Technical FAQ: How derailleur position impacts drivetrain friction
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Interesting articles about drivetrains. I still don’t get it, though. I believe any thoughtful rider would have figured out that a rather low gear on the big ring puts a strain on the derailleur that is not there on the small ring with the same gear. Compare 42/16 with 53/21, which is almost the same ratio. The pedaling is very smooth and unstrained in the smaller gear, while on the big ring, the chain feels too short, since the derailleur is stretched and there is a much bigger force applied on the chain. Just pick your bike and feel the difference. You must have felt it on the road, when you know it’s time to go down on the smaller ring, even if keeping the chain on the big ring with a similar ratio is associated with irrational pride.
PS: I was an elite rider for several years in my 20s, and I consider myself an experienced cyclist.
When you spin the cranks backward under no load with the chain on a 42/16 combination, it may spin backwards more easily, and hence with less friction, than when on a 53/21 combination, because the bottom pulley has not been pulled as far forward. However, no matter what that test indicates to you, the drivetrain friction is not less in that combination when pedaling hard.
The greater friction on the top section of chain created by the higher chain tension and increased chain articulation angles when using the smaller chainring and cog relative to larger ones causes more additional friction than the decreased lower chain tension (tension in the lower three spans of chain) saves by the derailleur b-spring not pulling as hard when the chain is on the smaller cog and chainring. As I discussed in this article , the tension on the top section of chain is greater at the same cadence and power output when on a smaller chainring than when on a bigger one. The articulation angles of the chain are greater over smaller chainrings and rear cogs as well.
Less chain tension means less chain friction, and less chain articulation also means less chain friction.
I really enjoy all of your discussions about the various minutiae of the many various aspects of cycling technology, biomechanics etc. It is all super informative and it’s a real treat how you are able to put something out there and then work with your “audience” to improve and enrich the discussion. I eagerly await each and every one of your Technical FAQ’s.
I had a couple of thoughts to follow up on the drivetrain losses discussion (which may reveal my own simple-mindedness).
- 1: Why are all of the calculations done with base 360 degrees (e.g., 360/52) to determine articulation angle? In the forest/trees analogy, I’m thinking about trees here – individual links. Between points A & D on your drivetrain graphic, the chain system only rotates through (roughly) 180 degrees (i.e., from horizontal going forward to horizontal going backward)? Certainly at least some of the links near point A will not reach point D in a single crank revolution (depends on length of chain, length of chainstays etc.). I tried this out on my gravel bike (which just happens to be conveniently hanging on the wall beside me). I put it in the big ring (46T) and the 23T cog in the back (same gear ratio as in your examples). There are 114 links in my chain and points D&G are roughly in the same vertical plane (D slightly farther forward). I positioned my master link at point A and cycled the crank through 2 full pedal revolutions – and the master link ended up just a hair beyond point D.
- 2: I believe the angles of articulation as the chain passes through the derailleur pulleys are quite variable, with much smaller angles with shorter chains and/or larger cog combinations (i.e., as the bottom pulley moves forward, closer to the crankset). How might this affect the calculation of losses as the chain passes through the derailleur.
I would answer your first question to say that, with a 52T chainring, you know that over those 52 teeth, the chain does a complete loop, or 360 degrees. So, each link is bent at each tooth to an angle of 360/52. If you are only going to look at 180 degrees, then you must also only look at the number of teeth it takes to make that reversal of chain direction from forward to back; this would be half the teeth on the chainring, so the angle each link is bent at each tooth would be 180/26, which is the same as 360/52.
All of this is getting lost in the trees and missing the forest, however. Don’t be concerned about how much chain is wrapped around the chainring—what matters is what angle a single link bends at when it goes from the straight span from the rear cogs to bent over the top chainring tooth, and that is equal to the number of degrees the ring encompasses, or 360 degrees, divided by the number of teeth on the ring.
In answer to your last question, I think that you are getting confused between chain-link articulation and chain wrap around the pulley wheel. Chain-link articulation is how much the link bends as it engages or disengages a pulley wheel (or chainring or cog). Chain wrap is how much chain is wrapped around the pulley wheel (or chainring or cog); it can be looked at as how many links are wrapped around the toothed wheel or how many degrees of the wheel’s circle the wrap encompasses.
More or less chain wrap around a pulley wheel (or chainring or cog) doesn’t change friction. Once the chain link bends at the engagement point and completes the one-link articulation, friction is created. As that link moves around the pulley wheel (or chainring or cog) it isn’t bending, and therefore no friction is produced until that link disengages from the pulley wheel. At that point, it articulates back to a straight chain line, and friction is created again. Said differently, when a single chain link enters onto a pulley wheel, friction is created. When a chain link rotates around the pulley wheel, no friction is created. When a chain link pulls off the pulley wheel, friction is again created. Whether a chain wraps 45 degrees or 180 degrees around, say, a 12-tooth pulley wheel, the chain articulation angle is always 30 degrees (360/12). It doesn’t vary, no matter how much chain wrap there is.
Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”
He holds a bachelor’s in physics from Colorado College.