Wednesday, August 8, 2012

Bike Mistakes - Part 2: Chain and gears

This is the second in a series on common mistakes on your bike...

One of the big ideas behind this series is to pass along solutions to the things that we commonly see in a shop; many of which can end up costing a lot of money to fix if left unattended. This time of year, as bikes are being brought back out of storage (many of which having been ridden hard and put away wet...) one of the things we see commonly is a worn out drivetrain.

So, the causes of worn drivetrains vary - but can all usually be traced back to a stretched chain. As you ride your bike; the chain stretches and the distance between the pins gets greater and often the bushings will wear as well. Over many miles of hard riding, the chain wears the gears to match this new spacing. Dirty, dry, or otherwise poorly maintained drivetrains only accelerate this effect.
This wear can be somewhat hard to gauge or measure and; as I have looked into them, most chain wear indicators on the market are still somewhat subjective - especially when measuring across several platforms (SRAM, Shimano, Campy and 8,9,10,11 speeds). For example: Shimano 10 speed chains measure at .75mm of stretch on some wear indicators straight out of the box - indicating that it is nearly time to change the chain even when it is brand new. This is due to variances in the size and diameter of the bushings (and potentially other dimensions) used in the different chains. So, take into consideration the number of miles, conditions, and care the chain has experienced when using these tools.

What is becoming one of my more favorite methods of measuring chains is actually relatively "old school". Because modern drivetrains are set to a 1/2" pitch (the chain pins are 1/2" apart on center), you can actually measure a chain with a ruler to check it's amount of wear. This method requires a great eye, steady hand, and ability to perceive very small distances: because it's basically 1mm of stretch we're trying to perceive over a 12" distance. (Yeah, I know I switched standards of measurement from empirical to metric there...) So, again; be careful about the placement of your ruler - but with the "0" mark on-center with one pin (preferably along the bottom of the chain as it is most under tension); a pin should line up exactly on center with the "12" mark. Any deviation from that indicates enough wear that replacement ought to be considered.

The Shimano "penguin" wear indicator is also quite effective - as it's more or less a "pass/no pass" type tool rather than trying to approximate the amount of stretch.  To me, it also seems to be the most consistent at gauging wear across multiple makes and models of chains.

Believe it or not; we actually had a chain come into the store last week which the pin was probably a good 1/8" entirely off the 12" mark. Not just 1/8" off center - but 1/8" entirely stretched from the mark. A 1 year old, Shimano Dura Ace 10 speed chain on the bike of a 100 lb. female! We found that it had not been lubricated at all. It was pretty clear what we needed to do on that one!

Excessive side to side movement when you take the chain in your fingers and flex it in-board and out-board is another acceptable condition to evaluate. But you'll want to have a good baseline for each chain's level of flex; as newer ultra-narrow chains have more side to side flexibility by design for better shifting. The newer Campagnolo 11 chains are very flexible compared to a Shimano or SRAM 8-speed.

Next condition to evaluate then is the wear on the gears themselves. The easiest thing to do (but not necessarily safest...) is to put a new chain on the bike and then go ride. A new chain on a worn drivetrain will skip severely when put under load. You may have to stand up while climbing a hill in the gears you most frequently ride to get the chain to skip; but this is when the system is under the most strain - so therefore your worst case scenario. You will also want to note that larger diameter (higher number of teeth) gears don't wear as quickly; so wear will most commonly show on smaller gears whether in front or back.

Visual inspection is a good indicator; but visible wear on the gears themselves often indicates a drivetrain so severely worn that nothing is salvageable. This wear will be visible as what is often referred to as "shark-finning" of the gears where they lose their more even profiles and start to take on a sharper appearance with longer, more oval-shaped valleys between the teeth.

Cassettes and freewheels (the gear clusters on the rear wheel) wear faster than the chainrings do. So they're the place to start when your drivetrain is skipping.  But a badly worn system will likely need a chain, cassette, and multiple chainrings; however the large chainring is usually the last to wear due to the greater amount of surface area.

Now, on to prevention. Step one: routine chain lubrication and maintaining a clean drivetrain. How and how often will depend upon the conditions you ride in, the lubricant you're using, and how frequently you ride. Too broad of a topic for here; so I'll recommend checking with an expert mechanic at a trusted local bike shop.  The most important thing here though is: there is such a thing as too much chain lube - especially if you're not wiping the excess off the outside of the chain. The lube only works on the inside of the chain - the outside doesn't need it. Wipe it off!

Step two: frequent new chains. A chain is significantly cheaper than a complete drivetrain and the labor involved to replace it. A commonly thrown around guideline is that you should be able to replace two to three chains before you have to replace a cassette or chainrings. I believe that under the right conditions; with proper maintenance, cleaning, and lubrication; and replacing chains at the right intervals it is possible to get more life out of a cassette than that.

Any great tips on preventive maintenance or other solutions? Share them in the comments section...

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