Power meters are a great way to measure the intensity of indoor (and outdoor) bike. Other methods of measuring effort such as heart rate or perceived exertion are useful but those methods can be more difficult to quantify and have a hard time harmonizing on shorter efforts. The problem with buying a power meter is the expense.
A possible replacement of a power meter is using your trainer and speed/cadence computer that is set up on your rear wheel. For triathletes, most of our intense efforts are set near our lactic threshold, roughly the effort you would be going at for your 40k time trial effort.
I’ve always found that my power on the indoor trainer is a little less then what I can push when riding outside, likely due to factors like overheating indoors. To simulate a 40k time trial effort, I use a 20 minute all out effort to try and determine my “threshold” watts.
Now if you don’t have a power meter, you can achieve the same effect using your speedometer. Assuming you are not changing the tension (setting) on your trainer, you can figure out your average speed for your 20 minute effort. You can then use this average speed as a guideline as to what your threshold effort. Here is a product that allows you to use your Ant+ device (like the Timex Global Trainer
) to monitor the speed of the rear wheel on the trainer. It’s called the Timex Bike Speed + Cadence Sensor
Let’s say you averaged 30 km/hr for your 20 min effort. You would then know that when cycling faster than 30 km/hr you would be above threshold. Every few weeks you should repeat the 20 minute time trial effort in order to reestablish your threshold pace.
Doing two times 20 minute effort, with 10 minutes of recovery in between and averaging the efforts would even be a better indication of threshold power, but it will also require a lot more mental discipline.
The poor man’s power meter relies on the fact that you are not changing trainer tension. You can’t transfer these numbers directly to outside as trainer speeds don’t duplicate outdoor speeds, because of wind, hills, etc. You can use this method for establishing a VERY effective indoor training sessions by knowing how close you are to your threshold power.