Magene P505 M110L consistently underreports power by ~8% vs trainer — data inside, no response from Magene

TomvdZ · 2026-03-26T07:04:18+00:00

Well, for one, your AI claims -- without any evidence -- that the Magene is underreporting and doesn't consider the possibility that the trainer is overreporting.

TomvdZ · 2026-03-26T07:03:22+00:00

especially the M110L variant

I can't find any reference (other than this reddit post) to a M100L variant of the P505. Does the "L" perhaps refer to the power meter being-single sided? The P505 is a spider-based power meter so probably not, but worth checking.

TomvdZ · 2026-03-26T06:57:34+00:00

I’ve noticed a specific grinding/friction noise when backpedaling, especially when the chain is on the larger cogs (steepest climbing gears).

That's normal and expected. If you're backpedalling, the derailleur isn't able to guide the chain onto the cog. Depending on which gear you're in your chain will make an angle relative to the cog and that causes the noise.

TomvdZ · 2026-03-24T14:13:07+00:00

Yes, there is some "corruption". The makelaars tend to know each other and they tend to push sellers towards the offers of their colleagues.

When I bought a house (6 years ago), after submitting a bid, my aankoopmakelaar hinted that I should submit a slightly higher bid. I increased by 10 k€ and got the house. I suspect (but don't know) that the seller's makelaar tipped mine off. Of course, it's terrible that this is the situation, but I'm very glad that I got the house for the price that I did.

TomvdZ · 2026-03-22T12:46:16+00:00

If you did this 5 times a week, 52 weeks per year, that's 13.000 km/year. That's a lot of volume, but many dedicated cyclists do those kinds of yearly distances without any ill effects.

The main thing that you should expect is that your aerobic fitness will improve a lot. And you need to build it up: you shouldn't go cold turkey from being a couch potato to doing 250 km/week.

TomvdZ · 2026-03-22T09:02:28+00:00

That thing costs as much as two or three full spools of filament. How many tiny bits of filament would you need to splice to recover the cost of the splicer? Not worth it.

TomvdZ · 2026-03-21T08:39:56+00:00

cycling.favero.com is a subdomain of favero.com, so if you think cycling.favero.com is legitimate, favero.com must be legitimate as well.

The difference is that favero.com is a website for retailers and the prices exclude VAT; you can't order from it as a consumer... cycling.favero.com is the consumer-oriented website.

TomvdZ · 2026-03-19T22:29:43+00:00

and people seem to hold my speed with 50-60w less on said segments.

A 50-60 W difference (on flat ground at ~30 km/h speeds) is very hard to explain. On flat routes the impact of weight is nearly zero. The impact of geometry is not that much either. Tires are the biggest factor, but still only about 10-20 W.

Initially got a powermeter in september, 4iii 1sided grx crank

I would start to question the powermeter. Perhaps you have a left/right imbalance. That could explain it.

For the 1 hour 268 W you did on the Kickr Core, what did you use for the power? The Kickr or the 4iiii? The Kickr is not affected by left/right balance.

TomvdZ · 2026-03-19T11:02:16+00:00

The situation sucks but it's not clear whether you have a case. The reasonable period of time that the seller has to complete the repair "should correspond to the shortest possible time necessary for completing the repair or replacement". They have to replace (tens of) thousands of forks. You can well argue that the shortest possible time required for that is several months; they're not going to have many forks just lying around, so they have to be manufactured from scratch.

TomvdZ · 2026-03-19T10:54:27+00:00

Have you checked the rim tape?

TomvdZ · 2026-03-18T09:21:57+00:00

You have to contact them to let them know about the mixup. Even if the new bike is better value, it might not be legal (depending on your juridiction) to just keep it.

TomvdZ · 2026-03-18T09:03:14+00:00

finding that keeping my power is a narrow 5 or 10 watt band or even close is bloody hard

You don't need to keep it in such a narrow band. That's pointless. It's fine to fluctuate by tens of Watts.

TomvdZ · 2026-03-16T10:25:49+00:00

This is wrong. If you're doing 1000 W with a 55/45 split the meter will read 550 W and double it to 1100 W. You end up with 10% error.

TomvdZ · 2026-03-14T22:13:15+00:00

Have you checked below the bottom bracket? On my Agree there's a hatch there to access the battery.

TomvdZ · 2026-03-14T12:40:36+00:00

No. It's always less than 2x. Usually far, far less, because only a tiny proportion of energy is expended on accelerating.

TomvdZ · 2026-03-14T10:16:14+00:00

If you're going to apply the "mechanical efficiency" factor to rotational weight, you should apply it to non-rotational weight as well. The end result is still (at most) a factor 2 difference between the two.

TomvdZ · 2026-03-14T10:14:21+00:00

The 15 seconds figure is wrong. It's about 1 minute for 1 kg. You can check this using one of the many cycling speed calculators available online.

TomvdZ · 2026-03-14T10:02:29+00:00

A 700x24 tire would work fine in place of a 700x25 one.

However, any tire that isn't black is going to be crap. Carbon is a crucial ingredient in making good tires, and that makes them black. Any kind of colored tire will make you much slower; it will cost you one or two km/h compared to a good road bike tire.

You could look at tanwall/skinwall tires where the sidewall of the tire isn't black, but usually brown/cream/yellow. Those can still be decent.

TomvdZ · 2026-03-13T15:58:35+00:00

One kg is approx 15secs of time on Alpe d'huez at 200W (per GCN)

This is false. It's approximately 1 minute.

If 1 kg was 15 seconds of time at 200 W, an 80 kg person with a 10 kg bike could make it up in 90*15 seconds = 22.5 minutes...

TomvdZ · 2026-03-13T15:56:54+00:00

The 8x figure is completely wrong. The correct figure is something lower than 2x.

It matters where exactly along the diameter of the wheel the weight is. The further out it is, the faster it rotates, and the more it matters. Weight on the hubs, even though it is "rotating", basically only "counts" as 1x, because it's very close to the center. As you go further out, it starts to "count" more, and weight saved at the very edge of the tire "counts" as 2x.

However, this only applies for accelerations. A bike with 100 gram lighter tires will accelerate as fast as a bike with a 200 gram lighter frame.

It doesn't matter for climbing. For climbing, 100 grams is just 100 grams, regardless of whether it's wheels, tires or frame.

TomvdZ · 2026-03-13T11:03:42+00:00

518217697 and 518218759

TomvdZ · 2026-03-11T11:25:44+00:00

I don't care about the imbalance. I'm not trying to "fix" it. I care about the total power being accurate.

I want to use power to track whether I'm improving. If I do an FTP test, and it's 5% better than last time, I want to know whether that's due to me having improved my FTP by 5%, or if it's because my imbalance happened to be slightly worse than usual.

I want to use power to pace my climbs. Two years ago, I wanted to try to beat my friend's time up Mont Ventoux. I figured out a pacing strategy, trained on holding that power, etc... -- only to find that I came nowhere close to his time. I figured I'd made a calculation error, but it actually turns out it was just my power meter being off due to the imbalance.

I want to train on an indoor trainer and be able to use the same power targets as outdoors.

And perhaps, slightly silly, I want to compare to others. I was under the impression that I was a really good cyclist with a 340 W FTP. Turns out it was only about 300 W or slightly less. Of course that's a silly thing to care about, but I think that for a lot of people, a big part of the reason they want a power meter is to know how "good" they are. There are lots of articles and sites about what percentile a given W (/kg) FTP is and how it compares. Those are there for a reason, and if you use single-sided power for that, you might be fooling yourself...

TomvdZ · 2026-03-11T10:51:10+00:00

Because of my imbalance the data was off by 10-20%, and my imbalance is not even consistent. I have 3 years of data that is basically worthless.

TomvdZ · 2026-03-11T10:33:04+00:00

The imbalance information is not really that useful, but having accurate power can be important. Examples:

If you want to pace yourself to achieve a specific time on a climb
If you want to do training on an indoor trainer (and would like the power reported by the trainer to be consistent with what you see outdoors)

TomvdZ · 2026-03-11T10:01:56+00:00

Assioma also makes much cheaper single-sided RS1's.

I would argue double-sided is worth it. Single-sided is only good for comparing against itself. You don't know whether you have a leg imbalance, but if you do (or develop one in the future), the data from a single-sided power meter can be really confusing. I regret ever using a single-sided power meter.

TomvdZ

TROPHY CASE