Power calculation

jgsi

I am using this site
http://bikecalculator.com/wattsUS.html

I have basic ave speed.. distance to input..
my 'problem' is that I put the last stats from a 15 mile segment of a 20 mile ride.. (the issue was .. I blew up...anyways thats digressing)

but the power calc indicated I was pushin out 340 watts..

I have to take that with a huge pinch of salt surely?

whats the accuracy with these types of generic formulas?

Anonymous

Not very. Take with more than a pinch of salt.

Alex_Simmons/RST

This site is better:
http://www.analyticcycling.com/

Then you'll realise that you need to make an awful lot of assumptions in order to estimate power. The most reasonable estimates will come from steep hillclimbs.

jgsi

Alex_Simmons/RST wrote:

This site is better:
http://www.analyticcycling.com/

cheers for that...
funnily enough tho , I am getting a similar figure using one the models...

the only way I have at my disposal every so often is to use an exercise bike in a gym which has a wattage output... I will put it in my diary to do a comparision test... but it's one of those fat assed Technogym bikes where I can manage about 15 minutes before my body yelling to get off 'cos you cant adjust riding position at all.

Anonymous

Unless you have a power meter on your bike, the best bet is just to forget training with power IMO. The only time you can rely on an exercise bike is if you always use the same one and nothing else. Woefully inaccurate but at least consistent...

Alex_Simmons/RST

JGSI wrote:

Alex_Simmons/RST wrote:

This site is better:
http://www.analyticcycling.com/

cheers for that...
funnily enough tho , I am getting a similar figure using one the models...

What assumptions are you using?

jgsi

[/quote]What assumptions are you using?[/quote]

Speed For Given PowerSpeed For These Parameters 8.51 m/s
Power 330 watts
Frontal Area 0.5 m2
Coefficient Wind Drag 0.5 Dimensionless
Air Density 1.226 kg/m3
Weight Rider & Bike 83 kg
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill 0.03 decimal

The ride stat was recorded as 15.5 miles at ave speed 19.89mph
I blew into a headwind , prob over 6mph and a grinding grinding slope upwards which got me

am I going way wrong here?

Mettan

JGSI wrote:

Speed For Given PowerSpeed For These Parameters 8.51 m/s
Power 330 watts
Frontal Area 0.5 m2
Coefficient Wind Drag 0.5 Dimensionless
Air Density 1.226 kg/m3
Weight Rider & Bike 83 kg
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill 0.03 decimal

The ride stat was recorded as 15.5 miles at ave speed 19.89mph
I blew into a headwind , prob over 6mph and a grinding grinding slope upwards which got me

am I going way wrong here?

Just for interest, what was your Avg over the full 20 miles, as opposed to the 15.5 mile "segment"? - (Not a point about power, just curious - if I'm being nosy, ignore).

TarmacExpert

JGSI wrote:

Speed For Given PowerSpeed For These Parameters 8.51 m/s
Power 330 watts
Frontal Area 0.5 m2
Coefficient Wind Drag 0.5 Dimensionless
Air Density 1.226 kg/m3
Weight Rider & Bike 83 kg
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill 0.03 decimal

The ride stat was recorded as 15.5 miles at ave speed 19.89mph
I blew into a headwind , prob over 6mph and a grinding grinding slope upwards which got me

am I going way wrong here?

That sounds like a very high power estimate. I did a lap of Richmond Park the other week with a PowerTap hub. Weight of bike and rider is the same as you, 83kg. I averaged over 21mph, but my average power was only 270W. At your weight, if you had an FTP of 330W, you would probably be good enough to be a Cat 1 rider in the UK.

Alex_Simmons/RST

JGSI wrote:

What assumptions are you using?

Speed For Given PowerSpeed For These Parameters 8.51 m/s
Power 330 watts
Frontal Area 0.5 m2
Coefficient Wind Drag 0.5 Dimensionless
Air Density 1.226 kg/m3
Weight Rider & Bike 83 kg
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill 0.03 decimal

The ride stat was recorded as 15.5 miles at ave speed 19.89mph
I blew into a headwind , prob over 6mph and a grinding grinding slope upwards which got me

am I going way wrong here?

Well:
1. 19.89 mph = 8.89 m/s

2. what bike/position were you riding? Do you really have a CdA of 0.5 x 0.5 = 0.25 m^2. That's like a CdA for a time trial rider.

3. Just checking - you were going up a 3% grade for the whole 15.5 miles at 19.89 mph?

4. Crr of 0.004. What tyres/tubes are you running? Road surface - smooth asphalt or rough road? 0.004 would be for very very good race tyres on a smooth road.

5. there was a headwind - what speed was the headwind?

6. temp, air pressure, humidity, altitude? (needed for air density calcs)

jgsi

Mettan wrote:

Just for interest, what was your Avg over the full 20 miles, as opposed to the 15.5 mile "segment"? - (Not a point about power, just curious - if I'm being nosy, ignore).

Not at all
the entire ride was 21 mile and after I managed to hit the lap button after I blew, I crawled in the last 4 mile .. making the ride ave 17.9... there was an initial lap of 1 mile or so where I have to get thru lights etc in order to get to the loop where I can push hard

Alex_Simmons/RST

I have to say, I'm having trouble believing a road bike ride at 19.9 mph up a 3% grade into a strong headwind for a bit more than 3/4 of an hour's duration.

The power required to do that would give you a power to weight ratio capable of winning alpine stages in the TdF.

jgsi

Alex_Simmons/RST wrote:

Well:
1. 19.89 mph = 8.89 m/s

2. what bike/position were you riding? Do you really have a CdA of 0.5 x 0.5 = 0.25 m^2. That's like a CdA for a time trial rider.

3. Just checking - you were going up a 3% grade for the whole 15.5 miles at 19.89 mph?

4. Crr of 0.004. What tyres/tubes are you running? Road surface - smooth asphalt or rough road? 0.004 would be for very very good race tyres on a smooth road.

5. there was a headwind - what speed was the headwind?

6. temp, air pressure, humidity, altitude? (needed for air density calcs)

Hopefully you can shed more light .
the bike weighs in at 9kg
CdA - I just took the default value, as I am not up on that
I rode on the drops and tried to keep to using gearing from 65" to 87"
The amount of gradient 120 meters of 'climb' .. so the profile a bit lumpy but not hilly
25mm tyres.. again I took default value for lack of knowledge
road surface more or less well surfaced tarmac - this is the UK after all
Headwind . yes ..always a nigglin one. Metcheck gave it as about 6mph yesterday
Air temp was warm 20c..humidity low tho' altitude not an issue..

this is getting addictive
:?

jgsi

Alex_Simmons/RST wrote:

I have to say, I'm having trouble believing a road bike ride at 19.9 mph up a 3% grade into a strong headwind for a bit more than 3/4 of an hour's duration.

The power required to do that would give you a power to weight ratio capable of winning alpine stages in the TdF.

.. for this reason, I am asking of course, I value the opinion

Alex_Simmons/RST

JGSI wrote:

Alex_Simmons/RST wrote:

Well:
1. 19.89 mph = 8.89 m/s

2. what bike/position were you riding? Do you really have a CdA of 0.5 x 0.5 = 0.25 m^2. That's like a CdA for a time trial rider.

3. Just checking - you were going up a 3% grade for the whole 15.5 miles at 19.89 mph?

4. Crr of 0.004. What tyres/tubes are you running? Road surface - smooth asphalt or rough road? 0.004 would be for very very good race tyres on a smooth road.

5. there was a headwind - what speed was the headwind?

6. temp, air pressure, humidity, altitude? (needed for air density calcs)

Hopefully you can shed more light .
the bike weighs in at 9kg
CdA - I just took the default value, as I am not up on that
I rode on the drops and tried to keep to using gearing from 65" to 87"
The amount of gradient 120 meters of 'climb' .. so the profile a bit lumpy but not hilly
25mm tyres.. again I took default value for lack of knowledge
road surface more or less well surfaced tarmac - this is the UK after all
Headwind . yes ..always a nigglin one. Metcheck gave it as about 6mph yesterday
Air temp was warm 20c..humidity low tho' altitude not an issue..

this is getting addictive
:?

OK, so you weren't climbing a long hill/mountain.

On flat terrain, with a CdA of 0.3m^2 and Crr of 0.005, with a direct headwind of 6mph you would average ~ 250 watts. That's ~ 3.4 W/kg.

jgsi

Alex_Simmons/RST wrote:

OK, so you weren't climbing a long hill/mountain.

On flat terrain, with a CdA of 0.3m^2 and Crr of 0.005, with a direct headwind of 6mph you would average ~ 250 watts. That's ~ 3.4 W/kg.

I'll take that