Is there a difference in efficiency between spinning the same gear ratio but being in the big ring vs the small ring? The big ring “feels” like I have a longer pedal stroke, but I’m not sure if it’s just my imagination. I feel more efficient in the big ring.
If you’ve got the same exact ratio, the big ring will be slightly more efficient because there is a tiny bit less friction in the chain when it’s bent around a larger radius. Also, the chain line may be more or less efficient in the big or little ring depending on what cog your in on the rear.
Other than that, no it’s exactly the same and should feel that way.
“there is a tiny bit less friction in the chain when it’s bent around a larger radius.”
Wouldn’t that also go for the rear cog as well? Seems to me that it would depend on the gear, as there’s a “point of diminishing returns” for a larger radius. I think that you’d get more benefit by going with a larger cog and smaller chainring than the other way around, as the change in radius per tooth is much greater, percentage-wise, for the cogs.
keeping gear ratio constant you are always going to get big ring/big cog = small ring/small cog, so no matter how you slice it the big ring is more efficient.
Ah, yes, you’re right. Need more caffeine…
keeping gear ratio constant you are always going to get big ring/big cog = small ring/small cog, so no matter how you slice it the big ring is more efficient.
Actually, you could spend some time with excel…
Look at chainspeed and then read spicer et.al.
All gears are not created equal…
G
keeping gear ratio constant you are always going to get big ring/big cog = small ring/small cog, so no matter how you slice it the big ring is more efficient.
Believe it or not, but a study on drivetrain efficiency published in the IHPVA journal indicates that although the smaller bending friction forces of a large gear are true, the majority of the drivetrain losses are inversely proportional to the tension in the drivetrain. This “tension dependent” loss overrides the bending losses, with the result being that for a given gear ratio and cadence, you actually have less drivetrain losses with a small ring/small cog choice.
Another way of thinking of it, and this is the way I understand Jens explains it, is that you get the best efficiency with the lowest chain speed for a given gear ratio and cadence.
Counterintuitive? Yes…but the data speaks for itself.
Compact cranks, anyone??
Maybe i need a little more explaining here, but it seems to me that for a given cadence at a given gear ratio, the chain speed is always going to be the same because chain speed at a constant resistance is only dependent on cadence?
Say eg. 45/15 vs 33/11 = 3.0 runout
in both of these gears the resistance load is equal, and if you pedal at 90rpm, the chain speed would be equal for both wouldn’t it? In 1 pedal revolution the chain is always going to move the same distance no matter what gear you’re in, or at least that’s what i thought. so where am i going wrong here?
Maybe i need a little more explaining here…
… if you pedal at 90rpm, the chain speed would be equal for both wouldn’t it?
nope.
you got some maths to do…
g
One pedal revolution in the small ring pulls less chain than one pedal revolution in the big ring. That’s kind of the point with a big and a small ring, isn’t it…?
This “tension dependent” loss overrides the bending losses, with the result being that for a given gear ratio and cadence, you actually have less drivetrain losses with a small ring/small cog choice.
Compact cranks, anyone??
Ummm…
Chain tension is greater with the small chainring/small cog combo;
Chain speed is greater with the large chainring/large cog combo;
Compact cranks are always going to give you more tension/less speed/tighter bending radius.
Ummm…
Chain tension is greater with the small chainring/small cog combo;
Chain speed is greater with the large chainring/large cog combo;
Compact cranks are always going to give you more tension/less speed/tighter bending radius.
yup and that is what you want…
from the cliff notes version here
The researchers found two factors that seemed to affect the bicycle chain drive's efficiency. Surprisingly, lubrication was not one of them.
"The first factor was sprocket size," Spicer says. "The larger the sprocket, the higher the efficiency we recorded." The sprocket is the circular plate whose teeth catch the chain links and move them along. Between the front and rear sprockets, the chain links line up straight. But when the links reach the sprocket, they bend slightly as they curl around the gear. "When the sprocket is larger, the links bend at a smaller angle," Spicer explains. "There's less frictional work, and as a result, less energy is lost."
The second factor that affected efficiency was tension in the chain. The higher the chain tension, Spicer says, the higher the efficiency score. "This is actually not in the direction you'd expect, based simply on friction," he says. "It's not clear to us at this time why this occurs."
g
Did they account for reduced tension on the non-drive side of the chain when running smaller rings/cogs? Was that published in Human Power or one of their other journals? (I couldn’t find it, but I got tired of digging after about six issues…)
yup and that is what you want…
I didn’t claim either setup was more efficient. I was just clearing up fundamental misunderstandings evident in some of the prior posts. (I didn’t mean to pick on Tom A.'s correctly stated post. Maybe I should have replied to one of the others…)
EDIT: BTW, thanks for the link. Interesting study…
So where does chainline fit into all this? Does it still matter? Is FD rub a bigger overriding factor than tension and speed?
i always preferred the big ring on the flats. seems to me we should be using “triple” BBs and setting up our FDs so that 39/11 is still a possible gear to use.
The chain in a bicycle drivetrain is remarkably efficient. If you perceive any difference with the smoothness between big ring/small ring combo with the same gear ratio it’s likely to do with the chainline more than anything else.
e.g., a 53X19 and 39X14 are essentially the same but the the chainline (influenced by the length of chainstays, cassette size, and other drivetrain factors) might be the reason.
e.g., a 53X19 and 39X14 are essentially the same but the the chainline (influenced by the length of chainstays, cassette size, and other drivetrain factors) might be the reason.
Except that the latter combination has lower drivetrain resistance on the order of 1-2 watts. Chainline, as Spicer found, isn’t all that important. Frictional forces increase with speed. That factor pretty much overwhelms all the others.
When I ride my TT bike with the 53/39 the SRM-Powertap difference is always greater than on the road bike with the 50/34. I actually put the compact on the TT bike for a while to see if the chainstay length was the issue. It wasn’t.
- jens