Gearing vs Weight

[Mark McM]

Originally Posted by morrisond

Can your muscles that are Fast Twitch muscles produce more power at high RPM than slow twitch muscles at low rpm?

Also when going up a hill due to the angle does that engage more of the slow twitch muscles?

Not quite. The names "Fast Twitch Muscle" and "Slow Twitch Muscle" aren't exactly misnomers, but they can be misleading. The two muscle fiber types were named based on how they respond to when stimulated. When stimulated (under no load), Fast Twitch muscles tend to contract faster, and Slow Twitch muscles tend to contract slower. But the more meaningful difference between the muscle types is in the energy systems that they use. Fast Twitch muscles produce power with more anaerobic processes, whereas Slow Twitch muscles produce power under more aerobic processes. Slow Twitch muscles are used in low force/power efforts, and they can sustain their output over longer periods of time. As the force/power requirement increases, Fast Twitch muscles start to be utilized more, but while Fast Twitch muscles can generate more force, their usage can be sustained only for shorter periods of time. So marathon running favor Slow Twitch muscle utilization, and power lifting favors Fast Twitch muscle utilization.

In terms of pedaling cadence, for a given power one can apply higher force on the pedals at a slower cadence, or lower force on the pedals at a faster cadence. But despite the legs moving faster at the higher cadence, higher cadences actually utilize more Slow Twitch muscles. This is because the muscle force (at a given power) is lower when the cadence is higher.

Originally Posted by morrisond

The reason I'm asking is that recently I was going up a climb standing and noticed that if dropped my Torso towards the bars it got a lot easier to pedal.

The forum's own member Ti Designs talks about this a lot. Our posture on the bike has a large effect on the range of motions and utilization of our muscles. When bending your torso over the bars, you are probably getting your gluteous muscles into a range of motion where they can generate more power. Search more on the forum for Ti Designs discussions about this.

[marciero]

Originally Posted by fourflys

my question is on a climb of the same pitch..

Would xx pounds of weight lost feel the same as gaining xx number of teeth on the largest cog?

I could certainly stand to lose xx pounds of weight..

If by "feel" you mean pedal force, one way to think about this is to consider the force at the pedals required to counter the gravitational force, as a function of weight, gearing, and gradient. This is given by

Fp = (wg)(R/l)(tf/tr)

where

Fp = force at the pedal, in lb
w = weight of bike and rider, in lb
g = gradient; that is, sine of the angle of elevation. That seems to be how it is typically defined.
l = crank length,
R = rear wheel radius
tr = rear teeth
tf = front teeth

The units for wheel radius and crank dont matter as long as they are the same.

Changing weight by dw would change the force

(dw*g)(R/l)(tf/tr)

While changing teeth on the rear from tr to tr' would change the force

(wg)(R/l)(tf)(1/tr' - 1/tr)

Equating these gives

tr'/tr = w/(w+dw)

So if you decrease weight by 5% it would feel the same as increasing rear teeth by a factor of 1/.95 or a little more than 5%.

Doing this with the front teeth would give a simple direct proportion.

We can also see that tr and w are proportional, so the same percent decrease in both would keep the force the same, for example going from 25 to 23 teeth-an 8% decrease- and losing 13.6lbs from 170 to 156.4 would leave the force unchanged.

It may seem odd that gradient drops out. But that is because we are dealing with proportions.