CompuTrainer/SpinScan on a 'Bent?

George · Aug 29, 2003

I posted this on the BROL HPV Racing site, but am posting it here too to get some response from a
wider, more diverse group if possible. Apologies to those of you who find it on both boards.

Anyone have experience using SpinScan with a CompuTrainer on a recumbent? I went to the National
Institute for Fitness ans Sports in Indianpolis today for a dynamic bicycle fit that included
analysis of my spin stroke with this equipment. The result that has me concerned is that SpinScan
indicated I was generating about 56 to maybe 59% of my power with my left leg. Way too close to
60/40 or 50% more with the left leg. I think the software determines left and right by the timing of
the cadence sensor on the left crank arm. On a DF the sensor is mounted on the left chainstay, and
the magnet passes the sensor at about the 9 o'clock position or about half way through the up/back
stroke part of the spin for the left leg. On my Strada we mounted the sensor on the main tube which
makes the magnet pass the sensor pass the sensor about 13 or 14 degrees before my 12 o'clock or top
dead center position. I'm looking for some insight into how this might affect the software's ability
to attributre power to each leg correctly. Any of you analytical or engineer types out there have
any thoughts?

Thanks,

George - trying to tweak the Strada a bit more

Risto Varanka · Aug 30, 2003

George <[email protected]> wrote:
: more with the left leg. I think the software determines left and right by the timing of the
: cadence sensor on the left crank arm. On a

How could it do that? Mail the software vendor and ask?

--
Risto Varanka | http://www.helsinki.fi/~rvaranka/hpv/hpv.html varis at no spam please iki fi

George · Aug 30, 2003

I sent an email to the vendor just before posting the original message here. I was hoping to find
someone here with experience using SpinScan on a recumbent.

As far as I can see, the cadence sensor is the only way it can tell left from right. If you are
spinning at a cadence of 90, the software must figure that 0.16667 seconds after the magnet passes
the sensor you are at the top of your stroke with the left leg - assuming the sensor is at 9 o'clock
for the left leg. Check my math though: cadence of 90 is 1.5 rotations per second, or 0.6667 seconds
per rotation. If the sensor is at 9 o'clock, 1/4 rotation past that will be top dead center or
0.66667 seconds/4. I thnk what the software was calling my left side stroke was really the bottom
half of my left and the top half of my right. Tried pedalling with just one foot and the software
did attribute some power to the other foot.

For now, I think I'm going to try a couple of days a week riding about every third mile using just
the right leg and see how that goes. Or maybe a third of a mile out of every mile?

George

[email protected] wrote in message
news:<[email protected]>...
> George <[email protected]> wrote:
> : more with the left leg. I think the software determines left and right by the timing of the
> : cadence sensor on the left crank arm. On a
>
> How could it do that? Mail the software vendor and ask?

Risto Varanka · Aug 30, 2003

George <[email protected]> wrote:

: As far as I can see, the cadence sensor is the only way it can tell left from right. If you are
: spinning at a cadence of 90, the software must figure that 0.16667 seconds after the magnet passes
: the sensor you are at the top of your stroke with the left leg - assuming the sensor is at 9
: o'clock for the left leg. Check my math though: cadence of 90 is 1.5 rotations per second, or
: 0.6667 seconds per rotation. If the sensor is at 9 o'clock, 1/4 rotation past that will be top
: dead center or 0.66667 seconds/4.

Then the software assumes that you are pedalling perfectly round circles with constant power input,
which is rather braindead.

--
Risto Varanka | http://www.helsinki.fi/~rvaranka/hpv/hpv.html varis at no spam please iki fi

George · Sep 1, 2003

Perfect circles? Well the pedals are spinning in perfect circles even if I have some foot movement
due to float in the cleats and changes in flex of the ankle - but the application does not assume
constant power output. In fact one of the more useful measurements it provides is your efficiency or
the variation in power applied throughout the circle. If you could spin with constant power
throughout the 360 degrees you would have 100% efficiency.

I'm still trying to figure out the effects of being about 80 to 90 degrees out of phase - don't
think it should affect the efficiency result, but I don't think the right to left difference is
accurate. I just don't know if the difference is more or less than indicated.

[email protected] wrote in message
news:<[email protected]>...
> Then the software assumes that you are pedalling perfectly round circles with constant power
> input, which is rather braindead.

Risto Varanka · Sep 2, 2003

George <[email protected]> wrote:
: output. In fact one of the more useful measurements it provides is your efficiency or the
: variation in power applied throughout the circle.

How on earth does it do that if it only has a simple cadence sensor?

: If you could spin with constant power throughout the 360 degrees you would have 100% efficiency.

Wonder what you mean with efficiency... Constant power is hardly most effective for **** sapiens.

--
Risto Varanka | http://www.helsinki.fi/~rvaranka/hpv/hpv.html varis at no spam please iki fi

Sticker Jim · Sep 9, 2003

As the bike is stationary at the time you are using the sensors, make some kind of small holder that
sits on the floor so that you can position the sensor in the same basic location and relationship to
the crank as it would normally be employed in.

"George" <[email protected]> wrote in message
news:[email protected]...
> Perfect circles? Well the pedals are spinning in perfect circles even if I have some foot movement
> due to float in the cleats and changes in flex of the ankle - but the application does not assume
> constant power output. In fact one of the more useful measurements it provides is your efficiency
> or the variation in power applied throughout the circle. If you could spin with constant power
> throughout the 360 degrees you would have 100% efficiency.
>
> I'm still trying to figure out the effects of being about 80 to 90 degrees out of phase - don't
> think it should affect the efficiency result, but I don't think the right to left difference is
> accurate. I just don't know if the difference is more or less than indicated.
>
>
>
> [email protected] wrote in message
news:<[email protected]>...
> > Then the software assumes that you are pedalling perfectly round circles with constant power
> > input, which is rather braindead.

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