Bigger rotors, wider tires and stopping distance

8 replies · 410 views

v.gcl
57 posts
#1Oct 24, 2022, 2:37 PMedited
I came across this channel a few weeks ago, it's incredible how simple and interesting he makes topics about cars and engines in general. If you like the subject or is simply curious about it, I fully recommend it. He also have some videos about steel, aluminum, carbon fiber and titanium, that can be applied to bikes, and it's explained way better than any bike related content I have seen.

The guy is really knowledgeable, but he released a video that left me on the fence... How this applies to bikes and different rotor sizes? 140, 160, 180 and 203mm?

https://www.youtube.com/watch?v=CyH5xOcsXxs

About thermal mass and dissipating heat I don't think there's much discussion, but how about braking power and general feel and control of the brakes/bike?

He is ignoring the leverage effect? He is talking in a context of stock cars that comes with brakes that can already lock up the wheels? If a 140mm rotor can lock the front wheel, bigger rotors will only make the brake levers softer? (which can be a desirable effect). Bigger rotors will increase the speed which the rotor rub against the pads, but this increases the friction?

How about modulation? I've always heard that smaller rotors have better modulation, and it's one of the reasons rim brakes lacks it.

So many questions...
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stu
646 posts
#2Oct 24, 2022, 5:50 PM
AFAIK different sizes of bicycle disc brakes use the same calipers, the same pads, and more or less the same rotor thickness. So a larger rotor should give you increased braking torque, thermal mass, and heat dissipation all directly proportional to diameter. A larger rotor will also be heavier and may cause more frame or fork flexing. I think modulation is more a function of pad material and stiffness in the cable/housing or hydraulics.

FWIW I'm using 180 mm rotors at both ends. I ride down a lot of long hills at low speeds so my brakes get plenty of use. I appreciate having lower force in the levers and better heat dissipation. I don't mind the few extra grams and my fork is rated for 180 mm rotors. I've also had excellent results with SwissStop E compound pads.
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boots2000
26 posts
#3Oct 27, 2022, 9:47 AM
With a gravel bike, braking power is usually not your limiter. You usually run up against the limit of traction or ability to handle bumps before braking gets you.
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v.gcl
57 posts
#4Oct 27, 2022, 2:13 PM
After doing some research, yes, bigger rotors have more stopping power. I found a comment of he saying that didn't included it in the video because in cars the difference is very small between available sizes.
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Mtnfun2
27 posts
#5Oct 28, 2022, 5:01 AM
Larger rotors give you more stopping power, but power is useless if your tire can't apply it. My fatbike can stop on a dime with 180 rotors, my gravel bike with 160 mm rotors and 45 mm tires is always a battle between not skidding the tire and slowing down. Thus Ianything over 160 mm would just add wgt
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bnystrom
459 posts
#6Oct 29, 2022, 8:20 AMedited
Exactly. Unless you're doing long descents with heavy, continuous braking, heat dissipation isn't an issue. The two limiting factors are tire traction and your ability to keep your weight back enough to prevent an endo. My gravel, fat and mountain bikes all have 160mm rotors and at 175#, I have no need for anything bigger (though heavier riders may). The only change I would consider is going smaller (140mm) in the rear on my gravel bike to reduce the tendency for the brake to lock. Even with careful modulation, it's easy to lock up the rear wheel on soft surfaces, especially when descending and getting bounced around.
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TheSubaruGuru
86 posts
#7Oct 31, 2022, 6:23 AM
Less powerful (smaller rotor) rear brakes certainly follows the automotive model as weight transfer to the front requires more stopping power, despite increased frontal mass hindering turning...something about the polar moment of inertia?
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stu
646 posts
#8Oct 31, 2022, 9:30 PM
TheSubaruGuru wrote:Less powerful (smaller rotor) rear brakes certainly follows the automotive model as weight transfer to the front requires more stopping power, despite increased frontal mass hindering turning...something about the polar moment of inertia?
Braking starts with transient where the weight takes some time to transfer from rear to front. In an automobile (or bicycle) with suspension the weight can't transfer until the front suspension compresses and the rear suspension extends. In a car with large rotational inertia about the pitch axis (side-to-side) it will take longer for that rotation to complete than in a car with lower rotational inertia (like a mid engine car). A suspension bike can rotate independently of the much more massive rider and a rigid bike hardly rotates at all since the tires can deform only a cm or so, for those reasons I don't think rotational inertia is a big deal with bicycle braking.

After the transient, steady state braking cannot exceed an acceleration which results in 100% of the weight on the front wheel. That acceleration will be greater if the wheelbase is longer and the center of mass is lower and farther back. That's why low, mid-engine race cars brake so well.

Turning a car is different. Transients are affected by rotational inertia about the yaw axis (top-to-bottom). The lower rotational inertia of a mid engine car will make it more responsive in quick turns. Transients are also affected by rotational inertia about the roll axis (front-to-back) as the car leans. Steady state is affected by weight transfer from the inside to the outside and by suspension issues which may cause the wheels not to be perpendicular to the road.

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bnystrom
459 posts
#9Nov 1, 2022, 6:03 AM
TheSubaruGuru wrote:Less powerful (smaller rotor) rear brakes certainly follows the automotive model as weight transfer to the front requires more stopping power,

It doesn't require more stopping power, the weight transfer to the front increases traction and provides more stopping power. Larger brakes take advantage of this. Conversely, that same weight transfer reduces traction at the rear wheel, so large brakes are both unnecessary and ineffective.
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