Welcome to our Ask a Mechanic column where our expert mechanic Daniel Slusser answers your bike maintenance questions. If you have a question for Daniel, please post it on our Facebook Wall or e-mail Daniel directly at firstname.lastname@example.org. In today’s entry Daniel reviews how to determine chain lengths for both road and mountain bikes and how to install Shimano chain pins correctly and safely.
How do you figure out what length your chain should be? I just got a new chain for my road bike and I was thinking of just matching the length of the old one. But what if you don’t have an old chain to go off of? How do you figure it out? From: Nathan
Chain, chain, chain. Make your chain the wrong length and you play the fool. Determining the correct chain length is easy. As you mentioned you could match the old chain but sometimes that is not an option if you are changing chainrings or cranks, or if you broke the chain and it had to be shortened, etc.
On a road bike you need to shift the bike into the small cog and the small chainring before removing the old chain. This is the gear combo that you will need to be in to determine the right chain length and will ensure that there is no unnecessary drag on the drivetrain when you are in the big ring and at the larger end of the cassette.
After removing the old chain with the proper tool, lay the new chain on these smallest gears and feed it through the derailleur so that both ends of the chain are underneath the chainstay. Pull the ends together until the chain no longer drags on the derailleur cage and mark or take a mental note how many links have to be removed in order to have a joined chain that puts the derailleur cage in this position. Essentially, you are trying to make the chain as long as possible without the chain dragging on the derailleur or sagging because the derailleur can’t pull it taught. Keep in mind that if it is a Shimano chain there is no master link so you need to take this into account in your measurements. The same goes for SRAM chains that do use a master link and will need to be a half-link shorter than a Shimano chain.
For mountain bikes the process is nearly the opposite. On a mountain bike you want the chain to be as short as possible to prevent chain slap and dropped chains. Start by removing the old chain and then shifting into the big chainring and the largest cog. Place the new chain on these largest gears and feed it through the derailleur and a chain guide if you are using one. Then pull the chain tight and make the chain as short as it can be but still be able to be joined and mark or make a mental note how many links need to be removed. Then subtract one link from the portion you have determined to remove so that the shortened chain is one link longer than it needs to be. This will allow for chain growth due to suspension articulation as well as accounting for the increased chain tension experienced during shifting when the chain is not fully engaged with the teeth of your chosen cog or chainring.
Make your chain the right length and you won’t long for another one.
My old bike that I have been riding for 4 years had SRAM Force and I always changed the chains myself without any hassle using their Power Links. Recently I got a new bike with Shimano Dura-Ace and after about 1200 miles it is time to change the chain. I know that Shimano chains are a little trickier to install with their master pins instead of master links. Any suggestions on how to install them? From: Octavio
For getting the right chain length just look at my answer to Nathan’s question above. When installing a Shimano chain, make sure the stamped logos are facing outward because the 7901 chain is asymmetrical and this will put the chain in the correct orientation. Having the open end pointing the right way, and ensuring the logos face outward, are absolutely critical to maximizing the strength of the chain. Always remove links from the closed end of the chain so that you can use the factory opened end to install the pin into. The factory opened end will likely have much cleaner holes in the links and they will have the proper diameter.
Once you are ready to insert the master pin, I like to first pull the chain off of the front chainrings and let it rest on the bottom bracket so there is no tension on it. Then I insert the pin from the drive-side and then use a Shimano 10-speed chain tool to push the pin into the chain being careful that the pin goes in straight. Also, make sure that the chain is not being pulled in any direction and is simply resting in the chain tool. Keep driving the pin into the chain until you feel that the tool gets slightly easier to turn just as the pin reaches its proper insertion. Go 1/4 turn past where this feeling initiates and that should put the pin right in the sweet spot.
Remove the tool and test to see if the chain moves freely at the master pin. If it does not, and the pin appears to be inserted all the way and is flush with the outer plate that is embossed with the Shimano logo, then take the links on either end of the master pin in your hands and gently flex the chain laterally in a plane that is perpendicular to the direction that the chain normally rotates on. This will seat the pin within the link and ensure that the links pivot smoothly.
All that is left to do is to snap off the guide portion of the pin by taking the end of the chain tool opposite the driving pin and insert the guide pin into the bore on the outside of the tool. Then twist the tool perpendicular to the normal rotation of the chain and the pin will snap off perfectly with no filing needed.
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.