Power = work/time
#1
Senior Member
Thread Starter
Join Date: Apr 2006
Posts: 267
Bikes: Cannondale CAAD8 with Ultegra/105, Ksyrium Elites, Powertap Pro, Toupe saddle, and Thomson X2 stem (gotta have something Thomson); Felt F65, Trek SU200, Trek 7300 FX
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
Power = work/time
Originally Posted by The Cyclists Training Bible
power = work/time
At the risk of oversimplification, in cycling "work" is essentially gear size and "time" is cadence. So if gear size is increased and cadence kept steady, power rises. Or, if cadence is increased (time per revolution of the crank is decreased) while using the same gear size, power also rises.
At the risk of oversimplification, in cycling "work" is essentially gear size and "time" is cadence. So if gear size is increased and cadence kept steady, power rises. Or, if cadence is increased (time per revolution of the crank is decreased) while using the same gear size, power also rises.
#3
Senior Member
Thread Starter
Join Date: Apr 2006
Posts: 267
Bikes: Cannondale CAAD8 with Ultegra/105, Ksyrium Elites, Powertap Pro, Toupe saddle, and Thomson X2 stem (gotta have something Thomson); Felt F65, Trek SU200, Trek 7300 FX
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
When I first read that, it seemed more than just oversimplified, but rather entirely wrong unless I'm missing something.
Last edited by Cyclologist; 12-15-07 at 04:01 PM.
#5
Registered User
Join Date: Jun 2006
Location: Gilbert AZ
Posts: 1,321
Bikes: Cervelo S5 SRAM Red -
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
Pt has a torque tube in the hub. as well as the Ergemo I believe has one in the BB.
#8
Making a kilometer blurry
PT measures torque and wheel speed to give you force and distance per time. From that you can calculate work and power.
#9
Knocking off the rust
Join Date: May 2006
Location: ICT
Posts: 349
Bikes: LeMond Tourmalet, Specialized Tarmac
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
I agree - he probably shouldn't have taken the risk.
Since work = force x distance, I can see where Friel equates distance with gear size. But reading that text you could infer that he sees it as a linear relationship. I think he's missing (or choosing to overlook for simplification) the relationship that force must increase with increasing proportions of distance over time (i.e. velocity).
It's a good book, but there are a few areas like that where it could have benefited by better editing.
#10
Burning Matches.
Join Date: Jan 2006
Location: San Jose, CA
Posts: 9,714
Mentioned: 9 Post(s)
Tagged: 0 Thread(s)
Quoted: 4077 Post(s)
Liked 1,002 Times
in
676 Posts
Originally Posted by Friel
Power = Torque * Rotational Speed
At the risk of correct explanation, torque is the amount of force you put on the pedals (multiplied by crank length) and rotational speed is your cadence. So if you put more force on the pedals (say, by going to a higher gear), power rises. Or, if cadence is increased while using the same gear size, power also rises.
At the risk of correct explanation, torque is the amount of force you put on the pedals (multiplied by crank length) and rotational speed is your cadence. So if you put more force on the pedals (say, by going to a higher gear), power rises. Or, if cadence is increased while using the same gear size, power also rises.
__________________
ElJamoquio didn't hate the world, per se; he was just constantly disappointed by humanity.
#11
Ho-Jahm
Join Date: Nov 2006
Location: Manchester, NH
Posts: 4,228
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 5 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
I agree - he probably shouldn't have taken the risk.
Since work = force x distance, I can see where Friel equates distance with gear size. But reading that text you could infer that he sees it as a linear relationship. I think he's missing (or choosing to overlook for simplification) the relationship that force must increase with increasing proportions of distance over time (i.e. velocity).
It's a good book, but there are a few areas like that where it could have benefited by better editing.
Since work = force x distance, I can see where Friel equates distance with gear size. But reading that text you could infer that he sees it as a linear relationship. I think he's missing (or choosing to overlook for simplification) the relationship that force must increase with increasing proportions of distance over time (i.e. velocity).
It's a good book, but there are a few areas like that where it could have benefited by better editing.
I can push a 53x13 on a flat surface at 90 rpm with no headwind.
Does my power change if my speed, gearing and cadence stay the same but now I'm going downhill? Or uphill? Or into a headwind? Or into a tailwind?
Obviously yes. There is more to power than gearing and cadence.
#12
Member
Join Date: May 2007
Location: Milton, WV
Posts: 32
Bikes: '08 Cannondale System Six,'08 Fuji Outland RC, early 90's Focus RS500, '05 Ironhorse Warrior Disc
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
Power is also going to depend on the weight of the rider (ie. when going uphill). We did power tests in physics, and this simplified explanation of power would only apply in a setting with zero friction from both rolling resistance and wind.
#13
Senior Member?
Join Date: Oct 2005
Location: Denver
Posts: 1,977
Bikes: orbea onix, Cervelo SLC, Specialzed Allez, Cervelo P3 Alu
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Likes: 0
Liked 1 Time
in
1 Post
power = work/time.
change some (fundamental physics)stuff around and you get
Power= torque*angular velocity.
Angular velocity varies directly with cadence, so
Power = torque*cadence to a constant.
This applies both at the cranks and at the hub on the wheel. The gearing system can trade out torque for cadence, but power will be the same under the came conditions (speed, incline, etc) regardless of cadence.
change some (fundamental physics)stuff around and you get
Power= torque*angular velocity.
Angular velocity varies directly with cadence, so
Power = torque*cadence to a constant.
This applies both at the cranks and at the hub on the wheel. The gearing system can trade out torque for cadence, but power will be the same under the came conditions (speed, incline, etc) regardless of cadence.
#15
Senior Member?
Join Date: Oct 2005
Location: Denver
Posts: 1,977
Bikes: orbea onix, Cervelo SLC, Specialzed Allez, Cervelo P3 Alu
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Likes: 0
Liked 1 Time
in
1 Post
on a hill, watts/kg are going to determine your steady state speed.
On a flat, watts/frontal area are what matters
For sprinting, it's some combo of the 2, but it's mostly positioning.
This is true even with air resistance. Generally it's safe to ignore rolling resistance in this type of discussion, because it's minimal (i would say negligible) compared to air resistance at high speeds, and it's again minimal compared to the force of gravity at low speeds.
Generally a heavier rider will have a higher overall power output, but a lower power/weight ratio.
This is itself a gross oversimplification, and outside of the professional peloton, it's mostly wrong.
On a flat, watts/frontal area are what matters
For sprinting, it's some combo of the 2, but it's mostly positioning.
This is true even with air resistance. Generally it's safe to ignore rolling resistance in this type of discussion, because it's minimal (i would say negligible) compared to air resistance at high speeds, and it's again minimal compared to the force of gravity at low speeds.
Generally a heavier rider will have a higher overall power output, but a lower power/weight ratio.
This is itself a gross oversimplification, and outside of the professional peloton, it's mostly wrong.
#16
Senior Member
~7% at 250 W for the same rider up a 5% grade. (per analyticcycling.com)
Last edited by asgelle; 12-15-07 at 06:57 PM.
#17
Senior Member
Join Date: Sep 2006
Location: Tucson, AZ
Posts: 260
Bikes: 1993 Specialized Allez, 2003 Gios A70 Ultralite, 2002 Rossin Synthesis
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
yes, rolling resistance is not negligible, as you indicate. it takes significantly more power for me to average 25mph on a still day than it does to average 15mph into a 10mph headwind.
#18
Senior Member
I'm not sure what there is to discuss.
FWIW, this is how I do trainer and roller interval sessions. I don't use a heart rate monitor or powermeter or anything, I just pick a cadence and gearing that has me feeling the right amount of pain, then use the speedometer to maintain wheel speed, and therefore, my power output, constant over the interval. If my speed starts drooping, then I know that my power output is drooping, and I need to step it up.
FWIW, this is how I do trainer and roller interval sessions. I don't use a heart rate monitor or powermeter or anything, I just pick a cadence and gearing that has me feeling the right amount of pain, then use the speedometer to maintain wheel speed, and therefore, my power output, constant over the interval. If my speed starts drooping, then I know that my power output is drooping, and I need to step it up.
__________________
Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
#19
Magnesium Dogmatic
Join Date: Jun 2006
Location: Long Beach, CA
Posts: 1,939
Bikes: Look 585 Ultra, Pinarello Dogma, Pegoretti Duende, Orbea, Cannondale Capo
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
Your power does not vary with the gear if your speed remains constant.
What varies with gear is the force and the distance equation as jayhawken said. The higher the gear, the more you need more force over less distance, with distance being the circumference of your pedal stroke.
What varies with gear is the force and the distance equation as jayhawken said. The higher the gear, the more you need more force over less distance, with distance being the circumference of your pedal stroke.
#20
Senior Member
#21
cab horn
Join Date: Jun 2004
Location: Toronto
Posts: 28,353
Bikes: 1987 Bianchi Campione
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 42 Post(s)
Likes: 0
Liked 25 Times
in
18 Posts
Next to nothing unless it's so tight that your derailleur breaks off. All you need is a fixed gear and moving the axle/back forth to see the effects yourself.
#22
Senior Member
#23
Making a kilometer blurry
Originally Posted by asgelle
Of course it does. Drive train losses are proportional to the tension in the chain and diameter of the sprockets. Now, how much of a change that makes is another story.
#24
Senior Member
Look at the original statement. Speed remains constant; therefore, power to the road is constant. Since drive train losses change (slightly) with gearing, the only way for speed to stay constant is for power from the rider to change. This has nothing to with how power is measured or even that it be measured at all.
#25
Administrator
Join Date: Apr 2005
Location: Hudson Valley, NY
Posts: 32,978
Bikes: Merlin Cyrene '04; Bridgestone RB-1 '92
Mentioned: 325 Post(s)
Tagged: 2 Thread(s)
Quoted: 11952 Post(s)
Liked 6,599 Times
in
3,465 Posts
I'm too tired for this.
__________________
See, this is why we can't have nice things. - - smarkinson
Where else but the internet can a bunch of cyclists go and be the tough guy? - - jdon