- This topic has 36 replies, 17 voices, and was last updated 13 years ago by njee20.
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cheapest way to measure power
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flangeFree Member
How do all
Been training using heart rate zones but want to start using power as Measure of improvement. Thought about a power tap bit they're mega money. Anyone know of any cheaper alternatives?
ahwilesFree Memberride up a hill, and time yourself.
mgh / time = power
m = mass of you + bike in kilograms
g = 9.81
h = height climbed in metrestime in seconds.
power in watts.
easy.
simonfbarnesFree Membermgh / time = power
m = mass of you + bike in kilograms
g = 9.81
h = height climbed in metrestime in seconds.
that gives an amount of energy in Joules, to get power you need to use the vertical velocity in m/s instead of h
ourmaninthenorthFull MemberOr a Kurt Kinetic turbo. It's calibrated, so with a rear wheel mounted sensor you can use speed displayed to work out power.
njee20Free MemberThe cheapest is probably the iBike Pro, that 'calculates' power based on a number of other variables without actually measuring it. The fact they seem to be discounted everywhere suggests they may not be all that! They don't work properly if you change position and things I believe. A guesstimate though!
Next cheapest is probably the Polar CS600, but it's rubbish, works off chain vibration, but looks clumsy and is notoriously inaccurate and temperamental.
There are a number of systems floating around, Ergomo did some cheapish ones using a bottom bracket, but they went bust pretty quickly!
Various turbo trainers include power, if you only want to train indoors, IMO that's not much use, I've had a Tacx I-Magic for years, and never really used the power. If you go for a Tacx you need one with a motorbrake for it to be decently accurate.
Power Taps are the best real world comprimise, the cheapest ones aren't that expensive, money well spent I reckon. If you happen to have a Garmin Edge 500 or 705 you can use that in place of the standard head unit, which is what I do, saves a decent amount of money.
Or… just go the whole hog and get a Quarq or SRM crank!
Edit: as an aside the Garmin software for analysing the data is rubbish, so reckon on playing with some other software to be able to use the output. I use Training Peaks WKO3, which is great, but expensive.
finbarFree MemberFinbar, on a phone so didn't get the image
That's probably for the best.
tronFree MemberUse a gym's wattbike? Only cheap if you already go to a gym though.
njee20Free MemberThe trouble with indoor ones is that you're obviously limited to certain workouts and measuring improvement is hard.
I want to do some hill reps or whatever, then go back and look at my output. Or use it for time trialling etc.
IanMunroFree MemberYou could get a garmin forerunner or edge, download the free sportstracks application and get one of the plugins that estimates your power output. Obviously not as accurate as a proper measurement system, but you do get lots of pretty graphs to ponder over 🙂
njee20Free MemberHow do they work? Presumably using mgh / time = power? Maybe I should try one, compare it to the Power Tap!
You can hire all the different systems for a month, try a Power Tap see if you actually like using it/benefit from it.
DWHFree MemberI'd agree with njee that the Polar CS600 is ugly, clumsy, temperamental and generally rubbish.
However, I also have a Taxc I-Magic but I do do use the power measurement facility and find it very useful to measure my fitness throughout the year.
But I am tempted by a PowerTap…
njee20Free MemberDo it, they're good.
I used to use the I-Magic power thing to 'test' myself on one of the courses. The problem is that a) the power is notoriously inaccurate if it's not a Fortius trainer with the motor brake and b) because the 'hills' are only simulated through resistance I sometimes found myself putting out lower power and thus going slower, although my power:weight was higher. Go outside and ride up a hill and you notice you go quicker.
molgripsFree MemberSecond hand powertap (the older wired kind) from ebay. Might be tempted to part with mine so I can upgrade.. not sure.. 🙂
They are excellent, but not just to see your progress. You find out what your power zones are and the power meter allows you to stick to them properly. So if your base zone is 180-220WW then you know you are spending 4 hours in that zone on a 4 hour ride. Without this you'll only be in it for 3 hours at best I reckon.
It's an absolute revelation when you first use one. The tendency to try harder on climbs and slacken off on descents is so psychologically ingrained that you'll have a really hard time keeping the power constant.
It's a big mental effort to stick to a certain power on a long ride, it's also physically brutal, but it's extremely effective training. 180W or whatever seems like nothing at all when you set off, but 4 hours later it is bloody hard.
EDIT: should also add that I have used the Polar power sensor. It was veryh nice to use, much better than the powertap, but fundamentally flawed. It measures chain tension via vibrations to get your power output, but when you change gears the chain line also affects the chain vibrations. So you'll be pedalling along in middle ring and largest sprocket at say 200W, you shift into the little (chain line improves) and at the same speed on the same gradient you're suddenly doing 160W.
I reckon it'd work fine on a singlespeed tho 🙂
njee20Free MemberExcept that the sensor mounts on the rear mech, it'd be fine if you never changed gear! The CS power sensors are marginally better than the old S-series ones, but still not a patch on the Power Tap.
wwaswasFull MemberThe trouble with indoor ones is that you're obviously limited to certain workouts and measuring improvement is hard.
I'd say that carrying ou tthe same test in the same environment was an ideal way to measure improvements? You want as few variables as possible.
You'll only want to do the test irregularly anyway – otherwise it's like weighing yourself daily when on a diet – you never see the improvement.
molgripsFree MemberThe rear mech sensor was just chain speed; the vibration sensor was on the chainstay.
Edit: Oh yes, see what you mean – it couldn't be installed on a singlespeed.. 🙂
njee20Free MemberI'd say that carrying ou tthe same test in the same environment was an ideal way to measure improvements? You want as few variables as possible.
That's true, but as power's a much more useful training tool than HR (or anything else) it seems a bit odd to fork out for an expensive turbo that does it, then only use it intermittently to confirm that you're doing the right thing on the road. If you have a Power Tap (or similar) you can use the data all the time to train properly, not just use it to confirm that you are training properly. If that makes sense…
molgripsFree Member+1. It's a tool for actually training (provided you understand what's going on) rather than just a performance tester.
You would benefit from a blood lactate test though to figure out where your zones are scientifically.
RealManFree Membersimonfbarnes, that would give power. Doesn't take into account rolling resistances, wind resistances, and any energy lost by the inefficiency of the bike.
njee20Free MemberAlthough there's plenty of guides online about how to do it such that you can monitor your progress over time anyway.
molgripsFree MemberDoesn't take into account rolling resistances, wind resistances, and any energy lost by the inefficiency of the bike
Or your muscles and cardiovascular system.
Plus, Barnes/awhiles have made a schoolboy error – that model suggests that riding along the flat would cost no energy at all, and therefore infinite speed would be attainable (relativity notwithstanding). Now that'd be a great way to travel!
DaveGrFree MemberIs there any benefit in getting an off road version of the Power Tap so it can be used off and on road ? You seem to be able to rent the road versions for a reasonable price but not the off road ones.
njee20Free MemberI don't think power is really much use off road, the terrain dictates your pace far more, and unless you ride on fireroads the whole time it's too irregular to be able to stick within zones. I think it'd be interesting, but not as useful.
If you use your MTB to train on the road, then the MTB one would be good, but otherwise I'd get the road one.
coffeekingFree Membertherefore infinite speed would be attainable (relativity notwithstanding)
You've not seen me on a road bike. Whoosh!
thisisnotaspoonFree MemberBarns' method works on a steep hill, you need a hill steep enough to have you in the granny gear at your desired cadence. The idea being that your going slowly enough for aero drag to be negligable.
More usefull for comparing yourself through the year, eg after every 3 weeks trainng + rest week, ride the same hill at the same cadence and heart rate and see how fast you are. The other method is to do a top speed test on a completely still day in the same position. But this is harder to reproduce accurately.
Another way is to sprint up a flight of stairs, and work out your sprinting power from that, or do a step test (same principle, lift your bodyweight through a distance and see how quickly you can do it to calculate power). The step test is better as you can do it continuously and plot power Vs heart rate.
simonfbarnesFree Membersimonfbarnes, that would give power. Doesn't take into account rolling resistances, wind resistances, and any energy lost by the inefficiency of the bike.
all of which probably amount to less than 10% at realistic climbing speeds and wouldn't invalidate comparisons.
DaveGrFree MemberI don't think power is really much use off road, the terrain dictates your pace far more
Tks, that's what I thought – might be useful to see the spread of power used off road and for how long to assist in planning your training.
birly-shirlyFree MemberI think the hill climb test is good for working out comparative changes in fitness. You can even do the same climb several times at different heart rates/intensities.
If you track changes in fitness this way – then I don't see many disadvantages to using HR as your measure for workouts from day to day. The main one would be if you're training at high intensity/short duration reps – when HR isn't a very reliable indicator of effort. But for steady state training – anything from tempo rides out to endurance efforts, I'd think HR zones would be fine. I also think that using any sort of power meter and trying to hold a constant power output on rough ground would be an exercise in frustration.
FWIW – I've seen the Polar power units come out quite well from a head to head test with SRM and Powertap. Not as good – but nowhere near as expensive. Consistency of reading was actually pretty good – though for some reason suffered if used on a stationary trainer.
njee20Free MemberThe underlying trouble with all the 'calculate your power based on hill climbing' methods is that it's totally pointless! Why not just use your time/speed!? If you do it quicker then you'll calculate your power as being higher, whilst the first, slower time may have been with a big headwind and the second quicker time at a lower power output with a tail wind.
The number itself is of little use.
It's the erratic nature of the Polars that they get panned for, poor road surfaces, changing gear etc all affect the output. If you don't already have a CS600, but do have an ANT+ Garmin then the Powertap Elite jobby is about the same price.
simonfbarnesFree MemberThe underlying trouble with all the 'calculate your power based on hill climbing' methods is that it's totally pointless!
I never said there was any point to it :o) It would be very sad only to be climbing a hill to find out how fast you can do it 🙁
birly-shirlyFree MemberI can't say I've had those issues with the polar, although I've heard of others who have. Set-up, possibly constrained by bike geometry, might be an issue.
I already had a compatible HRM and footpod for running, and for what I paid to add on the bike power module, I think it's a great package. Although, from what I can see, Polar no longer offers one watch unit that interfaces with both running time/distance and bike power.
ahwilesFree Membermolgrips – Member
Doesn't take into account rolling resistances, wind resistances, and any energy lost by the inefficiency of the bike
Or your muscles and cardiovascular system.
Plus, Barnes/awhiles have made a schoolboy error – that model suggests that riding along the flat would cost no energy at all, and therefore infinite speed would be attainable (relativity notwithstanding). Now that'd be a great way to travel!
no schoolboy mistakes were made, flange wants a cheap way to measure power, i gave it to him. he didn't say it had to be accurate, or even usefull.
cheap, accurate, pick one.
now, give me £200,000 and 2 years research, and i'll come back with some rough figures for power output to overcome rolling resistance taking into acount 2 or 3 variables.
njee20Free MemberIt would be very sad only to be climbing a hill to find out how fast you can do it
No, it would be training!
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