SRM vs. Computrainer

WARNING: If you don't like numbers and science and engineering and stuff, come back later...like in a day or two.

I've been saying that I was going to post some more technical posts or equipment testing, etc. Here is the first one. People ask a lot of times which powermeter is best. I think that the SRM is one of the best out there. It is also the most expensive. I bought mine used. The PowerTap is also very good. The Quark seems to be doing well in its first year. The Ergomo has some issues, and I'm not qualified enough to talk about them. The Computrainer (and I'll put the TACX trainer in there as well) is good, but you can't take it outside. The Computrainer is hard to beat for motivation if you are riding indoors. I've been riding one for almost 10 years. Andy Potts rides one. A ton of good athletes use them.

Here is a graph of my workout that I did on the Computrainer. This is the computrainer graph. As an aside, I pump my tire to about 105psi before I ride the computrainer. I also ride for 5-10 minutes before I do the computrainer rolling resistance calibration. Generally, I calibrate to a value of 2.5-2.9.



Here is a graph of the same workout taken from my SRM. You will notice a gap in the SRM file around 48 minutes. That is because I stopped to help get a thing or two ready for dinner for about 5 minutes. There are also a few spikes on this graph that are not on the computrainer graph. Those are for when I am calibrating the computrainer. I calibrate my SRM before each and every ride. The zero offset is steady unless there are wild temperature fluctuations.



You can't tell much from the graphs except for the fact that the two track pretty well. So, I've made a chart of the data between the two. I've put the average wattages for each of the intervals in the chart as well as the %age difference between the SRM and the Computrainer. Here is the chart.



Thoughts:
- I am sure that I do not use the Computrainer to the best of my abilities. I almost always ride a 'flat course' and do some intervals. I do not know if you can do 'lap' or 'split' times on it. So, I have had to pick out the intervals on the Computrainer file using CyclingPeaks. I could have done this using the raw data I am sure, but this will have to suffice for this blog post. That just means that I may have gotten a point or two at the front-end or back-end of some of the intervals that skewed the Computrainer data a little bit. In other words, it is not exact.
- The table above shows that the CompuT is generally 5-10% LOWER in power than the SRM. The SRM measures power at the crank. The CT calculates power at the rear wheel. You would expect the two to differ based on frictional losses between the chain, tire, etc.
- Conclusions: Whether you buy a CT, SRM, Powertap, or other powermeter, you really have to ask yourself 1 or 2 questions? 1-What is my budget?...A CT and Single Powertap are about the same $$...whereas an SRM and 2xPowertap is close to the same. 2-Do I want to train AND race with the powermeter?...race data is great data, but if you want to train AND race with a powermeter it is NOT going to be a CT. But, if you are going to ride almost exclusively indoors, then the CT is a very useful and worthwhile way to measure power. On race day you just go by perceived exertion or you just go HARD. If you want to be able to have power indoors AND outdoors, then the CT is NOT a good way to do it. The CT makes indoor riding much more tolerable in my mind since you have a screen to look at. Lastly, training with power is a great way to get faster...I would not become a total slave to the numbers, but you really are 'training' when you switch to using a powermeter.

Got any questions...my email is above.

That's all for now...as if it isn't long enough already.