Ask a Mechanic: Replacing Drivetrain Components and Adjusting Front Derailleurs

This is the fifth edition of a column we’re running here at Art’s Cyclery where our expert mechanic Daniel Slusser answers your bike maintenance questions.

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Question: How often do you recommend replacing your drive train components? From: Keith

This is a good question with a relatively straightforward answer. While there is no hard and fast mileage recommendation for this (too many variables including terrain, rider weight and strength, shifting preferences, compact vs. standard, etc.), there are quantitative indicators available to guide your decisions on this issue.

Park CC-2C Chain Checker Tool

Use the Park CC-2C chain checker tool to monitor chain wear.

Buy a chain wear checking tool so that you can stay on top of chain wear, as this is the main cause of drivetrain damage. Of the many chain checking tools available, I’ve used the Park CC-2 the most over the years. Using this particular tool, I typically don’t recommend replacement until the wear value reaches 0.7. Earlier replacement than this seems to be overkill since some new chains can measure as high as 0.5. With that said, after the wear reaches 0.9 it could be too late.

Once a chain is stretched beyond a certain point the teeth on your cogs and chainrings will be worn to match the new increased distance between pins on your worn chain. Sometimes this can be diagnosed visually by inspecting the chainring teeth for a shark fin shape. On the cassette the teeth will simply look too thin front to back. Another test is to pull the rear brake while applying pressure to the pedals while watching to see how much the chain climbs up the teeth. However, each of these visual tests often requires a trained eye to properly diagnose.

The only sure way to know if the cassette and chainrings need to be replaced is to install a new chain and see if it skips over the teeth under a load. When testing this be sure to do it at less than a few miles per hour while riding the brakes as a skipping chain at high speed can be very dangerous.

If your chain skips after replacement, make sure your b-tension screw on your rear derailleur is adjusted correctly. You want the derailleur to be as close to the cogs as possible without the upper pulley wheel touching the largest cog or positioned higher than the bottom of the big cog. Decreasing the distance between the upper pulley and big cog will increase chain wrap around the cog improving engagement and thereby prevent skipping. Correct adjustment here will also improve your shifting and drivetrain life dramatically.

If this doesn’t cure the problem and you don’t have the cash to replace the chainrings and cassette, then put your old chain back on and keep riding with it until the shifting deteriorates to a point that you can’t stand it anymore.Hopefully by that time you will have enough scratch to cover the needed repairs.

Last but not least, lube your chain! This is a simple thing you can do to prolong the life of all your drivetrain components and have a quieter running bike.Don’t be the guy on the group ride with the bike that sounds like a squeaky hamster wheel!

Question: My front derailleur never works right. It won’t downshift consistently and when it does, it drops the chain. How do I fix this? From: Joe

Nearly everyone struggles with front derailleurs including many professional mechanics. However, they are actually fairly easy to set up. There are only 4 adjustment parameters for a front derailleur: position, high and low limits, and cable tension. The secret to getting them right is to set them in the right order.

First, position the front derailleur the outer cage plate so it is between 1-3mm above the largest tooth of the largest chainring (I like to stay near the lower end of the adjustment range). Make sure the outer cage is parallel to the outer ring.

K-Edge Chain Catcher  Black

Chain catchers have become commonplace in the Pro Peloton and can actually enhance downshift performance.

Next, set your low limit adjustment (screw that is marked “L” ). Put the chain in the largest cog and smallest chainring. Release the cable anchor and let the cable dangle. If you have a road bike, adjust the limit so the inner portion of the derailleur cage is as close as possible to the chain without touching it. This will prevent the chain from falling off of the inside of the small chainring. If you have a mountain bike, adjust it so that the chain is positioned one third of the way between the inner cage plate and the outer cage plate. If you are confused, the chain should be closer to the inner plate than the outer.

Now set the cable tension. This will adjust where the derailleur ends up on the next shift. Pull the cable tight and anchor it to the cable anchor. Perform some test shifts. You want the front derailleur to shift up to the next chainring and have the inner plate end up as close to the chain as possible while in the largest cog out back. This will ensure that you have proper clearance between the chain and the outer plate of the derailleur when you are at the other end of the cassette.

All that remains is the high limit adjustment (screw marked “H”). With the chain shifted to the smallest cog make sure the front derailleur has about 1-2mm of clearance between the outer plate and the chain in this gear. Do a lot of test shifts to make sure that you don’t throw the chain off to the outside. If you do, then tighten the outer limit screw until this doesn’t happen anymore.

You are done!That wasn’t so bad was it? There are a lot of products on the market these days that aid in front derailleur setup. The most popular of these is the K-Edge chain catcher. With one of these installed you can adjust your inner limit screw out a bit to make downshifts a little easier. You don’t have to worry about dropping the chain with one of these so take advantage of the improved shifting performance that is available with them instead of using them only as insurance against a dropped chain.

Daniel Slusser is a professional bicycle mechanic with over ten years of experience. He holds a bachelor’s degree in business administration from HSU and a master’s degree in history from Cal Poly University. When he is not riding, wrenching, or writing he enjoys spending time with his wife and two children.