WTD: Someone with Power Meter to help calibrate my trainer

When drugs don't work: training and health tips!
ziglar
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WTD: Someone with Power Meter to help calibrate my trainer

Postby ziglar » Mon Oct 15, 2012 16:06 pm

Hi, I recently bought a 2nd hand Elite RealPower that I think is not putting out the correct wattage - I seem to be able to do 25mph on 2% gradient with 350W which most online calculators do not agree with. I contacted Elite who stated that it might need calibrated and that can only really be done with some kind of Power Meter which I don't own.

I live near Reading, Berkshire and wondered if there was anyone around who might be willing to lend me a powermeter for a couple of hours so that I can calibrate my trainer.

I would quite happily pay for any expenses incurred in getting it to me.

Cheers,

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T.M.H.N.E.T
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby T.M.H.N.E.T » Mon Oct 15, 2012 16:30 pm

You used an online calculator to gauge whether a trainer is accurately calibrated?

ziglar
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby ziglar » Mon Oct 15, 2012 17:01 pm

Not sure what the question is inferring but 'yes' - the online calculators give speed for a given power and slope assuming a fixed set of aerodynamic parameters (or power given the speed). I am, of course, assuming that my trainer also assumes the same aerodynamic parameters but I could be horribly wrong although I see no reason why the baseline for the online calculator and indoor trainer should be so different.

I would love to think that as a beginner I can pump out an average of 350W average for 30mins but somehow I think I would be deceiving myself.

If you know of any sure-fire way to achieve what I want then I'm all ears.

Cheers,

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GiantMike
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby GiantMike » Mon Oct 15, 2012 20:40 pm

EVERY device is inaccurate. It's the improvement that's important, not the absolute numbers.
my power improvement experiment blog

Rule number 100: It's your bike and your money and your time; do what you like with it and ignore other peoples' rules. Except this one.

ziglar
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby ziglar » Mon Oct 15, 2012 21:50 pm

You are almost certainly correct in that statement but it still makes sense to get the figures as accurate as they can be if at all possible.

An effect of the (in)accuracy of my trainer could be that if I were to participate in the online racing facility then a badly calibrated machine might give me an advantage or disadvantage which would skew the results although your comments regarding improvement would still be correct.

Does anyone else have data for a fixed set of conditions for their trainer e.g. speed attained for 0% slope at 350W? And how does that compare to the same conditions on the road (assuming no head head/back wind)?

Eyorerox
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby Eyorerox » Tue Oct 16, 2012 07:02 am

You could rent a powertap wheel?

neeb
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby neeb » Tue Oct 16, 2012 07:23 am

You could calibrate it once using a power meter, but it would probably not remain completely consistent from one session to the next after calibration. However, it would at least give you roughly realistic figures and if you were careful to keep everything constant (tyre pressure, etc) it might not drift too far off.

Another way to do it (without a power meter) would be to time yourself on a maximum effort up a long, more or less steady climb (at least 20mins, so difficult to find in the UK..), preferably on a still day (no wind). You could approximate your threshold power from that and then use this to (approximately) calibrate the trainer by doing a maximum effort of the same length on that.

Trev The Rev
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby Trev The Rev » Tue Oct 16, 2012 15:19 pm

Make sure their power meter is calibrated correctly.

kevin69
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby kevin69 » Wed Oct 17, 2012 12:14 pm

ziglar wrote:Not sure what the question is inferring but 'yes' - the online calculators give speed for a given power and slope assuming a fixed set of aerodynamic parameters (or power given the speed). I am, of course, assuming that my trainer also assumes the same aerodynamic parameters but I could be horribly wrong


i think you are horribly wrong.
Cycling quickly on the road, most of the effort goes into overcoming air resistance.
On an indoor trainer there is practically no air resistance, you are overcoming the resistance of
a fluid or magnetic flywheel.

So on the road, the aerodynamics of your position (and wind) are very important.
On an indoor trainer only your power output matters.

kevin

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amaferanga
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby amaferanga » Wed Oct 17, 2012 12:28 pm

kevin69 wrote:
ziglar wrote:Not sure what the question is inferring but 'yes' - the online calculators give speed for a given power and slope assuming a fixed set of aerodynamic parameters (or power given the speed). I am, of course, assuming that my trainer also assumes the same aerodynamic parameters but I could be horribly wrong


i think you are horribly wrong.
Cycling quickly on the road, most of the effort goes into overcoming air resistance.
On an indoor trainer there is practically no air resistance, you are overcoming the resistance of
a fluid or magnetic flywheel.

So on the road, the aerodynamics of your position (and wind) are very important.
On an indoor trainer only your power output matters.

kevin


I think the point he's trying to make is that the turbo should be calibrated such that riding at say 20mph up a 2% gradient on one of the Elite videos would be roughly the same as doing the same on the road. Not exactly the same, but in the right ballpark. Of course the absolute number really doesn't matter (apart from for willy waving), it's repeatability that he should be worrying about.
More problems but still living....

kevin69
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby kevin69 » Wed Oct 17, 2012 12:46 pm

ah, i think i see what he means now.

But i also agree that its consistency and repeatability that matters more than the number.

ziglar
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby ziglar » Wed Oct 17, 2012 19:14 pm

There is no disagreement from me regarding repeatability being more important than the absolute number but you would have thought that a 2nd hand, unused, still in sealed security tags etc resistance unit would be more accurate (maybe accurate is not the right word but at least in the same ballpark but maybe an average of 350W over 30 minutes is OK for a beginner and my resistance unit is OK :-) )

neeb
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby neeb » Wed Oct 17, 2012 19:51 pm

ziglar wrote: but maybe an average of 350W over 30 minutes is OK for a beginner and my resistance unit is OK :-) )

If you are a big bloke and physiologically naturally talented it's just about possible, but I suspect the unit is over estimating... :wink:

ziglar
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby ziglar » Wed Oct 17, 2012 20:02 pm

I'm 5'6, 11st10, nearly 55 years old and never done any aerobic/anerobic exercise in my entire life (except for some weightlifting when I was a teenager) so I must be naturally talented or maybe Lance Armstrong is my new best friend ;-)

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Alex_Simmons/RST
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby Alex_Simmons/RST » Wed Oct 17, 2012 21:55 pm

ziglar wrote:but you would have thought that a 2nd hand, unused, still in sealed security tags etc resistance unit would be more accurate

Why?

ziglar
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby ziglar » Thu Oct 18, 2012 06:00 am

I have to counter with 'Why not?' - looking at your signature you might be able to provide a better technical argument than I am able to do.

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Alex_Simmons/RST
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby Alex_Simmons/RST » Thu Oct 18, 2012 07:50 am

ziglar wrote:I have to counter with 'Why not?' - looking at your signature you might be able to provide a better technical argument than I am able to do.

Because a majority of trainers don't provide a consistent resistance when new (either during a session or from session to session), so I'm not sure why they would do so after that.

Even if you managed to determine a speed-power curve for your trainer, and were able to determine how it is affected as the resistance unit changes its operating temperature during a session, how are you then going to control the other major resistance factor - the tyre-roller interface - so that it is predictably set the same every time you use it?

Let's assume you control for tyre, tyre wear and tyre pressure. Then you are left with the press on force of the roller on the tyre (or rim for some trainers).

I have a trainer that actually measures the rolling resistance, and it doesn't take much at all to have that out by up to 50W, just a marginal change in the press on force on the tyre is all that's needed, certainly less than I can feel when setting it up.

As a possible solution, in your testing with a (Trev) calibrated power meter, it might be a good idea to test it over a range of different press on forces, and different operating temps (mostly the temp of the unit, not the room, although that's a factor too) which occur as the unit heats up, and see what impact it has on the roll down time from a specific speed.

Using the roll down time might be a crude way to calibrate press on force for future use, although there will of course be errors in measuring the time from speed X to speed Y (Y might be zero).

Trev The Rev
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby Trev The Rev » Thu Oct 18, 2012 10:32 am

Alex_Simmons/RST wrote:
ziglar wrote:I have to counter with 'Why not?' - looking at your signature you might be able to provide a better technical argument than I am able to do.

Because a majority of trainers don't provide a consistent resistance when new (either during a session or from session to session), so I'm not sure why they would do so after that.

Even if you managed to determine a speed-power curve for your trainer, and were able to determine how it is affected as the resistance unit changes its operating temperature during a session, how are you then going to control the other major resistance factor - the tyre-roller interface - so that it is predictably set the same every time you use it?

Let's assume you control for tyre, tyre wear and tyre pressure. Then you are left with the press on force of the roller on the tyre (or rim for some trainers).

I have a trainer that actually measures the rolling resistance, and it doesn't take much at all to have that out by up to 50W, just a marginal change in the press on force on the tyre is all that's needed, certainly less than I can feel when setting it up.

As a possible solution, in your testing with a (Trev) calibrated power meter, it might be a good idea to test it over a range of different press on forces, and different operating temps (mostly the temp of the unit, not the room, although that's a factor too) which occur as the unit heats up, and see what impact it has on the roll down time from a specific speed.

Using the roll down time might be a crude way to calibrate press on force for future use, although there will of course be errors in measuring the time from speed X to speed Y (Y might be zero).



I could alter the power required to maintain a given speed on a Tacx Trainer by 30 watts depending on how tight the roller was adjusted to the tyre.

ziglar
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby ziglar » Thu Oct 18, 2012 12:48 pm

Seem like quite a lot of variables that can have an effect.

Presumably then I would be looking for a trend in improvement rather than rely on a specific change between one session and the next?

If tyre pressure and temperature causes so much variation this must also have an effect on powers meters on road bikes?

ziglar
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Re: WTD: Someone with Power Meter to help calibrate my train

Postby ziglar » Thu Oct 18, 2012 12:50 pm

looks like I'd better start saving for that PowerTap ...


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