A little about Bike Derailleurs ~ (MTT 002)

Spring has officially kicked off in NW Montana. I mean, it’s been here for like almost three weeks, but I digress…


What I really mean is that the bikers are finally out. The temps have been high enough and the ground dry enough to invite our good old friends back out from hibernation. Actually, I’ve been bold enough to pull my bike out for the season myself. This of course included checking tire pressures and the break pads, and adjusting the seat, but otherwise I dove in head first!

This provided a few problems – namely my chain becoming unaligned and disconnected from the drivetrain. Meaning, I had to walk home and troubleshoot. After realigning the chain and ensuring everything was square, I determined it was the derailleur limits were too high. This allowed me to over-step the chain when dropping gears… oh well, life goes on.

So I fixed the problem and thought to myself “why don’t i provide a little info to help people prevent this in the future.”

So here we are, and here’s a small amount of info on bike derailleurs to hopefully help you if you have to troubleshoot (or better yet prevent problems on the trail). As a quick heads up I’ll be providing info on the rear derailleur as it will be the most likely culprit of mechanical issues.

First and foremost, the purpose of the rear derailleur is twofold: 1) to aligned and transfer the chain to the riders selected sprocket, and 2) take up or provide chain slack as the rider cycles through different sprockets. This means there are two main pulleys housed by the rear derailleur – the guide/jockey pulley, and the tension pulley.

The guide pulley is located on the topmost portion of the pulley cage (cage plate), and aligns the chain to the selected sprocket. The derailleur is maintained in its position by a combination of the pivot attachment and the spring located inside the derailleur itself. This spring is what allows the Tension pulley to rotate and take up the chain slack. The derailleur is typically designed to extend into the shape of a parallelogram which allows both stress relief and proper alignment during extension and retraction of the spring.


Next is the pinch bolt, which is what attached the shifting cable to the derailleur cage. As the biker cycles through his/her gears, the shifting cable will retract or tighten. This in turn will adjust the cage and allow both the guide pulley and tension pulley to do their jobs. The return spring, on the other hand, allows the derailleur to return to is resting position when the cable is extended/loosened.

To aligned the chain and guide pulley, the use of the barrel adjuster will provide fine tuning. If this is improperly aligned, it can cause the chain to skip and you’re gonna have a bad time (insert south park reference picture here). This is pretty easy to adjust, so a simple check will most likely suffice.

The limit adjusting screws are where my issues arose. There are two screws, typically labelled H and L, for your high (upper) and lower limits. this will allow you to fine tune your overall travel of the derailleur, in turn helping you prevent skipping the chain when you reach your highest and lowest sprocket settings.

One of the bigger things I forgot to mention is the point at which you mount the derailleur. Pretty standard on most bikes now, the mounting point is also a breakaway point – this saves money for the consumer as it prevents frame damage on spills that hit the derailleur.

So there’s some basic info on rear derailleurs. Hopefully it’s enough to get you a base knowledge in case you want to do a little tuning yourself. If you want to look at some examples, here are some from amazon

Shimano – 7 speed rear hub :https://amzn.to/2HJUVXf
Shimano – 8 speed MTN Bike hub: https://amzn.to/2rewxWC
Top Speed – Tourney 8 speed hub: https://amzn.to/2KrAIXS

Also, here’s a short video I made if you’re more into video. In the description is a source video for where I got some of the footage, if you want a more technical breakdown make sure to check it out.

Alright friends that’s all for this one…

Did that cover enough info, or would you like to know a bit more?

What would you like to learn about next week? Let me know down in the comments below!


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