4iiii Precision/Tacx Neo2 - different slope?

I recently purchased a 4iiii Precision to replace my Watteam Powerbeat G2, that stopped working. I had checked the Powerbeat against a Tacx Neo, and it was within 3-7 watts on all the steps of the Ramp Test. In a sprint, it was off a bit more, but still less than 3 % so I was satisfied.

I decided to do the same test with the 4iiii on a friend’s Neo2. The result was far less satisfying. TR controlled the Neo2 and I set my Bolt to record the 4iiii. I chose 20 second power, to minimize fluctuations and waited at least 30 seconds for the power to stabilize, after beginning each step.

I don’t have all the numbers here, but I remember enough of them, to paint a clear picture. For the warmup, TR set the Neo2 at 104 watts. The 4iiii showed 116. This continued the next steps, but the gap decreased, until around 230 watts, where they agreed. Then the gap started to increase, but in the opposite direction. On the last full step, the Neo2 was at 322 while the 4iiii showed 302!

This seem to indicate, that they have different power slopes. Has anyone seen something like that? Perhaps @GPLama?

Is the 4iiii faulty? The Neo2? I don’t think it is normal and certainly not within specs.

One quirk of this, is that if I use the Ramp Test, my ftp would be 225 with the 4iiii and 238 with the Neo2. That’s a fairly substantial difference. If I were to use the 20 min test, they would be much closer, since the two agree right around the point just above my ftp. If your ftp is much higher, these differences will be even larger.

What to do? The 4iiii power output can be adjusted using the app, but that won’t change the slope…

Interesting. I am not sure what is going on but I know that @GPLama’s video on the 4iiiis showed that it was pretty accurate and inline with the other meters he triangulated it against.

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Yes, they have a pretty solid reputation for “just working” and good accuracy, so I’m quite surprised.

I might have to do additional tests to be sure there is something wrong with the 4iiii. My friend has a set of Vector 3’s that I could put on my bike to compare. That might give a clearer picture, especially if they agree with the Neo2 (which I believe he says they do)

Is the 4iiii single or double sided? And were the WattTeams?

Ahh… good question, I forgot to mention that. They are both single sided. I’m aware of the potential issues with single sided powermeter, but I don’t think that is the issue here. The Watteam followed the slope of the Neo all the way, always being 3-7 watts higher, except for the sprint, where it was bit more, but still within the combined margin of error.

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The 4iiii Podiiium Dual on the FSA carbon crank tested really well. This is a different product to a 4iiii Precision on a Shimano crankset. It may use similar strain gauge tech… however the Shimano crankset proves problematic FOR ME in measuring accurate L+R data. My conclusion to date is that the right side design of a Shimano is a horrible, really horrible place to measure power under some conditions.

There’s a ton more to add here… but in short, it’s across the board. It’s a known issue. I’m an outlier. I don’t have any issues with pedals, spider based, or other non Shimano based power meters.

@Zahlmiac - See my video on how I compare power meters/trainers/etc here: https://www.youtube.com/watch?v=JhheDLXRM7I

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Hi Shane.
I have watched your video (and pretty much every other video you have made) and I am aware that I should use at least one other power meter, to get more solid data. I just took the easiest route, and since the Neo2 has a solid reputation of accuracy, I figured it would be a good comparison. I do have a friend who has a set of Vector 3’s, so I will try it with those too. I should probably also look at using the DCR analyser, to get the clearest picture.

Have any of the power meters you have tested been inaccurate, due to a “wrong” slope? I don’t remember seeing that in any of your reviews.

Wrong slope… not recently. A few years back a few SRMs and Quarq units need a nudge in the right direction. What I’m coming across more and more these days is inaccurate readings that’s not linear… so not a slope issue. It’s a ‘how and where they measure’ issue. Which is much worse.

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Are you comparing a left only 4iiii or a dual sided 4iiii?
Assuming it’s left only then you can’t really expect a fair comparison unless you know you’re pedalling at 50/50 L/R

Shane has gone down many rabbit holes recently on this.

As mentioned above, it is a single sided. I am aware of the potential issues with single sided powermeters, but I don’t think that is the issue here. The single sided Watteam followed the slope of the Neo all the way, always staying 3-7 watt above the Neo, except for the sprint, where it was a bit more, but still within the combined margin of error. That would imply, that my pedaling stays fairly consistent and my L/R doesn’t really change.

I have run my 4iiii against my Neo and it differs 3,5%. That’s most likely due to my left leg being weaker having had it in a cast for two months when I was younger. My Stages differs 4% but that can’t even be set to the correct scale which the 4iiii can. I have the scale factor on the 4iiii set to 1,035 so my outdoor riding is comparable with my Neo.

Yes, but that is not the problem here. If it read a little higher (or lower) all the up the power spectrum, I would be happy, since, as you mentioned, that is easily adjusted. The problem is, that below 230 watts, it reads higher than the Neo2 but above, it reads lower, so the adjustment function will not help.

Are you sure that your Neo2 is accurate then? Because I’ve seen more than one having issues with the Neo2 and power data.

No, as I have only done a test with the Neo2 and the 4iiii Precision, I cannot be sure which of them, if any, is correct. When I did the test of the Watteam Powerbeat, it was on a Neo 1, so I can’t even directly compare the two tests. I really have to borrow my friends Vector 3’s and do another test, recording all 3. Then I can also compare left power from the Vectors to my 4iiii. I will report back when I have done that. If it shows the slope of the Precision to be wonky, I will contact 4iiii.

Thanks for all the responses so far!

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I am quite interested to see your results from further testing. I have a 4iiii L and a Neo 1, and am quite unsure if I need to talk to 4iiii customer service because it reads almost 10% high which seems beyond any reasonable L/R imbalance.

I’ll need to collect more data myself…

Edit: I have to do more testing regarding the slope thing, can’t say much on it yet.

It’s hard to say without a third meter in there but I would bet the neo is correct. Just my two watt hunch

I’m seeing something similar to this on my Stages L G3 vs a Kickr Core. The Core was sent back and verified accurate against the distributors Stages and I was provided with the data files to back things up.

In my latest testing I have started gathering data using only one leg to pedal and then divide that by two.
At low watts the data matches pretty well and gives figures you’d expect to see given the +/- range of the two units. As I worked my way through the first 10 minutes of a ramp test things started to drift apart with the Stages reading higher than the Core. It was an unpleasant experience to say the least, but the data shows a similar drift to your findings.

Something is definitely amiss.

I was just about to post pretty much exactly the same post as yours. I have had a left sided 4iiii PM for a year or so. Just bought the Tacx Neo 2. Did a ramp test last week with TR connected to Neo while my 4iiii was connected to my Garmin. I noticed exactly the same thing as you. The 4iiii shows a higher reading at low wattage. Then at about 200 - 230 they become closer then start going the other way. The following are 30 second averages: 240(4iiii)/244(Neo), 252/260, 269/276, 280/290, 292/306, 304/323, 317/338, 330/354. Today I also saw them quite far apart at lower watts. 252 on the Neo was reading 230 on the 4iiii.

With regards to the big difference above 300 watts, I have really struggled with long 120% efforts this year. I usually do the first one then bail on the 2nd and then reduce to 115% and then don’t complete the workout. If the Neo is accurate, not the 4iiii, then I would have been attempting the intervals at 125%+ instead of 120%. When I did 120% efforts this week using the Neo, I completed the first four 2.5 min efforts at 120% before reducing a little, then putting it back up for the final 8th and 9th interval. I completed my first indoor long 120% ftp efforts session…just! :dizzy_face:

I have a 105 shimano 4iiii’s precision and a Cyclops H2. I have found them to be very close. But I only did one test with them. I will try again and report back.

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So I did a test today With the 4iiii’s and the Cycleops H2. Ive got the files in garmin connect. Now what do I do with them??? Sorry. New to power meters. Need some guidance.