It is generally accepted that disc brakes will offer better performance in the rain on a road bike.

But from reading many articles on the topic of disc vs rim brakes, I have yet to find any explanation of why this is the case.

The biggest issue with braking performance in the wet with rim brakes is that the rims will accumulate water and dirt which needs to be cleared off by the pads before they start to slow the bike properly. But to me it seems that discs should get just as wet and dirty.

So why are disc brakes so much better in the rain?

  • It's something with how between a rim brake and a disc brake, a rim has far more surface area to clear than a rotor, being a larger circle. Both see a complete pass through the caliper once per wheel revolution, but the individual spots on the rotor are passing by much slower, being a smaller circle. I'm bad at articulating why this results in the disc brake having an easier job keeping it clear. – Nathan Knutson Jun 27 '20 at 15:56
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    I suspect (but do not know) that the fact that in the rain, a rim is rolling adjacent to the ground, and is getting splashed/immersed on every rotation, so the pads need to keep clearing the rim on every rotation. – Adam Rice Jun 27 '20 at 19:58
  • Not adding another answer, but an observation is (especially for MTB bikes) disc brakes generate much more stopping power than needed to stop, allowing control and feathering. If they loose efficiency when wet, you can still easily generate more than enough stopping force. Rim brakes have less 'reserve' – mattnz Jun 28 '20 at 3:10

Discs have a much higher clamping force. For rim brakes, there's usually quite a bit of movement; you can run the brakes with 5mm clearance from the rim and it'll still work fine. With discs, the pads move a much smaller distance, allowing them to use higher leverage ratios. For example, people running hydraulic rim brakes found themselves crushing rims from all the power.

I imagine this allows disc brakes to have more authority while braking. Water isn't capable of forcing itself under the pads due to the high pressure. Discs also heat up more and can run more aggressive pad compositions (because rapidly wearing out an aluminum rim isn't an issue anymore).The best MTB brakes use sintered metallic pads, which still offer crazy power in the wet.

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    Spot on. Maximum permissible contact stress is the most important number when designing a break or a clutch. – Vorac Jun 28 '20 at 6:09

Firstly, one of the main reasons rim brakes fare worse in wet conditions is that the brake pads are often made of rubber or cork, which lose some of their grip when wet.

Disc brake pads, on the other hand, are usually made of a resin compound with added metallic powders; this composite material still retains most of its grip when wet.

Disc brakes are also probably more effective as they are designed to be compact and shielded – and thus less exposed to rain falling from above, unlike rim brakes which will have rain falling directly onto them.

I would imagine the fork also provides some shielding for the disc brake calliper.

Finally, rims are much more likely to accumulate water and dirt from the ground (compared to thin and perforated discs) due to the rim's proximity to the ground at the bottom of every rotation.

  • Makes you wonder why rim brake pads aren't made of sintered metal or organic material, like disk brake pads. – Criggie Jun 27 '20 at 23:08
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    @Criggie: it doesn't make me wonder. Rims do wear out, but they are not designed to be sacrificial, as a brake rotor is. The pads wear out faster than the rotor, but neither is intended to last as long as the much-more-expensive wheel rim. If I had a bike with rim brakes, I definitely would not want the brake pads made of the same material as my disc brake pads are. – Peter Duniho Jun 28 '20 at 0:43
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    Also (@Criggie) the clamping force you can exert on a solid steel disc is much less than on a channel made of thin aluminium - but you need this clamping force to get enough friction when you've got the harder materials of disc brakes and rotors. I've just tested with thumb pressure on a rim brake rubber pad compared to a (resin-bonded) ceramic disc pad against a scrap rim and the rubber grips much more for the same pressure. This need for more pressure from the same force also means less travel - not good with rims that can be well out of true, and probably bad with a dinged rim – Chris H Jun 28 '20 at 9:56

Because they have different kinds of brake pads that are optimized for all-weather performance rather than best dry coefficient of friction (this necessitates higher mechanical advantage to have more clamping force), because they have less heat capacity, and because the disc is further away from the road than a rim.

The heat capacity of rims is large. A rim weighs often more than half a kilogram, and transfers some of its heat to the tire as well. When braking, the heat generation is not at such a high level that the rim would immediately heat to the boiling temperature of water. I suspect that when starting to brake, during the first rotation of the rim the brake pads work like a squeegee, removing all but a thin film of water from the rim. At the second rotation of the rim, the localized heating evaporates away the thin water film. At the third rotation of the rim, you have full braking force.

The rim is very close to the road, thus it is likely to get wet from splash water. A disc brake disc, not so much.

Good rims are made from aluminum because (among other reasons) aluminum is soft, so the brake pads do not "polish" the rim like they would do for a steel rim, but instead cause it to wear faster, resulting in a rough structure. The rough structure is what helps with wet braking.

Even if the brake disc is wet from splash water, the different kinds of brake pads not made from rubber have a smaller difference in coefficient of friction between wet and dry states.

Also, if you brake often, you will have discs that are boiling hot (example: 100 kg rider braking at 0.67 g creates about 1400 joules of energy per wheel revolution, and 0.12 kg steel brake disc stores 55.2 joules per degree Celsius, so going from 20 C to 100 C requires only about three wheel rotations with braking). The heat capacity of those small discs is not substantial, and the heat dissipation is not efficient either: the discs being small compared to the rims. If a splash of water hits a boiling hot brake disc, it evaporates immediately.

In a certain sense, the rim brake is the best possible disc brake, having the largest possible disc size, and reusing an existing component as the brake disc. However, a practical rim brake must track a wavy rim to allow riding home after a spoke-damaging accident, so the rim brakes are restricted to simple mechanical devices, and thus, they have low mechanical advantage and thus must use brake pads that have a huge difference in dry and wet coefficients of friction in order to get the best possible dry coefficient of friction. Also, the largest possible disc size increases the risk of the discs being wet from splash water.

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