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Ok best value one?
When do they become worth the investment?
What kinda information do they give you and is it that valuable to a road rider? (watts...and what else?)
Any chit-chatter about them would be helpful.
I use a Powertap, (hub based system), I've found it an excellent tool for forcing me to train harder/at the correct intensity.
I've got fitter and quicker because of it, however I train rather than go for rides.
If you're not racing then I'd say they are a very expensive way to get new bits of data about your riding.
As with HR monitors, if you don't use the numbers correctly then there is little point to owning one.
powertap here too. using Golden Cheetah for analysis. had it for about 6 months now, the data is invaluable and way more informative than just heart rate.
If you're asking if they're valuable to a road rider, you more than likely don't need one.
Best analogy I've heard for those that don't really need them or generate the kind of power that makes owing one is:
"Like employing an accountant to tell you how poor you are"
The data you get from it and how you use the data to improve your fitness is what its all about. No point just riding around looking at the watts and not understanding whats going on. Try and have a look at training and racing with a power meter, really good read. I've trained with power for 3 years now and only the last year really got the hang of it. It takes a lot of effort to get the most out of it but if you're keen can be very worthwhile.
Oh and sorry I'm not a Roadie!
The best one depends on your needs. Power tap being a hub based system limits your wheel choice to the wheel with the power tap in it. This can be a pain as you have to use the same wheel for racing and training. A lot of people use a power tap as their 2nd power meter!! They are the cheapest proper power meter for the base model.
Quarq and SRM are slightly more hassle to change between bikes, but a major advantage compared to power tap is that you can change the calibration if you need to. Quarq is generally cheaper than SRM.
All you get is watts and cadence. So if you like tts for example, you can measure how hard you can go for 20 mins and an hour. Use this to gauge progress and your hour power to do your training zones. You can look use the meter to pace races or intervals, and retrospectively to see how well you paced them. You can also use software to give a record of your training stress to help you build up or taper as you desire. That's some examples but I would agree with the book recommendation above as I can't be arsed to type much on the iPad