consistency of resistance w Magnetic trainers

[Ken Roberts]

Does the amount of resistance force from a magnetic trainer change as the
trainer heats up?

If Yes, how quickly and how much might it happen if I were pedaling at 200
Watts? Is the resistance force fairly consistent for the first hour?

Like I've heard it claimed that the resistance of a fluid trainer could
decrease it the temperature of the fluid rises substantially during a
workout (Is that true?). And I sorta remember a from a physics course that
if a magnet is raised to a high temperature, its sub-regions can lose their
alignment -- with a permanent reduction in magnetism (Could that that
happen at 200 Watts for an hour with a praseodymium maget?).

The reason I wonder is because sometimes I've been surprisingly good at
achieving "negative splits" in my indoor interval workouts -- higher average
speeds in later intervals. I'd love to know whether that's from my
disciplined training or the heating of the magnet (without paying US$ 600+
for a power meter).

Thanks for any guidance on this.

Ken

 

[[email protected]]

Ken Roberts writes:

> Does the amount of resistance force from a magnetic trainer change
> as the trainer heats up?

> If Yes, how quickly and how much might it happen if I were pedaling
> at 200 Watts? Is the resistance force fairly consistent for the
> first hour?

> Like I've heard it claimed that the resistance of a fluid trainer
> could decrease it the temperature of the fluid rises substantially
> during a workout (Is that true?). And I sorta remember a from a
> physics course that if a magnet is raised to a high temperature, its
> sub-regions can lose their alignment -- with a permanent reduction
> in magnetism (Could that that happen at 200 Watts for an hour with a
> praseodymium magnet?).

> The reason I wonder is because sometimes I've been surprisingly good
> at achieving "negative splits" in my indoor interval workouts --
> higher average speeds in later intervals. I'd love to know whether
> that's from my disciplined training or the heating of the magnet
> (without paying US$ 600+ for a power meter).

Magnetic trainers use an eddy current brake in which electric currents
are generated in a metal disk that recirculate in vortices as it spins
in its magnetic field. These currents cause resistance to motion as
they circulate in the disk and trough electrical resistance generate
heat. You'll note that the disk is generally a non magnetic metal
with good conductance. It could be copper but is usually aluminum for
economic and weight reasons.

Electrical resistance in the disk changes with temperature but not
perceptibly for trainer application. This occurs as the disk becomes
hotter but rapidly achieves steady state at any fixed speed.

Jobst Brandt

 

[Phil Holman]

"Ken Roberts" <[email protected]> wrote in message
news:[email protected]...
> Does the amount of resistance force from a magnetic trainer change as
> the
> trainer heats up?
>
> If Yes, how quickly and how much might it happen if I were pedaling at
> 200
> Watts? Is the resistance force fairly consistent for the first hour?
>
> Like I've heard it claimed that the resistance of a fluid trainer
> could
> decrease it the temperature of the fluid rises substantially during a
> workout (Is that true?). And I sorta remember a from a physics course
> that
> if a magnet is raised to a high temperature, its sub-regions can lose
> their
> alignment -- with a permanent reduction in magnetism (Could that that
> happen at 200 Watts for an hour with a praseodymium maget?).
>
> The reason I wonder is because sometimes I've been surprisingly good
> at
> achieving "negative splits" in my indoor interval workouts -- higher
> average
> speeds in later intervals. I'd love to know whether that's from my
> disciplined training or the heating of the magnet (without paying US$
> 600+ for a power meter).
>
> Thanks for any guidance on this.

I've done many 1000s of miles, intervals and TT workouts on my trainer
and have never noticed or measured (power tap) any dropoff in resistance
at several different speeds.

Phil H

 

[John Forrest Tomlinson]

On Thu, 19 Oct 2006 13:25:46 GMT, "Ken Roberts"
<[email protected]> wrote:

>If Yes, how quickly and how much might it happen if I were pedaling at 200
>Watts? Is the resistance force fairly consistent for the first hour?
>
>Like I've heard it claimed that the resistance of a fluid trainer could
>decrease it the temperature of the fluid rises substantially during a
>workout (Is that true?). And I sorta remember a from a physics course that
>if a magnet is raised to a high temperature, its sub-regions can lose their
>alignment -- with a permanent reduction in magnetism (Could that that
>happen at 200 Watts for an hour with a praseodymium maget?).

You can test this with a coast down test -- have a speedometer on your
bike on the trainer and pedal to a certain speed. THen stop
pedalling and see how long it takes for the wheel to coast to a stop.
Then ride for awhile to let the system heat up, then try it again.

Note -- these results might also include changes in rolling resistance
of the tire.


--
JT
****************************
Remove "remove" to reply
Visit http://www.jt10000.com
****************************

 

[Ken Roberts]

John Forrest Tomlinson wrote
> You can test this with a coast down test

What a clever idea! I never thought of doing a "roll down" on an indoor
trainer. Thanks a lot, John.

I just tried it at starting at low temperature -- I'll try it later at the
end of a workout in high temperature.

> these results might also include changes in rolling resistance of the
> tire.

Good point. From my perspective I think that's an advantage. Since really I
want to (roughly) calibrate the heat-response of my overall system, not just
prove something about my magnetic device.

Ken

 

[Ken Roberts]

Jobst Brandt wrote
> Electrical resistance in the disk changes with temperature but not
> perceptibly for trainer application. This occurs as the disk becomes
> hotter but rapidly achieves steady state at any fixed speed.

Since I usually start my indoor sessions with a progressive warmup, sounds
like the force resistance should be fairly stable by the time I get into my
serious workout. What I wanted to know -- thanks Jobst.

Phil Holman wrote
> I've ... never noticed or measured (power tap) any dropoff in resistance
> at several different speeds.

Nothing like real-world experience for someone who's put the effort and
money into a measuring device -- thanks Phil.

So I'm thinking now I don't need to buy and install a good powermeter
myself. Since if the speed reading on my bike's speedometer while rolling on
a magnetic trainer show a vaguely linear function of my power into the
magnetic trainer on a consistent basis, that's a good enough measure of my
power output for my amateur training purposes. Does that make sense?

Ken

 

[Phil Holman]

"Ken Roberts" <[email protected]> wrote in message
news:[email protected]...
> Jobst Brandt wrote
>> Electrical resistance in the disk changes with temperature but not
>> perceptibly for trainer application. This occurs as the disk becomes
>> hotter but rapidly achieves steady state at any fixed speed.
>
> Since I usually start my indoor sessions with a progressive warmup,
> sounds like the force resistance should be fairly stable by the time I
> get into my serious workout. What I wanted to know -- thanks Jobst.
>
> Phil Holman wrote
>> I've ... never noticed or measured (power tap) any dropoff in
>> resistance at several different speeds.
>
> Nothing like real-world experience for someone who's put the effort
> and money into a measuring device -- thanks Phil.
>
> So I'm thinking now I don't need to buy and install a good powermeter
> myself. Since if the speed reading on my bike's speedometer while
> rolling on a magnetic trainer show a vaguely linear function of my
> power into the magnetic trainer on a consistent basis, that's a good
> enough measure of my power output for my amateur training purposes.
> Does that make sense?

It certainly does. You may want to ask one of your riding buddies (with
a power tap) to callibrate your trainer. Accepting a small variation for
different tires etc, its an inexpensive way to train at a known power
output.

Phil H

 

[Ken Roberts]

John Forrest Tomlinson wrote
> You can test this with a coast down test -- have a speedometer
> on your bike on the trainer and pedal to a certain speed. Then
> stop pedalling and see how long it takes for the wheel to coast
> to a stop. Then ride for awhile to let the system heat up, then try
> it again. Note -- these results might also include changes in rolling
> resistance of the tire.

Thanks for the great idea, John. It was harder to reliably implement
coast-down tests than I expected. After some "learning" days, here's a
summary of results from an indoor workout session with the rear wheel of my
road bike on a magnetic trainer:

* no significant weakening of resistance after first 30 minutes of pedaling.

* around 15% weakening of resistance during not more than 30 minutes after
cold start.

(Likely a stronger rider would heat up the magnet and tire rubber faster,
and so get thru that initial weakening period quicker than me.)

I'd be happy to get more insights about this, suggestions to improve
accuracy of measurement -- or to stabilize heat-response of the magnet and
tire rubber.

Of course I wish I could have a dedicated trainer with a high-inertia
flywheel to better simulate road riding. Please send money for both the
trainer and the larger apartment with space for it.

Ken
_________________________________________________________
sequence of measurements:
1) cold start.
.. lower resistance 20mph, coast-down time avg = 11.0 seconds
.. higher resistance 14mph, coast-down time avg = 4.3 seconds
2) after warmup intervals 4x 2-3 minutes with 1-2 minute rests between.
.. lower resistance 20mph, coast-down time avg = 11.8 seconds
.. higher resistance 14mph, coast-down time avg = 4.7 seconds
3) after 5 minutes for rest
.. lower resistance 20mph, coast-down time avg = 11.5 seconds
4) after 30 minutes continuous work
.. lower resistance 20mph, coast-down time avg = 12.5 seconds
.. higher resistance 14mph, coast-down time avg = 4.9 seconds
5) after 25 minutes continuous work
.. lower resistance 20mph, coast-down time avg = 12.5 seconds
.. higher resistance 14mph, coast-down time avg = 5.0 seconds

I did at least 3 "coast-down" measurements for each lower resistance 20mph
start, and at least 5 measurements for each higher resistance 14mph start --
then averaged the results.

I'd guess my pedaling output in this workout was around 175-200 Watts. I
don't have a power-meter, so this guess is based on perceived exertion
comparison with sustained hill climbs outdoors of known vertical and
horizontal distance, where I could estimate Watts. For sure my output to the
wheel was nothing like 300 Watts.

I found it somewhat difficult to get the starting speed exact for each
coast-down, because my indoor training set-up has much lower inertia than
riding out on the road. I got better at this. At first I was trying to get
repeatable starting spin from watching the cadence reading, but then I saw
that the wheel-speed measurement had faster time-response to variations. I
also found it tricky to press the Stop button on my timer at the exact
instant the wheel stopped.

Ken