Strava calculated power vs Powertap.

SHADOWMATE

Hi guys. First post ... so i'm a noob here. Just recently bought a Specialized 2013 Roubaix Comp Sport (first road bike) after messing with mountain bikes on and off for a few years (Still got a 2007 Spesh Rockhopper). I have a Garmin Edge 705 with cadence, speed and HR monitors which i use every time and have put 500 miles on the bike in about 5 weeks with it. I use Strava cos i think its pretty amazing and after a ride it gives me information on Power which their crazy magic technology has calculated for me. They say its pretty good compared to actual power meters with just minor differences etc, but my question is should i disregard it in favour of ACTUALLY GETTING a power meter.
Was looking at the Cycleops Powertap G3 but as its megabucks maybe the one lower down? I am properly hooked on road biking now and i am totally serious about cycling for losing weight and getting good n fit. (i am 6 foot 1 but weigh 17 stone, and i'm 37, but have been doing an average of a hundred miles a week when i'm on shoreleave) Can i honestly justify spending loads of money on information i get from Strava? How reliable is Strava information to use in my improvement plan. Plus if i got a Powertap already on the wheel, do i need the front aswell? Is that also a powertap or just a matching hub? Was looking on Evans Cycles as its on sale and i have a code from them too. Any feedback will be appreciated. Thanks guys.

danowat

SHADOWMATE wrote:

They say its pretty good compared to actual power meters with just minor differences etc,

They are wrong.

SHADOWMATE wrote:

Plus if i got a Powertap already on the wheel, do i need the front aswell? Is that also a powertap or just a matching hub?

Any front wheel is fine, doesn't need to match.

SHADOWMATE wrote:

They say its pretty good compared to actual power meters with just minor differences etc,
Was looking at the Cycleops Powertap G3 but as its megabucks maybe the one lower down?

You don't need to spend megabucks, either pick one up secondhand, or you can even get an SL+ from abroad for around £500.

majormantra

Strava power is not accurate enough to use for structured training. It probably provides a reasonable estimate for steep, windless climbs, but the rest of the time it's pretty useless.

That said, power is not essential for training anyway.

ShutUpLegs

Have you considered improving your pedalling technique first?

Trev The Rev

I. Without knowing your CdA, wind speed etc, you can't calculate power anywhere near accurately enough to use the power numbers for training. You would be better off just timing yourself up a regular hill.

2. You could hire a power meter - see how you get on with it. Some people find power meters more of a wind up than others.

3. People trained very effectively in the past, and still do, without power meters.

4. Before purchasing a power meter, read Racing & Training With A Power Meter by Hunter Allen & Andrew Coggan PhD. If you have not died of boredom or become bi polar when you have finished the book you will probably get on really well with a power meter.

5. I met a few wild eyed blokes living rough in the woods who's lives were ruined by their power meters, very sad, they lost their jobs, their wives left them and one of them tried to hang himself when he realised his FTP had not improved but his power meter needed re calibrating instead. The other had lost the power of speech because his computer crashed and lost six months power data.

6. In my opinion pedaling technique has little to do with cycling performance.

Herbsman

Trev The Rev wrote:

5. I met a few wild eyed blokes living rough in the woods who's lives were ruined by their power meters, very sad, they lost their jobs, their wives left them and one of them tried to hang himself when he realised his FTP had not improved but his power meter needed re calibrating instead. The other had lost the power of speech because his computer crashed and lost six months power data.

Heh.

jgsi

It's your money.
You don't need a power meter to lose weight thru cycling.
Since I returned to competitive cycling 5 seasons ago, I've only taken the first baby steps into training with a power meter (albeit VP with Trainer Road) for next season a few months back.... I'll see on payback in 2013.

Just ride.... or be prepared for someone to comment on the wealthy mamil generation nicking all the gear....

oh, loving Strava, but its power figures are all bllx

SHADOWMATE

Thanks for the feedback guys. Having digested everything you have all said, and concluded it would be wisest to hold off for now. I will get the book suggested (not on Kindle??) n see what i think afterwards. I'm also quite interested in the new Garmin left / right pedal power thing a me bob that's coming soon so will look into that.
I will try to get everything i can out of the stuff i've got for now. I am gonna have to get an exercise bike for on board though as it feels like i am detraining when i get back aboard (3 weeks about).
Actually i am working on my pedalling technique on rides as i do alternate pulling up x 4, pushing top x4, push x4, pulling bottom x4 sets but on the road its distracting (and people see me.) Getting an exercise bike for home (the boy and the missus will use it too) so i can practice on that.
Thanks again guys. This looks like a great community and i look forward to reading all of the info everyones put together. So....much....to....read....

indjke

I'd said that strava can give a good power estimation on close curcuit, when errors, caused by wind and incorrect elevation data, are compensated.

For example, strava estimated my best TT effort as 274W, and recent functional testing showed my FTP (measured 3 months later) must be near 270 W, close enough... :roll:

jgsi

indjke wrote:

I'd said that strava can give a good power estimation on close curcuit, when errors, caused by wind and incorrect elevation data, are compensated.

For example, strava estimated my best TT effort as 274W, and recent functional testing showed my FTP (measured 3 months later) must be near 270 W, close enough... :roll:

Your FTP is or isnt 270watts at a particular point in time. FTP is black and white.
"must be near" - what kind of testing protocol did you use?
I'll stick by Strava either being rubbish or flukey...

springtide9

So ignoring wind...

Looking at my figures, it varies a lot.

Strava on the flat with little wind it seems to overestimate. Strava seems most accurate on a moderate incline. On steeper climbs, it seems to sway the other way. But even the above statements are far too generalised to really enable a more educated guestimate.

So it's fine as a rough guide to seeing improvements over a long period of time, but it doesn't seem consistent enough to use as a guide for small improvements.

You can pick up the PowerTap Pro Rear wheel for around £600 new if you look around for offers/deals.

amaferanga

springtide9 wrote:

You can pick up the PowerTap Pro Rear wheel for around £600 new if you look around for offers/deals.

Or a Power2max crank for about the same.

Herbsman

amaferanga wrote:

springtide9 wrote:

You can pick up the PowerTap Pro Rear wheel for around £600 new if you look around for offers/deals.

Or a Power2max crank for about the same.

Tell me about these...

From what I've read, temperature changes cause shifts in power measurement. So riding in cold weather after the bike's been in a warm house all day would cause incorrect power values to be measured.

davidof

SHADOWMATE wrote:

I use Strava cos i think its pretty amazing and after a ride it gives me information on Power which their crazy magic technology has calculated for me. They say its pretty good compared to actual power meters with just minor differences etc,

given that the calculation of vertical covered usually overestimates by 15-20% or so I would say the power readings are not accurate. My max ten minute power is around 500 watts, I should be signed up by Sky if that were true. Some readings look more reliable but I think you'd be better off using HR for training that Strava power calculations.

amaferanga

Herbsman wrote:

amaferanga wrote:

springtide9 wrote:

You can pick up the PowerTap Pro Rear wheel for around £600 new if you look around for offers/deals.

Or a Power2max crank for about the same.

Tell me about these...

From what I've read, temperature changes cause shifts in power measurement. So riding in cold weather after the bike's been in a warm house all day would cause incorrect power values to be measured.

They've recently updated the P2M to include temperature sensors that compensate for temperature related drift, but this is actually only needed if you don't stop pedalling at all in the first 10-15 minutes to allow it to auto-calibrate. It would appear that the P2M was/is more affected by temperature than other power meters, but all power meters are susceptible to temperature related drift. My PowerTap would need ~10 minutes to acclimatise and my experience with my P2M suggests it takes about the same amount of time (and that's going from ~21 degrees indoors to ~8 degrees outdoors). I've not had the chance to do any direct comparisons since my PowerTap died before I got the P2M, but I'm confident that the numbers I'm seeing are reliable.

I have had a couple of issues with the auto-calibration messing up and giving low power numbers (sorted by doing a manual zero), but having contacted P2M and received a reply the same day, they have identified the problem and have an updated firmware for me. The unfortunate thing is though that I'll need to send it back to P2M in Germany to get the firmware updated, but I know that their turnaround is pretty quick (i.e. not weeks and weeks before they even look at your power meter as is the case with Paligap for PowerTaps). I've also had a software crash that required me to remove the battery from the P2M to reset it, which was pretty annoying. Again, P2M were very good and are going to test my unit when I return it for the firmware update to see if it's faulty and then either fix it or replace it if it is.

So the temperature drift issue is IMO now no longer an issue, but they're still not perfect. Mind you, the same can be said for PowerTap, Quarq and SRM - they all have issues. It seems that few people have had the issues that I've had, but the way P2M are addressing issues immediately is very reassuring. And knowing that I won't be hit by a bill for £425 in a couple of years time to keep the thing going (as was the case with my PowerTap) is also reassuring.

indjke

JGSI wrote:

Your FTP is or isnt 270watts at a particular point in time. FTP is black and white.
"must be near" - what kind of testing protocol did you use?
I'll stick by Strava either being rubbish or flukey...

Two minutes on each level, stepping up from 150W by 37,5W @ 75 rpm (dunno why so low, they said cadence, would it be 75 or 90, don't mean much)
Finished full 2-minute interval at ~340W at max HR of 210, they said this is enough :roll:
And my coach later said my FTP must be near 270W. I guess it was just estimation from MAP (80%, or 75% considering I could do some more seconds at higher power)
"Problem" is that I still haven't received full report based on all data (blood samples, gas analysis), otherways I could easily tell the exact number

Herbsman

amaferanga wrote:

They've recently updated the P2M to include temperature sensors that compensate for temperature related drift, but this is actually only needed if you don't stop pedalling at all in the first 10-15 minutes to allow it to auto-calibrate.

FFS, I was glad that last night that I was put off spending £700 on a power meter and £150 on a Garmin Edge 500.

Now I want to get one but can't justify spending the money....

willhub

I find in certain situations that Strava's power output readings seem accurate, as I roughly know the effort for 300W output, and if I feel I am sustaining around that power output then sometimes Strava is almost bang on. Obviously it's not accurate all the time so it's useless, if you're doing 300W into a 50mph heading you may well be doing 5mph, and Strava would have that down as something like 50w.

danowat

willhub wrote:

as I roughly know the effort for 300W output, and if I feel I am sustaining around that power output then sometimes Strava is almost bang on. Obviously it's not accurate all the time so it's useless, if you're doing 300W into a 50mph heading you may well be doing 5mph, and Strava would have that down as something like 50w.

I really don't know where to start with this one..............roughly, bang on, obviously not accurate......tell me, what exactly was the point of your post?

okgo

Lol!

It's a feature they should remove, that's the end of it really.

ShutUpLegs

amaferanga wrote:

Herbsman wrote:

amaferanga wrote:

springtide9 wrote:

You can pick up the PowerTap Pro Rear wheel for around £600 new if you look around for offers/deals.

Or a Power2max crank for about the same.

Tell me about these...

From what I've read, temperature changes cause shifts in power measurement. So riding in cold weather after the bike's been in a warm house all day would cause incorrect power values to be measured.

They've recently updated the P2M to include temperature sensors that compensate for temperature related drift, but this is actually only needed if you don't stop pedalling at all in the first 10-15 minutes to allow it to auto-calibrate. It would appear that the P2M was/is more affected by temperature than other power meters, but all power meters are susceptible to temperature related drift. My PowerTap would need ~10 minutes to acclimatise and my experience with my P2M suggests it takes about the same amount of time (and that's going from ~21 degrees indoors to ~8 degrees outdoors). I've not had the chance to do any direct comparisons since my PowerTap died before I got the P2M, but I'm confident that the numbers I'm seeing are reliable.

I have had a couple of issues with the auto-calibration messing up and giving low power numbers (sorted by doing a manual zero), but having contacted P2M and received a reply the same day, they have identified the problem and have an updated firmware for me. The unfortunate thing is though that I'll need to send it back to P2M in Germany to get the firmware updated, but I know that their turnaround is pretty quick (i.e. not weeks and weeks before they even look at your power meter as is the case with Paligap for PowerTaps). I've also had a software crash that required me to remove the battery from the P2M to reset it, which was pretty annoying. Again, P2M were very good and are going to test my unit when I return it for the firmware update to see if it's faulty and then either fix it or replace it if it is.

So the temperature drift issue is IMO now no longer an issue, but they're still not perfect. Mind you, the same can be said for PowerTap, Quarq and SRM - they all have issues. It seems that few people have had the issues that I've had, but the way P2M are addressing issues immediately is very reassuring. And knowing that I won't be hit by a bill for £425 in a couple of years time to keep the thing going (as was the case with my PowerTap) is also reassuring.

How easy are they to fit?

Herbsman

As easy as any other crank, I should think....

Alex_Simmons/RST

Crank based power meters are, cranks. So no more or less complex than fitting a regular crank. In the case of an SRM or Quarq, you do need to place a small magnet (supplied) in the vicinity of the inner plate of the power meter that trips the reed switch inside the meter (similar to a magnet on a spoke to trip a speed sensor). P2M uses a different technology to determine crank rotational speed and doesn't require this magnet.

ShutUpLegs

Thanks, just wondering if there was anything specific with the Rotor 3D cranks and the wanky BBright setup.

I emailed P2M today and they were keen to stress there would be no issues and they only take payment by bank transfer