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ShimanoXT RD-M8100 Rear Derailleur


Item # SHIU18I

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  • Black, SGS, 12-Speed ($118.99)
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Item # SHIU18I

XT RD-M8100 Rear Derailleur

It's been a little while since we've seen an update to Shimano's coveted XT group. In fact, the last time we saw big changes was when things shifted to 11-speed, which should make it no shock at all that Shimano's new XT RD-M8100 Rear Derailleur comes along with the launch of 12-speed XT. The new derailleur packs in a new, larger 13-tooth pulley, and carries on with the proven Shadow RD+ low-profile tech that keeps things tucked out of harm's way when the trail gets loose, wily, and filled with menacing boulders hungry to take a bite out of bike components.

The new derailleur comes in two versions, the M8100 and M8120. This particular model is built for one-by drivetrains, to be paired up with Shimano's new 12-speed cassette. It's capable of running both the 10-45t and 10-51t cassette, but won't fair well if you're hoping to run a two-by crank setup. The derailleur brings a new level of silence to the drivetrain thanks to decreased tension in the lower gear positions, plus an added bumper on the pulley cage. The shifting is a bit faster than on the 8000-series derailleur thanks in part to the larger pulley.

One thing that we're thankful has stayed the same through the generations is the stabilizer switch that we saw trickling down from XTR in the previous model. When engaged, this switch helps to eliminate chain slap and dropped chains by increasing spring tension. This is a valuable option for suspension systems that require chainstays and pivots located close to the rear derailleur. The RD M8100 keeps a good thing going with some of the previous model's improvements, including the direct cable path. Your shift housing can enter the stop on the derailleur without the traditional loop of excess housing that often was the victim of contamination in yester-years.

Shimano sticks with the old-reliable aluminum components in the XT derailleur. All of the linkage plates, knuckles, and cage sides are made from it, so while it may lack the carbon-fiber-laden aesthetic of the XTR line, it's more affordable, and if you're been running XT derailleurs as long as most of us around the office have, you know you'll get plenty of mileage out of it.

  • Shimano's workhorse rear derailleur goes 12-speed
  • Shadow RD+ with stabilizer switch reduces chain slap, chain drops
  • Lofty cage length is compatible with both of Shimano's 12-speed cassettes
  • Decreased tension at low gear keeps things smooth and quiet
  • New bumper on pulley cage reduces noise
  • Fast and precise shifting performance optimized for trail riding
  • Durable aluminum construction holds up to trail abuse
  • Compatible with 1-by-12 groupset, for 2-by applications reach for the RD-M8120

Tech Specs
Cage Lengthlong
Cassette Range10 - 51t
Gearing1 x 12
CompatibilityShimano 12-speed
Recommended Usemountain
Manufacturer Warranty2 years

Actual Weight

Black, SGS, 12-Speed

Actual weights are measured in-house by the Competitive Cyclist team.

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What do you think about this product?


>Rating: 5

Soooo Smooth

I've used it several times

I had about given up on Shimano but with the release of their 12-speed SLX and XT groupsets, they once again got my attention. Over the last few years my bikes have always had Shimano XT drivetrains, I've gone from a 1x10 to 1x11 and now 1x12. They nailed it with the 1x12. Yes, it took them quite a few years longer than SRAM but the wait was worth it. I am running the 51T cassette with a 32T chainring out here in Utah and that gives me all the range that I need. Right away I was climbing more efficiently and quicker than on my old 1x11 setup. So you might be asking, Why go with Shimano over the competition? Well, Shimano's Hyperglide+ design is the main thing that sets them apart. With this design you can now shift whenever you want. No need to unweigh the pedals slightly during a shift to prevent the chain and cassette from making terrible noises. The chain will move up or down the cassette without any fuss. It still seems strange to not ease up on the pedals before shifting but you can and the drivetrain is happy to get you into the gear you want. So there's that, and with Shimano you still have the ability to drop down two gears with one push. I love that feature and use it often. I'd say if you do not care about having the absolute lightest drivetrain setup then you will be very happy with Shimano XT for the money. Shimano XT has always been a great bang for the buck groupset and that remains true with the 8100 series. Pros: Shimano Shifters (Rapidfire Plus and I like shifting with my index finger), buttery smooth shifting up and down, quiet, easy to setup, the performance you get for the price.. There's even a mark on the inside of the pulley wheel cage to simplify setting the correct B-tension. Cons: A little heavier than SRAM GX. A con that I will happily live with for getting a better shifting drivetrain for less money. You need a MicroSpline freehub for the cassette. It was an easy swap for me with an I9 freehub. In the future MicroSpline will be much more common on new wheelsets so this will be less of a problem.

>Rating: 5

First Ride Impressions

I've used it several times

I was able to throw the XT 12 speed drive train on my MegaTower a couple weeks ago now and man does it know how to hold its own! What I noticed right off the bat is the set up was very simple! It only took about an hour to get it up and running. There are some amazing little details to the 12 speed components as well! My favorite being the rubber pad on the shifter for more traction (most companies leave it as polished alloy, which can be very slippery). After getting the first few miles under my belt I noticed how dead silent the shifting actually is! I would have to look down to make sure I did in fact shift, and it was always spot on. The other aspect that really has me excited about this stuff is that you can confidently shift under load. I have run Eagle drive trains on previous bikes since Shimano had not released a 12 speed group set yet, but I'm stoked they took their time, because they nailed it right off the bat! Pros: - Easy setup - Dead silent shifting - Ability to shift under load -Attention to detail (the little things that just make it easy to use) Cons: - It isn't the lightest 12 speed setup out there - Visual Design wasn't a big focus (is not a piece of art like xx1 Eagle)

>Rating: 5

Crank Away!

I've used it once or twice and have initial impressions

Had my first ride on the new XT 12 speed derailleur and I am drinking the cool-aid! Here is what I liked: 1. The install was a breeze. 2. The shifting is crisp and quick. Better than X01 I think 3. Reduced chain rattle is noticeable compared to X01 4. Looks pretty cool Here is what I really liked! 1. Shifting under load capabilities are no joke! I was cranking thru gears and this thing just took it! 2. The decreased tension on the chain when in the 51t really makes pedaling smooth when grinding up some steep stuff. A couple of things to keep in mind compatibility wise: This derailleur is built for the 1x12 systems and can take the 51t or the 45t cassette. If you want to run 2x you will need RD-M8120 and that can only take the 45t.