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Rotor Size Question.....

1K views 22 replies 8 participants last post by  andykrow 
#1 ·
I am not an engineer nor am I mechanically inclined. I was however, able to setup a pair of BB7's on my bike with ease. While I was setting up the brakes, a question popped into my head, and I just can't seem to comprehend the answer.

I know bigger rotors provide better stopping power. Bigger rotors are recommended for more aggressive riding and bigger riders. But my question is, why? Why does a bigger rotor give more stopping power? The contact surface area between the brake pad and the rotor remain the same regardless of the size of the rotor, so why does a 185mm rotor provide more stopping power than a 160mm? Or for that matter, why does a 203mm provide more power than a 185mm rotor?

Am I missing something here?
 
#2 ·
Two factors:

1. Lever Arm. The larger the rotor diameter, the longer the lever arm of the caliper acting on the hub. This can be simulated by spinning a rim at a high speed, then using your finger pressed alternately against the hub flange then against the rim to slow it.

2. Heat Dissipation. The greater rotor (obviously) consists of more material, which can provide additional heat transfer paths. Cool brakes equal happy brakes.
 
#3 ·
Speedub.Nate said:
Two factors:

1. Lever Arm. The larger the rotor diameter, the longer the lever arm of the caliper acting on the hub. This can be simulated by spinning a rim at a high speed, then using your finger pressed alternately against the hub flange then against the rim to slow it.

2. Heat Dissipation. The greater rotor (obviously) consists of more material, which can provide additional heat transfer paths. Cool brakes equal happy brakes.
I see... That is a very good explanation. So, although v-brakes are less than ideal, they are positioned in an ideal location, correct? Using your simulation, it is quite easier to stop a spinning wheel with your finger pressing on the rim, versus, pressing on the hub flange. So that being said, the v-brake is at the outermost limit of where any type of brake could have contact, thus making it ideal.

Hmmmm.....got me thinking. Weight issues aside, someone needs to test out a 26 inch rotor...maybe some sort of rim/rotor hybrid. And place the disc brake pad where a v-brake would normally sit.

I never thought about the heat dissipation. I guess that was kind of overlooking the obvious.

Thanks man!
 
#4 ·
rutkiller said:
...although v-brakes are less than ideal, they are positioned in an ideal location, correct?
'zactly.

But rims, seatstays / fork legs, and rubber pads are flexible compared to rotors, caliper bodies and disc sintered pads.

Take a look at the closest thing to a rim "disc" - a Magura HS33 hydraulic rim brake - and you'll see flexing frame parts and 2mm rim<->pad offsets conspire to rob the system of efficiency.

Set the HS33's pads much closer and only the truest of rims won't rub the pads. Remove all the flex from the stays & fork and you'll be riding a steel brick. Install harder pads and you'll quickly eat through your sidewall if you don't melt your tube first (which is actually possible with rubber pads and heavy braking).

Likewise, every time you increase the diameter of the rotor, you're adding weight exponentially. Consider that 8" stainless steel rotors already approach half the weight of 26" aluminum rims.
 
#5 ·
Excellent descriptions by S.N! Just one minor quibble: in practice, increasing the diameter of the rotor only adds weight *linearly*, since modern rotors are not solid, but have a fairly fixed-width pad contact strip around the circumference, with the rest of the rotor cut away except for relatively fixed-width "spokes" connecting the rim to the hub.
 
#6 ·
Mondoman1 said:
Excellent descriptions by S.N! Just one minor quibble: in practice, increasing the diameter of the rotor only adds weight *linearly*, since modern rotors are not solid, but have a fairly fixed-width pad contact strip around the circumference, with the rest of the rotor cut away except for relatively fixed-width "spokes" connecting the rim to the hub.
Since volume is proportional to the square of the radius, weight increases with the square of the radius, or 1/2 of the diameter. But it has that cutout area as you mentioned, so the relationship is somewhere in between linear and square. I am sure a geometry geek, I mean intellectual, can solve the equation exactly.
 
#7 ·
I'm embarrassed to admit I googled "exponentially" before posting that and thought I was on solid ground, but figured "what the hell, they'll correct me" if I wasn't. My thinking was that the brake track tends to grow in thickness, too, as diameter expands, though this isn't hard and fast.
 
#8 ·
We could say R1=the outer radius and R2=the inner radius. Therefore the area of pad contact area is pi(R1^2)-pi(R2^2)+(some constant for the rotor arms)*r2. We then see that it is indeed an exponential equation as the area increases in proportion to the square of the radius, rather than a linear equation which would be area increasing in proportion to the radius itself. This is not due to increasing brake track, (which I'm not sure occurs anyway), but due to the area of a circle being dependent on the square of the radius. Math nerds with less drug-addled brain cells than I can no doubt give us a rigorous proof.

For what it's worth the bigger benefit as I see it of a large rotor is cooling. I've always been able to easily lock a wheel wioth a 160mm rotor...
 
#9 ·
andykrow said:
For what it's worth the bigger benefit as I see it of a large rotor is cooling. I've always been able to easily lock a wheel wioth a 160mm rotor...
That is part of it, the other part is more material in square inches per rotation of the tire for the pad to grab and have friction with. There is nothing worse than going down a hugely steep ass section of Mammoth Mountain when you weigh 240 pounds wet, and smoking your rear brake pad on a 7 " rotor, knowing full well that in that loose scree, if you touch the front brake, the odds are high that you'll go down. The rear brake just fades away from heat, all the pressure in the world on the brake lever yields nothing in terms of increased braking force and slowing down.

So, the other benefit is not experiencing loss of braking power and brake fade with larger rotors on longer steep descents.
 
#10 ·
RandyBoy said:
So, the other benefit is not experiencing loss of braking power and brake fade with larger rotors on longer steep descents.
Which is due to not cooling properly. It can cause all sorts of problems...fade, glazing over your pads, boiling your fluid in a worst-case-scenario, burning the sh!t out of you if you touch the rotor by accident... I've definitely had that scary feeling with the 160mm!
 
#11 · (Edited)
ak - the equation does seem like polynomial growth, but if you work through your equation, the "squareds" cancel out, as I've worked out below:

Using:
R1 = outer radius
p = pad track width
R2 = R1 - p = inner radius

Then the area of pad contact is:
pi[R1^2-R2^2] = pi[R1^2-(R1^2 - 2pR1 + p^2)]
= pi[(R1^2 - R1^2) + 2pR1 - p^2]
= pi[0 + p(2R1 - p)]
= p * 2pi(R1-p/2)

In this last equation, (R1-p/2) is just the radius of the middle of the pad track, and 2pi times that is just the circumference of the rotor at that radius (which is directly proportional to the radius). Finally, multiplying the circumference of the middle of the pad track by the width of the pad track (p) gives the pad track's area, which is directly proportional to R1, the outer radius of the rotor.

Thus, the pad track area increases linearly with rotor radius. As for the "spoke" area, from what I've seen it looks like the spokes remain the same width in bigger rotors, and just get longer, which would make them linearly increasing in area, too. However, I haven't gone out and actually measured the spokes on different rotors, so I could be wrong here.

PS - thanks to the mtbr crew for forcing me to use some neurons!
 
#15 ·
Mondoman1 said:
ak - the equation does seem like polynomial growth, but if you work through your equation, the "squareds" cancel out, as I've worked out below:

Using:
R1 = outer radius
p = pad track width
R2 = R1 - p = inner radius

Then the area of pad contact is:
pi[R1^2-R2^2] = pi[R1^2-(R1^2 - 2pR1 + p^2)]
= pi[(R1^2 - R1^2) + 2pR1 - p^2]
= pi[0 + p(2R1 - p)]
= p * 2pi(R1-p/2)

In this last equation, (R1-p/2) is just the radius of the middle of the pad track, and 2pi times that is just the circumference of the rotor at that radius (which is directly proportional to the radius). Finally, multiplying the circumference of the middle of the pad track by the width of the pad track (p) gives the pad track's area, which is directly proportional to R1, the outer radius of the rotor.

Thus, the pad track area increases linearly with rotor radius. As for the "spoke" area, from what I've seen it looks like the spokes remain the same width in bigger rotors, and just get longer, which would make them linearly increasing in area, too. However, I haven't gone out and actually measured the spokes on different rotors, so I could be wrong here.

PS - thanks to the mtbr crew for forcing me to use some neurons!
Man...there are some smart cookies on this website. I was just looking for a simple, no nonsense answer....like...."cooler rotors."

haha... You guys rock!
 
#18 ·
Increased lever arm is only part of the answer, increasing the diameter of the rotor drastically increases the swept area of the brake - the amount of rotor exposed to the pads in a single revolution of the wheel.

A greater swept area means you burn off more energy (friction force x distance swept) when you apply the pads to the rotors, consequently you get better braking.

... course it's been a while since university, so this could well be nonsense, but it seems to make sense :)
 
#19 ·
Kyoseki said:
Increased lever arm is only part of the answer, increasing the diameter of the rotor drastically increases the swept area of the brake - the amount of rotor exposed to the pads in a single revolution of the wheel.

A greater swept area means you burn off more energy (friction force x distance swept) when you apply the pads to the rotors, consequently you get better braking.

... course it's been a while since university, so this could well be nonsense, but it seems to make sense :)
Hmmm... I think that factors in to heat dissipation, but not stopping force (directly). But managing heat is ultimately important during extended use.

Pad material & size don't change, nor does rotor material, so coefficient of friction remains the same.
 
#20 ·
Speedub.Nate said:
Hmmm... I think that factors in to heat dissipation, but not stopping force (directly). But managing heat is ultimately important during extended use.
It's definitely a factor for heat dissipation, but if you think in terms of conservation of energy, you build up a ton of kinetic energy hammering down the hill, if you want to slow down you have to burn that off.

Wind resistance is going to account for some of it, but the only other way to burn off that energy is by applying the brakes and turning most of it into heat.
Speedub.Nate said:
Pad material & size don't change, nor does rotor material, so coefficient of friction remains the same.
Indeed, however the distance moved by the rotor disc relative to the pad is much greater, therefore the amount of work done by the pad is greater and so you burn off more energy/slow down quicker.
 
#21 ·
But increased distance from the axle increases torque, so the pad actually is applying less friction to equal the stopping power of a smaller rotor.

Considering you can lock brakes with any size rotor, the issue is not if you can stop more quickly with a larger rotor - you can't - that's simply a function of how much traction you have. The advantage is in cooling and less finger pressure required for a given amount of braking.
 
#22 ·
andykrow said:
Considering you can lock brakes with any size rotor, the issue is not if you can stop more quickly with a larger rotor - you can't - that's simply a function of how much traction you have.
Ah, but whilst this is true under ideal conditions with cold rotors and pads, as things heat up and you lose braking efficiency you retain higher energy dissipation with a larger rotor both because of swept area and increased surface area which speeds cooling - as well as reducing heat buildup in the first place, because, as you say, you need a lower amount of pressure to build the same torque about the axle.
 
#23 ·
I see it the same way. Once you overheat the small rotor system you're fvcked and the comparisons are out the window. Brake fade is the reason I first switched to big rotors in the first place!

While I never had a brake fade out so hard I couldn't lock the wheel up, I certainly had 160mm rotors go from one finger lockup to three finger sphincter-pinching scariness when I first started hardcore downhilling.
 
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