4iii Left crank power meter - Gotchas when having power data first time outdoors

With some keen prices on 4iii left crank power meters in the UK at the moment: I’ve decided to buy one. This will be my first power meter on any of my bikes. Currently my training is a mix of a smart trainer and outdoors. Outdoors I’ve just gone with RPE and heart rate.

I’d just like the hear some top tips and gotchas to avoid when moving from my current training setup / methods to having power data available both indoors and out.

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PowerMatch indoors if you can ride the same bike. Thats about it.

Yes it’ll be same bike, how do you powermatch?

In TR its the default setting if you pair trainer and power meter.

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Powermatch as above.

Shouldn’t be far enough off to require a new FTP but you might need a couple weeks of adaptive training before things feel ‘right’ again in terms of workout intensity.

If you’re anything like me you’ll start to want power on other bikes too. I now have 3 LH 4iiii cranks and feel totally locked in to the LH system - I’ll either need to switch all my bikes at once or risk having inconsistent data across them. (This is more problematic than switching from spider to pedals or whatever, because in those situations the discrepancy should be a consistent percentage, where L/R power balance tends to change depending on intensity, fatigue, etc so you can’t just scale things up or down by a fixed amount.)
So just keep that in mind before you buy your second PM!

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I would also be wary you’re not overusing your left leg more than right because that’s where the power meter is. I’ve been accidentally doing this over the past 2 years and it’s hard to get 50/50 balance now, even though my right side is usually stronger. Enjoy the power meter! Agree with the comment above, you’ll likely want power on all your bikes

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Calibrate before every ride. Made that mistake once.

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Only thing I can think of is check clearances, but unlikely to be a problem. Think they might have info on their site iirc.

Can I jump in and ask a question as I think it’s related and I’m in a similar position albeit with assioma power meter.

Should there be such an obvious power difference between indoors and out? (In my situation I ride different bikes inside and out, wattbike atom inside, power pedals outside)

Power meters are all over the place.

Yes, someone will reply in the very next post saying something like “I have 8 million PMs and they all read within a watt.”

Please ignore that.

PMs really do vary a lot from manufacture to manufacture, then from unit to unit. Ask TR, ask any coach with many athletes and years of experience. They are not the ultra perfect devices many believe them to be.

If possible, use the same PM for indoors and outdoors. That way, you will at least have a chance at stable data. Remember, you will generate more power outside. That is almost universal, at this stage. It is for numerous reasons, discussed elsewhere.

My personal recommendation is power pedals. Favero’s, being the current recommendation from most coaches etc. If you have to use more than one set of pedals, you can attempt to adjust each pedals settings in the app, to line them up.

Good luck.

I have owned two 4iiii one-sided power meters, although none of my bikes have one at the moment. (I have a road bike with a Quarq DZero.)

First of all, independently of what I’m going to write below, this was a very good purchase. A power meter is going to make you faster, and it gives more bang-for-the-eh-pound than upgrading from 105 to Ultegra or some such.

Still, here are some experiences from myself.

  1. When I first got my 4iiii on my road bike, it took a 2–3 weeks for it to settle in. Initially, the numbers were very, hmmm, flattering. Apparently, I did multi-hour rides with an average power >400 W and had sprint power that would make me competitive with the best of the world. Funnily enough, the speeds didn’t quite match with the power output :grin: After that settling in period, the power numbers were good, though.
  2. I developed a severe one-sidedness with my left leg: I reckon my body found out that rather than raising my power by 2 W and distribute that equally across both legs, it could just lift power in my left leg. I think I am somewhat of an outlier, but it is something you should keep in mind.
  3. I initially bought a lightly used 4iiii power meter from ProBike Kit. Unfortunately, it failed after about 2 weeks. Got a refund, so no harm, no foul.
  4. When I got my new road bike with a Quarq DZero, I checked the power numbers and they were all very close (if you account for the left-right imbalance.
  5. Apart from being left-sided, the only limitation that I came across was the relatively low cadence limit of 160 rpm. In cadence drills I exceed that number by quite a bit. But since this is an all-out effort, it is fine. It is just that your power drops to 0.

Yes most of my power PRs are on one of my three 4iiii cranks (only one of the three where I sent my existing crank to them for factory install, and the pod is slightly nearer to the pedal than where they put them on the off-the-shelf units). It’s enough of a difference that I’ve noticed it, and if I’ve had that bike on the trainer for a while and then switch to one of the others I’m aware that my next few workouts are going to feel a lot harder, but in terms of outside riding or anything else without ERG mode it’s close enough that I don’t have to make any adjustments or anything. I did once try to scale them all to match my Kickr but that was more trouble than it was worth - powermatch is a far better solution!

My partner said my left leg looks bigger than my right, and the fact my Kickr reads lower than my 4iiii would indicate a bit of imbalance too, so sometimes I make a conscious effort to work my right leg harder than my left, but otherwise I’m not overly concerned about this. Yet.

Good point in terms of ‘gotchas’ as OP asked: you also can’t do right leg only drills on the trainer if using Powermatch. Got to either switch to your trainer as power source, or at the very least disable ‘auto pause’ in the TR app to ensure that when power drops to zero the workout continues.

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I’ve got a 4iiis PM on my road bike on the whole I’ve found it good and robust but as with all power meters/data I would advise against becoming too focussed on it when the conditions don’t allow and accept that sometimes you won’t be able to get in the zone (a steep downhill etc). I think TR’s advice was to accept it to and when you can get back in zone hit it, don’t exceed it to get your interval average into zone.

Good advice, plenty of lanes I ride in where trying to hit a number on the gps isn’t going to end well. I’ve got a good selection of lanes where I know how long I can push it for without interruption. Dow hills usually isn’t one of them other than a few of the B roads.

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I’ve been quite disciplined with heart rate when riding my Z2 rides, letting ego go when passed on uphills etc. hopefully I will now be able to enjoy more outdoor rides being disciplined with power in the same way.

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I have 2 4iiii left only power meters. Originally had 105 on my road bike, but got an Ultegra to match the groupset. I now use the 105 on my gravel bike (with GRX everything else).

My n=1 is that I’ve dual recorded and haven’t seen significant enough difference to my Original Hammer to ditch my permanently set up turbo bike in favour of swapping on and off. They were both within the two margins of error even taking into account total power v left only, and different measuring points. Consistent enough power numbers for me anyway, and having a permanently set up bike helps my consistency more than getting down in the weeds of absolute accuracy of power.


I would also advise on calibrating before every ride, especially when the temperature varies. I did a colder ride this weekend and it was definitely showing 20-30W more than I was putting in. Calibrated via my Garmin and worked fine; make sure to also wait for “Torque” to appear on the Garmin calibration screen before riding.

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What’s great about power match is you really don’t need to worry about trainer spin downs anymore. TR will read the power from your meter and adjust the trainer so that you’re putting in the proper power based on your actual PM

I’m not regretting getting a 4iiii at that time, it was by far the most affordable and sensible option. But if given the choice, I’d strongly argue for something dual-sided.

I was able to fix my left/right balance for the most part by simply adding L/R balance to my indoor workout screen on my Wahoo. I’d just glance at it and focus more on my right (weak) leg. Across a season that took care of it for the most part. Still, it is something to keep in mind.

Still, overall having a one-sided power meter is way better than having none at all: you can pace your efforts much better and track TSS when outdoors, for example.

Yup. I was fortunately on a dumb fluid direct drive trainer at the time, so this wasn’t an issue. But I think you have to either switch to resistance mode or rely on your trainer’s power meter.

Due to my setup at the time, I could use cadence and gearing as an indication, but it certainly wasn’t as good as measuring power. Also keep in mind that any drills done with your left leg will show double the power, so if your target is 120 W, you must aim for 240 W displayed.

I second that. AT least my 4iiii’s (which were previous-gen units) seemed to deviate more than my Quarq. And despite Auto Zero I do religiously zero all my power meters before every ride.