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

[Alex321]

thank you for your input Sometimes I think there are too many gears. I'm not averse to riding in the middle chain ring and I often find I have to do that to cope with head winds or gradually rising roads. For me there is an issue, however, in that the increase in cadence required to maintain my road speed is generally more than my legs can cope with. For example, riding the big chain ring/small rear cog at my natural cadence of 75rpm gives a (theoretical) road speed of 27.3mph. To reach that same speed riding the middle chain ring/small rear cog requires a cadence of 96rpm ... and my legs just don't go that fast and never have done.

If you can ride on the flat in 50/11 and 75rpm for more than a few minutes, then you are exceedingly fit, and could expect to be winning most club TT races.

Most of us will rarely use the top gear on the flat, and very few of us "older" riders could maintain much more than 20mph for very long. I think you ,may be expecting to be able to do too much.

 

[Bristolian]

Your weight loss is more than just remarkable. Well done.

But I'm afraid that there is no mechanical magic. Being in the same weight range as you are in the moment (down from 120something) and in the early 50s I can comprehend your experiences: keeping the cadence is similar difficult as pushing higher gears, but different. Probably it is a bit easier to work on the higher cadence, but with patience. Then shorter cranks could be helpful to spin lighter gears a bit faster. Assuming that your position on your bicycle is good, I'd try to deliberately choose a slightly lower gear on a section where you can comfortably ride and try to reach your usual speed - for example if your usually riding around 25km/h on 39:18, choose 50:24 instead. Over time it will become easier.

Hope that helps.

E.

Hi EckyH, and thanks for your input The weight loss is less remarkable when you factor in the 4 years it has taken, but thanks for the compliment. Ditto for your journey as well.

I'm realistic enough to know there's no magical bullet that'll turn me back into an 18 year old version of me - heaven forbid - but I could do with a little mechanical advantage to make life easier Even as a very fit teenager (soldier, competitive cyclist, cricketer, runner) my legs absolutely refused to work at high speed. They've not gotten any more cooperative over time

 

[cyberknight]

If you can ride on the flat in 50/11 and 75rpm for more than a few minutes, then you are exceedingly fit, and could expect to be winning most club TT races.

Most of us will rarely use the top gear on the flat, and very few of us "older" riders could maintain much more than 20mph for very long. I think you ,may be expecting to be able to do too much.

My default flat speed is 20 -21 mph and that's 50 tooth and middle gear on an 11-28 cassette cadence around 80-85 rpm

 

[MichaelW2]

Crank length is part of the total gear ratio from foot to rim, but you select crank length for ergonomic reasons.

Short legs->short cranks.
Short upper leg/lowrt leg ratio-> short cranks.
For the same rider power output, short cranks at high revs are equiv to long cranks at low revs. Long cranks at same revs make a bigger circle to pedal.
Once you have selected appropriate crank length, you should select gear ratios that suit your power output. A significant change in crank length needs a slight change in sprocket ratios.

 

[MichaelW2]

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

Try that thought experiment with 10cm cranks and 30cm cranks. Sheldon Brown accounted for that difference in his "gain ratio":

https://www.sheldonbrown.com/gain.html

 

[screenman]

Try that thought experiment with 10cm cranks and 30cm cranks. Sheldon Brown accounted for that difference in his "gain ratio":

https://www.sheldonbrown.com/gain.html

He is wrong in what we were talking about, regardless of crank size one revolution is one revolution.

 

[presta]

it's harder to factor in crank length

It's no harder than: Chainring teeth x Wheel radius / (Sprocket teeth x Crank length)

One revolution is just that regardless of crank lenght.

'One revolution' isn't a definition of the input. The energy supplied at the pedal is force x distance, and the force and distance both change when you change the crank length. Changing the crank length changes the ratio of force to distance just as changing gears does, and the overall gear ratio from pedal to road surface is the distance travelled by the periphery of the wheel divided by the distance travelled by the pedal:

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

If you're going to leave out the crank length then the same argument applies to the wheel.

No it doesn't, and no it isn't.

The gear ratio has a specific meaning, and it is the ratio between number of revolutions of the input and number of revolutions of the output.

https://www.collinsdictionary.com/dictionary/english/gear-ratio
https://dictionary.cambridge.org/dictionary/english/gear-ratio
https://www.merriam-webster.com/dictionary/gear ratio

The length of the lever driving the input gear has no bearing on the ratio, but it does affect the effort required to turn the input gear.

If you're going to define bike gears as just the sprocket and chainring you have to leave out the wheel, if you define bike gears as the overall ratio that the rider experiences, then you have to include the wheel and the crank. At the moment we have the harebrained 'gear inch' system that's neither one nor the other.

The point that nobody seems able to grasp on here is that changing the crank length changes the distance that the pedal travels as well as the force on the pedal, and energy at the input is force times distance, just as energy at the output is.

Yes, which is what the gearing is there to control..

The ratio of force at the output of a machine to force at the input is its Mechanical Advantage, the ratio of distance travelled at the input to distance travelled at the output is the Velocity Ratio, with Mechanical Advantage being the Velocity Ratio multiplied by the efficiency. Gear ratio is the reciprocal of the Velocity Ratio.

Crank length is part of the total gear ratio from foot to rim, but you select crank length for ergonomic reasons.

There are all sorts of factors that affect the choice of wheel size too, but that doesn't change the fact that wheel size is part of the overall gear ratio. The problem with 'gear inches' is that it acknowledges the part the wheel plays in the gearing whilst ignoring the role of the crank. It's just plain irrational and inconsistent.

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

That's because the 'gear inch' system excludes the crank length. You're making a circular argument: "Gear inches leave out crank length therefore crank length doesn't affect gear inches".

The problem with gear inches is that it's an incomplete definition of the bike gearing because it omits one of the four variables that determine it.

 

[screenman]

"The point that nobody seems able to grasp on here is that changing the crank length changes the distance that the pedal travels as well as the force on the pedal, and energy at the input is force times distance, just as energy at the output is." I grasped that many years ago.

So at what point does the legs or force change the gear ratio? Which is what was being talked about when I jumped in.

 

[Alex321]

If you're going to define bike gears as just the sprocket and chainring you have to leave out the wheel, if you define bike gears as the overall ratio that the rider experiences, then you have to include the wheel and the crank. At the moment we have the harebrained 'gear inch' system that's neither one nor the other.

I don't care in the least what "gear inches" are. I was talking about gear ratios.

And those are well defined. If you want to define ratio as something else, go ahead, but don't expect people to accept it.

The point that nobody seems able to grasp on here is that changing the crank length changes the distance that the pedal travels as well as the force on the pedal, and energy at the input is force times distance, just as energy at the output is.

Can you point to anything anybody has said which suggests they "don't grasp" that?

I think we all understand it perfectly well.

Which is why we have all agreed that the effort required varies with trh crank length.

The ratio of force at the output of a machine to force at the input is its Mechanical Advantage, the ratio of distance travelled at the input to distance travelled at the output is the Velocity Ratio, with Mechanical Advantage being the Velocity Ratio multiplied by the efficiency. Gear ratio is the reciprocal of the Velocity Ratio.

No it isn't.

Gear ratio is the ratio between the gears.

If you have a 50- tooth ring and a 25 tooth sprocket, then the gear ratio is 1:2.
It really is that simple.

There are all sorts of factors that affect the choice of wheel size too, but that doesn't change the fact that wheel size is part of the overall gear ratio.

No. It isn't.

You clearly don't understand what a gear ratio is. Read those three definitions I posted links to. They al say the same thing, in slightly different words, and NONE of them agree with you.

 

[roubaixtuesday]

Changing crank length, as @Alex321 says, does not change the gear ratio.

It does, however change the force required at the pedal to generate a given force at the tyre/road interface (though not much in reality, given the range of cranks available).

It also changes the angle your knee goes through with each pedal stroke.

All three things matter to a cyclist; you can argue about the relative importance.

 

[mickle]

The force that can be applied at the pedal will vary depending on the applicable angle of the legs. That angle will change over the period of a pedal stroke (and vary more for longer cranks) and is not a fixed value through the full pedal stroke. Plus the relative energy cost is lower for the shorter cranks. With shorter cranks it is easier to maintain a higher cadence. As per link above with shorter cranks you can put out higher power numbers.

How does 'leg angle' affect the force that can be applied to the pedal?

And how might it apply to recumbents?

 

[Ming the Merciless]

How does 'leg angle' affect the force that can be applied to the pedal?

And how might it apply to recumbents?

It’s affects the max force that can be generated by the legs through the pedal axle. Plus what percentage of that force is accelerating the pedal axle. You cannot uniformly generate the same max force through all leg angles. Thus is you’re going to talk about torque you can’t ignore what forces can be applied consistently through the full pedal stroke. Plus the relative energy cost of many pedal strokes (of different length axles) as a result.

But unless you know the biology as well as the maths you’re not going to get very far.

 

[MichaelW2]

How does 'leg angle' affect the force that can be applied to the pedal?

And how might it apply to recumbents?

Many short riders are riding long cranks. Their upper leg and hip travel through a very high range of motion , beyond the normal efficient range.
Riding too small cranks is not so bad but still places the range of motion outside optimum.

 

[mickle]

Many short riders are riding long cranks. Their upper leg and hip travel through a very high range of motion , beyond the normal efficient range.
Riding too small cranks is not so bad but still places the range of motion outside optimum.

It’s affects the max force that can be generated by the legs through the pedal axle. Plus what percentage of that force is accelerating the pedal axle. You cannot uniformly generate the same max force through all leg angles. Thus is you’re going to talk about torque you can’t ignore what forces can be applied consistently through the full pedal stroke. Plus the relative energy cost of many pedal strokes (of different length axles) as a result.

But unless you know the biology as well as the maths you’re not going to get very far.

Yeah. But. What about *leg angle*? Where are you measuring it?

 

[Ming the Merciless]

Yeah. But. What about *leg angle*? Where are you measuring it?

The usual leg joints unless the bones are broken