Posts tagged #Powertap

What's What With Watts...

A few days ago I published a post on choosing a power meter and, though it seems to have been well received, I have been asked some follow up questions.
Firstly it seems that there is some interest in knowing what kind of power output is attainable and how does your output compare to others. To try and answer this I have broken down some previously published data and averaged out the results into four rider groups. Lastly, so that it is possible to compare riders of differing weights, I have broken down the power output into watts per kilo of body weight
(a kilo =2.2lbs)

1 Minute Burst( watts per kilo)
Maintained (watts per kilo)
Pro Tour Rider
Cat 2 Rider
Cat 4 Rider
Regular Recreational Rider

The higher wattage figure in each category is what the average rider, in that group, can reasonably expect to sustain for 1 minute. The second, lower figure, is the average output for a usual distance ride in each group.
These figures are by no means written in stone and I have done my best to average everything out with the intent of giving you some kind of baseline in each group. There are plenty of riders, in every category, that produce figures well outside of the norm and if you want to compare your wattage to your favorite tour rider then a five minute search online will glean you a set of figures to use. Top sprinters are capable of producing around 1200 watts output over the last run to the line and top climbers are getting close to 525 watts on extended climbing stages. Tour guys love to show off their power figures I have found…

The Second questioned, that has been asked more than once. Is there a way to calculate watts without a power meter?
Well, kind of. The first thing to remember is all the variables that act against your forward motion, as mentioned in my original post, the terrain, weather, equipment etc; all act against you gaining an accurate set of data from anything other than a power-meter but, by choosing a flat course and a mild day with no adverse wind conditions, a fair judge of power output can be recorded.
Using the table below, again I have taken an average of many power output readings to try and lessen inaccuracies; you can see what kind of power is required to propel you along at a given speed. The data used in the calculations all came from drop bar road bikes using tires from 18 to 25 wide. There may possibly have been the odd 28 in there too but not enough to make much of a difference.
The following is the chart that was produced.

Speed,(kmh)                                     Watts
20                                                           78
25                                                           123
30                                                           186
35                                                           273
40                                                           378

It is interesting to see how the wattage required to increase your speed grows substantially the faster you go. At low speeds the wind resistance plays very little part in holding you back along with other resistance factors. However the faster you go the more wind plays a part. This is because the resistance increases in line with the square of the forward velocity. All pretty technical but this is the basic factor that we have battled with since we decided to start going places on something else other than our feet. The bottom line is that the better you get the harder it is to get even faster.
Posted on September 17, 2012 .

Choosing a Powermeter

First things first, what does a power-meter actually do and why do you need one? Traditionally cycling performance has been measured by using a basic sensor fitted to the bike which tracks and monitors speed, distance and, more recently, cadence. Within those features we also have average speeds, max speeds attained, etc. Whilst this information is good and for most people who just want to keep a general record of miles ridden and average speeds probably all that is necessary, however the data and feedback does not take into account ride conditions such as headwind, tailwind, altitude or gradients. More importantly there is no measure of effort from you, the rider.
                 Lots of riders nowadays use some form of cardiac feedback, usually in the form of a band with a sensor worn about the chest and, though this data is useful, it still has large gaps and omissions plus it is wildly inaccurate at times. Again, outside factors of terrain and climate, altitude, whether you had a late night or two shots of espresso before you started all have effects on your base line. There is also a significant lag between pedaling output and your hearts increased beat rate.
                 So this is where a power meter takes over. It really is the only accurate way to gauge and compare performance. Power-meters measure power output in wattage and, more importantly, remove all the variables from the data. As an example; you completed a ride last week in fair weather and kept up a steady output of 285 watts for two, twenty minute training intervals. Today you did the same ride, this time in driving rain and a headwind but still completed two, twenty minute intervals, your distance covered was much shorter but your power output was 288 watts. You did better, but your ordinary computer would show that as a bad day. With the power meter this is accurate usable data, no need to discount the day or make notes to allow for bad weather. Watts are an accurate measurement of your performance regardless of all the changing forces acting against you. The end result is a phenomenal training tool and record of improvement.
                 During racing and endurance events it is a great source of feedback for maintaining calorie intake to match output and also keeping a comfortable pace for a long endurance event.
So, now that we have covered what a power meter can do for you we shall take a quick look at your main choices.  The benchmark device is probably the SRM range of crank mounted power meters. Very reliable, accurate and the company has been producing meters for many years. Sadly though, what limits their popularity is the price. While any of these devices are not cheap, setting up a bike with an SRM unit is going to run around 3 grand. Ouch. 
Next in the line-up is Ergomo, these guys produce a solid, bottom bracket device which you can install with a crank-set of your choice. Accurate and the company produce the usual range of data analysis software. Ergomo is cheaper than the SRM meters but still a little more expensive for a full system setup than the third choice.
 Power-Tap from CycleOps is about the cheapest option for a reliable power meter system and, for that reason, it is this system I shall concentrate on here. Still not cheap but they do have an option below a $1000 which is considerably less than the SRM. Even the top of the line G3 ceramic is less than the next player in the market and for that reason alone Power tap has become a very popular choice.
I shall start at the top with the G3 ceramic. This hub is the world’s lightest power meter and the complete hubs weigh in at a mere 315g. Next we have the standard G3, basically the same hub but without the ceramic bearings. The weight is still excellent at 325g and you can always upgrade to a ceramic bearing at a later date if you feel the need. The G3 series hubs are a complete makeover from the original power tap hubs. They have a significantly reworked body and allow for much easier servicing. One of the main differences and to my mind the most important one is the increased gap between the flanges. This dimension is very important and affects greatly the final strength of the wheel. The G3 series hubs increase the gap by 5.6mm over the original models and this makes it possible to build a very stiff and strong wheel, the dimensions are much the same as any road race hub in fact.
The original design is still available; it is called the Power-Tap Pro. With a sticker price of less than 900 a hub it is by far the cheapest meter option out there. Each year it gets a basic makeover to keep the internals up to date but shape and external dimensions have changed little. The pro is a fine piece of equipment and has accuracy the same as the G series, all the hubs have great accuracy, to within +/- 1.5%, and the company produces a great range of analytical software, it is quite a bit heavier than the G3s but it is also a lot cheaper. Over the years we have built a lot of them up and I always recommend getting one with a higher spoke count. With the closeness of the flanges on this hub we need to get as much strength as we can from the spoke number. You are not going to be making massive weight savings opting for the 24 spoke, not when the hub itself is 450g, so do yourself a favor and opt for the 32. With a 2x pattern and maybe an offset rim or a deep V at least, it can be built up quite strong and stiff.
When used for training purposes and accurate performance recording the standard Power-Tap Pro is a fine choice however, if like many you are looking for a wheel to race with I would suggest opting for the G3 series. These hubs are much the same as any of the many race hubs that we use and with the extra flange width I can build a super race wheel that gives you feedback as well.
A final thought on some other options. Recently there have been some forays into the power meter market by a few well known players. Over the past few months I have read some interesting press releases with regard to pedal devices. On the face of it a pedal option sounds a good idea, easy to transfer from bike to bike for one. But some of the price points that I have seen are ludicrous and I would like to see them in action first with some good field testing and data collection behind them. Some of the accuracy reports I have seen are a little disappointing, but it is early days. For the moment I suggest the Power-Tap , for best accuracy and bang for the buck.
Posted on September 12, 2012 .