Power vs Gear selection

wacka

Just a question out of interest.

Would the power required to drive a gear in the large chainring and a large sprocket be more than the power required to drive a gear in the small chainring a small sprocket if both gearing equates to the same speed at same cadence?

slowbike

I always feel it is easier to go faster in the big ring than the small ring - not that I can see any logic to it as the gear-inches could be the same and all the bearings are rotating at the same speed and the power required to go that speed is the same ... or is it?

NANS295

I believe your question is if you select a sprocket on the cassette that results in the same gear inches when in the big and small chainrings, would there be a difference in energy used (and cadence remains the constant). The answer is no, you would use the same amount of energy.

wacka

NANS295 wrote:

I believe your question is if you select a sprocket on the cassette that results in the same gear inches when in the big and small chainrings, would there be a difference in energy used (and cadence remains the constant). The answer is no, you would use the same amount of energy.

That was the answer I was after, cheers mate was just out of interest!

amaferanga

madasahattersley wrote:

Wouldn't be too quick to believe him. A study I found says a perfect chain line in 98.6% efficient, and the worst possible chain line in 81% efficient. That's a HUGE difference. That means you'd basically have to put in 15ish% more power to maintain the same speed.

I'd love to see that study because that sounds like nonsense.

jibberjim

NANS295 wrote:

I believe your question is if you select a sprocket on the cassette that results in the same gear inches when in the big and small chainrings, would there be a difference in energy used (and cadence remains the constant). The answer is no, you would use the same amount of energy.

There's a small advantage from larger rings so 53x23 would be better than 39x17. (due to losses from friction in the chain around the ring)
However there's also a small advantage from straighter chain lines so 39x17 would be better than 53x23 on normal bike set ups.

It's most likely that the 53x23 is actually better even with the http://hpva.org/HParchive/PDF/hp50-2000.pdf has lots of info.

But in reality the differences are tiny, madasahattersley 81% efficiency is a complete red herring, no study has ever found that, they've found chains which are that inefficient, but those are extremely low tension chains, such that you would never find on a bike with a derailleur (unless it was knackered). The difference in actual chainline is much smaller. See the paper above.

lawrences

i wasn't aware that the hardwood plywood and veneer association had conducted studies of chain inefficiencies.

NewTTer

amaferanga wrote:

madasahattersley wrote:

Wouldn't be too quick to believe him. A study I found says a perfect chain line in 98.6% efficient, and the worst possible chain line in 81% efficient. That's a HUGE difference. That means you'd basically have to put in 15ish% more power to maintain the same speed.

I'd love to see that study because that sounds like nonsense.

Agreed utter tosh

diamonddog

lawrences wrote:

i wasn't aware that the hardwood plywood and veneer association had conducted studies of chain inefficiencies.

bernithebiker

I did once read that Fabian Cancellera chose his gearing carefully for a TdF TT, so that he would spend as much time as possible with the straightest chain line…….

nickyboy zz

NewTTer wrote:

amaferanga wrote:

madasahattersley wrote:

Wouldn't be too quick to believe him. A study I found says a perfect chain line in 98.6% efficient, and the worst possible chain line in 81% efficient. That's a HUGE difference. That means you'd basically have to put in 15ish% more power to maintain the same speed.

I'd love to see that study because that sounds like nonsense.

Agreed utter tosh

And the maths is tosh too. To offset the effect of 81% v 98.6% efficiency would require 21.7% more power input

sungod

op - general rule is you're better on bigger chainring+sprocket than smaller ones, for most riders there's probably little practical difference, but someone doing a tt would value even a couple of seconds

loads on drivetrain efficiency here...

http://djconnel.blogspot.co.uk/search/label/drivetrain

...for instance this paper...

http://www.ihpva.org/HParchive/PDF/hp52-2001.pdf

...there are so many variables and models, draw your own conclusions