A roll-down test in the rain on the same stretch of road - what’s the problem? It has been done in the dry with similar constraints.
Well at 76kg I couldn’t actually stop on a wet descent of the Grand Ballon with DT-Swiss carbon wheels and SwissStop pads but the year before on the same descent in the same weather on DuraAce C24 (with aluminium brakepads) I could stop just fine, ow and I was even a little heavier. And yes I guess I bring the weather from Belgium to France.
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Those conditions would have to be the same for all tests, and even you could guarantee that the conditions for that test–surface water, rate of rain fall, surface drainage, contaminant content, et al–were all exactly the same, such a test would only cover one set of conditions, not a wide variety of conditions. At best, you could only draw conclusions about braking in those specific conditions.
You also have to be able to accurately measure braking distance as well apply the same amount of braking power each time and modulate the braking power the same way each time. That will mean there will be some, maybe a lot, of measurement uncertainty. Sure you could increase the number trial runs to reduce that uncertainty, but then you have to think about how consistent a human can modulate braking power over many tests, especially as braking feel my vary significantly over all that testing.
Consider the time effort Ronan and his droogs put in for their tire testing. IMHO, their tire testing was less complicated and less time consuming than an equally well thought out brake testing regime would be. I also think Ronan’s results would have a smaller uncertainty than similar brake testing…maybe by a lot.
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I can imagine test rigs that would be more repeatable than a rider on a bike, but then folks would argue about how they applied to the real world.
There is a GCN YouTube video titled Rim Brakes Vs Disc Brakes: Which Really Are Better? which I think captures what you are looking to see. It is nearly impossible to make a test like this scientific (this kind of contents is fundamentally edu-tainment) but you can get a sense for pros and cons of Rim v Disc in dry and wet conditions.
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Probably too cynical. And I can be pretty cynical. As others have noted, I believe the issue is more the difficulty and expense in figuring out how to do (and then doing and publishing) such a test. Given we have first principles reasons to believe discs are more effective, commercial reasons to want to sell them, a lot of qualitative feedback from riders indicating the first principles reasons are borne out in the real world, and commercial signal that people are indeed buying discs — it’s not clear who would be capable of and interested in funding and doing that kind of scientific test.
As far as a sniff test goes though, that GCN video @GregS dug up is pretty good! And I think you’ll find most riders here agree the result is directionally correct based on personal experiences.
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If you narrow the question to “What pad works the best in the wet for me on my bike”, testing could get a lot simpler. I’d probably find a hill for coast-down/constant speed, set up a couple of garden sprinklers to ride through, and have a marked braking point. You’d need a protocol for cleaning the pads and rims after each run to avoid cross-contamination or buildup. But you could ignore some of the other variables because you weren’t trying for a universally applicable result.
There’s often a difference between “What is the best X?” and “What is the best X for me?”
I tried SwissStop BXPs, and did not like the feel when dry compared to the stock Ultegra 8000 pads. They may have been better in the wet(no data), but I rarely ride in the rain and the tradeoff wasn’t worth it for me.
One difficulty of testing pads is ensuring a uniform bed-in. Rim brake pads pick up aluminum from the rim and that changes the braking characteristics.
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Interesting. I am perfectly fine with those pads in the dry, albeit I don’t ride much in the rain. The bike in question does see some spring and early winter usage. The BXPs have been fine there for me as well.
Most of the times I’ve got caught in the rain have, so far, been BXPs with Hed Jet Blacks, and they did not brake well at all.
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Well, top end bikes, and most mid-range bikes, come with disc brakes nowadays, rim brake options tend to be pretty limited.
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No question that rim brakes and carbon wheels don’t mix in the wet. The GCN test also used carbon wheels. But the original question applies to alloy rims.
Worth a study ten years ago maybe
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I didn’t suggest that disc brakes had to be hydraulic.
From experience, 1) hard anodized rims don’t stop at all in wet weather, sometimes not in dry weather either, 2) in wet grass, good luck stopping any rim brake bike, and 3) in heavy fog with a rim brake you can pull your levers and hope that the bike will stop where a disc brake might take a second but you can be pretty sure it’s going to stop and in a repeatable manner. My Super Record rim brakes work wonderfully on my shiny aluminum rims (as good as any disc brake) when the sun is shining but I’m not riding that bike in the rain. Too many years of bike commuting tells me disc brakes are better for all weather riding.
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In my personal experience, careful brake pad selection will yield superb breaking performance, even with carbon rims. Having earned both Physics and Engineering degrees, I experimented at length and will share my data if you wish me to.
I lived in Colorado for 7 years, riding a bike with Dura Ace 7900 calipers and Shimano pads on Velocity alloy rims. Coming down in the freezing rain from Mt Evans., Colorado stands out. Yes, the brakes worked all the time predictably.
Then, I tried carbon rims and the SwissStop Black Prince brake pads. Far far superior braking, the Shimano pads on alloys suck in comparison, in terms of modulation and absolute braking power (such as stopping from 40mph on a 14% downhill incline going into a hairpin). I am a certified trackday instructor, and have no problems riding like it’s a trackday with my carbon rims. The newer carbon rims are wider, this means I can ride with lower air pressures, thereby improving wet traction and having excellent and predictable wet braking.
Going back to alloys…Try the SwissStop BXP…I used these on a very wet Tour of Flanders in 2024, with great results. Yes, I did hose off the rims when I got a chance, to clean off the muck.
Having owned a disc-braked bike since 2004 and being a year-round commuter, I routinely experience the rotors getting wet, thus having no initial braking power. IMHO, it is a myth that only rim brakes suffer from this in the rain. Just anticipate it, drag the brakes if needed and ride on! 
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Thanks M Karan. I would be fascinated to see your data!
Please take your snow back 
2024 Flanders ate my nearly-new KoolStop Salmon rim brake pads (and ate my 100-kgs buddy’s brand new Shimano disc brake pads to the point where the pads’ spring made sparks fly ^^).
KoolStop brake pads, good alu rims and good brakes work in the rain, but discs are much more comfortable (and in a peloton where everyone is on discs, people brake later and harder than they used to - in any weather).
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As promised, my notes (copied from my Instagram post 
)
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Cork (Alligator) pads: nonexistent braking power, even in dry conditions, drag shoe on pavement to stop. Perhaps appropriate for flat-landers, very gentle on rims.
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Shimano for Carbon wheels: reasonably good braking power, modulation and rain performance. My choice for the rear wheel’s caliper.
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Scribe Cycling pads (not pictured). Reasonably good in the dry, a bit more powerful than the Shimanos. Not recommended, no rain performance.
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Dark Pads: a bit like the Shimanos, but more initial breaking power, predicable in the rain. Mediocre modulation, at times akin to using wooden brake pads.
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Campagnolo: Great initial bite point, fairly good modulation, very soft (quick wear rate). Not enough power if you like diving into corners. Great for rear wheel caliper. Before tossing them, use them on your alloy rims, great dry performance with them.
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Swiss Stop Black Prince: Top in every sense. Great initial bite point, excellent modulation, great rain performance. Powerful and confidence-inducing. Make you feel like Daniel Ricciardo late-brake diving into a corner, a trackday driver’s choice. Almost too much braking power for rear wheel use. My choice for the front caliper.
Testing was performed using the following combinations:
- Shimano 6600 caliper (insufficient clamping power, really need Dura Ace calipers for either alloy or carbon fiber rims for performance in either rainy or hilly conditions)
- Shimano 6600 shifters with 7800 calipers
- Shimano 9100 shifters with 7800 calipers (far more potent combination than the one above)
- Shimano 7900 shifters and calipers
- Shimano 9100 shifters and calipers
Wheels used:
- Synchros RL1.1 (by DT Swiss)
- Scribe Cycling 3850
- Shimano Ultegra (alloy brake track)
Notes:
- Carelessly jamming on the brakes with the 7900 or 9100 calipers and the Black Prince may result in an endo or get you rear ended. These combinations are ferociously powerful and obviate disc brakes, at least for a male rider with average hand grip strength. Braking distance is limited by tire traction, not braking power.
- Anyone with low to medium finger strength might consider the Black Prince, as the other pads require a LOT of finger strength to produce requisite braking power when descending (especially with a heavy or loaded bicycle).
- Lever reach adjustment seems even more important when not using Black Prince, given the other pads’ inferior initial bite and modulation.
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I think I used about a 1/3 of the pads in Flanders.
What I tried to relay in the notes I just posted is that disc brakes somewhat eliminated the hand strength factor, something my female friends appreciate. Strong rim brake performance demands:
- Very clean rims (wipe brake track with pure alcohol, not lotion) is what I use. Especially if one uses automotive car wash on their bicycle.
- Effective clamping force with low finger effort (hence the listing of which calipers worked well).
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Thanks for this - most interesting. General message seems to be that discs always stop you. While some caliper/rim/pad combinations are also excellent, others perform much worse than discs. I am very happy with my Ultegra/Swiss Stop/alloy rim combination and any slight reduction in performance is more than offset by the other advantages of rim brakes. Perhaps I would think differently if I raced in the Alps, but I don’t. I dread the day that I have to replace the bike.
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