Let's talk rotors

Discuss light weight issues concerning road bikes & parts.
spud
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Joined: Sat Feb 07, 2009 5:52 am

by spud

YM, while I don't disagree with most of what you said, a few caveates -
The front center on MTBs is MUCH longer than any road bike. Center of mass relative to front axle location is also much more advantageous on MTB, therefore you can develope quite a bit more braking power before lifting the rear wheel
On MTB, generally there's not as much difference between static Mu and dynamic Mu as on road. The upshot being, you can feel more secure accessing maximum braking power, and occasionally exceeding it, on the MTB. THis comes into play because not all braking is done straight line, often there is lateral acceleration to it as well. On a road tire, you're gripping, right up until the time you are not, then you are on the ground, on your side. MTB offers a chance to recover from that, thus allowing most people to access that capability.
With regards to grip from rubber/surface contact, we know it's not purely based on normal loadingxMu. If that was true, fat drag racing tires wouldn't exist. There is some sticky interaction between the rubber compound and the surface. You can make the claim that with the compounds we use, it's not much of a factor.

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nycebo
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Location: New York, NY

by nycebo

youngs_modulus wrote:
Sat Mar 09, 2019 12:04 am
I often hear people say things like “mountain bikes stop faster than road bikes because the contact patch on a road tire just isn’t big enough.” That’s not true. First of all, the area of a contact patch has almost nothing* to do with how much grip a tire can generate. More importantly, neither tire is traction-limited: both will pitch you over the bars (barring snowy or wet metal surfaces) before they’ll lock the front wheel.

Yep; that’s pretty much right. A 160mm disc produces a little more braking power for a given lever pull force than a 140mm disc because the caliper is farther from the center of rotation, but the bigger rotor doesn’t let you stop you any faster.

Rather, it dumps heat more effectively into the air because it has more surface area than the smaller rotor. The bigger rotor also doesn’t get quite as hot after a given stop because it has more thermal mass to over which to distribute the energy that pad friction dumps into it.

If your rotor isn’t overheating, then there’s not much reason to switch to a bigger one; it won’t stop you any faster in an emergency. (It will reduce lever effort by a small-but-perceptible amount, which some might prefer).
Great post. 100% agreed, and this coming from a guy at 175 lbs who has NEVER had an issue with the feel of 140mm rotors on my now buried Dogma. I replicated the same on my new bike, but your post has me thinking that maybe I should buy a 160mm for the front, largely owing to your comment on center of rotation. Then again, the aesthetic in me hates the imbalanced look of 160/140. ;-)

by Weenie


Karvalo
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by Karvalo

nycebo wrote:
Mon Mar 11, 2019 9:46 pm
Great post. 100% agreed, and this coming from a guy at 175 lbs who has NEVER had an issue with the feel of 140mm rotors on my now buried Dogma. I replicated the same on my new bike, but your post has me thinking that maybe I should buy a 160mm for the front, largely owing to your comment on center of rotation.
I think you may have taken the exact opposite message from his post to what he intended.

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nycebo
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by nycebo

Karvalo, I just didn't word it correctly. I'm thinking of laying down the heat on descents in the mountains. Tickling the lever with my finger would be easier...is what I aimed to reflect. Further, larger rotor, quicker heat dissipation....owing to more thermal mass and surface area.

Either way, I still prefer smaller. I've never actually overheated a rotor. Does it begin to feel sluggish and one can still come to a stop to let it cool? Or, perish the thought, does it just cook the fluid in the mineral oil and the brakes just vanish. In my car on the track, I could always feel the brakes begin to fade but still had time to stop. Going downhill on a bike in the mountains? Oh boy.

jih
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by jih

Curious: other than weight, what are the downsides to larger rotors?

dcorn
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by dcorn

jih wrote:
Wed Mar 13, 2019 5:21 pm
Curious: other than weight, what are the downsides to larger rotors?
Looks? My Aeroad has 160mm rotors front and rear and still looks awesome. They don't look large or out of place on the bike. I couldn't imagine having them smaller, especially on the front. Plus, I'm 200 lbs and I like to fly down hills, so I'll take all the braking I can get.

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Dan Gerous
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by Dan Gerous

dcorn wrote:
Wed Mar 13, 2019 5:49 pm
jih wrote:
Wed Mar 13, 2019 5:21 pm
Curious: other than weight, what are the downsides to larger rotors?
Looks? My Aeroad has 160mm rotors front and rear and still looks awesome. They don't look large or out of place on the bike. I couldn't imagine having them smaller, especially on the front. Plus, I'm 200 lbs and I like to fly down hills, so I'll take all the braking I can get.
Maybe they catch side winds more, especially Shimano rotors that have much more surface area? 160mm vs 140mm is probably not enough to make much of a difference though.

My bike with discs came with 140mm front and rear, I changed the front to a 160mm as the smaller front was too easy to heat, I can't feel the difference in weight... or in side winds! :mrgreen:

dricked
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by dricked

I switched mine to 140mm. My area has rolling hills and very few long extended braking areas. The 54g weight savings was the main reason but there’s no downside in performance for me to worry about.

jfranci3
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by jfranci3

Dan Gerous wrote:
Wed Mar 13, 2019 6:43 pm
dcorn wrote:
Wed Mar 13, 2019 5:49 pm
jih wrote:
Wed Mar 13, 2019 5:21 pm
Curious: other than weight, what are the downsides to larger rotors?
Looks? My Aeroad has 160mm rotors front and rear and still looks awesome. They don't look large or out of place on the bike. I couldn't imagine having them smaller, especially on the front. Plus, I'm 200 lbs and I like to fly down hills, so I'll take all the braking I can get.
Maybe they catch side winds more, especially Shimano rotors that have much more surface area? 160mm vs 140mm is probably not enough to make much of a difference though.

My bike with discs came with 140mm front and rear, I changed the front to a 160mm as the smaller front was too easy to heat, I can't feel the difference in weight... or in side winds! :mrgreen:
160s have more surface area, but less surface area density (outside of finned Shimano and SRAM). There could be less pressure, since the air has more places to go. Not sure if this would make a noticable difference.

youngs_modulus
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by youngs_modulus

jih wrote:
Wed Mar 13, 2019 5:21 pm
Curious: other than weight, what are the downsides to larger rotors?
Aero drag. I generally buy manufacturers’ data suggesting that discs don’t add much drag. But for a given shape, drag is proportional to frontal area, so a smaller diameter equals less frontal area and thus less drag. It’s probably a very small difference, but hey—you asked.

Also, as has been mentioned before, smaller rotors have less thermal mass than larger rotors, so they heat up a little more for a given stop.

youngs_modulus
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by youngs_modulus

jfranci3 wrote:
Wed Mar 13, 2019 8:37 pm
160s have more surface area, but less surface area density (outside of finned Shimano and SRAM). There could be less pressure, since the air has more places to go. Not sure if this would make a noticable difference.
No, the only thing that matters for side force is surface area. Strictly speaking, the pressure is the same, but the force is larger because force = pressure * area.

jfranci3
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by jfranci3

Were talking about a compressible gas. There are efficiencies involved with letting the old are bounce off the rotor as new air comes on board. Assuming the same surface area and the air hitting it sideways, the air will compress a lot more on the more enclosed area. The more open rotor will allow the gas to bleed off to the side of of the surface easier.

TobinHatesYou
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by TobinHatesYou

jfranci3 wrote:
Fri Mar 15, 2019 3:33 am
Were talking about a compressible gas. There are efficiencies involved with letting the old are bounce off the rotor as new air comes on board. Assuming the same surface area and the air hitting it sideways, the air will compress a lot more on the more enclosed area. The more open rotor will allow the gas to bleed off to the side of of the surface easier.

I'd wager airflow around the rotor is so turbulent, this basically doesn't matter. The cross-section relative to wind direction has a much more profound effect on aerodynamic drag.

youngs_modulus
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by youngs_modulus

jfranci3 wrote:
Fri Mar 15, 2019 3:33 am
Were talking about a compressible gas.
Well, sort of. Compressibility matters a whole lot more when you get into the transonic range. I don’t get close to Mach 1 myself, but maybe you’re in better shape than I am.
jfranci3 wrote:
Fri Mar 15, 2019 3:33 am
There are efficiencies involved with letting the old are bounce off the rotor as new air comes on board. Assuming the same surface area and the air hitting it sideways, the air will compress a lot more on the more enclosed area. The more open rotor will allow the gas to bleed off to the side of of the surface easier.
Respectfully, and with apologies to Wolfgang Pauli: that’s not right. That’s not even wrong.

You’re now guessing at fluid dynamics. If that worked, we wouldn’t need wind tunnels or computational fluid dynamics.

That said, there’s absolutely nothing wrong with guessing or trying to learn more about these complicated, math-heavy subjects. That’s how I figured out I wanted to become an engineer, so I’d be a hypocrite if I discouraged someone else from doing the same.

by Weenie


youngs_modulus
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by youngs_modulus

TobinHatesYou wrote:
Fri Mar 15, 2019 4:16 am
I'd wager airflow around the rotor is so turbulent, this basically doesn't matter. The cross-section relative to wind direction has a much more profound effect on aerodynamic drag.
I agree with you, which probably shocks no one. But still, yeah, discs and the spiders they use to connect to the hub aren’t exactly designed for laminar flow.

But the laminar/turbulent question might be a bit of a red herring. The drag equation includes the expression CdA. These terms suggest that aerodynamic drag is proportional to the coefficient of drag (Cd) times area (A). Two discs of different diameter apps but the same design likely share a coefficient of drag, but the smaller disc has less frontal area, so it has less drag.

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