Need new 8-speed RD
 

My 8-speed RD is close to being toast and it is need of replacement. I am finding it hard to find, actually, next to impossible. what i have is a Sora 8-speed and am using it on a 50/39/30 with 11-28 cassette. i believe this translates to a medium cage (i measured ~85mm from jockey wheel to jockey wheel if that matters).

assuming i cannot find an exact replacement i've begun looking at other RDs. what i have read is i should stick with the same brand and if possible same group set. some sources i have read say a 9 or even 10 speed will work fine, and others claim otherwise (mostly found in other forums). i'm pretty sure that shifting is dictated solely by the shifter and not the RD.

also if a 10 speed can work the 8-speed chain may be too fat. but i've also read that in this case a 9-speed chain could/might work because it is the outer dimensions of the chain that changed and not the inside.

the reason i am replacing it is due to it being bent. i've already replaced the hanger and that helped a bit. the bike has been in a crash or two the last being last year. while it does shift it could shift better.

Get any semi modern Shimano compatible 8-9 speed RDER.
Going to a "mountain" RDER would allow you to use larger cogs in case you need lower gears.

Yes. An 8,9 or 10 speed Shimano road triple or GS, excluding 4700. A mountain 8 or 9 speed (not 10) would work and as mentioned above, give you the option to use a 32T if you ever needed one. The easiest to find might be a 9 speed Deore. https://www.jensonusa.com/Shimano-De...ear-Derailleur

If you want to stay with the same group set (Sora) than you can get Shimano's current version.. labeled as RD-R3000, which has the following specs:

• Drivetrain Speeds: 9 (will work equally well with your current 8 speed shifters)
  
• Cage Length: Short, or Medium (You will want the medium)
  
• Maximum Sprocket: 32T (short cage); 34T (medium cage)
  
• Minimum Sprocket: 11T
  
• Maximum Front Difference: 16T (short cage); 20T (medium cage) (Your crankset is 50 large 30 small, a 20T difference)
  
• Total Capacity: 37T (short cage); 43T (medium cage) (you need at least 37T [50-30] + [28-11])
  

The medium cage is available from Universal Cycles for $37.99.


You can also use the current Claris rear derailleur, model RD-R2000, one step down from Sora but only $26.99 for the medium cage. Or any current 9 speed Shimano derailleur. Can't use Shimano MTB 10 speed derailleurs or road 11 speed derailleurs as neither is compatible with you 8 speed Sora Shifters. Hope this helps.

^^^ +1 Better price!

I have this derailleur and it does a good job on 8 speed.

https://www.amazon.com/SHIMANO-RD-M5...042&th=1&psc=1

The RD motion is determined by both the shifter cable travel and the derailleur ratio. Here some links with some good information: blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-drivetrain-compatibility/ http://www.bike.bikegremlin.com/2016...compatibility/

Different rear derailleurs move by different amounts for a given amount of cable pull; how much lateral motion a derailleur exhibits for a given amount of cable pull is sometimes called the "actuation ratio" of the derailleur. If you're keeping the shifter and cassette the same, then you'll need to find a derailleur with the same actuation ratio as your existing one, or else the drivetrain will not be able to index the gears properly.

The good news is that your old derailleur has an actuation ratio of 1.7, which is VERY common. All Shimano 8/9-speed rear derailleurs use this ratio, as do Shimano 10-speed rear derailleurs made prior to 2015 (when Tiagra 4700 was introduced). So you've got loads of options.

When buying a new rear derailleur, you should try to get one that's specified to handle your biggest rear cog, and the chain wrap of the drivetrain. Your big cog is 28T. Wrap is calculated by:
(teeth of biggest chainring - teeth of smallest chainring) + (teeth of biggest rear cog - teeth of smallest rear cog)
Which is your case is:
(50-30)+(28-11) = 37
So a big cog of 28T and a chain wrap of 37T. On Shimano's product pages, they refer to these specifications as "low sprocket max" and "total capacity", respectively.

Anyway, you're in luck: pretty much all current Shimano derailleurs that are compatible with your drivetrain are able to handle these requirements, even the short-cage road versions.

If you care about being minimal and want the highest-quality current option, the Sora RD-R3000-SS is probably a good bet. But going with the RD-R3000-GS would be a better option if you want to leave yourself headroom to make the drivetrain wider and don't mind adding an extra 5-20g to the bike.
The lower-end Claris versions of those derailleurs, RD-R2000-SS and RD-R2000-GS, would also work.
You could even use 8/9-speed MTB derailleurs, if you want to give yourself a ridiculous amount of headroom for drivetrain customization. The "trekking" variants like the RD-T3000 and RD-T4000 would look somewhat natural as part of a Sora drivetrain.

The M591, one of the most universal RD out there, works with anything road 10 speed and down, and anything MTB 9 speed and down, good choice!

Thanks everybody. really confirmed what i had read but it is nice to hear from all of the experts...

the first link was really a good read and will be bookmarked for later reference. Unfortunately the second link did not work.

am curious to know where you got this info. the first link (above) i read had this info too. the closest i got to a spec was https://productinfo.shimano.com/#/archive and i could not find that information. i wonder what else i am not knowing about. i no longer have the data sheet for the RD so i cannot reference that, but i did have the data sheet for the FD but did not find anything like that listed. the first link above implied that this might have been measured along with cable pull.

glad i have options. i need to wait for an order to be canceled before i order. i pulled the trigger too soon on a new BB, different story.

I'm not sure what the original sources are; I learned it from the same kinds of stuff being posted in this thread, not from Shimano spec sheets.