Dual vs. single power meter advice please
 

I returned to riding a few years ago. Now 60 years-old but in okay shape. Each year I have ridden more and more, a few centuries, etc. I have improved but really slowly. So this year I want to use a coach. probably through Training Peaks. Anyway, I think a power meter will be a helpful tool but am not sure that the extra cost of a dual pedal system vs. single pedal or solo crank is worth it for me. I'm looking to become a stronger rider because I think I will enjoy it more. Any advice? Thanks.

Many will tell you that single leg is fine, but others will point out that L/R leg output isn't symmetrical for everybody all of the time. You really have to decide for yourself if you're okay with working with half of the story and guessing the rest. Honestly, I don't feel like a potential discrepancy between single (doubled) and dual would make a practical difference for me, but I'm just the kind of person that would rather know for sure.

Anywho, that in mind, if I were to go with a single leg solution, it would be one that's upgradeable to dual without spending too much more than going dual off the bat. At this point, Stages, 4iiii, Pioneer all have single-to-dual upgrade paths.

What I ended up doing, though - Powertap C1 chainrings. As of Mar 1st, Powertap/Saris is only selling them direct, but they bumped the price from $699 down to $349. This is a total power solution (with estimated L/R output, should you want that) for a very reasonable price, as long as your crankset is compatible (5-arm, 110 BCD without hidden bolts). My crankset wasn't compatible, but I tracked down an older Shimano crankset for less than $100 to pair with the Powertap. Do I like it? I don't know, I haven't received it yet - should be here tomorrow, I think.

I was discussing single vs. dual sided power meters with a Stages owner this morning. His main complaint was that his single-sided Stages often gave wonky results when he did low cadence (50-70 RPM) intervals.

If you think of pushing pedals like lifting weights it helps kind of clarify the differences in the two different approaches of obtaining power readings.


If your pedaling was bicep curls a single sided meter will tell you how much weight you're lifting and for how many reps, but the accuracy may suffer a little because the meter assumes both arms are lifting equally.


A double sided meter will tell you the same data but will also allow you to see which arm contributed more to the lift and the weight reading will be slightly more accurate.


To me the end goal is pedaling harder and going faster for longer, so a single sided meter gives me plenty to go on. I added it because speed isn't always a consistent way to measure progress since I ride mostly in groups, the number of pulls vary, the routes vary, and the conditions vary.

I don't think there's anything magic about knowing your L/R balance, but having data from both legs is important (to me). The C1 sounds like a fantastic deal and I'd jump on that in a heartbeat!

Pedaling asymmetry is a real thing, and it's a real thing that varies with your cadence, your power output, and your level of fatigue. For many people, the asymmetry is small and not very variable. For some, it's large and variable. The thing is, almost no one knows which group they're in ahead of time. Some people think that if they don't know, they can safely assume that they're pretty even and that it doesn't vary much with cadence, power, or fatigue. Others think that if their friends (or friends of friends) are pretty even, then they must be, too. You might be lucky in this way, and if there were a large difference in cost between single and "dual" sided power meters, that might be a good bet to make.

It also turns out there are accuracy and consistency differences between meters but, frankly, most people don't do things that require much accuracy or consistency so they don't notice it. That's because most people only use their power meters for training their FTP, and training FTP is one of the *least* demanding things you can do with a power meter. Nonetheless, sometimes a few people do have issues (like, "why is my power different on this other device? Which one should I trust?" or "I just bought a new power meter, and it doesn't agree with the old one"), and we generally buy power meters to help answer questions and resolve problems, not to raise new ones. Once again, if there were a large difference in cost, this might still be a good bet.

However, right now, a Power Tap G3 hub with a perfectly acceptable DT Swiss rim is $500; if you have the right crank, you can get a Power Tap C1 chainring meter for $350. Both measure total ("dual-sided") power. Both are either cheaper or the same price as single-sided cranks or single-sided pedals; for example, right now the 105 version of the Stages costs $530. With the hub, you don't need an extra speed sensor. Depending on the rear wheel you currently have, the PT hub can actually weigh less than your current setup (especially if you're currently riding with a separate speed and/or cadence sensor).

With the current pricing on the Power Taps, you can have accuracy should you ever need it and still save money. People think that swapping pedals or a crank between bikes is easy, but swapping a wheel is even easier. The only real disadvantage of the PT wheel that I can think of is if you frequently travel between places where you want to ride a bike and you want power data for those rides. It's a lot easier to stick pedals or a single-sided crank into your bags than a wheel. That's a specific use case that not many people face. If you have a setup that is compatible with the C1 chainring, that'd be an even better deal.

Here's a plot from a paper published last year (2017) that compares accuracy ("trueness") and precision ("consistency") of power meters.

http://anonymous.coward.free.fr/wattage/maier2017.png

Left side only power might not be as accurate due to leg imbalances, but it's still good enough for a lot of uses. I'm tracking my ride efforts, and using it for pacing. Not for structured training. I've gotten a pretty good idea of my best power at 1, 5, and 10 minute efforts. My power numbers have matched quite close on steady climbs to the calculated power for that speed on a few on-line bike power vs speed calculators.


For example, dcrainmaker.com posted this 30 second averaged chart, comparing Powertap hub, pedals, and chainring meters to the 4iiii left side meter. They track pretty close. (The 4iiii looks like it reads high on some peak efforts. I see this when going hard from a stop sign, sometimes getting high readings for a couple of seconds.)


https://media.dcrainmaker.com/images...12/image23.png



And power readings jump around a lot from one second to the next. That's why bike computers show 3 second or 30 second averages. If my readings are accurate within 5% or so, that's good for me.

Here's a ride charted in the fantastic, free Golden Cheetah software. Each dot is a 3-second average.
Then, the Golden Cheetah Critical Power graph of this season's rides, with the best effort at all different time periods from 1 second to the longest ride time. The current ride shows as a line in the graph.

There is no y-axis scaling in the second graph; how is anyone supposed to ridicule your power output without numbers?

I agree, though it depends on the rider and what his or her uses are.

That's really interesting. If you dig back in his archives, when he tested the first version of the Garmin Vectors he had a PT, a Quarq, the Vectors, and a Stages on his bike. The Stages read lower than the others at peak power. At the time, we thought it meant that Ray slightly favored his right leg at high power.

lol, I feel bad saying this, but was wondering where the power numbers were too. Heck, since the poster has a power meter and knows their way around Golden Cheetah, it's probably pretty good.

That's so cute.

I used a single sided PM (Garmin Vector S) for about 6 months. I added the second measuring pedal "just because". I am pretty consistently 48/52 and I don't think that what I learned from the extra pedal carried much value. But I still like that better.

There was one interesting exception where I had a knee injury and my cycling went pretty one-sided for quite a while. (like 56/44 - the 'other' way). So it isn't like there was no value to me.

dave

Given that there are dual-sided options that are cheaper, what's the argument in favor of single-sided?

Single sided is strictly price and I am not aware of any single side options that are more expensive than the same thing in double sided. Of course model XYX-single might well be more expensive than model AB-Dual. But # of sides is hardly the only variable in that case.

For me I wanted the flexibility of pedals and Garmin was it at that time.

dave

ps. I guess there is the ZWIFT option of only pedaling with one leg and alternating legs (but not sides somehow) :):)

It scaled between "Sad" and "Good try"

Golden Cheetah was pretty easy to get started with. Just upload your ride recordings, and you get a bunch of automatic charts and season trends. Later, you can edit or create new charts quite easily.

The new affordable crank meters that measure both legs look good. There wasn't anything like that last year.

I do like the single crank meters for their stealth size, extreme light weight, and simple CR2032 battery replacements. I'd think they will drop again in price from the new competition, maybe down near $200?

The least expensive single-sided meters tend to be left crank arms and pedals, so I guess the appeal would be ease of install and portability. With the crankarm, you could easily cover multiple bikes as long as you were running the same type/length of crankset (all 172.5mm Hollowgrams, for instance) on each bike. The pedals, on the other hand, are damn near universal. Even though I have a definite preference for Speedplays, the $800 Powertap pedals were very tempting from that perspective - in under two minutes they could be moved to any bike in the house, including the wife's Dutchie bike with wicker handlebar basket.

Thanks all for the advice. I have narrowed it down to a PowerTap G3 hub or PowerTap P1 pedals. Because I am having a new wheel built the difference in price between the wheel with DT Swiss 240 and G3 hub is only about $200; that seems the way to go.

This is easy, Single = low cost (typically) and normally consistent data, if you are using this with training programs, it doesn't matter if there is a left right difference, it just matters that you always have that left right difference. The downside, of course, is it will be hard to relate and compare your data, use services such as best bike split, or switch to a different power meter like a smart trainer.

Double = high cost (except G3 hub, C1 chainring, and Power2maxNeco) accuracy + consistency.

So, if you only intend on training with that power meter for the next couple of years the cheapest possible, proven consistent (like stages with built in temperature drift control), single leg power meter will work exactly as good as a double.

If you like to hop between your power meter and a smart trainer, or multiple bikes with different power meters (think dirt and road, most power meters are usually not swap-able between them), or want in-depth comparative analysis against other people with double power meters, a double will likely better suit you.

I ended up going with Assioma duo pedals myself, after originally ordering the single. But now that I have them I regret spending the extra money (but don't tell my wife this). Once you know your L/R balance what more do you need to know about it? Unless you are totally one-sided and are aiming to correct it, you won't use that data again. It turns out my balance is between 50/50 and 52/48 L/R. I had knee surgery in my right knee last year and that would explain the entire difference. How much accuracy does a 63 yo rider like myself really need? Not much.

Timely thread, I've spent the last few weeks researching the same. I think I've decided to go single sided Garmin Vector 3 because I like the option of quickly swapping between bikes, my winter training consists of dumb rollers, and I like the look of the Vector the best out of the pedal options.

I’d check the the problems with the vector 3s and the battery door fiasco on their website forum before buying v3’s. I was gonna upgrade my 2’s but now I will wait for a solution to the cheap battery housing or a new version of the pedal.

A good power meter is accurate to +/- 2 %, sometimes better. If you're only measuring one leg, you don't really know the maximum error of the thing. (Leg imbalances tend to change with cadence and output.)

It'll get you close enough for training and dieting purposes, but I personally don't see the point of buying an exacting measuring device if it isn't exacting.

But I'm a software developer by day, so I tend to think in terms of "it's either correct or it's not."

On my trainer tonight I was L/R 50/50 again. I really should've just bought the Uno.

Thanks for the heads up on that, I have not looked into it yet.

Been using one for two years. About to buy a second one since the price is so low.

Mine's been great.