Assuming you get the same gear inches using the large chainring and rear sprocket and the small chainring and rear sprocket.
Which one is most efficient.. Using the large or small chainring?

The combo with the straightest chain line.

^^^ This.

Thread closed.

I would think the efficiency is the same. It depends on the actual ride for me. If I'm in a group ride I like to try to stay in the big ring so I don't have to upshift for surges and sprints.

If they are in comparable chain deflection, the smaller front ring will result in slower chain speed which will reduce the energy required to drive the chain.

Therefore it is a balancing act between chain deflection and chain speed/power losses.

Lennard Zinn seems to think the big/big combo is more efficient. Mathematically, they're both the same, of course.

Let's do this right and determine if it matters instead of embarking on discussions of math and physics.

Why on earth would we want to do that? Much more satisfying to really get into the weeds about something that doesn't make a lick of difference.

This is the 41!

The only real difference you'd notice is that a larger ring will produce a lower chain tension and thus a longer chain life, plus a longer life for the rings and cogs, since more cogs contact the chain at a time. Over thousands of miles, you'll notice slightly longer chain life. On the tandem, I've been running the big ring down to the 3rd cog, the 26T. Tandems are hard on drive chains, and I notice the difference.

Get a power meter wheel, a crank that can take 46, 50, and 52 tooth chainrings. (Alternatively have the rings custom made to fit the crank.) Set the bicycle as a singlespeed with a straight chain line and do test runs with the following gear combinations: 50x25, 52x26, 46x23. Either do constant speed runs and report power or constant power and report speed. This should give you the answer on which one is more efficient.

Every study of drive train losses I have ever seen over many decades say that big-big is more efficient than small-small. (On the track, this is both a choice that can be made and important, so it has been studied.) The studies I have seen all assumed a straight chainline, As posted above, chain deflection can easily matter more than the cog size. (The lower chain speed of smaller chainrings is combined with higher chain tension and therefore more friction as the pins rotate coming off the cassette/FW/cog teeth under full load. Also the chain/ cog tooth forces are higher, making for higher losses. You can observe the cassette/FW/cog teeth wearing faster riding small cogs, visible proof that you are doing more work.)

Ben

A friend did a little test a few years ago. He mounted a SRM and a PT on the same bike with a Frankencassette so that larger cogs were outboard of the smaller cogs (so the smaller cogs were inline with the smaller chainring and the larger cogs were inline with the larger chainring). I plotted the difference in the power recorded by the SRM and PT as he shifted through gears.



I've indicated the rings and cogs used for each section in the top panel. You can see that 54x16 is bracketed by 42x12 and 42x13. The middle panel shows his power output -- steady at first, then a ramp up for each gear combination. The bottom panel shows the difference between the SRM and PT power. What you can see is that more power gets through to the PT with the larger chainring, i.e., fewer losses.

The 54-15 doesn't look much different from the 42-12.

I don't have the links to data that Mr. Chung does, so you can equate my comments as anecdotal. Chain line, and side loading, is a red herring in chain efficiency, and chordal action of the chain results in more efficiency loss than side loading. IIRC, on 1/2" pitch chain, you start to see it more at 13T and smaller cogs.

If you mean a larger chainring (and smaller cog) I think you have it backwards.

The larger chainring combo means a smaller cog, which is harder to rotate, so the chain tension is higher...

Over how long? advantage to 2 bigger Chainring/cog sets is the tooth wear is slower because there is more to share it

.. and the flexing the chain friction is lower because of the larger diameter .



NB.

Belt drive sprockets are fairly large , because the carbon belt will break sooner if forced to wrap around too small a Sprocket.

New here... what is SRM and PT

At the same cadence and crank torque (which means at the same power), a larger chainring means higher chain speed. Since power is not only cadence * crank torque but also chain speed * chain tension, higher chain speed for the same power means lower chain tension.

SRM is a crankset power meter. PT is a rear hub power meter. In theory you can measure chain power losses by comparing power readings at the two.

They're power meters. The SRM measures power at the crank spider, the Power Tap measures power at the rear hub. That means the difference between the two readings tells you about losses between the spider and the hub.

I know I've said this before, but thanks for sharing your test results and knowledge here.

Nothing you will ever notice...and that is coming from a weight weenie!

You have four variable here; can you express it as an equation?

All force transferred along the chain that is normal to the chain-pin axis is the most efficient from a load-distribution standpoint. Forces are evenly shared by both inner and outer links, and side-load to the gear teeth is minimized. Any oblique angle of force in the chain line will apply more pressure on either the inner, or outer link, increasing material deflection, as well as more side-contact with gear teeth, increasing heat, and increasing friction. Reducing the force in the chain will decrease friction in the pins and gear teeth at normal and oblique angles, but minimally with the gear teeth, as there will be more contact with gear teeth under side-load.

All that taken into consideration, however, still means the difference will still be immeasurable with the +/- 2.5% accuracy error of most consumer-available power meters.

Yes.