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Daily Archives: September 28, 2021

How do you know when a rear derailleur is going bad? And what to do about it.

First, let’s get into how to know your rear “mech” or derailleur is going bad. This is a very simple assessment. Complexly. The derailleur will become increasingly more difficult to “dial in” to a point it will shift well going up or down the cassette, but not both (unless you’ve got it set just right – then, a short while later, that won’t work, either).

There’s only one big problem: the same diagnosis applies for about a dozen other problems in shifting as it pertains to the drivetrain. Worn chain rings, worn cassette, worn chain, worn master link, loose chainring bolts, too much tension in the cable, too little tension in the cable, a kink in the cable, dirty shift cable housing, old cable housing… sweat or dirt that clogged a cable housing ferule, a dirty shifter, sweat or sports drink that leaks down to and gums up the cable guide below the bottom bracket housing… and that’s just a good start!

The point is, the only way to really know it’s your derailleur is to make sure all of the above items are eliminated first. If you’ve got those issues well under control and your shifting is still suspicious (and you’ve got five to 20 years on a derailleur, it’s a fair bet your mech is tired. I like to be in the perfect gear at any given moment, so I shift a lot. It makes sense that I’ll only get eight years out of a derailleur. Give or take.

Now, I know some will replace their drivetrains every few years. I know one guy who lives in the UK where it rains a lot, and changes out his drivetrain yearly. I can get five to ten years out of a drivetrain, but I don’t ride much in the rain, either. Usually, cleaning the drivetrain or replacing cables and housings will do the trick for any shifting issues I’ve got.

Recently, though, I’ve had to replace the derailleur on the Trek and the Venge is up next. First, there’s no question the chainrings are bad, so those have to go as well. However, it’s quite easy to tell the Ultegra rear derailleur is on its last leg. I’ve got a new stainless cable, new housings, new ferules all the way back to the rear mech. It shifts like butter at the shifter… except it’s almost impossible to dial in at the rear barrel adjuster. The derailleur is going.

So, the answer is to hop on down to the bike shop and order a new 10 speed Ultegra rear derailleur, right? Wrong. You can’t get them anymore. Not new, anyway… unless you’re willing to pay $70 for the new part and $150 for shipping (I’m not kidding). And a used mech from eBay will likely get me into the same mess I’m already in… So, the answer is a new 105 10 speed rear derailleur. They’re still made and sold new and run about $45 to $60 before shipping. Mine is on the way, with the new chainrings (I got about five years out of the current chainrings) and the order was big enough I didn’t pay for shipping.

Now, ordering the rear derailleur isn’t perfectly simple – nor is ordering the chainrings.

First, for the derailleur, you’ve got to decide on a short or medium cage. If you’re using big rings (52 & 53 tooth) with a corncob cassette (say 11-23), you’re going to want a short cage. With a compact setup (50/34 or smaller) with a bigger cassette (say 11-28 or 11-32), you’re going to want a medium cage which will allow for a bigger cassette for those climby days. I’ve covered chainrings elsewhere. For derailleurs, I really had to hem and haw over the Venge but I ended up going for the medium cage. I could have gone short, but I wanted the option for when I’m older to use a bigger cassette (currently 11-25 or 11-28 as the mood and amount of “up” suits me).

New chainrings on the Trek – the same are coming for the Venge

So, that’s the first option. There is a second… but it’s sketchy. And I’m going to try it after the new derailleur gets here.

The second option is to rebuild the old derailleur. You can purchase kits that include a new spring and grommets to refurbish an old mech, and there are plenty of videos on the web that show how to accomplish this. There’s also, for the real adventurous, a third option. The part that holds the replaceable spring has two holes. One for less tension and one for more tension. If the spring is worn out, and mine likely is because my derailleur is obviously clean and well-lubed because I take care of my stuff, common sense suggests I should be able to switch spring holes to add a little more tension to the spring which should get me a few more years out of it (?).

I’m not going to mess with that until the new mech is here and installed, though. I’ve got about another month of riding left on the Venge. I’m not about to shelve the bike until the cold weather has me storing it for the winter.

More to come…