Advice on renting a power meter

chakattack

I'm thinking of renting a power meter instead of renewing my gym membership while I wait and save up for a pedal based system in the future.

I have a Garmin 500 in the post and don't want to mess about with changing cranksets etc. for SRM so I think I'll go with a powertap wheel of some sort.

Given that the wheels available are basic open pro rims with powertap hubs I'll probably only use it for training and flat/low priority races. With this in mind is there any advantage to choosing the powertap pro+ or sl+ hubs over the more basic elite model? Is weight the only difference?

I'll have the garmin for hr/cadence/speed so just need power data from the powertap.

Any thoughts or advice welcome.

billy.fish

If you are going to rent a powermeter the best data you will get is from racing not from training alone.

How else do you actually get to see what you can or cannot do in real world racing and change your training to suit?

I do agree that a PM is a better investment than a gym membership for most, but make sure you are the kinda person that will benifit from one.

Go buy TARWAPM mk2 and read as much of it as you can, then you should have a better idea.

FWIW i think its the best investment you can make after a coach.

Beasty

billy.fish wrote: »

Go buy TARWAPM mk2 and read as much of it as you can, then you should have a better idea.

TARWAPM mk2 - in case anyone's wondering

I agree that the power meter should be used for both racing and training - I know more about my limits, and what I need to work on to push them out further from my racing data than from any training I did last year

chakattack

Thanks lads.

billy.fish

Never a problem. Soon you will be indoctrinated into the church of Power. It is good.

Blessed are the W.kg-1

chakattack

My biggest problem is that Joe Friel tends to scare me off training hard and while I feel fresh I don't have the satisfaction of knowing I've trained as hard as I should. With a PM I can let the numbers tell me when to go hard and when to go easy.

markdrayton

The Pro+ weighs *about* the same as the more pricey ones but has an alloy axle and (I believe) an older, possibly inferior, bearing design. Some people have had issues with the relatively bendy axle causing brake rub. I have a Pro+ and I've never noticed this but if you're awesomely powerful either take the weight of the steel-axled Elite+ or the price of the SL+.

The Coggan/Allen book and the wattage list are your best friends. Friel does seem very long/steady distance based whereas Coggan/Allen will have you doing L3/L4 during the winter. This gets my vote -- quicker and more stimulating than plodding.

pgibbo

chakattack wrote: »

I'm thinking of renting a power meter instead of renewing my gym membership while I wait and save up for a pedal based system in the future.

I have a Garmin 500 in the post and don't want to mess about with changing cranksets etc. for SRM so I think I'll go with a powertap wheel of some sort.

Given that the wheels available are basic open pro rims with powertap hubs I'll probably only use it for training and flat/low priority races. With this in mind is there any advantage to choosing the powertap pro+ or sl+ hubs over the more basic elite model? Is weight the only difference?

I'll have the garmin for hr/cadence/speed so just need power data from the powertap.

Any thoughts or advice welcome.

Where are you going to rent from? I was looking in to this yesterday and saw that Cycle Power Meters have a waiting list on rentals for PTs. Cheers.

chakattack

I didn't look that hard to be honest. Guess I'll have to sign up for the waiting list.......:mad:

mo_bhicycle

I had a quick look at cyclepowermeters.com, there seems to be a 3 month minimum contract.
I'm a bit confused by their pricing, the PowerTap Elite+ wheel only rental is £11 a week, which works out at approx £189 for 3 months. However, what is the £89 listed on the "PowerTap Elite+ wheel only rental" page.

Anybody know what the actual price for 3 months (assuming that is the minimum) rental is?

Also, for the sake of comparison, where's the cheapest place to buy an Elite+ wheel or hub?

Beasty

mo_bhicycle wrote: »

I had a quick look at cyclepowermeters.com, there seems to be a 3 month minimum contract.
I'm a bit confused by their pricing, the PowerTap Elite+ wheel only rental is £11 a week, which works out at approx £189 for 3 months. However, what is the £89 listed on the "PowerTap Elite+ wheel only rental" page.

Anybody know what the actual price for 3 months (assuming that is the minimum) rental is?

Also, for the sake of comparison, where's the cheapest place to buy an Elite+ wheel or hub?

The £89 is 4 weeks rental at £11pw plus £45 deposit

over 13 weeks you would pay 13 x £11 = £143 (after return of your deposit)

Check wiggle and CRC for alternative purchase prices

mo_bhicycle

Beasty wrote: »

The £89 is 4 weeks rental at £11pw plus £45 deposit

over 13 weeks you would pay 13 x £11 = £143 (after return of your deposit)

Check wiggle and CRC for alternative purchase prices

Thanks Beasty ... total maths fail on my part

chakattack

markdrayton wrote: »

The Pro+ weighs *about* the same as the more pricey ones but has an alloy axle and (I believe) an older, possibly inferior, bearing design. Some people have had issues with the relatively bendy axle causing brake rub. I have a Pro+ and I've never noticed this but if you're awesomely powerful either take the weight of the steel-axled Elite+ or the price of the SL+.

The Coggan/Allen book and the wattage list are your best friends. Friel does seem very long/steady distance based whereas Coggan/Allen will have you doing L3/L4 during the winter. This gets my vote -- quicker and more stimulating than plodding.

Thanks Mark,

Not sure how "awesomely" powerful I am but that's what the power meter is there for.....a dose of reality

Lumen

chakattack wrote: »

Not sure how "awesomely" powerful I am but that's what the power meter is there for.....a dose of reality

A power meter just ferments different lies.

"GoldenCheetah says my threshold power is only X but I think it is actually Y because I wasn't in a proper race mindset and the ambient temperature was too low and the elastic of my bib tights was stealing my power".

mloc123

Keep in mind that Garmin data is largely junk when it comes to HR and Power.
Either get a joule headunit also or go wired... IMO

Lumen

mloc123 wrote: »

Keep in mind that Garmin data is largely junk when it comes to HR and Power.

References please, specifically w.r.t. power data.

edit: OK, I searched Wattage and now my brain hurts (login required).

I did some calculations on the uniform cadence approximation.

Consider the case where rotation rate varies, but torque also varies
to keep applied power constant. Power is the product of torque and
rotation rate, so this assumption can be stated:

τ = P / ω

where ω is the rotation rate, and τ is the torque.

Let's then take the simplest possible case: ω = ω₀ ‒ Δω for half the
stroke duration (time, not angle), while τ = P / ( ω₀ ‒ Δω ), and for
the rest of the pedal stroke ω = ω₀ + Δω, with τ = P / ( ω₀ + Δω ).
Obviously, the average power is P, since I am assuming that the power
is always P.

But what would Cinqo conclude? It assumes the average power is the
average rotation rate multiplied by the average torque. Average
rotation rate is simply ω₀. Average torque's a bit more complicated,
though:

average τ = P [ 1 / ( ω₀ ‒ Δω ) + 1 / ( ω₀ + Δω ) ] / 2

= P ω₀ / ( ω₀² ‒ Δω² )

So if I calculate the power from this, where PQ equals the Quarq-
effective power, I get:

PQ = P ω₀² / ( ω₀² ‒ Δω² )

= P / ( 1 ‒ Δω² / ω₀² )

So if Δω / ω = 10%, then the error is 1%. If Δω / ω = 20%, then the
error is 4%. In each case the Cinqo overestimates power.

billy.fish

ZZZzzzzzzzz.....

It all boils down to two things: push harder or pedal faster.

Assuming you dont get a reduction in one while the other increases, you get faster*

Get one, see how you go, ask questions. 90% of he science behind it is of no interest to 99% of the people.

Always willing to leand a hand to people having a bit of knowledge, but mostly, start collecting data. You'll need at least 3months of solid data before you can do anything interesting with it.




*this is not _entirely_ true

chakattack

mloc123 wrote: »

Keep in mind that Garmin data is largely junk when it comes to HR and Power.
Either get a joule headunit also or go wired... IMO

Garmin delivered today....thanks for bursting my bubble :cool:

What's your definition of junk? Read marginally high or low?

ktz84

mloc123 wrote: »

Keep in mind that Garmin data is largely junk when it comes to HR and Power.
Either get a joule headunit also or go wired... IMO

What's wrong with the lyc if data accuracy is your main beef with the garmins?

As regards the joule, which is a bike computer afterall, is grossly overpriced therefore anyone buying it has more money than sense IMHO

Beasty

ktz84 wrote: »

As regards the joule, which is a bike computer afterall, is grossly overpriced therefore anyone buying it has more money than sense IMHO

I take it that if you have such views on the Joule, you have actually used one?

Having Garmins (705, 500, 800) and a Joule I can confirm their purchases have left me with less money than sense (others may disagree, but I believe I am better placed than them to comment)

Some spend thei money on flash bikes, others on fast wheels, or computers or cycling clothing. I spend mine on all these. Yes, it may appear extravagent, but so what?.

The Garmins give pretty much identical power data to the Joule. The benefit of the Joule is it can provide additional power data "real time", such as normalised power, TSS and IF - I find this particularly helpful when racing, as it shows me if I need to back-off, or if I have capacity to push harder

ktz84

You are no better to comment than I am. What I said was that the Joule was grossly over priced. If every feature works as advertised then it still means it is grossly over priced as far as I'm concerned.

What is so special about a joule that it commands such a high retail price? A few training metrics. For that you lose functionality you can get elsewhere. When you compare its featureset to those of other bike computers then I'm afraid it just doesn't add up to that cost and just because some people buy still doesn't mean that it well priced.

It could sell that computer and half the price or a bit more than that in my opinion would sell 10 times as many.

Let's not forget that it is the power meter that does all the really important stuff. The joule is just there to represent that information.

ktz84

Why not build your own

http://chomsky.shampoo.ca/goldenembed/
http://groups.google.com/group/goldenembed?lnk=

GPS will make this an interesting project.

markdrayton

chakattack wrote: »

Garmin delivered today....thanks for bursting my bubble :cool:

What's your definition of junk? Read marginally high or low?

I don't know of a problem with Garmin units and power/HR data. If mloc123 has a reference I'd like to see it.

I vaguely remember the first 705s didn't smooth the realtime power at all so it was very spiky (the raw numbers from the meter jump about a lot) so quite hard to use. IIRC this was corrected a long time ago. The Edge 500 can display live power raw, smoothed over 3s and smoothed over 30s.

I've used the original Powertap head unit and an Edge 500 for quite some time. My fitness and the numbers each have produced are the same.

The Edge 500 fits my needs pretty much perfectly. The only issue I had was pre-2.40 firmware versions losing ride data now and then. This hasn't happened since 2.40 was released (and 2.60 out now). The Joule has extra, like TSS and NP, but these are generally of more use in post-ride anaylsis. WKO can generate them for you or you can even do it by hand.

billy.fish

Mark. issues comes down to the sampling rate used , never mind the heart rate 'calculations' that they attempt.

good thing is they a pretty much consistently wrong so its all good.

have papers somewhere on it but no idea where.

pgibbo

Sorry to interrupt the flow on the Garmin vs Joule but I was wondering is there anywhere else other than Cycle Power Meters that rent Power Meters? Cheers.

markdrayton

billy.fish wrote: »

Mark. issues comes down to the sampling rate used , never mind the heart rate 'calculations' that they attempt.

But surely the *sampling rate* (ie, the frequency at which raw torque values are emitted) is determined by the meter's transmission rate, not the head unit? If the meter emits a value every x seconds you're never going to collect more than 60/x samples per minute. If the head unit's recording rate is much lower than the transmission rate as you'd lose some precision in the stored data but my understanding is that the Powertap's transmission rate is somewhere around 1s, and the PT and Garmin head units will both record at 1s. The raw data data should be the same on each. Of course, I work for neither company nor have any insider information, but given the data transmission rate is always the same (same hub) and both units can record at the same rate I see no reason why one would be worse than the other. Certainly in my n=1 experience the numbers from both units are comparable.

As for HR calculations, AFAIK the only one the Garmin does (and PT?) is the standard guesstimate-from-age formula. This doesn't hold for a huge number of people but it's a market requirement -- every HRM does it so Garmin (and PT?) would be at a disadvantage not to follow along.

billy.fish

As far as i remember, would need to dig out the paper so dont hold this as 100% , its not that the transmission rates differ (how could they) but the way that the 1 second average is calculated.

FWIR the Garmin almost takes a snapshot every 1second rather than a rolling 1s summation. Cant remember though.

Its an issue we've come across inhouse trying to develop software to link several sensor streams together, one of which is a PT the other an SRM. Reasons why are kinda odd, somewhat of a mind exercise, but purpose underlaying it.

markdrayton

billy.fish wrote: »

FWIR the Garmin almost takes a snapshot every 1second rather than a rolling 1s summation. Cant remember though.

Okay, I can believe there are differences. But did the paper suggest that one head unit is more accurate or useful than the other, which is kinda where this all started?

billy.fish

You're making an assumption i read the entire thread aren't you Mark

No they were just talking about the rates and how its handled.

Personally i still use my LYC and love it. Joule would be my preference due to the real time NP for time trial events, not to fussed on the GPS side of things as i generally know where im going , if not i just bang then 405 on (hence the sensor combination thingy).

So in short. No.

Mine is better though.

Lumen

Ah, LYC = little yellow computer, i.e. Cycleops/Saris/Powertap head unit.

chakattack

pgibbo wrote: »

Sorry to interrupt the flow on the Garmin vs Joule but I was wondering is there anywhere else other than Cycle Power Meters that rent Power Meters? Cheers.

I rang them and they got more in stock over xmas and there is no longer a waiting list.

They just haven't updated the webpage.

Now for lots of reading and the appliance of science......