August 14, 2008
DIY: Tuning Your Derailleur
By VWCabrio82
First off I would like to say that this is not the type of thing you want to just tear into. The drive train system on your bike is a precision piece of machinery. If you are even remotely worried about messing with your drive train, don't do it. Take your cycle to a shop for professional maintenance.
There are many parts that have to be working in sync for the chain to smoothly shift from 1 gear to another and back again. You can see from this picture here, a typical 9 speed transmission:
From this picture you can see 4 of the 5 adjustment points on your rear transmission, starting at the top screw. This is called the B screw. This adjusts how far the top pulley wheel is from the cassette. Next screw down is the H screw this adjusts how far from the top pulley wheel moves across the cassette towards the outer edge of your frame, or how far across the smallest cog of your cassette, the chain will move. Next is the L screw this adjusts the same as the H screw but it adjusts toward your wheel, across the largest cog. And finally there is the thumb adjuster, or cable tension, screw. The fifth adjustment point is on your gear selector. It is also a thumb adjuster, or cable tension screw.
Now let's look at some causes of issue. If you are finding that there is a clicking sound when you pedal there is probably a minor adjustment to be made. If the chain skips over the rear cassette as you pedal there is most likely more involved, and if nothing happens as you shift your selector from one gear to another... I can almost bet you need to get your wallet out, its part replacement time!
If it is just clicking after a shift or before a shift. First thing you should consider, is this a newer bike or newer drive train? If the answer is yes perhaps your only issue is the cable has now stretched slightly from its original size. If the bike is new, check where you bought it first, most shops provide a free tune up with the purchase of your new cycle. It would be much better to go that avenue for repair if you have never done this before. Now back to the clicking often found when "This thing worked fine yesterday!" There is most likely going to be one of two problems that appear more than others. As mentioned above if your bike is new or if there is a new cable recently installed, it is probably just cable stretch. Most times you can adjust the stretch by turning the thumb screw out in small 1/4 of a turn until the chain is shifting smoothly. The other issue that is most common is the derailleur hanger getting bent. Although getting bent sounds like a great time, it does nothing for your transmission system. Most times this hanger can be straightened and there is a proper tool for it. I have tried eyeballing it myself and have had plenty of "luck" doing it this way. The hanger is the small, usually aluminum, piece of metal that the derailleur attaches to the rear hub of the bike. This hanger is a delicate piece, and they are usually made of a soft metal to prevent your derailleur from taking too much abuse. It can become damaged simply by laying your bike down on its side. Sticks and stones cause the most damage, even shuttle runs can cause issues.
When straightening out the hanger you can put the appropriate Allen key in the bolt attached to the hanger, proceed to use it as a lever to straighten out the hanger. You should never grab and twist or pull on the derailleur to straighten out the hanger, this can cause permanent damage and a new derailleur.
Well that is the most common stuff out of the way. Let's say that your transmission does not fall into these issues, or you have tried both of these steps without any progress. My next step is to start from scratch. What I do is loosen off the nut holding the cable to the derailleur, and allow the cable to totally relax. When you do this the derailleur will contract to the smallest sprocket on the wheel, adjust the chain to allow for this as it may cause damage to your chain or derailleur. Move the front derailleur and chain to the middle ring and then set both of the thumb screws two full turns out from the bottom by turning the screws all the way in then turn them both out 2 full turns.
Now you want to check the derailleur alignment with the final sprocket, you want to see the top pulley wheel lining up with the sprocket use the H screw to adjust the derailleur accordingly. Next use your thumb to push the derailleur to the inside sprocket as far as it will go... adjust the chain to allow for full extension. You want to see the top pulley align with the big sprocket, while holding the derailleur tight you can adjust the L screw to get it to line up properly. Next step would be to allow the derailleur relax back to the small sprocket, adjusting the chain to allow for full movement. Set your rear gear selector on the highest setting, or hardest to pedal. Now thread the cable through the pinch nut, firmly pull the cable and tighten the nut, you can use pliers while being sure not to kink or damage the cable.
Now it's just down to fine tuning. With the rear wheel off the ground rotate the chain and sprockets. Your chain should still be in the highest setting, or the small sprocket. Turn the H screw in until you start to hear a clicking sound. When you hear this turn it out until the clicking stops. Next select the lowest gear or your rear selector. This should move the derailleur to the big sprocket or your low gear. You want to do the same here, if the chain goes right off into the spokes you need do turn the L screw in. If it is not going into the big gear at all you need to turn the L screw out to allow selection of the desired gear. Now you want to turn the L screw in until you hear clicking... then back it off as you did the H side. Now cycle through your gears and see if there are any issues. Are things working well? No, alright we need to do some fine tuning. This is where the thumb screws come in to play. As you are shifting down to a lower gear, if nothing happens you need to increase the tension on the cable. This can be done by turning the thumb screw out you will want to do this in small bits at a time, 1/4 turns. What you do from this screw makes a big difference in the amount of travel on the derailleur. Also keep in mind what you adjust on the low side affects the high side. This is where experience comes into play, if you are making the cable longer to increase tension to shift down, your highest gear may no longer be able to be selected. This will have to be adjusted out by using your H screw.
There are also other variables, chain wear is a big problem as is sprocket wear. If your chain is old it is most likely stretched and will not seat properly in the sprockets of your rear or front cogs. There is no amount of adjusting that will make this issue any better, a new chain is in order. There are tools that measure this without removal, or you can measure it yourself. This most times, requires removing the chain from the bicycle and measuring out 20 half links at 254 mm, when the chain reaches 256 mm it is at 7% wear and needs replaced. If the chain is not replaced, damage to the sprockets may ensue. You can see some wear appearing here on the ends of the teeth on this sprocket.
Sometimes visiting the shop for a tune up is not a bad thing to keep your chariot in good working order. If there is any doubt in your mind as to what is going on with your drive train, take your cycle into the local bike shop and have it attended by an experienced mechanic, trust me it will be the best couple of bucks you spend. Getting off the couch is hard enough we don’t want to make cycling a miserable chore as well.
Posted by O9man at August 14, 2008 12:00 AM