One for the boffins?...

Porkyboy · June 2007

Hi

Can anyone explain this to me? One of the sessions I do is an hour on a ergotrainer keeping my HR at about 80% of MHR, in my case about 150BPM. My results from last night are shown below with the results from a previous session shown in brackets, the previous session was a month or so ago.

Effort length - 1:00:00 (1:00:00)
Average effort HR - 148BPM (147BPM)
Average effort power - 238W (240W)
Average effort cadence - 90RPM (91RPM)
Average effort speed - 28mph (24.3mph)
Effort distance - 28m (24.27m)

My question, I'm crap at "sums", is this:

If you ride with a HR which is pretty much identical and produce an average power which is pretty much identical for the same time period, one hour, no weather factors (ergotrainer) why is the speed faster and the distance further? I'm not sure but is it possible that I've used a higher gear and therefore gone further and faster for the same apparent power/HR?

Because I don't understand the power/time/distance/speed relationship very well will fitness improvements just make you more efficient (same power and time producing more distance) or does your power always have to be higher for a given time to go further and faster?

or...

Do you think there a measurement problem? I doubt this, I'm using a calibrated CompuTrainer and am pretty careful about setups etc.

Sorry to be a bit thick but I can't get my hear round this.

PB

What do you mean your legs are hurting? Give it some welly man!

andyBcp · June 2007

The only thing I would say Porkyboy, is that if you are able to ride to a targeted power output, do it and forget about HR, as this will rise throughout the session. Trying to hold a targeted HR will produce a diminishing power output.

http://www.teamvelosportif.co.uk

Porkyboy · June 2007

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by andyB

The only thing I would say Porkyboy, is that if you are able to ride to a targeted power output, do it and forget about HR, as this will rise throughout the session. Trying to hold a targeted HR will produce a diminishing power output.
<hr height="1" noshade id="quote"></blockquote id="quote">

Hi Andy

Thanks for that, a phenomenon I've observed, cardiac drift I think I've seen it called, largely caused by muscle fatigue and dehydration is my understanding. I'd still like to hear if you or anyone else can throw some light on my observations. I'm been trying to understand the relationship between power, speed, ?gearing, HR etc, to get these things clear in my mind.

If a car/electric motor/cyclist produces a constant power output of say 300W (very small car!) for one hour will the speed and distance covered always be the same or what will influence the speed and distance if power and time remain the same? It seems to me that if a cyclist produces 300W for a given HR over 2 sessions months apart and in session 2 goes further faster something has changed. Clearly it's not power output and it's not a higher HR so what is the something? Efficiency? Strength?

I'll get to the bottom of this! Seems to me that if you ride an ergotrainer at a fixed wattage/load at a similar HR if distance and speed rise for a given power then improvements somewhere are taking place, but where [?]

PB

What do you mean your legs are hurting? Give it some welly man!

domtyler · June 2007

Something doesn't add up there. Are you sure you haven't been fiddling with the settings again? [;)]

~~~~~~~~~~~~~~~~~~~
Porridge not Petrol
________
Motorcycle tires

Porkyboy · June 2007

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by domtyler

Something doesn't add up there. Are you sure you haven't been fiddling with the settings again? [;)]
<hr height="1" noshade id="quote"></blockquote id="quote">

Yup, I'd confess if I had!

PB

What do you mean your legs are hurting? Give it some welly man!

domtyler · June 2007

Well for the same cadence and the same power output you should have gone the same speed, if you were in a higher gear then for the same cadence you would have had to have produced more power, everything else being equal. Can you simulate an incline on your machine? If you were going slightly downhill on the second trip compared to the first then you could have pushed a higher gear at the same power output but gone faster and further. I cannot think of anything else that would produce these results.

~~~~~~~~~~~~~~~~~~~
Porridge not Petrol
________
MOTORCYCLE TIRES

BeaconRuth · June 2007

The first thing you should do is remove HR from the puzzle. HR varies with fitness and other factors - riding with the same HR as a month ago cannot be assumed to lead to the same power, speed or anything else.

If you really are generating the same power for the same duration then you are doing the same total amount of work (power = work/time). If all resistance factors are constant than you can expect to travel the same distance for the same amount of work. (Work = resistance forces x dist)

My conclusion: there's something wrong with the set-up or the algorithms the Computrainer is using. You can't go 4 miles further for the same power against identical resistance. I take it the turbo was set in the same mode for both sessions?

Porkyboy · June 2007

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by BeaconRuth

The first thing you should do is remove HR from the puzzle. HR varies with fitness and other factors - riding with the same HR as a month ago cannot be assumed to lead to the same power, speed or anything else.

If you really are generating the same power for the same duration then you are doing the same total amount of work (power = work/time). If all resistance factors are constant than you can expect to travel the same distance for the same amount of work. (Work = resistance forces x dist)

My conclusion: there's something wrong with the set-up or the algorithms the Computrainer is using. You can't go 4 miles further for the same power against identical resistance. I take it the turbo was set in the same mode for both sessions?
<hr height="1" noshade id="quote"></blockquote id="quote">

Thanks Ruth.

Yup, same mode. Your conclusion is the same as mine [:(]

I'll investigate with the company!

Good training in any event.

PB

What do you mean your legs are hurting? Give it some welly man!

BeaconRuth · June 2007

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Porkyboy
If a car/electric motor/cyclist produces a constant power output of say 300W (very small car!) for one hour will the speed and distance covered always be the same or what will influence the speed and distance if power and time remain the same? <hr height="1" noshade id="quote"></blockquote id="quote">The only thing that will alter the speed and therefore distance is the resistive force acting.
<blockquote id="quote">quote:<hr height="1" noshade id="quote">It seems to me that if a cyclist produces 300W for a given HR over 2 sessions months apart and in session 2 goes further faster something has changed. Clearly it's not power output and it's not a higher HR so what is the something? Efficiency? Strength?<hr height="1" noshade id="quote"></blockquote id="quote">As in my previous post, HR is a confusagram, but the constant power should inidcate the same speed/distance unless something else has changed. It can only be changed by 'error' (wrongly/differently set up) or because the Computrainer is functioning in an unexpected way............

chriswcp · June 2007

Perhaps you were more aero the second time on the turbo[:D]

oldwelshman · June 2007

Distance covered? I did not realise they moved? [:D]
Maybe you lost some weight also [:D]

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meenaghman · June 2007

Was your rear tyre at the same pressure ? if it was less pressure, then less resistance if your trainer is similar to mine in setup. Also a worn tyre could have less resistance.

SteveR_100Milers · June 2007

Same conclusion as Ruth. Same power under same conditions = same speed or distance over a fixed time. One possible variant of course is resistance, more resistance lower speed and distance covered, but you said all the bike setting were unchanged.

Power (Watts) = Energy (joules)used per second
Energy (joules) = Force (Newtons) x Distance (Metres)

The force you apply as a torque to the pedal cranks is equal and opposite to the sum of all of the system resistances: friction (and normally wind resistance which in normal cycling is the biggest factor. Distance is the distance that you move the object against the resistive force, i.e. how far you move. This is effective power at the wheels. There is a power output at your crank, which is a factor of pedal torque and cadence. Pedal torque is measured in Newton metres, and is the force of your foot on the pedals muliplied by the length of the crank arm. The maths is all very similar to comparing BHP figures for cars.

Time! Time! It's always too long and there's never enough!

BeaconRuth · June 2007

Was thinking about this while out training just now......

Are you using the Computrainer in a constant-power mode? That is, the turbo ensures that you're generating the same power whatever cadence you use? If you are then our assumptions of constant resistance are flawed. By definition the turbo must be altering the resistance you're pedalling against, in order to generate the same power at different cadences. Now, you say you used the same cadence, but if you were using a different gear, then the turbo would set itself a different resistance in order to achieve the same power....... and your speed would be different. So what we need to know is - were you using the same gear?

blackhands · June 2007

Whats the margin of erroor for the computrainer - as the SRM or the Lode ergometer is =/- 2% I doubt its anything like that as we wouldn't be paying about œ15,000 for the SRM. (I believe that the SRM lab cranks which cost œ20,000 are +/- 0,5%. You have shown a difference of 13% which is within the range +/- 6.5% which might be acceptable for the computrainer.

I'm not sure how Computrainers measure power, if indded they do. I suspect that power is calculated based on flywheel speed and parameters you give it such as weight,

Porkyboy · June 2007

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by blackhands

Whats the margin of erroor for the computrainer - as the SRM or the Lode ergometer is =/- 2% I doubt its anything like that as we wouldn't be paying about ‹¨«15,000 for the SRM. (I believe that the SRM lab cranks which cost ‹¨«20,000 are +/- 0,5%. You have shown a difference of 13% which is within the range +/- 6.5% which might be acceptable for the computrainer.

I'm not sure how Computrainers measure power, if indded they do. I suspect that power is calculated based on flywheel speed and parameters you give it such as weight,
<hr height="1" noshade id="quote"></blockquote id="quote">

Blackhands

I use the Pro lab version. The specified accuracy is +/- 2.5% as stated in the technical appendix of the manual. the unit is used in many clinical as well as sports applications. I also contacted the manufacturer and am enclosing information returned to me today below which I hope will be of interest:

"As for Blackhands question on accuracy -- what you pay for in power meters is the number of strain gauges... with the assumption being "the more strain gauges you get, the higher the degree of accuracy 'potential'." I stress the word potential because even a 12-strain gauge device can be as inaccurate as a 2-strain gauge device if they are mistreated or uncalibrated.

For what it's worth our engineer/inventor of all we make, is very well versed in strain gauge technology... so much so that he refuses to use them in anything we make. He says they are very unpredictable without a lot of constant checking. We use a dynamic dynamometer to calibrate and check our products, which can check for accuracy in a real-time use mode. Most strain-gauge devices are only be checked statically once they have been built. Dynamic forces are far different than static ones. Strain gauges can change for a lot of reasons: Heat, glue failure/age (the glue that attaches the strain gauge to the surface it is mounted to), unpredictable tensions applied to the unit when installed (chain rings, spindle bolts, pedals, spoke tightening (PT), etc. etc.) that differ from when it was initially calibrated. Without a "gold-standard verification device" (a dynamic check), it is anyone's guess what the accuracy of a strain gauge power meter actually is. A recent study I was told about conducted on 20 Powertap power meters found some to be way off -- far outside their 2.5% accuracy. I recall numbers in the 19% range and such.

For us, we are a "table lookup" device, where the Load Generator produces load based upon an electric current applied to the system, which is being read from with a table imbedded within the micro. The software has tables that say for a particular speed the drive to the electrical system should be "X" value. This table also accounts for temperature changes, acceleration, weight, etc., as well. In Ergo mode it is done similarly, but instead of speed being the factor it is the target wattage that drives the current and RPM is the variable. We do a 100% check on critical components within our Load Generator to make sure every Load Generator is like the next. They are then calibrated to known point at an "average" speed and wattage of a typical cyclist. They are then cross checked at a different point to make sure there is no drift from the load characteristic of the generator itself. Lab units are checked even further at more points. In all tests the table drives the Load Generator to a known "current" which should produce a known "load" and that is then checked and proven by the dyno calculating the values all match. It's a totally fool-proof method. There are, still, two things that can cause our system to "misread" -- 1) improper rolling resistance calibration procedures and 2) tire slip, which can't be measured. We've received units back from the field after years and 1000's of hours of use and they are EXACTLY as they were when they left, which proves the system does not drift with age. The only time we see changes is when something has failed. Replacing the failed component always brings the unit right back to where it was. You would expect things to change if some component failed -- not just because you used it.

So, suffice it to say, our accuracy is pretty darn good." 

PB

What do you mean your legs are hurting? Give it some welly man!

Porkyboy · June 2007

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by BeaconRuth

Was thinking about this while out training just now......

Are you using the Computrainer in a constant-power mode? That is, the turbo ensures that you're generating the same power whatever cadence you use? If you are then our assumptions of constant resistance are flawed. By definition the turbo must be altering the resistance you're pedalling against, in order to generate the same power at different cadences. Now, you say you used the same cadence, but if you were using a different gear, then the turbo would set itself a different resistance in order to achieve the same power....... and your speed would be different. So what we need to know is - were you using the same gear?
<hr height="1" noshade id="quote"></blockquote id="quote">

Hi Ruth

Yes, using the turbo in constant power mode, it calls it manual ergo mode, you set the wattage you want to pedal at and it keeps it at that by automatically varying the resistance (30 times per second I believe) in response to your cadence and effort variations.

I strongly suspect that I rode in a higher gear at the 2nd test, because I felt I was able to I guess, whilst maintaining the power output and HR where I wanted them. This is why I thought the differences observed might be genuine and why I drew the car analogy. My belief is that a car producing 300W and being driven in 1st gear will go less far more slowly that a car producing 300W and being driven in 4th gear, though I may be wrong!

There is no doubt at all that the resistance gets lower on the CompuTrainer in ergo mode as the cadence goes up. Interestingly though it is just as hard work, as it should be, no matter what the cadence though this is not true of for example the Tacx Flow which I have also used. On the Flow 300W at a cadence of say 80RPM feels like a significant effort but at a cadence of 95 feels one heck of a lot easier, too much easier.

Regards.

PB

What do you mean your legs are hurting? Give it some welly man!

chriswcp · June 2007

You know the real answer don't you?

Get off the trainer and onto the road.[;)]

Self confesed number geek says you've got too many numbers.

BeaconRuth · June 2007

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Porkyboy
Yes, using the turbo in constant power mode, it calls it manual ergo mode, you set the wattage you want to pedal at and it keeps it at that by automatically varying the resistance (30 times per second I believe) in response to your cadence and effort variations.<hr height="1" noshade id="quote"></blockquote id="quote">There's your answer then. The turbo is changing the resistance constantly and the speed/distance readings will be a lot of nonsense.

andyBcp · June 2007

I would say that I have found that speed remains the same as power, when using different gears.
It is only the cadence that changes.
Then again, as I only use PT and SRM, these devices have probably given me a very inaccurate understanding of constant power output, according to the people at computrainer, as they both use strain guages[;)]

http://www.teamvelosportif.co.uk

FrankM · June 2007

I don't have a Computrainer but if you need to input your weight I reckon that's the variable that's changed. You'll produce less power if you're lighter, but weighing less will have no (negative) impact on your speed/distance, and should in fact make you go faster/further.

term1te · June 2007

Could it be that in the algorithm the trainer is using takes into account the law or diminishing returns? For every mph faster you go you need to add more effort than for the last one. Imagine I undertake two sessions, one steady pace and one variable, with sprints and slower recovery periods. My average HR, power output, cadence, etc., could be the same, but if they were, I would have covered less distance in the variable circuit than at constant speed.

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BeaconRuth · June 2007

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Termite

Could it be that in the algorithm the trainer is using takes into account the law or diminishing returns? For every mph faster you go you need to add more effort than for the last one. Imagine I undertake two sessions, one steady pace and one variable, with sprints and slower recovery periods. My average HR, power output, cadence, etc., could be the same, but if they were, I would have covered less distance in the variable circuit than at constant speed.
<hr height="1" noshade id="quote"></blockquote id="quote">I think we got to the bottom of the problem, Termite. He was using the turbo in a constant-power mode, but using a different gear. By definition he will therefore go at a different 'speed' for the same power output, because the turbo has adjusts the resistance to give the same power at whatever cadence is used. He used the same cadence - so with a bigger gear this means a greater 'speed' - though his power was the same.

One for the boffins?...

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