Smaller chain ring or shorter cranks ... or won't it matter?

[presta]

Technically, it doesn't alter the gear ratio at all - one revolution of the pedals still produces exactly the same distance travelled.

Technically, it does alter the gear ratio. As I said above, with shorter cranks your foot travels a shorter distance, and the gear ratio is distance travelled at the output divided by the corresponding distance travelled at the input.

Are you channelling Sheldon
https://www.sheldonbrown.com/gain.html

Brown's just proliferating more unnecessary definitions, all that's needed is:

Chainring teeth x Wheel radius / (Sprocket teeth x Crank length)

This then is the true gear ratio overall, a simple number that includes all the relevant factors, and is dimensionless.

 

[roubaixtuesday]

Technically, it does alter the gear ratio. As I said above, with shorter cranks your foot travels a shorter distance, and the gear ratio is distance travelled at the output divided by the corresponding distance travelled at the input.


Brown's just proliferating more unnecessary definitions, all that's needed is:

Chainring teeth x Wheel radius / (Sprocket teeth x Crank length)

This then is the true gear ratio overall, a simple number that includes all the relevant factors, and is dimensionless.

One ratio to rule them all and in the darkness bind them?

I love the smell of unevidenced assertion in the morning

 

[screenman]

Are you channelling Sheldon
https://www.sheldonbrown.com/gain.html

I do not read his blurb.

 

[wafter]

Technically, it does alter the gear ratio. As I said above, with shorter cranks your foot travels a shorter distance, and the gear ratio is distance travelled at the output divided by the corresponding distance travelled at the input.

I think both points hold some validity depending on where you choose to take the input from.

Many consider the gear ratio only as a function of the chainrings and cassette; rightly or wrongly... IMO this is reasonable to an extent as it's harder to factor in crank length (I guess you'd need to convert it to an effective circumference expressed in the same half-inch pitch units the the teeth represent on the chainwheels if you wanted it to play nicely with the typical way of calculating ratios based on sprockets and chainrings.

On top of that crank length tends to vary much less than tooth counts on various parts, while for the complete picture you'd want to be factoring in tyre size too - which I guess is where the idea of "gear inches" (or some other "distance per rev" metric) comes in handy as a unit.

 

[screenman]

Technically, it does alter the gear ratio. As I said above, with shorter cranks your foot travels a shorter distance, and the gear ratio is distance travelled at the output divided by the corresponding distance travelled at the input.


Brown's just proliferating more unnecessary definitions, all that's needed is:

Chainring teeth x Wheel radius / (Sprocket teeth x Crank length)

This then is the true gear ratio overall, a simple number that includes all the relevant factors, and is dimensionless.

So you are saying a shorter crank turns a 15 tooth sprocket into something different? One revolution is just that regardless of crank lenght.

 

[wafter]

So you are saying a shorter crank turns a 15 tooth sprocket into something different? One revolution is just that regardless of crank lenght.

It alters the torque transmitted to the crank, and in turn into the rest of the drivetrain.. so considering that the gear ratios available on-bike are necessary to match the rider's available torque at the crank, arm length / its effect on torque input from the rider becomes a relevant factor.

 

[screenman]

It alters the torque transmitted to the crank, and in turn into the rest of the drivetrain.. so considering that the gear ratios available on-bike are necessary to match the rider's available torque at the crank, arm length / its effect on torque input from the rider becomes a relevant factor.

I don't disagree with that; what I disagree with is the statement that it changes the ratio. Out of interest, do you have a selection of bikes with different-length cracks?

 

[Bristolian]

"Top of the gears" means 50:11 or one gear lower? That would be real high gears (which I couldn't use even in the flat for more than 30 seconds).
We don't know at which speed you're cycling and which gears in particular you mean. It would be helpful if you could provide us that information.

If I get that right than one reason might be the gap between your expectation of yourself and your actual fitness in regards to strength endurance (power for high gears) and speed endurance (high cadence in lower gears). Both have to be trained.

But there are other factors which can lead to difficulties: people with shorter legs may find it easier to cope with shorter cranks. If you have the possibility to borrow a shorter crank with a compatible shaft, then give it a try. I switched from 170mm cranks to 175mm cranks on my commuter bike and I don't notice that much difference.

E.

I didn't mean THE top gear. Sorry for any confusion but back in my youth "keeping on top of the gear" meant being able to hold a steady cadence/speed in whatever gear you were in.

I'm aware that the cause of the problem is my fitness but at nearly 70 years of age and 108kg in weight (down from 136kg) that's not going to improve significantly so I'm looking for a mechanical way out.

 

[mickle]

I should add that a 75 inch year ratio remains the same, no matter what length crank you have on.

Technically yes. Practically, because your crank/lever is shorter it requires more effort to push it around. Effectively raising your gear ratios. Gear inch charts can't tell the whole story if they don't take crank length into account.

 

[Ming the Merciless]

More power with shorter cranks. Have a gander

https://pubmed.ncbi.nlm.nih.gov/11417428/

 

[screenman]

Technically yes. Practically, because your crank/lever is shorter it requires more effort to push it around. Effectively raising your gear ratios. Gear inch charts can't tell the whole story if they don't take crank length into account.

It does not change the ratio, it may or may not change the effort required, a 52x13 is 108 inches on a 700c wheel I think, changing cranl length makes no difference to that equation.

 

[mickle]

I have six bikes here, one running 150 cranks, one 165 the rest 172.5. The saddle height is slightly different, I have never felt the need to lower the bottom bracket, hearing wise I have not got a clue I just pedal, switching from one bike to another feels no different. The reason I have experimented with different lengths is to try and relieve a hip problem, this was at the advise if a physio trained bike fitter. In time all my bikes will have shorter cranks.

Presumably you've raised the saddle by 22.5mm on the bike that has 150mm cranks? The pedal axle is now 22.5mm higher at the bottom of the stroke than it was when equipped with 172.5mm cranks. *If* it bothered you, the only way today remedy it would be to change your frame to one with a greater BB shell drop. From a frame design POV 22.5mm is a significant difference.

 

[screenman]

Presumably you've raised the saddle by 22.5mm on the bike that has 150mm cranks? The pedal axle is now 22.5mm higher at the bottom of the stroke than it was when equipped with 172.5mm cranks. *If* it bothered you, the only way today remedy it would be to change your frame to one with a greater BB shell drop. From a frame design POV 22.5mm is a significant difference.

That is exactly what I have done, it handles no different to before, so why would I do that. Different styles of bikes have different height bottom brackets already and have done so for as long as I can remember.

 

[screenman]

My main reason of shorter cranks was to give more room at the top.

 

[mickle]

The simplest way of visualising the equation is to take the chain and sprockets out of it and refer back to high wheelers (ordinary/penny farthings) which is what we're referencing when we mention 'inches'. An eight inch crank has a different relationship to a 64" wheel than a six inch crank on a 64" wheel. The wheel size (AKA the ratio) remains the same but the effort to turn the wheel is greater with the shorter cranks. Many early bikes and trikes had adjustable pedals for this reason.