No, but it’s not accurate either. There’s only so many ways we can tell you it’s not accurate. Would wearing a white lab coat help? Charging you a fee? Wearing glasses?
We can’t tell you how inaccurate your trainer is because if they could figure that out then it would be accurate wouldn’t it? You can only measure it against something else.
Easy to get the wrong answer. More difficult to get a consistent accurate reading within a few percentage points.
Here’s the thing Michael you seem to want someone to convince you otherwise. You know like those times when someone asks me if they should buy a car or a motorbike and I know that they’ve already made up their mind and they just want me to tell them to do what they’ve already decided.
If that’s the case, then sure, let me reassure that it’s really easy to measure power and the Reebok FR30 is, as you suggest, as accurate and repeatable as an experiment done by a class of high school students. Hope that helps.
It is a magnetic resistant unit so it just measuring the mag field or the electricity required to generate the resistance. Mag fields will fluctuate a bit (ever see the Northern Lights…dancing all over the place). Reasonable to assume Reebok is not using a super high-quality magnet that is stable. Some fluctuation. I used to have a mag trainer (way before Zwift…decades) and was quite thankful fluid trainers came as the magnetic ones had varying resistance in the pedal stroke. Very noticeable.
Accuracy at 100-200 watts is probably semi-reasonable…something like +/- 10%. But the peaks up to 2000 may creep in depending facators: environmental, electrical, etc…
If you race, you will have to seek races that allow non-acceptable power meters.
One thing you can do is ride the same course once a week and track Power, time, hr and see if you can track stability.
So it’s not setting the resistance accurately, consistently due to how the magnet is controlled and it’s in the region of 10% error at steady speeds. That’s useful to know. Thanks everyone.
Sorry to be pedantic but I was trying to understand specifically why it was inaccurate and by how much because maybe I have to buy an expensive bike. Let me write to Reebok, they probably won’t tell me anything though.
Because of the way magnets degrade I can see that after a while it’s not going to be great but I was hoping that having had it less than a year and not used it much that it might be accurate to within a percent or two at the moment for 99% of what I’m doing. Or is the magnet just not great from the start out of the box if it’s cheap? Just that riding along at 200W when it’s really 180 or 220 isnt acceptable.
In my experience, I would not find magnets at this level to be overly accurate. I do not think it has degraded.
Note I am assuming an actual magnet. If it is an electrically induced magnet … well my undergrad physics was over 4 decades ago…I just do not remember.
It’s interesting. What I thought aswell is that it could be that variations in the effort of the cyclist affect the resistance that the magnet provides or the trainer’s ability to maintain that resistance? It could throw it off if there are big enough surges or quick changes in what the cyclist does.
What I said in my last post I don’t think is right now because I realised that because it can generate and hold a very high resistance easily then pedalling won’t diminish it causing negative feedback in some way.
Also this 10% variability in the magnetic field at steady state doesn’t seem right to me for this bike as when I consider the rides on it then even ±2.5% I think I would notice it fluctuating at say 200W (195-205 ). It doesn’t feel like the magnetic field/resistance fluctuates in a way that I notice even. Certainly you would notice more than a few percent.
So then you consider temperature of the parts and how much that affects riding. On a regular bike once the bike heats up ( rather than legs warming up ) is it more than 1% easier to pedal? Is it the same type of parts on these bikes?
Power accuracy is basically the top feature of a power meter, or smart bike, so any company that sells an accurate one will have the variability listed in the specs for the product quite prominently. If the manufacturer does not list the power variability in the specs anywhere you really have no idea on its own how accurate it is or is not. But you’d need to assume it’s just not in that case.
They might not have even individually calibrated these at all to save costs, or used some very cheap sensors etc. so while it ‘could’ be accurate. I would say if you really care about the numbers and you have a device that showed you doing 2000W for 5s on the top end (world tour sprinter level numbers), I would just assume the bottom end is also not reliable… by what percentage? Nobody knows without comparing it to a known power meter that is calibrated.
If it’s at least consistent then the numbers don’t really matter that much because if a workout is too hard you can reduce the power target and then you’ll have a good setpoint. You just can’t really compare your numbers to other people with this product and expect to see the same thing outdoors or on another trainer.
Let me get a power meter and measure it. I was trying to reason it out and so I could get to a point where I could be confident that its only a few percent rather than 10 or so and then persuade myself to spend £3000 on a new bike. Thanks.
Not sure where you are, but you could try hunting down rental power meter pedals (you’d need cycling shoes though that can mount the cleats on though). Curiously, for whatever reason, in the US there are tons of bike rental options, but don’t see anything much in terms of power meter pedal rentals. Or buy them and return them if you find that they’re miraculously spot on to the power readings your spin bike shows.
Yeah because then the pedals cost more than the bike lol.
Interestingly tonight I rode the Pretzel for 90 minutes with my Coros watch which has all the fitness data on the app and it came out at 927 calories vs 944 that Zwift reports from the bike.
Yeah, a good power meter is not really cheap, but they will likely be more accurate (or at least publish their accuracy tolerances so you know what you’re getting). Calculating power from HR is gonna vary pretty substantially I would think. So watches won’t be that good of a comparison unfortunately.
I think if you mainly care about getting a good workout you can use an inexpensive bike and mainly be concerned if it stays consistent between workouts, but if you want to be able to compare your numbers it will be hard to do without a device that has a published accuracy. Those tend to cost more than that bike for sure - but they also have a specific standard they are targeting. Maybe look into a used one?
I’ll try and borrow the pedals. Just out of interest do people on Zwift care a lot about accuracy if they are into the virtual racing aspect where I would imagine it needs to be within a fraction of a percent otherwise the results might be a farce? Even then, only allowing meters/bikes with 1/100th of a percent accuracy, if thats possible, at best still might mean it’s not entirely clear who won a race? I’m thinking my idea of accuracy might not be the same as other peoples. I’m ok with ±3% say. If it’s under that I won’t buy a new bike.
Honestly I don’t think ±3% is bad for a Zwift race - that’s not too far off the stated accuracy of the Wahoo KickrCore at ±2%. I think the issue becomes when folks are putting out 2000W at the line to win gold when really they’re putting out 1000W or less, or sitting in Zone 2 at 400W for the entire race.
Others might have different opinions.
There are hardware restricted races, which remove zPower riders, and I think some power based trainers as well, but I don’t actually know the list of what is blocked there.
Edit: I mean to clarify - if you win a race with a 5s power of 2000W there will likely be someone who notices that.
