shimano 105 di2 RD R7150

All you need to know about rear derailleur

Rear derailleurs have bling factor, and also dictates how your bike ride, along with gearing. A bad derailleur, either front or back, easily makes you detest your bike.

Yet there is a lot of nomenclature, slang, love and hate, mechanical parts and legends involved, it works amazingly simple: In fact, it’s a movable chain guide, and that’s all about it.

As the name suggests, what it does to “derail” the chain into another cog. It is generally operated by Bowden cable, in engineering speak, but electronic versions are getting more common, albeit with slow adoption due to huge price difference. Weirdly, it may even be operated by hydraulics, like some Rotor groupsets.

What is long cage? short cage?

When buying a new drivetrain, or even a new chainring for your crankset or new rear cassette, you should consider derailler capacity.

Personally, I never ran into such problem; like my short cage rear derailleur is not enough to shift a big cassette. Why? Never trust common sense! I like reading manuals. So should you, as drivetrains getting more complex then cars.

Every derailleur, except a few, 3$ Chinese ones rated for 7 speed rear cogs, have a “capacity rating”. Say, 40T. What this means? Actually, that is pretty easy:

1.Note your cranksets chainrings, like 36-24, 48-36-26, 40-26, etc. Pick the largest one and the smallest one. Say, for, 48-36-26, its 48 and 26. Subtract small one from the big one (so you dont get a negative number!); 22. So, our crankset have 22T difference, as you might have guessed, “T” means “Teeth”.

2.Note your biggest and smallest cog on your cassette. If you have a 11-34 cassette, its 34-11=23T.

3. Add both= 22 + 23 = 45T.

Your derailleur should have at least 45T capacity. Generally, a smaller difference, like 2-3 teeth, may work.

Longer cages obviously have more capacity.

What if you use a long cage with a road cassette like, 11-25? It might still work, depending on your chainrings, but since your rear derailleur won’t be that tight enough when in small chainring – small cassette cog combination, you need to cut the chain fairly short. Otherwise, rear derailleur tend to go back too far, not stretching chain tight enough, besides, chain will not wrap the cassette cogs enough.

When in doubt, get a long cage derailleur if you’re riding a touring or MTB bike of some sort. The logic behind long cage derailluers are pretty basic: you may want to change your close ratio 36-26 front chainrings to 38-24; and install a bigger cassette. When I need to buy a new derailleur, I consider my front chainring difference as 14T; whatever it is. This is easy: I’ve never come across a MTB front derailluer that can handle over 14T, at least officially advertised. Road cranksets are generally have 16T difference as a standard, like 50-34, 52-36T, so road fronts are 16T. That’s why you may see MTB components on first gravel / cyclocross bikes with road front derailleurs, because these bikes generally had road cranksets, or “weird” chainring sizes as 46-30t.

Shorter cages have their advantages, too: They shift a bit faster, but this is quite subjective. This can be true because cage will have less flex; considering all other parameters equal. On technical terrain, or laymen terms, if you’re riding in places with big stones, you’ll have less risk of breaking a derailleur. I’m not a big technical terrain fan. I like riding long distances, preferably on some countryside roads, but sometimes dive into forests. I’m a careful type. I never broke a derailleur cage by hitting a rock. And don’t think a few centimeters will make a big difference, if you’ re not a competitive MTB rider. I’m not.

Shimano names their longest cages with SGS. GS are medium cage derailluers; while SS are short cages.

Not every series, not every brand makes medium cage derailluers, and its harder to find a short cage derailleur for MTB. Logic is pretty easy: you need a big range when riding on mixed terrain; uphill or downhill. If you need a short cage MTB derailleur, your best bet is obviously Shimano Saint or Zee rear derailleurs; for downhill or dirt jump bikes.

In SRAM side of things, things are a bit of hard to get: Currently, SRAM makes XO1 DH and GX DH series for downhill.  XO1 DH is Saint, while GX is Zee equivalent. What is weird about these are, they use 7 speed rear cassettes, with 11 speed chain! PG-720 is a 7 speed, 11-25 cassette. Of course, you cannot use your grandpa’s 7 speed shifters with these. X01 seems to have an 10s option, which I did not came across. Both XO1 and GX seems to have medium and short cages ONLY, which makes sense. Must also note that both SRAM and Shimano, downhill (gravity in Sram speak) specific rear derailleurs are for 1x cranksets. I doubt these will with 2x cranksets, even close ratio ones like 36-26. They look even shorter than their road version ones. I successfully used a Sora on a complete XT drivetrain for quite a long time.

What is derailleur clutch ?

I don’t know why it is called a clutch. It does not do what a clutch does. It rather acts like a ratchet torque spanner, but obviously not, too!

Shimano rear derailleurs with “Shadow Plus”, or “Shadow+” designation does have clutch. If you have a 9 speed drivetrain, you’re out of luck; because “clutch” madness started with 10 speed. Some people go over crazy with the clutch, and retrofitted clutches to 9 speed derailleurs, by cannibalising 2 healthy rear derailleurs; practically making the 9 speed clutch derailleur the most expensive rear derailleur.

What does it do? There is something crazy here: those clutches have done a good job keeping the chain tight. Do you know where chain tightness is really critical? Dual suspension bikes and 1x cranksets! Not anywhere else. Quite strangely, it is almost never advertised as a great breakthru for dual suspension bikes. Instead, what the “magazines” told you that, it stops chain slap. Clutch is only needed when you have a huge cassette with 1x drivetrain, so in a 2×10 drivetrain, a clutch derailleur is expensive, complex, prone to wear and heavy.

Chain slap is nothing critical at hardtails, UNLESS you don’t have a narrow-wide 1x chainring. You either need a chain guide, or a clutch derailleur to keep your chain where it belongs, if you jump. Even a bit.

These clutches minimize derailleur cage moving to front of the bike, under tension or sudden shock like landing on wheels. Of course, to some degree (otherwise, the derailleur will break).

Generally, these derailluers have also stronger springs.

They say, SRAM clutch performs better, and I know Shimano’s “lock” tabs have a bad habit of breaking under slight pressure. I’d wholeheartedly believe this: Srams always been a worse front-shifter, so they “invented” 1x – that’s why they also needed the rear derailleur clutch.

It’s nice to have a clutch derailleur, in theory. If your clutch doesn’t break. And they will fail. I don’t like the idea of adding one extra point of failure to my bike. I usually cover long distances, and it wouldn’t be funny to walk 30 kms with your bike. Yet I’d definitely buy one for a dual suspension bike, where my car would be parked on the finish line.

One popular question about clutch rear derailleurs are, “does clutch add friction, or we lose some watts there?”. The answer is obvious: clutch does not pull chain towards the rear. Otherwise, it would add some friction. So its basically the ordinary spring tension that acts on friction here; not the clutch itself.

On pulleys…

The upper pulley on a rear derailleur, the guide pulley, guides the chain to cassette sprockets. The one below is the tension pulley, which keeps the chain in tension. Lower pulley is called the tension pulley, because the tension applied to chain comes from the cage pivot spring, but its the lower pulley that tensions the chain.

Not all rear derailleurs have 2 pulleys: Japanese Suntour, went bankrupt and now owned by a Chinese company that does not produce premium groupsets anymore, released 3 pulley rear derailleurs back in the day. When they jumped into MTB’s, they released MounTech rear derailleurs with 3 pulleys to handle extra capacity needed – instead of resorting to a longer cage. They produced various models with 3 pulleys…

There is a huge literature on pulleys. How big is better? Bearings or bushings? Which bearings? Does Ceramicspeed worth it?

My take on pulleys is quite simple. I generally use Shimano XTR rear derailleurs. They come with bearings on pulleys, which is a abomination. Why? These tiny cassette bearings are so small that, balls inside them are pinheads, and the rings, especially the inner rings, resemble tin. They have elastic seals on both sides (2RS) and I have to replace them after every rainy day. I hate seeing bearings on pulleys. Tiny bearings never work.

I have a bag of those tiny bearings. Bushings are superior. A note to bike equipment makers: if you ever need to use bearings in tiny places, use needle bearings instead. They last long, easy to maintain and cheap.

Mind you; anything that free spins without load does not mean it has low resistance. Bike parts are under load, constantly. Bike parts are not fidget spinners. A hub spinning without load a longer time, or a pulley spins like a fidget spinner does not they equally perform well under load. In fact, they may have worse friction under load. Any bearing with smaller balls, looser tolerances and light oil will spin better. That smaller balls will try to dig into bearing cages, light oil be useless and lose tolerances mean less life.

Bearings in pulleys is a bad idea, because its the first place water, mud, sand will penetrate. Bushings installed with thick marine grease keeps water and dust outside, for a long time, while in bearings, it becomes a thick and abrasive paste that renders those tiny and weak bearings useless.

What about bigger pulleys? The idea behind bigger pulleys is that, if the chain bends less, it will have less friction. So, in theory, bigger pulleys are better. To some degree, at least: Ridicilously big tension pulleys does not leave much space for guide pulleys. This doesn’t make sense. So, huge tension pulleys are marketing gimmick. But later models Sram rear derailleurs have bigger pulleys, as well as better pulley placement. I find Sram rear derailleurs (MTB models) superior in many aspects to Shimano.

Rear derailleur compatibility

So, what makes rear derailleurs so discrete; a 9 speed derailleur won’t work on a 8 speed drivetrain?

If you dive into the specs, a standard, Shimano compatible rear hub takes 8,9, or 10 speed cassettes. What does it mean? All 8,9, and 10 speed cassettes have the same width!

…it really doesn’t makes sense, right? A rear derailleur specced at 8 speed, can move along at all 10 cogs.

Indeed, it can! If you have an old style, friction shifter like the one in old road bikes, a 8 speed derailleur will work on a 10 speed cassette, or vice versa.

The only difference is the teeth width of the pulleys, which can be changed.

This is a case of planned obsolecence.

With indexed shifting, shifter must match the rear derailleur. That’s not the whole story: a 10 speed Shimano derailluer wont work with a 10 speed Sram shifter. Also, 10 speed MTB derailleur, any brand, won’t work with road shifters, any brand. (Brifter, STI, whatever you call it)

With 9 speed, things are easier: a 9 speed MTB and road drivetrain is %100 compatible: a 9 speed road shifter will shift 9 speed MTB rear derailleur; provided both being Shimano.

I plan to write on this a lot; because there are interesting pitfalls and opportunies here!

When to replace a rear derailleur ?

A primitive rear derailleur probably outlast you, while modern ones not.

There are many occassions the rear derailleur will be considered out of service:

  • Broken or excessively bent cages. Generally, for XT or XTR, cages are available as replacement but not worthwhile.
  • Worn knuckles; so derailleur have excessive side movement, more then 3-4mm. This will cause misshifts. With 1x drivetrain or bad shifting habits, knuckles worn out prematurely.
  • Broken clutch. These things are repairable sometimes, or you can dismiss the clutch altogether. Yet it is a flaw.
  • Electronic rear derailleurs with stripped gears or electronic component failures.
  • Simple failures like broken springs can be fixed, but if your derailleur is old or cheap, this is not a viable fix, as finding the exact spring is no easy task.

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