Calipers rule and discs shouldn't win

Discuss light weight issues concerning road bikes & parts.
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dynaserve
Posts: 272
Joined: Fri Jul 18, 2008 8:25 pm
Location: UK

by dynaserve

I've spent some time on my Force disc brakes now on my commuter and they're great in the wet. In the dry I see no extra power or modulation vs a good set of calipers+pads and I'd even say calipers are better, which got me thinking...

(This must have been said before, so sorry if it has but I've not seen it)

Perhaps one of the reasons discs work so well for cars + motorbikes is that the rotor diameter is proportionally large vs the rim diameter. On a 700c road bike the rim is far larger than the disc so you have far more brake surface per wheel revolution: A quick calc indicates around 3 to 4 times the surface area on a 700c rim brake surface vs a 160mm disc.

This seems to explain why dry braking seems better on rim brakes and why discs brakes seem to chew up pads.

Are there any rim vs disc test results out there?

by Weenie


JunkyCollegeBiker18
Posts: 22
Joined: Wed Apr 27, 2016 11:35 pm

by JunkyCollegeBiker18

I'm thinking about buying carbon clinchers in the near future, but I'm waiting to decide on if I want a disc set or not.

I don't think its questionable that discs offer better stopping power in every scenario. If you're riding aluminum rims, I really don't think it would be worth switching over to discs, especially if you're satisfied with how they ride in the wet conditions. If someone lives in a flat area and only rides on the pavement, I see no reason they would need discs.

Having ridden both, I would only recommend disc brakes if someone were in the market for a brand new road bike or lives in hilly conditions. I would prefer to have a set of carbon disc clinchers because they will last longer than rim-brake rims and you don't have to worry about overheating during a long descent.

As far as the science, the disc rotor being in the center of the wheel as well as the material is why it has more stopping power than a rim-brake. A 160mm rotor can achieve the same stopping power as a smaller rotor with less torque. Car rotors are tasked with the duty of slowing down a couple of tons of moving weight. If a 165lb/75kg rider is moving at 24 mph/40kph, they'll have a momentum of 3000 kg per meter per second. Each bike disc should stop half of that, so 1500 kg/m/s per disc. A 4000 lb/1800kg car moving at 24mph/40kph has a momentum of 72000 kg per meter per second. 4 wheels stopping that momentum comes out to 18,000 kg/m/s per wheel. The reason cars started putting on larger rotors is due to more surface area to dissipate heat and increase the lifetime due to face. I can't find any correlations between rotor size and stopping power,

Vermu
Posts: 213
Joined: Wed Dec 22, 2010 6:39 am

by Vermu

JunkyCollegeBiker18 wrote:As far as the science, the disc rotor being in the center of the wheel as well as the material is why it has more stopping power than a rim-brake. A 160mm rotor can achieve the same stopping power as a smaller rotor with less torque. Car rotors are tasked with the duty of slowing down a couple of tons of moving weight. If a 165lb/75kg rider is moving at 24 mph/40kph, they'll have a momentum of 3000 kg per meter per second. Each bike disc should stop half of that, so 1500 kg/m/s per disc. A 4000 lb/1800kg car moving at 24mph/40kph has a momentum of 72000 kg per meter per second. 4 wheels stopping that momentum comes out to 18,000 kg/m/s per wheel. The reason cars started putting on larger rotors is due to more surface area to dissipate heat and increase the lifetime due to face. I can't find any correlations between rotor size and stopping power,


The bigger discs (on cars) usually are combined with multipiston brake calibers thus increasing brake pad size leading to increased power due larger contact area.

But yeah, I would definetly get discs for my all-around bike and winter/wet riding. For road? No, but then again I live in flats

tarmackev
Posts: 232
Joined: Sat Jan 15, 2011 9:59 pm

by tarmackev

Hilly where I live, no comparison on 15% descents in the wet. Discs all the way.
Same hills in the dry, slight edge to discs.
On the flats again wet weather discs are better, in the dry, no real difference.


Sent from my iPhone using Tapatalk

gewichtweenie
Posts: 152
Joined: Fri Sep 25, 2015 3:12 pm

by gewichtweenie

dynaserve wrote:I've spent some time on my Force disc brakes now on my commuter and they're great in the wet. In the dry I see no extra power or modulation vs a good set of calipers+pads and I'd even say calipers are better, which got me thinking...

(This must have been said before, so sorry if it has but I've not seen it)

Perhaps one of the reasons discs work so well for cars + motorbikes is that the rotor diameter is proportionally large vs the rim diameter. On a 700c road bike the rim is far larger than the disc so you have far more brake surface per wheel revolution: A quick calc indicates around 3 to 4 times the surface area on a 700c rim brake surface vs a 160mm disc.

This seems to explain why dry braking seems better on rim brakes and why discs brakes seem to chew up pads.

Are there any rim vs disc test results out there?


on rim vs disc its about the braking acting at a distance far from the rotational center...essentially rim brakes have advantage of leverage.
size of surface is more about heat dissipation

youngs_modulus
Posts: 512
Joined: Wed Sep 20, 2006 1:03 am
Location: Madison, WI USA

by youngs_modulus

You're only considering one part of the lever. Yes, rim brakes have a better leverage ratio from the braking surface to the contact patch. But the leverage of rim brakes is worse than discs in terms of the lever-pull-to-pad-travel-ratio. Disc brakes (especially full hydraulic disc brakes) produce much more force at the pad than rim brakes do.

It's on the order of ten times more. Since friction at the pads is equal to normal force (clamping force) times the coefficient of friction, a ~10x increase in clamping force more than makes up for discs' leverage disadvantage from the pad to the contact patch.

That's why the overall leverage ratio for hydraulic discs (braking force per degree of lever travel) is higher for hydraulic discs than for cabled rim brakes. That said, Sram's hydraulic rim brake (and Magura's before that) also have (slightly) better overall leverage ratios than cables rim brakes because they don't "waste" lever travel by stretching cables and compressing housing.

The large-diameter discs on cars and motorcycles aren't so much about bringing the braking surface closer to the rim as they are about accommodating things like wheel bearings while also providing enough thermal mass to absorb repeated braking from, say, 100 mph to 40 mph for a race car/motorcycle or 60 mph to zero in a street car/motorcycle. In fact, motorsports racers don't switch to bigger discs to get more power; any brake will lock a wheel. They switch to bigger discs because they can absorb/dissipate more heat, thus preventing brake fade.

Street poseurs, however, often use bigger discs in the mistaken belief that they will be able to stop their cars faster. Bigger discs will lower pedal effort, but for a given car, peak deceleration depends on how sticky your tires are, not the diameter of your discs.

The upshot is that good caliper brakes absolutely don't slow you down faster than good disc brakes. Both can lock the rear and catapult you over the bars. Peak braking force is the same, but most find hydraulic discs easier to modulate.

Lower effort generally means better modulation, since fine motor control (muscular effort and finger position) is better when effort is low.

"Modulation" is an entirely subjective quality; you can't measure it. People used to weaker brakes sometimes try hydraulic discs and come back saying "The power is great, but there's no modulation!" They're just not used to stronger brakes. People said the same thing when road bikes switched from sidepull to dual-pivot brakes.

Edit: clarity

gewichtweenie
Posts: 152
Joined: Fri Sep 25, 2015 3:12 pm

by gewichtweenie

of course. hydraulic pressure is another factor.

just clarifying the primary advantage as it relates to diameter of braking surface which in this instance is the leverage, not the surface area.

in fact back in the days (they prob dont make em anymore), the best braking performance on a bike...and most often on downhill bikes... were hydraulic rim (v-)brakes.

they still might be used on some trial bikes today but i dont keep up

by Weenie


mattr
Posts: 3534
Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr

Magura HS series brakes, still around, much sought after, top end ones no longer made.

Had a set on my MTB tandem BITD (HS33 IIRC)

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