To the mathematicians amongst you.

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Exactly the same
 

swansonj

Guru
This is directly applicable to the timing chain on tandems. I was chatting with the proprietor of my lbs about this and we agreed the trend seemed to be to fit smaller rings these days, counter to the argument of marginal efficiency gain. But it saves a bit of weight and a bit of chain, and reduces the risk of grounding the captain's ring on bumps. Or maybe it just looks better.
 

Vegan1

Guest
It's be a 30*30 due to the slightly lesser weight.
 

Ajax Bay

Guru
Location
East Devon
Pasting from @ColinJ 's link:
"Efficiency is higher when using larger sprockets because the chain benefits from a less extreme radius of rotation – chain links going around corners cause greater frictional power losses. The take away here is that if you can achieve the same gear using a large chainring + larger sprocket combination than a small ring + smaller sprocket combination it is a worthwhile consideration. The efficiency difference between an equal gear that involves the 24 sprocket and the 13 sprocket can be worth 1-2 watts when riding in the 200-400 watt range."
The next bullet is:
"“Cross Chaining” (riding with the chain at angles between the chainring and rear sprocket) really hurts efficiency – it’s pretty obvious [Edit: such a phrase often flags up that the science is dodgy ime - see below.] that frictional losses increase. [Edit: The authority the article quotes (Spicer - link below) says that the losses are rather low.] You may intuitively avoid this scenario because it can be noisy and to avoid dropping a chain but efficiency of power transfer is another key consideration."
But I have separately read (link below) that the drop in efficiency when cross-chaining (say at 2.5 degrees - large/large, say) is about 0.5% ie about the same efficiency difference gained by going large to third from large (say 50/21) as opposed to 34/14 (say) - NB same ratio.
So large to quite large is more efficient (same ratio/gear length) is more efficient, and indeed (heresy) large to large is just as efficient, even with the cross-chaining losses, than small to whatever (same ratio). Just consider whether to do this when you know there's a gratuitous advice giver on your wheel (see other thread) and make sure your chain is long enough to be content with large/large. And be prepared for your chain to rub on the cage. And if you need to change down further, perhaps unexpectedly, be prepared for a 'double change' ie front and rear together or accept a 32% change in ratio (compact) with the sudden drop onto the small chainring.
People might find this interesting reading.
"Human Power" Number 50 Spring 2000
TECHNICAL JOURNAL OF THE IHPVA

the article:
On the efficiency of bicycle chain drives
James B. Spicer and others
 
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