fully interchangeable,.

There are two factors to consider when choosing the rear derailleur: max cog limit and capacity limit.

Using a cassette with 32 or more teeth will necessitate a MTB RD (this is independent of how long the cage is).

If the capacity of your system [(largest chainring + largest cog) - (smallest chainring + smallest cog)] exceeds the total capacity of a short-cage MTB RD (GS), you'd be best off using a long-cage version (SGS). Shimano XT GS RD has a capsity of 33T. 50/34T crank x 11-32T or 12-32T cassette would have a capacity need of 37 and 36, respectively, so you'd be best off with the SGS version, which has a 46T capacity.

It's a good idea to replace the chain whenever the cassette gets replaced.
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Old 08-04-10, 10:03 PM   #10
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One more time... Your Dura-Ace 7800 triple RD WILL NOT WORK.

It will not work because of tooth capacity or cage length but because it was never intended to be used with a large sprocket like the 32t you want to use.
32t is (now with Rival and APEX was)mountain bike territory and thus you need a mountain bike RD that can physically go up to a 32t sprocket (or higher).

Purchase ANY Shimano MTB RD with a long cage (SGS) and your system will run smoothly although I suggest sticking with a SLT, XT or XTR unit depending on your budget. You don't need the shadow versions and do not get a "reverse pull" low-normal version either. Stick with a traditional top-normal unit. You can use either a 9 speed or newer 10 speed "dynasys" unit, it doesn't matter since they both work. 9 speed RDs will be cheaper now. One last thing, try to find one that has a adjustment barrel oif your frame doesn't have adjustment barrels on the down tube (like Giants)

Also don't forget to pick up a new 10 speed chain to allow the larger sprocket.

As for cassette, pick any 10 speed cassette you want in the right tooth configuration. Shimano has a 11-32 XT Dyna sys cassette and SRAM has their 1070 11-32. Either will work since SRAM and Shimano cassettes and chains are interchangeable.

I have installed this type of setup on at least 20 different road bikes this year in 9 and 10 speed configurations on customers bikes. It works.

You do not need to use a RIVAL or APEX RD to make this work. Doing so will cause to you buy a new set of shifters.
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Old 08-04-10, 10:13 PM   #11
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Ok, so I'll just get a PG-1070(12/32) + a SRAM long cage. Now do I have to go with all SRAM, will a PG-1070 work with a Shimano 10spd MTB long cage?

Last question, I can still use my 7800 STI shifters, right? Replacing shifters is costly.

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One more time... Your Dura-Ace 7800 triple RD WILL NOT WORK.

It will not work because of tooth capacity or cage length but because it was never intended to be used with a large sprocket like the 32t you want to use.
32t is (now with Rival and APEX was)mountain bike territory and thus you need a mountain bike RD that can physically go up to a 32t sprocket (or higher).

Purchase ANY Shimano MTB RD with a long cage (SGS) and your system will run smoothly although I suggest sticking with a SLT, XT or XTR unit depending on your budget. You don't need the shadow versions and do not get a "reverse pull" low-normal version either. Stick with a traditional top-normal unit. You can use either a 9 speed or newer 10 speed "dynasys" unit, it doesn't matter since they both work. 9 speed RDs will be cheaper now. One last thing, try to find one that has a adjustment barrel oif your frame doesn't have adjustment barrels on the down tube (like Giants)

Also don't forget to pick up a new 10 speed chain to allow the larger sprocket.

As for cassette, pick any 10 speed cassette you want in the right tooth configuration. Shimano has a 11-32 XT Dyna sys cassette and SRAM has their 1070 11-32. Either will work since SRAM and Shimano cassettes and chains are interchangeable.

I have installed this type of setup on at least 20 different road bikes this year in 9 and 10 speed configurations on customers bikes. It works.

You do not need to use a RIVAL or APEX RD to make this work. Doing so will cause to you buy a new set of shifters.
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Old 08-04-10, 10:26 PM   #12
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Quote:
Originally Posted by Aero-X View Post
Ok, so I'll just get a PG-1070(12/32) + a SRAM long cage. Now do I have to go with all SRAM, will a PG-1070 work with a Shimano 10spd MTB long cage?

Last question, I can still use my 7800 STI shifters, right? Replacing shifters is costly.
As I said before, Shimano and SRAM cassettes are FULLY interchangeable--either will work regardless of the shifter or RD (assuming everything else is compatible with each other).

However, SRAM RDs and Shimano rear shifters are NOT compatible. If you get a SRAM RD, you will need to replace your shifter and they are, as you said, costly.

Here's exactly what you need to do:
-Get either of the 10-speed cassettes you listed (doesn't matter which).
-Get a Shimano MTB RD in the "SGS" version.
-Get a new 10-speed chain and size it using the big/big + 1" method: http://sheldonbrown.com/derailer-adjustment.html#chain (or however else you want to size it as long as you have enough links to use big/big).
-Done.

Last edited by JiveTurkey; 08-04-10 at 10:29 PM. Reason: The obligatory sheldon link.
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Old 08-04-10, 10:31 PM   #13
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Thanks! All clear now.

Quote:
Originally Posted by JiveTurkey View Post
As I said before, Shimano and SRAM cassettes are FULLY interchangeable--either will work regardless of the shifter or RD (assuming everything else is compatible with each other).

However, SRAM RDs and Shimano rear shifters are NOT compatible. If you get a SRAM RD, you will need to replace your shifter and they are, as you said, costly.

Here's exactly what you need to do:
-Get either of the 10-speed cassettes you listed (doesn't matter which).
-Get a Shimano MTB RD in the "SGS" version.
-Get a new 10-speed chain and size it using the big/big + 1" method: http://sheldonbrown.com/derailer-adjustment.html#chain (or however else you want to size it as long as you have enough links to use big/big).
-Done.
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Old 08-04-10, 10:36 PM   #14
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Cool. Good luck. Have fun with that low gear. If you really want to get low, the setup described above will also handle a 34T cassette and it won't affect the rest of the gears much.

If you haven't played around with this gear calculator yet, give it a shot: http://sheldonbrown.com/gears/

I find MPH @ a given RPM (e.g. MPH @ 90 RPM) to be the most intuitive output.
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Old 08-04-10, 10:38 PM   #15
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Competitive Cyclist has XTR shadow on sale for $160. Seems to be a good deal. Will this work? It's a shadow.

http://www.competitivecyclist.com/mo...308.457.1.html

Quote:
Originally Posted by JiveTurkey View Post
As I said before, Shimano and SRAM cassettes are FULLY interchangeable--either will work regardless of the shifter or RD (assuming everything else is compatible with each other).

However, SRAM RDs and Shimano rear shifters are NOT compatible. If you get a SRAM RD, you will need to replace your shifter and they are, as you said, costly.

Here's exactly what you need to do:
-Get either of the 10-speed cassettes you listed (doesn't matter which).
-Get a Shimano MTB RD in the "SGS" version.
-Get a new 10-speed chain and size it using the big/big + 1" method: http://sheldonbrown.com/derailer-adjustment.html#chain (or however else you want to size it as long as you have enough links to use big/big).
-Done.
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Old 08-04-10, 10:38 PM   #16
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never mind...
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Old 08-05-10, 01:24 PM   #17
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I was checking out some online bike stores. It seems all current XT/XTR rear derailluers are all 9spd. Would they work with SRAM PG-1070 10spd cassettes?
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Old 08-05-10, 03:22 PM   #18
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Quote:
Originally Posted by Aero-X View Post
I was checking out some online bike stores. It seems all current XT/XTR rear derailluers are all 9spd. Would they work with SRAM PG-1070 10spd cassettes?
Yes, with a few exceptions, all modern Shimano rear derailleurs use the same cable-pull ratio. The RD doesn't care how many stops there are in the shifter, it will go where the shifter tells it to.

Someone mentioned above to make sure you have a cable barrel adjuster somewhere if you get a Shadow RD. The Shadow does not have a barrel adjuster at the RD.

Last edited by JiveTurkey; 08-05-10 at 03:27 PM.
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Old 08-05-10, 06:36 PM   #19
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Be careful for a second with this because I believe there may be a change in the newest shimano 10 speed mtb rear derailleurs and they may not work properly with shimano 10 speed road shifters. There is another thread about that somewhere on here. So it will be cheaper to just get a "9 speed" mtb rear derailleur and it will work for sure, too.
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Old 08-05-10, 07:01 PM   #20
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Quote:
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Be careful for a second with this because I believe there may be a change in the newest shimano 10 speed mtb rear derailleurs and they may not work properly with shimano 10 speed road shifters. There is another thread about that somewhere on here. So it will be cheaper to just get a "9 speed" mtb rear derailleur and it will work for sure, too.
Wow, after some digging in my parts bin. I found a matte gray XTR RD-M952 SGS long cage and a matching CS-950! This is a 95~00 era 9spd RD. Will it work with PG-1070?
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Old 08-05-10, 09:59 PM   #21
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Wow, after some digging in my parts bin. I found a matte gray XTR RD-M952 SGS long cage and a matching CS-950! This is a 95~00 era 9spd RD. Will it work with PG-1070?
Perfect! That RD is exactly what you need for your proposed setup.

Put it on, get a new chain (10speed) and hit the road!
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Old 08-06-10, 12:02 AM   #22
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Quote:
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Be careful for a second with this because I believe there may be a change in the newest shimano 10 speed mtb rear derailleurs and they may not work properly with shimano 10 speed road shifters. There is another thread about that somewhere on here. So it will be cheaper to just get a "9 speed" mtb rear derailleur and it will work for sure, too.
Yes, I haven't been keeping up on this. It used to be just the obligatory pre-1997 or pre-9-speed Dura Ace exception.
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