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WR304 said:
The drop outs have been getting increasingly frequent so it looks like the hub batteries are going. There were several in today's ride so I'm going to try replacing the batteries with new ones. The batteries needed for the Powertap hub are:

2 x 357 batteries for the hub
I replaced the batteries in the Powertap hub and it looks like that was the problem. There were no data drop outs at all in today's ride.:)

The original batteries only lasted for approximately 60 hours of riding. I'm hoping that starting with fresh batteries means the hub will last a bit longer before they need replacing again.

One thing that's really annoying me is the barometric altimeter on the Cycleops Joule 2.0 head unit. There was about a week where the outdoors temperature increased and the altimeter recorded positive altitude values. The outdoors temperature has dropped back down towards 0c and the altimeter is at -572 feet again. As it can't be calibrated to the correct starting point that means no altitude data is viewable in WKO+, even for the hilly rides.:madman:

I've been seriously thinking about getting a Garmin Edge 800 as a replacement but all the threads about corrupt Garmin Edge 800 .fit data files which can't be uploaded, loss of connection to Powertap hubs, cracked touch screens etc have me wondering whether it would actually be any better than the Joule 2.0. :skep:

Garmin Edge 800 corrupt data
https://forums.garmin.com//showthread.php?t=15674

Garmin Edge 800 loss of connection to Powertap hub
https://forums.garmin.com//showthread.php?t=15826

Garmin Edge 800 cracked touch screen
https://forums.garmin.com//showthread.php?t=14493
 

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The Punk Hucker
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I love my Edge 705, couldn't it be a good alternative? The 800 doesn't have that many interesting new features honestly.

WR304 said:
I replaced the batteries in the Powertap hub and it looks like that was the problem. There were no data drop outs at all in today's ride.:)

The original batteries only lasted for approximately 60 hours of riding. I'm hoping that starting with fresh batteries means the hub will last a bit longer before they need replacing again.

One thing that's really annoying me is the barometric altimeter on the Cycleops Joule 2.0 head unit. There was about a week where the outdoors temperature increased and the altimeter recorded positive altitude values. The outdoors temperature has dropped back down towards 0c and the altimeter is at -572 feet again. As it can't be calibrated to the correct starting point that means no altitude data is viewable in WKO+, even for the hilly rides.:madman:

I've been seriously thinking about getting a Garmin Edge 800 as a replacement but all the threads about corrupt Garmin Edge 800 .fit data files which can't be uploaded, loss of connection to Powertap hubs, cracked touch screens etc have me wondering whether it would actually be any better than the Joule 2.0. :skep:

Garmin Edge 800 corrupt data
https://forums.garmin.com//showthread.php?t=15674

Garmin Edge 800 loss of connection to Powertap hub
https://forums.garmin.com//showthread.php?t=15826

Garmin Edge 800 cracked touch screen
https://forums.garmin.com//showthread.php?t=14493
 

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WR304 said:
The drop outs have been getting increasingly frequent so it looks like the hub batteries are going. There were several in today's ride so I'm going to try replacing the batteries with new ones. The batteries needed for the Powertap hub are:

2 x 357 batteries for the hub
The ability to replace the batteries in a Powertap is the reason they are superior to SRMs. You have to send SRMs away to get the batteries replaced, which
a) Takes a fair bit of time
b) Is stupidly expensive ($200+)
c) The north american company that does it, isn't very good at calibrating them.
 

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LMN said:
The ability to replace the batteries in a Powertap is the reason they are superior to SRMs.
Doesn't Alison's husband work for SRM now? Maybe you could get him to work on that...?
 

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And I know this is petty but, the edge 500 and 800 win me over due to the mount almost all by itself.

I just could not stand that sliding clasp thingy on my 705. I broke two of em in frustration trying to get the damn thing off of there.

The twist mount is infinitely better IMO.
 

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The Punk Hucker
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MisterC said:
And I know this is petty but, the edge 500 and 800 win me over due to the mount almost all by itself.

I just could not stand that sliding clasp thingy on my 705. I broke two of em in frustration trying to get the damn thing off of there.

The twist mount is infinitely better IMO.
I've been abusing mine (3 on 3 bikes) without a problem for as long as the 705 came out. Some folks are rough with their mounts, I've read numerous stories of them breaking but never understood how they did it.
 

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PissedOffCil said:
I love my Edge 705, couldn't it be a good alternative? The 800 doesn't have that many interesting new features honestly.
I'm dithering badly about what a good power meter head unit option would be. They all seem to have some disadvantages. You don't really know how easy they are to live with until you actually get to try them out.:(

The thing I like in theory about the Garmin Edge 800 over the Edge 705 is that it has a touch screen instead of a joystick. The joystick for navigating menus and displays on the Cycleops Joule 2.0 is fiddly and a nightmare to use when wearing winter gloves.

With devices like these Garmins you're very reliant on firmware updates. When a newer model comes out the older ones often become a lower priority for updates. A newer model seems like it would be more likely to have ongoing support. The argument with the Garmins seems to be that it takes a few years before their products actually work properly.

LMN said:
The ability to replace the batteries in a Powertap is the reason they are superior to SRMs
Changing the Powertap hub batteries would be a lot easier if they'd designed the hub so the plastic tool can unscrew the cover without needing to remove the brake rotor first.

I was a bit hasty thinking the new batteries had solved the issue. There were two dropouts in yesterday's 2h30 ride and three dropouts in today's 2h45 ride so it's still happening. The dropouts appear to be totally random whilst out in the middle of the countryside. I don't know if it's the head unit or the hub? On the plus side the dropouts are all short like the one pictured in post #75 so they're only losing a few seconds of data each time. When riding I'm not noticing the dropouts as they're short gaps. They only show up when reviewing the data post ride.

One thing I'm trying to understand is why there's a clear gap between my climbing power output and flat power output. It's not the same as Battle Duck's thread because all of my data is whilst riding seated.

http://forums.mtbr.com/showthread.php?t=676171

The two graphs below are extracts from today's ride (The altimeter had gone wrong so there's no altitude data). The seated climb is up a 6.6% average gradient hill. I was using the 23T rotor granny ring. The flat ride section is the final 10 minutes of the same ride home on the 44T round big ring where I was flat out and going as hard as I could. For a similar average heart rate and average cadence I managed 240 watts average climbing but only a poor 215 watts on the flat.

There's got to be an obvious explanation.:)

Pictured below: Differences in seated climbing power output up a 6.6% gradient hill and flat power output from today's ride.
 

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bholwell said:
Doesn't Alison's husband work for SRM now? Maybe you could get him to work on that...?
Ha! No. I don't work for SRM, or any other cycling related company. Wish I could help. I could recommend some good angioplasty catheters though. :D
 

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whybotherme said:
Ha! No. I don't work for SRM, or any other cycling related company. Wish I could help. I could recommend some good angioplasty catheters though. :D
No, sorry, not you- Alison Dunlap's husband.
 

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One of my big reasons for getting a power meter was for comparing ride intensity and trying to evaluate whether I was tired or not.

The problem being that after a few weeks of solid riding my exercise heart rate response would be very low, even though I felt good. I'd end up ignoring the heart rate monitor most of the time as I simply didn't believe what it was telling me.

http://forums.mtbr.com/showthread.php?t=673646

My hope was that the power meter would give a more objective measure that I could rely on. So far this has been working quite well. When riding the drop in performance when you're tired, both in terms of Intensity Factor and average power is very noticeable. A quick glance at the report is all it takes to show which rides were good and which weren't so far this year.

The WKO+ report below shows the rides that I've done so far in January 2011. The red line is average heart rate whilst the yellow line is average power for the same ride. You can see immediately that most of the rides have been fairly solid, apart from three which have much lower average power. The two days marked with a 1. are active recovery rides, whilst the ride marked with a 2. is the one mentioned in post #80 where I blew up badly. When you view the data in a report these stand out clearly as being different from the others. If you blow up it's reflected in the data.:)

I mentioned that I found heart rate an unreliable tracking measure and you can see that pattern emerging here also. For the first week or so the red heart rate line is higher than the average power line, but then the trend reverses mid month and the red heart rate line falls below the average power line. It's most marked at the point marked 3. on the report where I was doing 2h30 each day. My average heart rate per ride drops badly to as low as 148 bpm, even though I'm still feeling ok and able to maintain the average power output of previous rides.

The Intensity Factor for most rides was in the .85 to .93 range (assuming my FTP is right...) I've left IF off the report below because you can't adjust the axis scales in WKO+ reports. All the different lines and bars end up in a difficult to view mass. It's quite a serious omission from the WKO+ software.

The power meter is useful for ensuring active recovery rides are in the right zone too. It's all too easy to end up going harder than you'd think. Keeping the power output low enough on an active recovery ride seems to involve bottom gear whenever there's any sort of slope at the moment.:)

Pictured below: My Average Power and Average Heart Rate trends for January 2011 so far. It shows that my heart rate and power output aren't always closely related.
 

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It's possible to use recorded power data as a way of deciding the best pacing strategy for a ride or climb. Going as hard as possible isn't always the best option.:)

Burning "Matches"
A "match" is an elusive term used by riders, and coaches within the bike racing world. When you burn a match, you have done a hard effort. It's an effort that in which you had to dig deep, or you had to really push yourself.
...
Why do you need to know what a match is? Well, you as a rider, start out the day with a full set of matches in your matchbook, but every time you go hard, do an attack, have to hammer over a hill, you burn one of your matches. All of us have different size matchbooks, but nobody has an infinite number, so it's important to burn your -matches- at the right time during a race or in training. Otherwise you are left with an empty matchbook and then your chances of performing well have been drastically reduced."
Hunter Allen

http://www.peaksware.com/articles/cycling/what-is-a-match,-by-hunter-allen.aspx

When reviewing power meter data you can look through and note where the hardest efforts were. A theme in many of my rides was the observation that hard efforts were often followed by a sag in power output as I struggled to recover, losing more time than the hard effort had gained.

One of the worst of these was part of a short ride that I had done on 13 Jan 2011. I got to the second hill of the day and went hard up the first gentle 5.7% gradient section at 230 watts. The climb then ramps up steeply mid way to 10% gradient. I went up the steep section as hard as I could, peaking at 291 watts for 42 seconds, putting me deeply into the red "anaerobic zone" according to the Coggan power zones. As a result I had to slow down to try and recover. I could only manage 195 watts average power output for the remainder of the climb, which is a 4.5% gradient. It took a long time to recover after the climb also.

I did the same ride today. Remembering what had happened last time I decided to try a different strategy for the climb. I could see from the power data that the steep section looked like I'd burned a "match" as the 291 watts power output was 132% of my (estimated) 220 watts FTP. This time I decided to try and pace myself up the steepest section using the power meter instead of going flat out. I knew I'd lose some time on the steep section but hopefully make it back later.

Comparing Climbing Pacing Strategies
The graph below shows the climbing power output from 13 Jan 2011 (magenta) overlaid with the climbing power output from 25 Jan 2011 (yellow). It's easier to see if you zoom the picture in your browser.

The four dotted lines are the edges of the different power zones. Anything above the upper dotted line is "anaerobic zone". The first section of the climb is highlighted in green. I rode the lower section of the climb at a similar average power output on both days. 13 Jan 2011 was slightly quicker here because I carried more speed through the bend that leads onto the climb.

The steepest section of the climb is very different however. Instead of going competely flat out I used a lower gear. I was still trying hard but only in the "anaerobic zone" for 19 seconds at 272 watts. Half the time when compared to 13 Jan 2011. I was only 4 seconds slower too despite riding at a lower intensity!

The benefits of this approach are apparent in the time and power output for the top section of the climb where I was able to maintain the power output at 232 watts on 25 Jan 2011 as opposed to 195 watts (and struggling) for the same section on 13 Jan 2011.

I was also able to recover quicker after the climb and keep the pace high for the rest of the ride. The final totals for the entire ride being 1h 36 min at an average of 188 watts for 13 Jan 2011. 25 Jan 2011 was 1h 34 min and 36 sec at an average of 200 watts. All that time difference was gained in the section of ride after the climb pictured below as I felt able to keep pushing. The two rides had been very even until then.:)

Pictured below: Variations in pacing when climbing on two different days. Yellow is 25 Jan 2011, Magenta is 13 Jan 2011. Zooming the picture in your browser makes it easier to see both traces.
 

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I've been trying to compare the energy expenditure (kilojoules) as calculated by my Powertap with the energy expenditure (kilo calories) that was calculated by my Polar RS800CX heart rate monitor. I wanted to see how comparable the energy expenditure calculation was between the two devices. In theory you can simply say that 1 kilojoule measured by the power meter is equal to 1 kilo calorie because of efficiency losses (instead of converting the kilojoules into calories).

The kilo calories burnt calculation of a heart rate monitor is highly reliant on the initial user values that you enter into the heart rate monitor (age, weight, exercise level, Max HR, VO2 Max etc.) The energy expenditure is then estimated using a formula based on these values. As a result the kilo calories burnt figure calculated using a heart rate monitor can be wildly inaccurate - often over estimating the numbers of calories burnt.

Have a read of this web page and try out the calculator to see how the estimated kilo calorie usage numbers change when you use different VO2 Max and age values (age in this formula adjusts your maximum heart rate). If you use an estimated maximum heart rate and estimated VO2 Max for the settings (as opposed to knowing your actual maximum heart rate and having a lab tested VO2 Max) then it introduces errors into how the heart rate monitor calculates energy expenditure.

Calorie Expenditure Calculator:
http://www.braydenwm.com/calburn.htm

Whilst I was looking at various articles the interview discussing energy expenditure below was linked. I've posted it here because it seems relevant when considering what a Powertap can be useful for.:)

Full Discussion:
http://www.bikeforums.net/showthread.php/179758-calories-burned-seem-low

**************************************************************************************************
Energy In, Energy Out: What's the Magic Equation? (Part I)

A few weeks ago I was going through a thread on slowtwitch.com discussing caloric expenditure on the bike and the accuracy of the kcal numbers on our HRM's. I always assumed that the variables in caloric expenditure could never be accurately determined by heart rate alone; temperature, hydration, force, etc all have a direct effect on energy needs, right? In order to get a better understanding on our energy needs I contacted Richard Stern*, cycling and multisport coach of cyclecoach.com, Gordo Byrn of coachgordo.com, and my coach, Lauren Maule.

I'll begin this series with Richard. Reading his in depth explanations on the slowtwitch thread - "Fat loss question" are what set me on this quest. I began my inquiry by asking Ric simply, why is the Polar computer inaccurate and by how much?

Stern: Energy Expenditure (EE) is related to power output, as EE = power x time. This is actually the mechanical energy used to drive the bike but is actually close enough to actual EE. The mechanical energy is actually in kj (kilojules), but can be converted to kcal (kilo calories) -- the more usual metric for nutrition.

Supposing, that a cyclist averaged 200 watts (W) for 3-hours, the mechanical energy (ME) would be: ME = power (/1000) x time (secs) = (200/1000) x (3 x 3600) = 0.2 x 10800 = 2160 kj

However, 1 kcal is equal to 4.18 kj, thus, 2160 kj is equal to 517 kcal. Obviously, this would be a very low EE for a 3-hr ride. Interestingly, efficiency during cycling is ~ 20 - 25% in trained riders, so these two figures approximately cancel, and we can then say that 2160 kj ride is approximately a 2160 kcal ride.

Efficiency (thermodynamic efficiency), is a measure of the actual mechanical work accomplished divided by the input of energy. This is mainly affected by cadence in cycling. Paradoxically, efficiency is *highest* at lower cadences, and lower at higher cadences. This is because energy is required to just move the legs (with example, no chain on the bike) and higher cadences require more energy. However, as power output increases, efficiency also increases. Therefore, topographical conditions have an effect on efficiency as you tend to pedal slower uphill and faster on the flat.

The biggest retarding force in cycling is air drag (hence the reason why we all try to get as aero as possible). If for example, you have two bikes a TT aero one, and a standard road bike, at the same speed under the same environmental and topographical conditions you'll need to produce less power on the TT bike than the road bike.

Heart rate can vary for a multitude of reasons, e.g., temperature, topographical conditions, humidity, fatigue, stress, caffeine, etc. Heart rate can vary quite dramatically even at a constant power output. As an example, when I complete 20-min TT intervals at the same power (ridden indoors on a trainer) my HR can vary by ~ 15 to 20 b/min depending on the session the day before, what I've done just before, fatigue, etc. Heart rate therefore becomes 'unreliable'.

Therefore, knowing power is the only realistic way of calculating energy expenditure, as trying to estimate it EE from e.g., HR is just too variable. Thus a HR monitor won't give an accurate or useable EE for cycling as there are too many variables not accounted for.

KP: Without the benefit of having a power meter, how can one assess their caloric expenditure while cycling (assuming they use an HRM)?

Stern: Basically, unless you want to do a reasonable amount of analysis it's pretty much impossible to estimate EE without a power meter, and get a meaningful answer.

KP: I read on Gordo's site that Molina is of the opinion that the average well trained athlete burns 100 kcal per mile and an elite runner could burn as little as 50 per mile. Do you agree? Can you speculate on how this translates with swimming?
Stern: It's generally easier to calculate EE in running because speed varies less than in cycling and position isn't as important. However, running economy does vary between individuals.

KP: What do you suggest for your cycling and multisport athletes that you coach who lack power meters?
Stern: I don't suggest that they attempt to track EE. It would be very unreliable data.

KP: Do the foods consumed and their micronutrient content affect the balance of fat used to other fuel sources? How can one maximize fat usage as opposed to glycogen and protein?

Stern: Other than by training and getting fitter (i.e., increased VO2 max, increased LT) there is little or nothing that can be done to affect substrate usage. As we get fitter, we use more fat at a given intensity compared to glycogen. As the duration increases there can be a shift to greater fat usage. As relative intensity increases there's a greater use of muscle and liver glycogen (i.e., for an Olympic/sprint you'll be exercising at a higher intensity than an Ironman distance event, and using more carbohydrates).

During exercise, it's important to keep taking in carbohydrates and fluids. There's unequivocal evidence to support the use of carbohydrate during events longer than one hour (and some evidence for less than one hour). Taking in 0.7 g carbohydrate / kg body mass has been shown to unequivocally extend performance (e.g., 30 to 60 g / hr).

During exercise fluid intake is crucial to prevent dehydration: this can be achieved with 150 - 350 mL (milliliters) of a carbohydrate-electrolyte solution every 15 - 20-mins, starting at the beginning of exercise. The carbohydrate-electrolyte solution should be 4 to 8% and contain sodium. The electrolytes stimulate the thirst mechanism and can help prevent hyponatraemia in susceptible people.

KP: Can one determine optimal power to weight ratio without a power meter? Some try so vehemently to lose weight they end up costing themselves power in the process. Can one determine their optimal power to weight ratio by doing say, a specified hill or TT course over a period of weeks and comparing weight, time's, effort level, and HR?

Stern: Within the confines of losing typically small amounts of mass (e.g., ~1 or 2 kg) it will make little or no difference to performance. Unless you have a large amount of fat to loose, then it's virtually always better and easier to gain power compared to losing weight (as a primary goal). In fact, with a lot of the riders that I coach, whilst training intensely for increases in e.g., LT power output or power at VO2 max, they end up losing some fat due to the increased training energy expenditure. During flat ground cycling there's little to be gained by loosing small amounts of weight and even on hills it will only add a very small amount of time. Power to mass ratio is of more importance when e.g., cycling uphill.

More to follow in the next installment.

*Richard Stern is a cycling/multisport coach in UK and retains the copyright on the material used in this interview. For more information, visit www.cyclecoach.com or click on the banner below.

***************************************************************************************************

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There was a short article about the Fisica ANT+ sensor key on the DC Rainmaker blog.

It lets you use ANT+ to connect sensors such as the Powertap hub to your iphone or ipad and use it as a head unit.

http://www.dcrainmaker.com/2011/01/weekend-and-wedding-update.html

http://www.dcrainmaker.com/2010/05/first-look-at-wahoo-fitness-ant-iphone.html

http://www.wahoofitness.com/Fisica/Wahoo-Fitness-Fisica-Fitness-Sensor-Key.asp

The example was of overlaying your live power data on the screen with a video. It might be interesting if you do a lot of indoor training and want some extra motivation.;)

Pictured below: DC Rainmaker live power data overlaid on a ipad.
 

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WR304 said:
One of my big reasons for getting a power meter was for comparing ride intensity and trying to evaluate whether I was tired or not.

The problem being that after a few weeks of solid riding my exercise heart rate response would be very low, even though I felt good. I'd end up ignoring the heart rate monitor most of the time as I simply didn't believe what it was telling me.
A lot of the things which I'm finding when riding with a power meter appear fairly obvious. What it does is provide the supporting information to actually prove it either way - whether I really was being lazy and riding around at a low intensity as the heart rate monitor was saying, or whether I was trying hard like my perceived exertion indicated.

The graph below is an updated version of the chart from my earlier post. This one has the rides grouped into weekly blocks and is up to 30 January 2011 for five full weeks. This style of report hides the detail of individual days but it does give an overview of how my heart rate and power output has changed over the month. It appears to largely confirm what I'd suspected - that even when my heart rate response is low I can still have good pace, be trying hard and improving.:)

In the final week of January 2011 I set my best peak power records (for January) at 5 seconds, 1 minute, 20 minutes and 60 minutes which was encouraging. Without Monday's active recovery ride included the final week to 30 January 2011 was also the best overall (178 watts average, 146 bpm heart rate and an IF of 0.87 for 11h 45 min!). It's still a way off October 2010 but shows progress.

At the same time however my average heart rate has fallen significantly over the same period - from an average of 166 bpm in the first full week of January 2011 to an average of 144 bpm in the final week of January 2011. If you were looking purely at the WKO+ heart rate zone summaries for the last few weeks then you'd conclude that I'd just been doing lots of long steady distance riding without any intensity, which is a misleading picture.

I need to sort out what my real FTP is though. I'm still using 220 watts for the FTP figure used to calculate these IF and TSS figures. I think it may need revising upwards for February. I'm nowhere near being able to manage even 220 watts for a full hour in actual riding though. The best I've managed so far this year was an average of 202 watts for an hour.:(

Pictured below: My weekly totals for average power and average heart rate to 30 January 2011. The power output stays constant but heart rate falls steeply.
 

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I am in the prosess of buying a powertap for my road bike.

Am I wrong in thinking the functions on the SL+ and Pro+ are the same but the SL+ is carbon/lighter? I already have a garmin 500, so i`m just looking at hubs.

I have both 26 and 29 inch wheels on my MTB`s and most of my intervals are done on the road so I think that is the best option for me.

I train watt on my fortius in the winter and would like to do it outdoors during the season too.

There is a lot of cool info been posted up in this thread and i`m looking for a nice gift from my wife for my 40th birthday this year. :D
 

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The Punk Hucker
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fux said:
I am in the prosess of buying a powertap for my road bike.

Am I wrong in thinking the functions on the SL+ and Pro+ are the same but the SL+ is carbon/lighter?
Marginally lighter, that's the difference. I got the Pro+ because it's a training wheel after all.
 

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fux said:
I am in the prosess of buying a powertap for my road bike.

Am I wrong in thinking the functions on the SL+ and Pro+ are the same but the SL+ is carbon/lighter? I already have a garmin 500, so i`m just looking at hubs.
You currently don't get a choice of models for the MTB Powertap. The only option is the most expensive SL+ if you use disc brakes.:(

The difference in price between the new Powertap road models appears to be purely down to differing construction and materials used. You're paying for a part carbon hub body and alloy axle / freehub, rather than different internal electronics.:)

http://www.cycleops.com/products/power-meters.html

Road Powertap SL+
•15mm Alloy axle and free hub body
•Carbon/alloy hubshell
•Hub weighs 412 grams

Road Powertap Pro+
•15mm Alloy axle and free hub body
•Alloy hubshell
•Hub weighs 466 grams

Road Powertap Elite+
•15mm Steel axle and free hub body.
•Alloy hubshell.
•Hub weighs 583 grams

If you're looking at road Powertaps for a training wheel then I think I'd choose the Powertap Elite+. The reason being that it has a steel freehub body which will be more reliable as it won't be damaged by Shimano cassette splines. Weight doesn't really matter for a training wheel.

The MTB disc version of the Powertap SL+ hub has a steel freehub body according to the product page:

MTB Powertap Disc SL+
•Alloy axle with steel freehub body
•Carbon/alloy hubshell
•Hub with rotor weighs 680 grams

There doesn't seem to be any difference in the number of strain gauges or accuracy between the different wireless Powertap hubs. (+/-1.5% claimed accuracy for the newer Powertap hubs.) So far as I can tell there's no difference between the data accuracy whether it is recorded by a SL+, Pro+ or Elite+ hub.

If you were looking at SRM crank power meters then they have different numbers of strain gauges in different models. It makes the more expensive SRM models more accurate, adding something besides just different weights.

Powertap Accuracy (2005)
http://www.active.com/cycling/Articles/SRM_and_PowerTap__How_accurate_are_they_.htm

Powertap Accuracy (updated January 2011)
http://biketechreview.com/reviews/power-meters/61-power-meter-review?start=1

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