Is disk brake effectiveness mitigated by tyres losing traction under strong braking?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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).

Answer 4

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.