Shimano RD-M640 Zee Shadow Plus 10 Speed Rear Derailleur

It's awesome to get the same build/design quality of the pro-downhill level derailleurs at a non-pro price. This thing is fairly bulletproof in it's build quality, has a great clutch system to keep my chain from dancing, crazy easy to install with a new chain, and shifts smooth. Being it's not original equipment to my rig, my cassette is such that I have to really watch out for straining my transmission (running a 2x chainring). YES, it's an older bike, but just try and catch me on a downhill. It's not what you have, but how you use it!

This was a lot cheaper when I bought it a few years ago. It is just great. Shame how much it costs now. It's almost still worth it. 10 speed Shimano is basically dead but I'm locked in and loving it. Shifts every bit as good as 10 speed 105, and has the clutch. Super low profile too. Almost looks like an 80's derailleur with the tiny cage. Perfect for a suburban commuter or anything else really. I hear it's been discontinued, another terrible decision by the bike industry.

we Great derailer. It works with my 34 tooth cassette and 30 tooth chainring. Select the part number for 36 tooth cassette. The photos shows it fitting and a 20 inch wheel diameter kids bike. The cage clears the fat 20x 2.8” tires. The derailer keeps the chain on the gears over bumps even with clutch off. There is a slight increase in pulley friction drag with the clutch on and in the largest cassette gear. So my kid rides with the clutch off. Still is a large decrease in chain slap over the stock derailer.

Install was a snap and I had it adjusted in less than 5 minutes. The secret to a quick set up is to click your shifter into 10th gear then mount the deraileur and install the chain. Using the 10th gear limit screw, adjust the deraileur until it pedals perfectly in 10th gear. Then, pull the shifter cable tight with pliers and tighten the shifter cable bolt on the deraileur. After, shift through the gears and use the adjusting nob on the shifter to fine tune it. Done! 5 minutes, perfect shifts. Another nice thing about the derailleur is if you look down from the top of the bike, the derailleur doesn't stick out past chainstay. This makes it much more difficult to damage the derailleur in case of a get-off. Very nice derailleur, smooth and quiet. Seems to be very good quality. I'll know more as I put more miles on it.

Love this smooth shifting 10 speed derailleur on my Trek 920. I did have to shorten my chain to 110 links from 116 links, but it smoothly works with a 11-36t cassette and a 38t front chain ring. Love the shadow plus clutch, no chain slap and only a couple chain drops on washboard roads with my non-narrow wide front chain ring.

I was new to mountain biking and this was going on my Cannondale catalyst 4 hardtail. For the money, this is one of the best derailleurs you can get, and I can say that with confidence. shifts extremely smooth on flat and on slight inclines, for more harsh inclines it will still shift it just doesn't like it too much. However, behind that and the relatively small range cassette. I have no complaints. if you are looking for something for the trails where there will be a lot of climbing you might want to steer away from this but if you are up for building some serious muscle it is good. Like I said in the title I have run this since 2020 and have had zero problems with it. The durability is also extreme, it can take a real bashing and it has taken several rock hits and come out okay.

What can I say? This derailleur along with the Z shifter work flawlessly on my mountain bike. If all you need is 10 speeds I highly recommend it

As a few other reviewers have mentioned, this is an excellent product for kids bikes like my 6 year old sons 20” bike. My son really liked the gear indicator on his previous setup, so I wanted to pair it with an indicator shifter, and ended up with the SHIMANO Deore SL-T6000 Shifter. This short cage derailleur is great for kids, way fewer rock strikes and a clutch to reduce the chance of throwing a chain. Wish I would have done this upgrade directly instead of an in between Alivio setup.

Es un muy buen componente, llevo usándolo más de dos meses y ha resultado de muy buena calidad

Using the free ride version on my folding bike (11-32) due to the short cage. Shifting performance is great, short and precise. Not cheap but worth it.

It's well made, nice and compact and although the specs say that the biggest rear cog it can handle is a 36T it can actually take a larger 42T cog especially on a hardtail bike (due to a lack of suspension induced chain growth). I am running this derailleur on a 14-42T rear gearset. Another thing that can be done, either by use of a spacer or by drilling a hole, is to make this ten speed derailleur work with a 9 speed cassette/freewheel. I currently have it working as a 9 speed derailleur. It's a lovely derailleur and it's compact size makes it ideal for use on small wheeled bikes such as folding bikes. I hope Shimano continues to produce it.