Can anyone check my logic/math on this and make sure I’ve thought it through correctly? Also has anyone done similar and were there any unexpected outcomes?
I have 3x 4iiii LH crank arms on different bikes. I’ve always known I had a slight left-heavy power imbalance, but this year I started to suspect I was actually training myself to exaggerate it by subconciously relying even more on my left leg to gut out hard intervals in ERG mode. So I bought Assioma Duos for my road bike which is also the one I do most of my indoor training on. I’m still going to use the 2x LH GRX cranks on my gravel bikes though, so I’ll need to do some work with power scaling.
Since I got the Assiomas I’ve been continuing to use the numbers from my 4iiii Ultegra crank on my head unit and for training, but I’ve been dual-recording the Assiomas on a separate device for comparison.
In the DCR analyzer I can see that the LH crank tracks pretty well with the LH pedal, so I think my adjustments are only going to have to be about L/R balance.
I’ve never directly compared the 3 crank arms to each other but have always felt that the numbers were useably similar from a pacing and training point of view. So I’m happy to treat them as equal for now.
On average across all the data I’ve captured on the Assiomas I am 53/47 LR, but it’s definitely a bigger difference during recovery/easy pedaling than it is during a hard interval. Obviously I can’t include this in the scale factor so my instinct is to just scale my 4iiii units based on a 53/47 average and know that perhaps on gravel I’ll get a low reading during a big effort and a high reading during recovery.
If my imbalance is in fact trained rather than natural then hopefully over time I’ll see those numbers even out and perhaps in a few months/years I can start scaling the 4iii cranks back up closer to 100%. If not then at least I know.
Using the calculations laid out HERE, it looks like my case is as follows:
53-47=6
1/1.06=0.9434
So I need to set the 4iii cranks at a scale factor of 0.943
And if my last AI FTP was 273w on the LH crank then I need to multiply by the same scale factor and change it to 257.547 or 258w
Then I can start using the Assiomas as my primary/only source of power on the road bike and trust that the gravel bikes will be functionally similar if a bit less precise.
I’m just about to end my season and build my training plans for next year, so this feels like the right time to do this - will keep power PRs consistent by season etc. I think I might keep the power crank on my road bike for a bit and continue dual recording (but with the pedals on my head unit and the crank as secondary) so I can validate what I’ve done, but within a few rides I should be able to sell that crank secondhand. At some point I’ll also want to fit the Assiomas to my GRX bikes and double check those comparisons too, but I think it’s just too much for me to try to do all of that testing and comparing at once - one thing at a time!
Anything I’ve missed or misunderstood? Anyone else done similar and how did you find it (other than the ego hit of ‘reducing’ your FTP)?
You have the exact same numbers as me (circa 2021), a 53/47 split. You also have the exact same equipment (both Assioma pedals and a 4iiii crank)!
Math looks good and you should expect similar results from either power now.
The ego hit to the FTP is real. No effect on real life, but it still hurts. Btw, I got my split down to 52/48 after a year of single leg based exercises and that’s “good enough”.
My numbers gradually got worse and worse, never really realised how bad it was until I saw 39:61 then it hit me my right leg is almost twice as powerful as my left. I made a lot of changes, changed fit with help of velofit (raised saddle 2cm increased reach), got new more comfortable shoes and insoles and made sure my trainer was level (it wasn’t) and now I am gradually moving in the right direction, 45:55 today. Anyway moral of the story look into why, don’t expect the imbalance to even out by itself
I think the problem you may (will?) encounter is that L/R power imbalances are not consistent across time and / or effort. Even if you L/R splits tend to be close from one ride to another doesn’t mean that they are consistent within that ride. I routinely see different splits between harder efforts, higher cadence rates, etc. I also tend to see different imbalances overall between gravel and road rides.
Which is all to say it is a bit of a crapshoot…take your best guess and don’t sweat the details. At the end of the day, you’ll know if you are getting faster, if you are tired, etc.
there is a really easy way to check you math here:
if for instance you have a balance 53/47 L/R a dual sided simply sums the reading to get a total, this say at an output of 100 watts left is reading 53 watts. Thus doubling 53 (as the left only does) would read 106. Therefore is you apply your factor 94.3% (0.943) you get close at 99.9 watts. The real problem is that this is only a partial solution. I have 4iiii PMs and have bit the bullet and upgraded to dual sided. Then it is not quite that big a guessing game. As for taking a global average I would suggest looking at a period at an effort more in line with what is important to you, ie say longer durations at a certain wattage, then I might fine tune things for that case because I doubt your factor applies across all situation and thus I would develop the factor for the specific use case. So say for instance you did a period (x minutes) at xxx watts that is the left right ratio I would use for cases for training. The L/R balance will change over power and time with fatigue.
I suspect it has a lot to do with a right side inguinal hernia developed during first lockdown, being frightened of making it worse during the 18-month pandemic wait for non-essential surgery, and then the surgery itself 13 months ago
(There’s a really slight fit thing I’m keeping an eye on with my fitter too. And appreciate the heads up on other biomechanics factors as well!)
Yeah I’m very aware that L and LR power meters are just fundamentally not the same kind of training tool. If it weren’t for the fear that I was making an imbalance worse by having LH only I probably would have stuck with that setup on all bikes. As it is, I think the real goal here is to get great numbers for my interval training (Assioma) and then have a close-enough reference point on the other bikes that I can calibrate my RPE in a race effort, plan fuelling effectively, and do some high-level analysis after the fact.
Yes I’ve been trying to think through something like this so thanks for highlighting. I can absolutely see how to apply this on a time trial bike for example, but on a gravel bike used for 4-13 hour events I’m not sure what exactly I’d tailor for. I guess in those situations I’m using the PM for:
Getting a sense of just how much I’m going to pay for this hard effort later (start effort, chasing an attack, etc.)
Knowing when to let a group go and stay in tempo or sweet spot on a longer climb.
Keeping power up in a respectable part of z2 during the long slog solo bits (or an endurance training ride).
Pacing pulls when working with a group.
Those are all so different and none of them are that precise, so I’m sort of inclined to stick with @Power13’s advice and not sweat details. Even if I was going to do a gravel TT or whatever would I even be any better off adjusting my scale factor than I would be by just doing a bit of event-specific training on that bike in the leadup so I know what that bike’s numbers are like regardless of whether they’re accurate or not?
Yeah, I think those are reasonable, attainable goals…you should get enough infor from the LH crank, combined with RPE, HR, etc to effectively pace / fuel yourself.
I would also add that I don’t think worrying about L/R imbalances is worth the effort, certainly not to the point of trying to “correct” it. Barring needing to rehab from an injury, I have yet to see any definitive need to correct a natural power imbalance, not have I ever seen anything indicating a ~50/50 balance is “ideal”.
