Importance of Derailleur Compatibility on New Drivetrains

 One of the best features of many new component groups is the ability to run a wide range of chainring and cassette setups.  1x, 2x, and even triple crank setups are possible within the same component group, and cassettes range from 28t to as big as 52t.  

All this availability has made it more important to ensure you're using compatible parts, but sometimes it's difficult to know the correct derailleur to use with the cassette you're using.  I hope this article helps understand why.

Shimano relies on the upper derailleur pulley tracking as close as possible to the cogs.  This is why their rear derailleurs have a low tolerance for cassette sizes.  Let's take Deore XT M8100 series 12 speed rear derailleurs for instance.  There are three versions, each with a specific purpose:

GS- Short cage, designed for a single chainring

SGS - Medium Cage, designed for a single chainring

M8120 SGS - Long cage, designed for a double chainring

All of these derailleurs look very similar and can easily be mistaken for each other.   Looking at the specs at bike.shimano.com, we see that there's very specific cassette compatibility:

GS- 10-45 12 speed cassette

SGS: 10-51 12 speed cassette

M8120 SGS: 10-45 cassette with a maximum chainring difference of 10 teeth

Of course, there's some cross compatibility that will work, but there's some that won't work at all.  For instance, the M8120 SGS derailleur can easily be used on a single chainring setup and 10-45 cassette with no problems.  If you attempt to use it on a 10-51 cassette it MIGHT shift if you turn the 'b adjust' screw all the way in.  However, shifting will suffer in the smaller cogs because the upper pulley will track too far away from the cogs.  Likewise, an M8100 SGS derailleur will shift on a 10-45 cassette but the gap between the pulley and the cogs will not stay constant, and shifting won't be as quick and smooth as it should.  You should never use the short cage GS derailleur with a double crank - the capacity (the difference in cog sizes plus the difference in chainring sizes) is too small.  When you size the chain according to Shimano's instructions, the chain will be too long when using the small chainring and smaller cogs because the derailleur cage isn't long enough to take up the slack.

All this is true because each rear derailleur is designed with a specific geometry that tracks the cogs at a precise angle.  This keeps the pulley-cog distance constant, ensuring smooth and accurate shifting.  If you mix up the compatible parts,  shifting will begin to suffer.  Sometimes it's an acceptable amount, sometimes it could slip and mis-shift.  This is why Shimano 'requires' a specific derailleur for each setup.

SRAM approaches the issue a little differently.  The position of the upper pulley on the cage allows the pulley to track different range cassettes.  However, they rely on proper chain length to do this.  Even though an Eagle rear derailleur will work on both a 10-50 and 10-52 cassette, it's best to size the chain to the specific cassette.  If you use a chain that's sized to a 10-52 cassette with a 10-50 setup, the upper pulley could track too closely to the cog when the 'b-adjust' screw is adjusted properly.  You could possibly have some grinding in the larger cogs.  If you were to adjust the 'b-adjust' screw in to compensate, then the upper pulley won't track the rest of the cogs properly and you could end up with unreliable shifting.  Likewise, if you use a chain sized for a 10-50 cassette on a 10-52, it might shift to all the gears, but the upper pulley will track farther away from the smaller cogs and shift accuracy will suffer.  Beware though - running a chain that's too short could break the derailleur, especially if you have a full suspension bike.

In many of these cases, the drivetrain may shift acceptably.  From a mechanic's point of view, it's our job to make it shift as smoothly and precisely as possible so we recommend using the correct setup to achieve that.

SRAM's AXS standard road derailleurs are compatible with 10-28, 10-33, and 10-36 cassettes with either single or double chainrings; the Xplr road derailleur is compatible with 10-44 or 10-36 cassettes and a single chainring.  Like before though, proper chain length is critical for precise shifting.

I hope this helps clear up some confusion about compatibility issues that aren't straightforward.  Thanks for reading!

My Top 5 Tips to Make Your Bike Ride Smoother

  I've worked as a mechanic in the bike industry for over 35 years.  I get asked for bike care tips a lot so I thought I'd pass along my top five.

1. Find a mechanic you trust and have them check your bike regularly

I know - exactly what you'd expect a mechanic to say (I also follow this advice with my car.)  Once you find a good mechanic, your loyalty should be rewarded with a smooth-running, problem-free ride.  If you log thousands of miles, ride in poor conditions, or are generally hard on equipment you may need more service than normal and it's good to have someone who knows your unique needs.  If you return to the same mechanic time after time, they'll have a personal interest in your experience when you ride.  That'll show every time you get on your bike.

I'd also recommend calling ahead to make an appointment.  Sometimes it's just the polite thing to do, but many shops are overwhelmed in the summer.  Making an appointment means you'll likely spend less time without your bike.

2. Oil your chain with a good quality chain-specific lube

Lubricants that are made specifically for chains are designed for the high load and contamination-rife drivetrains on bikes.  There are also many lubes available that won't make a greasy mess.  Silca Super Secret truly can't be beat.  It's a liquid wax that doesn't make a mess and runs super smooth, but it's pricey (well worth the money).  Rock n Roll Gold is a good, more affordable alternative.  Try to avoid using thin lubes like Tri-flow on your chain - they wear out quickly and attract a lot of dirt.  And WD-40?  It's not really a lube, it's made to displace water to prevent rust.  I like it to clean chains, but it will only last a few miles if you attempt to use it as a lube.

For more scientific research on chain lubes, see zerofriction.com

3. Pay attention to air pressure

This means that you may need to do some trial and error to find the correct pressure.  There's a lot of information going around the industry about what 'correct pressure' really is.  I've been running 90psi in 700x25c tires for over 25 years, even when everyone recommended 120psi.  To me, the ride quality on the rough roads was more important than 1.5 watts saved at higher pressure.  I also found that if I ran less pressure, I would pinch flat my tubes.  Tubeless road tires might allow for even lower pressure, as would a bit wider tire (if it fits your frame).

Also, better quality tires offer much better ride quality.  Period.  Everything you read about Vittoria Corsa and Continental GP5000 tires is true - they're both great tires.  The downside is that they're nearly $100 each now, but consider it an investment in your riding experience.

Want more scientific information about tire pressure vs rolling resistance?  Check out bicyclerollingresistance.com.  He offers a lot of information about how tire pressure affects rolling resistance.  

4. Preventative maintenance is better than waiting until you have a problem

Bikes don't break down when they hang in the garage.  They break down at the worst possible moment, usually climbing a big hill.  Routine preventative maintenance will catch problems before they lead to a ruined afternoon ride.  Preventative maintenance will make sure your derailleur hanger is aligned properly for better shifting, check your chain for wear or damage, and check your cable condition.  When was the last time you replaced the batteries in your wireless shifters?  Are your stem bolts tight?  Spokes tensioned properly?  Don't wait to find out during the grand fondo you've trained for all year.

5. Replace your cables.

Cables and housing wear.  Although they do last a long time, nothing shifts better than new cables.  Cables wear grooves inside the housings, increasing friction.  I replace my cables about every 2000 miles, my housings every 4000 miles.  Don't ride that much?  At least have your cables lubricated.  It makes a big difference in shifting quality.

You shouldn't be surprised to hear me say this: better quality cables shift better.  My choice for all of my bikes is Jagwire Pro slick polished cables.  This is the best working cable I've ever used.

There you go, my top five tips.  There's more where these came from, but these are the tips I tell everyone first when they ask.  I hope they help you have a great ride!