Yes your right. I was thinking about the common pairings, you could in theory put a 48/38 on a 130 BCD crank as apparently there is a 130bcd 48 tooth chainring out there. You’re missing out on the ability to go down to a 34.
Looks like i have a 5 bolt chainring on the bike at the mo. Quick search of the current spec gives me:
| Crank | FSA Gossamer, 36/52 |
|---|---|
| Cassette | Shimano 105 12x25, 11-speed |
There’s a 52/36 available now on PBK with 172.5mm cranks. Just had an email with an extra £20 off from PBK for orders over £150 as well … which is annoyingly exempt for this product 
Still looking at my options, this is as good as it gets i think. my bike shop man can get me a discount on an Avio PM, but then i have to switch the chainset to Shim, so that’s a new 105, plus something to do with the BB conversion. I’d be looking in excess of £300 for that option, for a single sided PM with 2% accuracy loss (when the 4iiii is only 1%). So this FSA/P2M seems to be the winner.
I think i still have £50 on account with the bike shop, so hoping he can fit it for me for that as well. I’m not mucking about with that stuff!
It’s just a lot of money, which is holding me back!
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Would the PM being sold here work on your crank? Can’t remember what crank length you’ve got.
Stages FSA SPM1 172.5 Left Side Crank Arm Power Meter £190.00
There’s also someone selling the Ultegra double sided Stages for £350 on FB, you’d then need to adapt the BB but shouldn’t be too expensive.
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Haven’t really got into it as not in the market but will park this here for anyone who is:
£100 seem reasonable for a Gen1 stages PM with left side FSA crank? I’m the seller, not really sure what to ask for given its age.
Absent anywhere else to put it, reports are that Stages have gone out of business.
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Yeh I saw that. Binned off all of their staff. (Less) power to them, I suppose.
Poet has just mentioned on the purchases thread
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I am about to pull the trigger on assioma uno, not sure getting duo is worth the extra cost. I appreciate that duo is more accurate but uno should be close enough. Any reason a slow old fella should buy the duo?
Last time I read anything on this it was that usually is some discrepancy in power but the harder you work the less that discrepancy.
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My SRM’s were single sided but appear to have been accurate in comparison to newer dual sided PM’s
Only thing for me is that I have a massive discrepancy in my left leg, usually regardless of effort and the dual sided reminds me of it!
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Harder work brings more discrepancy for me.
I’m usually 49.x:50.x but that can gradually drop to 46:54 when I’m working hard (and tiring).
If you’re happy accepting that it’s within 3-5% ballpark, and realise actually the only comparison you ever want to do is with your own numbers then need to decide if you think your imbalance will ever potentially change in future (injury, strength training, maybe a change in position between bikes might do it?)
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If you have a large discrepancy, and the single side were showing what the double side are now, that just means that one (presumably the older single side) were out by a similar %
EG if old showed 200W based on left leg pedalling 100W, that should be 195 or 205W on the newer ones.
Reality is I still don’t think any PM is that close still. My SRAM spider, Assioma Duos and Kickr Core are all quite different when I get above 350W (but track similar to 250W)
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Do you have access to a Wattbike at a local gym by any chance? They measure and report on left/right balance and a couple of rides at various intensities and when fatigued might ease your mind on making the right decision.
Ultimately though, you’re measuring a number and then looking to increase that number or ride at a related number for a period of time, so as long as your goal isn’t to compare FTPs down the pub I think single sided will be fine.
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Sorry, it was poorly worded.
In general the SRM’s averages\max etc. were relatively consistent with what I see on dual-sided these days, e.g. Assioma’s, Wattbike, Giant Power, I don’t recall the SRM’s trying to estimate the L\R balance but the ride average seemed about right.
Across the 2 PM’s & Wattbike and at least one V expensive lab bike I’ve regularly had around 43/57 L/R imbalances. Although this seems a bit closer these days, on tonight’s easy ride it was around 47/53.
Maybe the single leg work I did because of the hamstring problem (also left leg) might have reduced the imbalance.
When running my chest strap seems to have almost 50/50 assuming it is accurate so seems more cycling specific.
Defo agree re relative improvement being the #1. Also, you can ‘upgrade’ the single to duo at a later stage if you felt like there was a discrepancy.
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What else do you use a PM for?
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One other suggested issue is that your body/brain might actually start to “game” the numbers and introduce an imbalance over time.
Almost certainly not an issue if you’re predominantly riding outdoors (you’d never be looking at the numbers frequently enough for that to occur) and probably the same with free riding/racing in zwift, but if you’re doing lots of erg workouts in zwift/TR/etc then it’s actually quite possible that staring at a number will ultimately lead your brain to subconsciously find a way of hitting that number more easily over time. And that can mean a slightly higher output on the left and lower on the right for the same physiological overall “cost”.
I’m not sure it has ever been proven in practice and might just be rumour, but the theoretical risk makes sense to me. I know there was a period in time when I used a watt bike very regularly, and without ever having a particular focus my peanut force diagram smoothed out over time, probably because it was data I had right in front of me that my brain read in and adapted in response to
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I think this is a valid concern. After previously using dual sided SRM/Quarq and having close enough to 50/50 splits, I have done all my hard sessions on single sided stages over past 5 years and i think on hard sessions I definitely favour the left (reporting) leg. I have now recently been back on Assioma duo/quarq and found I am down to around 60/40 in favour of the left leg…
n=1 and all that
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Yes, not even subconscious in my case.
I originally bought Unos after a year or two of riding on gym wattbike, and couldn’t manage the ego hit of just 5-10W and would deliberately push harder on left leg 
Luckily I bought in good sale before COVID, sold them second hand for what I’d paid.
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I ordered my assiomas from cyclepowermeters.com, had an email last Tuesday to say they had been dispatched but still received nothing. I have tried calling them but there’s no answer. 
ETA: spoke to them this morning. Parcel Force, the wankers, apparently tried to deliver it on a few days, including when I was WFH. They left it at my local post office on Friday but didn’t tell me. I can now measure how shit I am.
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