WTD: Someone with Power Meter to help calibrate my trainer

ziglar

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,

t.m.h.n.e.t

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

ziglar

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,

GiantMike

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

ziglar

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

You could rent a powertap wheel?

neeb

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

Make sure their power meter is calibrated correctly.

kevin69

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

amaferanga

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.

kevin69

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

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

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...

ziglar

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 ;-)

Alex_Simmons/RST

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

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.

Alex_Simmons/RST

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

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

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

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

Trev The Rev

ziglar wrote:

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?

Tyre pressure will affect speed and how much power is transfered to the road or lost to rolling resistance but the power meter will have measured the power at the crank, pedal or hub before the tyre comes into play. Power meters can be affected by heat but this is not a major problem. The major problem with power meters is reliability (but that is my personal opinion).

ziglar

Should have realised that .. doh

Maybe this should be a new thread (there might already be one) but what are the major pros and cons of pedal, crank and hub power measurement.

Thanks,

Alex_Simmons/RST

ziglar wrote:

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

It will have an affect on the speed you get from a given power output, but it doesn't affect the measurement of power itself* (which is made upstream of the tyre).

* unless of course it's not a power meter, but some means of estimating power based on speed and other assumptions.

moonshine

I did a "test" on a club mates Tacx flow using my (trev) calibrated PT SL+.
The outcome was the Flow over read by up to 30% for any given wattage, reporting approx 400w power for a 300wPT effort.
We also had doubts about drift in the session & session reproducibility for the reasons outlined by Alex.
In short, we came to the conclusion the Flow was virtually useless as a serious training tool and was more of a gimmick.
My club mate is looking at buying a "proper" PM now instead.

charlie_potatoes

I have an old Tacx ergo trainer. I did the same 11 mile run with a 'hill' in the middle on Tuesday and Wednesday and my average power output only changed by 1 watt. This was consistent with the variation in calories used and time taken to complete.
I'm not sure how accurate it is but the power outputs seem to be broadly what I would expect for someone of my mediocre abilities
I do notice that my average speed is higher than when I am on the road but assume this is due to lack of wind resistance.
I would say that the 350 watt figure for 25 mph/2% seems exaggerated.

My trainer doesn't give % for hills but grades them 1 to 9.
Ziglar if you post some figures for your trainer at 0% gradient then I and others could maybe do the same? This might help to give a general comparison?

ziglar

I'll post some figures this evening

Alex_Simmons/RST

Speed on a trainer is meaningless. All that matters is intensity and time.

Trev The Rev

moonshine wrote:

I did a "test" on a club mates Tacx flow using my (trev) calibrated PT SL+.
The outcome was the Flow over read by up to 30% for any given wattage, reporting approx 400w power for a 300wPT effort.
We also had doubts about drift in the session & session reproducibility for the reasons outlined by Alex.
In short, we came to the conclusion the Flow was virtually useless as a serious training tool and was more of a gimmick.
My club mate is looking at buying a "proper" PM now instead.

If you set the pressure of the roller on the tyre to the maximum you get more realistic power readings (I did on mine anyway) but the Tacx Flow does not give realistic power figures. It is reasonably repeatable though if you always set the roller to tyre and tyre pressure the same.

ziglar

Here are the figures for the RealPower:
Slope Power mph

0% 150W 16.3
200W 20.0
250W 23.5
300W 25.6
350W 27.3

2% 150W 15.9
200W 19.0
250W 21.3
300W 23.2
350W 25.0

However, I am beginning to see the shortcomings of absolute measurements and will concentrate on the trend over time.

charlie_potatoes

Alex_Simmons/RST wrote:

Speed on a trainer is meaningless. All that matters is intensity and time.

ziglar wrote:

However, I am beginning to see the shortcomings of absolute measurements and will concentrate on the trend over time.

I haven't had a chance to get on my trainer yet but having read all the posts on this thread it seems that the important thing is that the machine is reasonably consistent.
Makes sense

I will post up the figures though just out of interest.

ziglar

Got a cheap Powertap off ebay. No doubt it will give a completely different set of readings and as pointed out I will still have no idea which is more correct as it too may be horribly calibrated ... C'est la vie.