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So I understand that disk brakes are superior to rim brakes and this leads to faster stopping particularly in wet weather.

I have wondered though, in wet conditions when you wish to stop as fast as possible doesn't the superior stopping effect of the disks result in the tyre losing traction ? At some point the benefits of being able to stop the wheel moving is presumably overcome by the tyre being unable to maintain traction on the road surface.

I'm assuming a decent quality tarmac surface and commuter type tyres on a day that is wet enough that rims and rim brakes would be compromised to some extent.

I realise there are a ton of variables here but I presume somebody has looked into this and I was curious to know.

Thanks.

  • In my experience disc brakes don't grip as hard as rim brakes - with rim brakes you can easily get thrown over the handle bar, discs brake "softer". Thus the loss of traction is rather seldom (again, in my experience). – Erik May 3 at 8:04
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    Basically what you have described is why many riders feel that disc brakes are unnecessary and have been pushed on us by the bike industry. – Andy P May 3 at 8:11
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    You're coming up with the idea of anti-lock brakes in your head. This is why they are used on all new cars and many new motorcycles. – JPhi1618 May 3 at 17:17
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    @Erik: That makes sense, since disk-brake pads apply braking force much closer to the hub and therefore exert less torque on the wheel per unit brake pressure than rim brakes do. – Sean May 4 at 20:28
  • @JPhi1618: And all aircraft (their original application, decades before they made the jump to road vehicles). – Sean May 4 at 20:28
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Ideally, the brakes should be able strong enough to break traction otherwise you will not be able to brake maximally (i.e., just before the threshold of losing traction). Rim brakes can do this, even in wet weather, if set up correctly. The real advantage of disc brakes in wet weather is other aspects of how the brakes perform.

From personal experience, disc brakes have superior wet weather performance for the following reasons:

  • Modulation: as stated above maximal braking is when you are on the threshold of losing traction. Good hydraulic disc brakes are typically renowned for their feel and modulation (i.e., enacting finite changes in brake force), letting you approach those thresholds with more control.
  • Response time: while a well set up rim brakes can generate strong braking forces, the response is often delayed in the wet due to the need to first clear the rim of water before effective braking can occur. In a similar manner disc brakes must clear water from the rotor, but this tends to occur much more quickly due to rotors being better protected than rims and the slots in the rotors giving water a place to go when the bake pads are engaged. This gives disc brakes responsive brakes even in down pour conditions.
  • Wear/longevity: rim and disc brakes both use some type of brake pad, and in both cases these brake pads will wear faster in the wet. That said, brake pads associated with rim brakes tend to wear proportionately faster due the rim braking surface being closer to the road causing it to get dirtier quicker than a disc brake rotor which is close to the wheel hub.
  • Structural integrity: Disc brakes have a dedicated braking surface (i.e. the disc rotor), where as with rim brakes the braking surface is built into the rim. Wearing the braking surface of rim brakes can eventually result in rim failure without regular inspections and replacements. Where as rim brakes require a wheel replacement or rebuild, with disc brakes wheels should theoretically last longer as it is fairly trivial to replace a disc rotor. Furthermore, damage to a rim will affect braking for rim brakes, but not disc brakes.

In dry weather and/or clean conditions there is arguably less of an advantage to disc brakes compared to a well set up rim brake.

  • The disc also heats up more than rims, evaporating rain water under heaving braking. Additional the focus on 'Well setup rim brakes" - disc brakes are more likely to be setup well, where rim brakes are often poorly adjusted. (I have had perfectly adjusted rim brakes become poor adjusted in less than one ride (wet, volcanic soil trail wore 3/4 of new pads in 30km of riding....) – mattnz May 3 at 0:06
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    @mattnz if I don't put in the caveat about mythical well set up rim brakes, I can expect serious repercussion from the rim brake aficionados. Stack exchange is supposed to be fun, I don't want to get doxed and swatted over my braking preferences. – Rider_X May 3 at 3:11
  • I would add that not all disc brakes are hydraulic, especially on cheaper road/gravel/commuter bikes (which may be the OPs case). – Vladimir F May 3 at 7:39
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    I would like to add that rim brakes are affected by a bend in the rim (caused by hitting a stone for example). Disc brakes are not affected by this. – Deruijter May 3 at 12:59
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    @Deruijter thank you for the suggestion, I added as a more general note to the final bullet. – Rider_X May 4 at 13:03
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You are correct that the maximum deceleration is limited by the maximum force the tires can generate against the road surface. If the brakes can cause the tires to reach that point (usually just before the tires start slipping) they have sufficient maximum power.

The problem is the braking force required to keep the tires at the point they generate maximum deceleration force changes as the bike velocity and road surface changes. What makes brakes powerful in a real sense is the ability to modulate the braking force to keep the tires at the changing point of generating maximum deceleration force as the bike slows down and stops (this is essentially what automotive ABS systems do).

Disc brakes initially appeared on mountain bikes, where there are a lot of advantages aside from braking power: they remove design constraints on the wheel rims and front and rear suspension structures and provide for better tyre clearance.

  • How long until bike manufacturers start selling anti-lock braking systems for bikes? :) – Jeremy Friesner May 5 at 1:12
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Another advantage of disc brakes is they are made a lot more powerful than needed to lock up the front wheel, allowing low hand pressure to get to wheel lockup. I can lock my MTB rear wheel with just my pinky finger.

This is a significant advantage mountain biking, and I imagine in some road situations, where you need to have close to maximum braking for extended periods. The reduced effort means you less fatigue so can brake for longer while maintaining modulation, giving control.

Anyone who has mountain biked with good brakes and gone to a bike with a cheap set of brakes appreciates just how much faster good brakes allow you to ride (without crashing).

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    You're right, but the question's more about tyre and traction on the road, rather than a direct comparison between the brake formats. – Criggie May 3 at 10:26
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In my experience if the brakes are well set up, rim brakes are very effective for little outlay. Disc brakes I find are more controllable and I find myself rarely using the front disc unless I need to endo / 90 spin or 180 spin the rear round.

Both have advantages and disadvantages.

I do see a lot more bikes now coming standard with discs so maybe that's what the future holds.

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    Note that OP was specifically talking about tarmac riding with commuter tyres. Probably not doing a lot of endos. – Criggie May 3 at 5:52

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