Changing gearing when changing crank length

[mikehhhhhhh]

I've recently changed crank length from 167.5mm to 160mm based on the advice of my bike fitter.

That's also involved a change of drivetrain. I've changed from SRAM 10-33 / 48-35 to Shimano 11-34 / 52-36 but the easiest ratio is identical, so I don't think it's a factor.

When climbing higher gradients in the easiest gear, it feels more difficult, I'm guessing because the peak torque figures are higher because of that shorter crank, hence the gravitation towards higher cadences with shorter cranks (I'm up ~10rpm)

What I'm trying to work out is the torque figure at a given weight/gradient/crank length/gear ratio/cadence to understand when I head off on a climbing holiday whether the weight I've lost is enough to compensate for the lower crank length or I need to go for smaller chainrings.

I'd rather avoid smaller chainrings and losing even more top end, hence wanting to quantify rather than just take a blind stab at it.

I know torque is pretty easy to calculate given power and rpm, but it's that weight/gradient component I need. Is there a calculator/modelling software out there that can work this out for me?

[wheelsONfire]

Well, a 52T is harder to push than a 48T. I have been using 48, 50 and 52T and i would say it's a big jump going for a 52T.
If torque through longer crankarm really is a deal, then i would say that 52T will feel really heavy.
Just look at this, how would your cassette equalize the resistance. What would your cogs on a cassette need to be if the resistance you feel, going from 48 to 52T be.
Quite alot. 1 cog on the chainring equals 3 on the cassette.
I have tried to figure if going to 165mm would feel alot different than a 170mm version.

[JoO]

With this formula you can compare the Gain Ratio.
How much your wheel moves for every unit of foot movement.
https://sheldonbrown.com/gain.html

[bobones]

I know torque is pretty easy to calculate given power and rpm, but it's that weight/gradient component I need. Is there a calculator/modelling software out there that can work this out for me?

http://bikecalculator.com/ will allow you to calculate power required for given speed, gradient and weight.
https://www.gear-calculator.com/ will allow you to compare gear ratios and provide speed for given cadence.

[Karvalo]

When climbing higher gradients in the easiest gear, it feels more difficult, I'm guessing because the peak torque figures are higher because of that shorter crank, hence the gravitation towards higher cadences with shorter cranks (I'm up ~10rpm)

I think what you're getting at is that peak force is higher to produce the same torque (torque = force x distance).

What I'm trying to work out is the torque figure at a given weight/gradient/crank length/gear ratio/cadence to understand when I head off on a climbing holiday whether the weight I've lost is enough to compensate for the lower crank length or I need to go for smaller chainrings.

All that sounds way too individual to you as a rider for you to be able to find any rote calculator to do the job. I mean, what else has happened while you've lost weight? Have all your other performance figures stayed the same? Unlikely.

The only 'calculation' I think that might help what you're trying to do is that 160/167.5 is about 1/1.05. Does that mean you want a 1.05x higher cadence and lower gear? Maybe, but it sounds like you've already self selected a bigger change than that.

Ultimately you'd be best just figuring out from your riding what cadence you want to keep above, and what gearing you need to achieve that at the speeds you think you'll be able to climb at, wouldn't you?

[TobinHatesYou]

This is the rub with going to shorter cranks. If you want the same feel/feedback through your legs while putting out 300W@90rpm with shorter cranks, then you necessarily have to be pedaling at a higher cadence. There’s no way around this, either adapt to riding at higher RPM with a lower gear ratio or use more force at the same gear ratio.

[mikehhhhhhh]

With this formula you can compare the Gain Ratio.
How much your wheel moves for every unit of foot movement.
https://sheldonbrown.com/gain.html

That helps, thank you.

I think what you're getting at is that peak force is higher to produce the same torque (torque = force x distance).

You're absolutely right, I meant force, not torque.

Ultimately you'd be best just figuring out from your riding what cadence you want to keep above, and what gearing you need to achieve that at the speeds you think you'll be able to climb at, wouldn't you?

I think you're right, I probably over-thought the weight reducing the effort to maintain a speed at a given gradient reducing the torque, but the fact of the matter is that I'm likely to be making similar power, so that weight decrease is more likely to translate to more speed and thus the same torque (And slightly higher force with shorter cranks) rather than me maintaining the same speed as before.

This points to needing to goto a 50-34 to maintain a similar force. Doh

This is the rub with going to shorter cranks. If you want the same feel/feedback through your legs while putting out 300W@90rpm with shorter cranks, then you necessarily have to be pedaling at a higher cadence. There’s no way around this, either adapt to riding at higher RPM with a lower gear ratio or use more force at the same gear ratio.

This bit is clear, the question is whether I can get away with the ratios I have now, which I was already close to the limit of previously or whether I need to compromise on top end and take some smaller chainrings.

[Karvalo]

I think you're right, I probably over-thought the weight reducing the effort to maintain a speed at a given gradient reducing the torque, but the fact of the matter is that I'm likely to be making similar power, so that weight decrease is more likely to translate to more speed and thus the same torque (And slightly higher force with shorter cranks) rather than me maintaining the same speed as before.

I still think you're over complicating things by trying to reverse engineer the force term. You already appear to know what effect the crank length change has made to your ideal cadence. If you know your cadence and your probable climb performance, you can choose your gearing.

Ultimately there's no difference to how you'd choose gearing in any other scenario, right?

[DaveS]

It's not that complicated. You need about 2t less up front or 2t more in back to get a 4-5% lower gear ratio. A 50/34 should work. Either that or do nothing and accept that you lost 4.5% in mechanical advantage at the low end.

[pdlpsher1]

This is the rub with going to shorter cranks. If you want the same feel/feedback through your legs while putting out 300W@90rpm with shorter cranks, then you necessarily have to be pedaling at a higher cadence. There’s no way around this, either adapt to riding at higher RPM with a lower gear ratio or use more force at the same gear ratio.

This is not true. It's far easier to push a higher force on the pedals at the same exertion 'feel' when the pedaling circle is smaller. The leg muscles are more efficient at producing force at smaller hip and knee angles. A good analogy is when you're on the Stairmaster and it's easier to take smaller steps vs. giant steps. When I switched from 170mm to 155mm cranks I had expected my cadence to increase to compensate for the lower torque on the shorter cranks. To my surprise my cadence remained the same, while power remained the same or higher. This can only mean the force on the pedals are higher than before. But the overall exertion 'feel' is better, meaning I felt it was easier to produce the same or higher power. If someone feels it's difficult to push the same power on a shorter crank, he should revert back to longer cranks. He doesn't benefit from shorter cranks. So the answer is a 'no', one doesn't need to worry about a lack of low gearing when using shorter cranks.

[pdlpsher1]

It's not that complicated. You need about 2t less up front or 2t more in back to get a 4-5% lower gear ratio. A 50/34 should work. Either that or do nothing and accept that you lost 4.5% in mechanical advantage at the low end.

I'm a Strava user and use Sauce for post ride analysis. Buckhorn/Stove Prairie/Rist wall/Glen Haven are my favorite places for my climbing science experiments. Come ride with me and we can chat about science. I'll send you my contact info. via PM.

[Karvalo]

This is not true. It's far easier to push a higher force on the pedals at the same exertion 'feel' when the pedaling circle is smaller. The leg muscles are more efficient at producing force at smaller hip and knee angles. A good analogy is when you're on the Stairmaster and it's easier to take smaller steps vs. giant steps. When I switched from 170mm to 155mm cranks I had expected my cadence to increase to compensate for the lower torque on the shorter cranks. To my surprise my cadence remained the same, while power remained the same or higher.

I'd be wary of making blanket physiological statements based on personal experience alone. AFAIK the (admittedly very limited) research done on this topic finds that most people do what the OP did, which is naturally increase their cadence by riding in a lower gear.

[Lina]

This is not true. It's far easier to push a higher force on the pedals at the same exertion 'feel' when the pedaling circle is smaller. The leg muscles are more efficient at producing force at smaller hip and knee angles. A good analogy is when you're on the Stairmaster and it's easier to take smaller steps vs. giant steps. When I switched from 170mm to 155mm cranks I had expected my cadence to increase to compensate for the lower torque on the shorter cranks. To my surprise my cadence remained the same, while power remained the same or higher.

I'd be wary of making blanket physiological statements based on personal experience alone. AFAIK the (admittedly very limited) research done on this topic finds that most people do what the OP did, which is naturally increase their cadence by riding in a lower gear.

It's well known fact that it's easier work for muscles when they're doing smaller range of motion. Anyone can test it themselves by going to the gym and seeing how much they can bench or squat with full rom and partial rom. The difference is HUGE. Now it's debatable if this has a big enough effect between the range of motion that one would benefit from/experience when switching to shorter cranks.

[TobinHatesYou]

This is the rub with going to shorter cranks. If you want the same feel/feedback through your legs while putting out 300W@90rpm with shorter cranks, then you necessarily have to be pedaling at a higher cadence. There’s no way around this, either adapt to riding at higher RPM with a lower gear ratio or use more force at the same gear ratio.

This is not true. It's far easier to push a higher force on the pedals at the same exertion 'feel' when the pedaling circle is smaller. The leg muscles are more efficient at producing force at smaller hip and knee angles. A good analogy is when you're on the Stairmaster and it's easier to take smaller steps vs. giant steps. When I switched from 170mm to 155mm cranks I had expected my cadence to increase to compensate for the lower torque on the shorter cranks. To my surprise my cadence remained the same, while power remained the same or higher. This can only mean the force on the pedals are higher than before. But the overall exertion 'feel' is better, meaning I felt it was easier to produce the same or higher power. If someone feels it's difficult to push the same power on a shorter crank, he should revert back to longer cranks. He doesn't benefit from shorter cranks. So the answer is a 'no', one doesn't need to worry about a lack of low gearing when using shorter cranks.

I’m not talking about the aerobic effect or the “overall” feel. I’m talking about the force vector that goes into turning the pedals. That MUST increase at the same cadence and gear ratio with smaller cranks, so skeletal and neuromuscular strain he feels at any particular gear ratio and cadence will be higher. So of course you basically must compensate with higher cadence and lower gearing, and in some cases that really throws things off.

[pdlpsher1]

Neuromuscular strain is higher but it feels easier with shorter cranks due to the smaller range of motion. What really matters is the perceived exertion, no?