Pedaling style and spinscan, what does it all mean

[Frank_Day]

A long time ago I was given some actual pedal force data comparing a rider using PowerCranks and the same rider going back to regular cranks. These are substantially different because he was only marginally trained with the PC’s so he quickly reverted back to a “normal” way of pedaling, putting some back force on the upward moving pedal. The problem was he pedaled the PC’s at 81 rpm and the regular cranks at 88 rpm. This makes direct comparison of the two data points difficult.

Here is that graph again

So, I have tried to modify these numbers to make the comparison more valid and to help one to better understand what spinscan really is and how it doesn’t do a very good job of telling one whether one is really “pedaling in circles” or not. I decided I would try to do an analysis at 80 rpm. Since both trials were done at 250 watts I knew I had to make each rider do the same amount of work per one revolution if they were at the same cadence and same wattage. I was lowering the cadence for both riders, one from 81 rpm and the other from 88 rpm. I made the assumption that both riders would be able to push harder on the down stroke with a lower cadence and that the PC rider would be able to pull up a little more on the upstroke and the regular cranks rider would not be quite so negative on the upstroke. So, I adjusted these numbers by the ratio of how much I was slowing the cadence and then applied a small fudge factor (1.059) to get the areas under the curve to be identical.

These are the resulting curves:

Spinscan combines the right and left crank torques into one graph. Here is what we get if we assume the right and left cranks are identical

The spinscan number is the ratio of the average torque divided by the maximum torque. For any given power the flatness of the curve really only has to do with what one is doing across the top and bottom because the higher it is there, the lower the peak has to be to keep the area under the curve the same. It says zero about what one is doing on the upstroke.

Anyone care to comment on my analysis and on the assumptions I made in manipulating the data to get here?

[jefeloco]

Why manipulate very old data? Spinscans are everywhere. Why not get someone, or several riders to get on the Spinscan, with and without powercranks. It is not that hard to pedal with the same cadence- Or same power, if you want to see what that looks like.
Then you get an apples to apples comparison instead of Abstract Math.

[Frank_Day]

Why manipulate very old data? Spinscans are everywhere. Why not get someone, or several riders to get on the Spinscan, with and without powercranks. It is not that hard to pedal with the same cadence- Or same power, if you want to see what that looks like.
Then you get an apples to apples comparison instead of Abstract Math.
The purpose of this was to try to explain where the spinscan comes from and what it does and doesn’t mean. Here I have real pedal force data that needed a small amount of manipulation to make “everything” the “same”. Riding a pair of PowerCranks on spinscan will tend to raise someones spinscan number about 10 points (depends upon where they are starting from) and this little exercise got an 8 point increase. When one just gets on a CT and sees that increase it is not obvious where it is really coming from. I think this exercise helps explain it.