Dr. Paul Smeulders, PhD. Inventor, ErgVideo.com and Intellicoach.ca

Dr. Paul Smeulders, PhD. Inventor, ErgVideo.com and Intellicoach.ca

Below is a well-written piece by ErgVideo creator Paul Smeulders, that best-explains just what happens when you ‘Bog Down’ on a given section of a wattage-based CompuTrainer workout. Read carefully, but I think you’ll leave with a better understanding of just what happens when you feel that strain getting too hard.

…Replying to a misunderstanding regarding the CompuTrainer’s application of Load.

The CT in ERG or MRC mode (RW note – MRC is what we use at the Cycling Center of Dallas) operates as a feedback control system where the setpoint target is your output power.


Power is expressed as the sum of power components due to Rolling Resistance (has a speed dependence) and Braking Force (torque) x speed.


When you are rolling along slowly, where the power due to Rolling Resistance is greater than the setpoint power target, the CT braking via electromagnetism just turns itself ”off”, since any further braking at that speed would cause power output to go up, even further from the setpoint. The CT is trying to regulate you always to the setpoint, as a function of Speed and the known braking force and calibrated Rolling Resistance.

Thus, when your Rolling Resistance (RRC) is high (press-on is high) it requires you to go faster before the braking comes on at all. The power due to Rolling Resistance is higher for a given speed. I recommend a press on force of 2-2.5 in erg mode just so you can feel load changes in the 80-120W ranges more easily. (RW note: we use 1.0 -2.2 RRC in our class because of the different trainer stands we use, and the fact that we can’t cinch them down at that angle as well.)

When your wheel speed goes up beyond a point where the RR power is below the setpoint target, the CT magnets turn on, with regulated current. This is controlled carefully so that while you may see overshoots etc, the CT ensures you stay on target rather well. One need only examine the performance file from a controlled ERG or MRC workouts to see all of this.

Now consider the case where the Rolling Resistance power is low and we are trying to ride at a hard, high wattage, like 350W. Let’s suppose that at the speed we are riding, the Rolling Resistance is only 100W (I am making these numbers up for the sake of clarity). So the CompuTrainer has to apply braking at that speed of 250W, and for simplicity, let’s just say it works out to X newton-m, so X*V1 = 250W, where V1 is the radian speed of the roller at this wheel speed so there is a direct relationship. Again, a ton of constants are being left out or wrapped-in if you will.

Now you start to fatigue and SLOW DOWN, to say 10% slower, so 0.9V1. Lets suppose this new speed means the power lost to RR is now 90W. The CT wants you at 350W total STILL, so it must account for 260W of total power dissipation, and we are at a lower speed than before! Solving for the new torque Y, if

X*V1= 250 and Y*0.9V1=260, then Y=260X/(.9×250)=1.15X

So that simply means that the torque or braking force MUST GO UP (in this example by 15%) in order to hold the setpoint power when the speed goes down by 10% (a rider fatiguing), and this particular press on force that I just manufactured for simplicity. We are only talking about proportional relationships here.

A=B*C

If A is to be held constant and B goes down, then C must go up. It is that simple,and only complicated more by the Rolling Resistance a little bit.

This has the impact of holding you to your setpoint target power
regardless of speed or gear, and fails when you go so slowly that Rolling Resistance is the only contributing factor. Please note that this is NOT a discussion of accuracy. That is, if the CT has an inaccurate view of what it the setpoint power of 350 watts is, (due to miscalibration or other failure), it will do a great job of holding you to that inaccurate view of what is 350W. Accuracy with the CompuTrainer is a whole other affair.

The INCREASE in torque when a rider is failing to hold the power target is EXACTLY the correct response we want in erg or MRC mode (remember the POINT is to force a target power out of the rider). The only other thing that could be done for a rider failing to hold the target of 350 W is to start
driving the pedals faster FOR HIM. That would not do, since now YOU are not putting out the power at all, but only a fraction of your target total, and the CT would be saying “there there, let me pedal for you, everything is good, just sit there”.

The impact of this control in erg mode and ergvideo accordingly, is that you really fight hard to keep the pedals rolling. You do not have to carefully TRY to keep a target power precisely, as if staring at your SRM on a trainer. Just pedal, and the CompuTrainer (and all other automatic ergs) will adjust the torque according to your speed (and Rolling Resistnace) so that you ARE putting out the target power. Outputting less than target wattage means torque MUST come up, and it will crush you, indicating your failure to output the target power.

I have an article on my forum (not trying to get people over there…I have it locked down for awhile just as a FAQ now) with some more insights into the relationship of RR and how erg mode works. They are  here http://forum.ergvideo.com/forum/viewtopic.php?f=8&t=24 and here
http://forum.ergvideo.com/forum/viewtopic.php?f=8&t=44 (hmm, will these show as clickable links?)

Thanks,

Paul Smeulders, inventor, ErgVideo.com and Intellicoach.ca

Visit my homepage for ergvideo, some funy promo videos to watch and understand the product: http://www.ergvideo.com

So there you have it, a good, detailed, but still basic enough to understand article about how power works on the CompuTrainer. You’ll get stronger, better workouts with CompuTrainer, ErgVideo, and Coaching through the Cycling Center of Dallas!!!!