Are all equivalent gear-inch set ups equal?

BUICK

By which I mean, if you have two different set ups with the same overall gear inches, but one has a bigger chainring on the front (assume we are talking fixed here) are they equal in terms of efficiency? Or is there a difference due to 'leverage' of a bigger chainring? Or will friction affect the 'best' choice?

I'm guessing there won't be a BIG difference, but am interested to know peoples thoughts on this

andrewgturnbull

Hi there.

Technically a set-up with a bigger chainring and bigger cog at the back will be more efficient, as the chain wraps round a larger radius. But the differences are measured in fractions of fractions...

Then on the other hand, the more efficient big-big combination will weigh more, which may make a difference under certain circumstances.

In reality it doesn't make any difference that anyone could notice.

Cheers, Andy

BUICK

So, big/big wins because of more efficient transfer of energy? I suppose that's probably safer too given that it will put less strain on the chain with the pressure being distributed over a greater area. On the other hand, if you've got a nice strong chain the small/small combo would make for a lighter set up with less friction you would think...

meagain

Six o' one, half a dozen of t'other, springs to mind!

robbie the roadie

Very negligable.

markos1963

Biggest difference is wear, small cogs are always prone to higher wear rates. Anyone who owned a Suzuki GS550 motorbike in the 70/80's will attest to as the gearbox sprocket was tiny and the chainwear was terrible.

topdude

What about the effect the size of the chainring has on leverage.
A large ring will reduce the leverage of the crank arm,
A small ring will increase the leverage.
Therefore for the same overall gearing more effort will be required to turn a large ring than a small one ?
Or will it :?:

SteveR_100Milers

topdude wrote:

What about the effect the size of the chainring has on leverage.
A large ring will reduce the leverage of the crank arm,
A small ring will increase the leverage.
Therefore for the same overall gearing more effort will be required to turn a large ring than a small one ?
Or will it :?:

No. The leverage (or torque) is a fucntion of (leg) force x crank length. chainring size has no effect on the force transmitted to the rear sprocket.

John.T

What about the effect the size of the chainring has on leverage.
A large ring will reduce the leverage of the crank arm,
A small ring will increase the leverage.
Therefore for the same overall gearing more effort will be required to turn a large ring than a small one ?
Or will it

A small ring will pull the chain harder (with more force) but due to the smaller cog will not put any of this into the back wheel. It will increase the tenion on the top of the chain and may increase wear. For any given gear ratio the same power is transmitted.

SteveR_100Milers

sorry but it doesnt matter how big your chainrings are, the same force is applied to the chain for a given crank torque. Remember Newtons laws!

Power is a function of force (leg x crank) x velocity (cadence) unsurprisingly, and is the same at the wheel less any frictional forces in the transmission. There might be ever so slightly more friction in certain chainring and sprocket combinations, but I'd be surprised you could even measure these differences reliably with powertap or SRM cranks etc let alone feel the difference.

John.T

It is the law of levers that applies here. The laws of motion do not come into it. The rings are a form of lever that can provide a constant leverage. If the crank is X long with Y force applied to it then if the radius of the chainring is 1/2 X then the force applied to the chain is 2Y. Similarly if the radius of the chainring is 1/4X then the force appllied to the chain is 4Y. The only thing this affects is the tension in the driven section of the chain. Provided the gear ratio remains the same the power transmitted to the rear wheel remains the same as the cog size difference cancels out the leverage difference at the crank. I totaly agree that you could never measure any differences between big/big and small/small combinations. You just use the most appropriate one.