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Of course, cheapest yet best, what the hell do I need? Bike is Spark 2018 with Sram GXP PF integrated / shell 41x89.5 with Sram X1 1000 Eagle GXP Boost PF QF 168 crankset.
Seems a bloody minefield... thanks.
If you want lowest price, then Stages or 4iii as a single sided power meter may be good enough. If you are using a turbo trainer, then you may find that this measures differently from the single sided power and hence you may not get the most from it.
Then it escalates in price from there. The most flexible solution would be the SRM X Power pedals, not sure if they are available yet thought.
Do you need power on an MTB? I'm not convinced I'd get much from it TBH - for XC racing the measurement would be so spikey. If you are doing all training on it, either on flat or long climbs then there's some use there but HR is just as useful for those steady efforts
(disclaimer - I don't have a powermeter, but have trained to power on Zwift so understand the utility)
As richard says, easiest/cheapest is probably a left crank meter.
Spider meters (like Quark) are about twice the price I think.
The SRM and IQ2 SPD pedal meters aren't out yet.. but you could do this - and be able to swap between bikes !
I've got on well with a power 2 max on the road. You can by a spider that will fit a SRAM mtb crank.
https://www.power2max.com/en/product/ngeco-mtb-sram-single-power-meter-boost/
Given how good I've found the Assiomas on my road bike, if I was looking to add a power meter to my MTB, I'd be inclined to mod a set with Xpedo pedals as per the video that timb34 posted above.
Looks pretty straightforward; the only downside I could see is that the guts of the powermeter might be a little vulnerable to pedal strikes off rocks etc, but I'd struggle to imagine what useable data you'd get from a power meter if you were riding on more technical terrain where the force through the pedals is changing from moment to moment?
Not at all the cheapest as you will be buying cranks and a BB too, but the Quarq has been faultless on Mrs Yak's bike. All her training is on the mtb in all weathers.
I recently posted a 'what PM thread' on here...
Well, specifically was asking about Avio Powersense LH crank one.. £150 fitted
I got one in the end; posted my Ultegra crank off on Wednesday, and got it back Saturday am with a PM glued on..
It works... yeah it's the first PM i've had, but the data makes sense, and the unit links with Garmin/zwift easily enough.
I'm kinda tempted to send off the XT crank on my single-speed, just for data "shits and giggles"!
Anyway..I guess the Avio is a good shout. The 4iii is similar too, but think you have to BUY their crank/unit as one.
DrP
I've had my fair share of Stages power meters on all of my bikes. All have failed in one way or another. Reliability and consistency is rubbish. I wouldn't buy another with my own money
I've got a LH stages on XTR. Frankly it's shocking. Regularly goes out of calibration and I return to a new best 20 minute power of 954 watts or something.
Assiomas on road bikes are faultless. Mod the time pedals.
Edit: Also get rid of the garbage GXP BB / crank whilst you're at it. Awful design.
There are some spider power meter options that might be worth investigating....
http://www.sigeyi.cn/en/col.jsp?id=113
or
https://xcadey.com/product/xpower-s-sram-3-bolt-104bcd/
I use a Powerpod on my mountain bike. It's a lot cheaper than other power meters and can be easily transferred to other bikes. It's a bit fiddly to set up but the support is really excellent and it's surprisingly accurate when you compare it to other strain gauge based technologies.
It is a minefield - there is less choice than on the road, and they cost more. I gave up.
Crank - stages do one that would fit. It'll be 2x the price of a road stages. I don't think 4iiii do one that'll suit you. Ditto avio (although if they do, it would be a bargain because their pricing structure is so flat).
Spider - power2max like @ac282 said. expensive though, especially with the "pay for accuracy" add on.
Pedals - they've been imminent for ages, but still vapourware for now. the iqsquared ones might be great one day. the SRM ones (launched, but hard to get?) are very expensive. at least they'd move easily between bikes. that assioma hack is quite popular on the internet.
Road powermeters really are a normal consumer product now. MTB isn't quite there. And, like @richardk said, the utility is less obvious than on the road too. If you do get one, can you post back? The internet is awash with info on the road, but there's much less about MTB about...
I bought a Quarq Sram XX1 Eagle Dub SL Boost to replace the almost identical non power meter crank set on my Oiz.
One thing that I had not taken into consideration was that the powermeter element has a deeper profile than the original crank set so when I installed it the outer edge of the power bit of the chain set was touching the swing arm in the area of the BB.
Fortunately I resolved this by placing a slightly thicker shim, by about 1.5mm, on the drive side which pushed the chain set away from the frame and there was enough slack in the bearing adjuster to accommodate the extra width.
Great bit of kit though, I'm very pleased with it.
I have 3 stages and a pair of vector 2's. one set of stages is on an xtr crank on my scale - I have never had any issues. vectors eat batteries at an alarming rate. All read within sensible amounts to each other and my kickr.
I would not put power pedals on a spark if you ride rocky terrain. I was at afan a couple of weeks ago on my spark and the pedals took several really big strikes. on a set of xtrs I was getting to the point I was starting to back off in case they were mashed. I'd be really concerned if it was something like £800 I was smashing into rocks
I also see to be an outsider that all my stages have been fine, for years, I think two of them are mk1's and a mk2 (with revised door I think)
I miss riding with a power meter on the spark. For climbs like whites in afan which are about a 30min ish effort you can definitely pace yourself to power if you watch your average watts - yes you spike all over the place as its technical and not like a road climb but it is definitely useful, more than HR to know if you are going to blow up before the top, or if you should have extra in the tank.