azza_333

I was wondering if anyone knows of any testing results, comparing 160/160mm, 140/140mm, and 160/140mm rotor performance with hydraulic calipers.

subgrade

This GMBN video suggests that bigger rotors deliver more stopping power, however, the experiment is not very precise, and the difference is within 10%. The numbers start after 3:30 in the video.

In any case, there is not much point in having bigger rotors on the rear, as the traction will be lost very soon and most of the braking is done on the front wheel.

CargoDane

I run 180 mm on the back and 203mm on the front. And I use Saints DH brakes. Now, granted, they'd probably be overkill for a normal length bike, but as mine (the Omnium) is longer than a normal bike, if I do an endo, it is me going over the handlebars landing on the cargo platform, rather than the entire bike endoing.

What is also a tad ridiculous is that the 203mm disc takes up just under half of the diameter of the front wheel (front = 20"). The front brake is ridiculously powerful - especially with weight on the bike (because a lot of that extra weight is taken by the front wheel, increasing grip). That's how I like it. You can never have too much stopping power. Assuming you can modulate said power.

Physics will tell you that a larger diameter rotor will be more powerful. It's not simply a matter of heat dissipation as a certain someone claims. It's about leverage.If it was about heat dissipation, they could just make the, say, 140mm rotor larger inwards, even make them completely solid, thus giving more area and more mass to the rotor. That's not how disc brakes works.

On the back, though, I don't need a 203mm disc. A 160mm would probably be plenty of braking power, but I decided that it's only a little bit more weight to have a 180mm .

Gary in NJ

If you ride flat terrain and are a reasonable weight, the 140mm rotors are probably fine, especially if you are a weight weenie. However if you ride hilly terrain where you have to control your speed with the brakes for a few miles, you will want the larger brakes to dissipate heat. Once you cook your brakes, the fun is over.

CargoDane

Hard on. That's how to use brakes on long downhills to stop cooking them, regardless of rotor size. Don't do the slow stops. Brake late, hard, and short. A bigger rotor helps there because it has more leverage, meaning for the same force (friction) applied to the rotor, the larger one will provide more stopping power. As for heat dissipation, I have finned brake pads (larger surface area). Those pads receive the same amount of heat as the rotor, yet has much smaller surface area. It's not the better heat dissipation of a bigger rotor that stops you from cooking your brakes.

cxwrench

I worked on a bike w/ Saint brakes the other day...holy Jesus...those things work!

For the OP: I'd think 160fr/140rr would work great for most people on a road bike.

CargoDane

Haha, yeah, they're insane. In the manual it says something about them not to be used on the roads, and if doing so you need to be extra careful. I absolutely love them.

cxwrench

Heat dissipation is most definitely part of it. That's why Shimano, for example, does all the fancy stuff like finned pads, Ice Tech rotors, and ceramic pistons. That's also why carbon brakes exist for cars and there is super high boiling point brake fluid, fancy brake ducting, and Mercedes spent millions designing their wheels. Larger rotors have more leverage, so more 'power'. All the other stuff makes sure the 'power' is there for more than 1 or 2 braking events.

Kapusta

I believe he meant it is not ONLY about heat dissipation.

It is also about mechanical advantage (Reason #3 in the link you provided).

CargoDane

On the pad. I know heat dissipation is a thing. I have an "ICE" shimano rotor on the front. What I'm saying is that that is not the reason bigger rotors are better. Boiling brakes is when the pads transfer too much heat to the oil. Hence finned pads.

A bigger rotor is better because it has more leverage which means that for a given force it will stop you quicker. Or for a given brake length, you will need to apply less force than on a smaller rotor.

It is friction that stops you.

I mentioned the small front wheel on my bike.As such, having a smaller front wheel will mean that the rotor goes through more revolutions for a given brake distance, but it also means that the lever compared to the circumference of the wheel is much larger than rotor size alone will give you. That said, all that can be somewhat ignored, because no one use brakes like that. It's about stopping the mass traveling at X speeds. Friction does that, and yes, friction creates heat. A bigger rotor means that that friction exerts more force to stop you for a given input (i.e. a given amount of friction) because the lever - that is: The force lever from the axix to where the force/friction is applied - is longer.

If it was all about heat dissipation, you could simply make a very holed surface (to maximise surface area), but make it a thick disk, and only make it, say, 3 inches in diameter.

I don't like rim brakes over disc brakes, but the only reason they work (so-so), is because the lever arm where the friction is applied is so far away from the axis. If you attempted to use the same level of force on, say, a 120mm disc, it would stop your bike very, very slowly. That is because the force arm is so short.

Heat dissipation is obviously a thing, because where you have friction, you will have heat. That's a byproduct of stopping power though.
People have exploded their tyres going downhill with rim brakes.

One of the ways to ride more efficiently, faster, and taking care of your rims or hydraulic disc brakes is to brake late and brake hard. Hovering with your hand on the brakes to take top of the speed is a surefire way to overheat things, and your average speed will be lower to boot.

gsa103

Larger front rotors helps balance braking forces front and rear. If you apply the same level pull, the greater leverage of the front rotor means that you'll get more braking on the front. That's a good thing, because the rear will tend to brake traction first due to weigh shifting. The one possible exception would be tandem bicycles, where you want as much brake as you can get.
I run 180/160 on my mountain bike and I find that it's very balanced. Even with the larger front rotor, the rear will break traction first. My commuter has 160/160, and I wish it had a little stronger front brake.
For a road bike where you care about weight 160/140 is a good balance.

CargoDane

My emphasis.
There you go again with the nonsense.
On loose gravel over hard surface, obviously even a worn and mal-adjusted rim brake will get the job done. On ice, even a pad-less brake will probably suffice.
My brakes are DH brakes. And as my bike is used mostly on asphalt with a load on it, those brakes are great. They stop much better than most anyting else.
What will seemingly come as a shock to you is that you don't have to grab a handful constantly. You can actually modulate how much force you apply - you don't have to apply all of it every damn time.

Friction stops you. Force applied at a longer distance from the axis will mean that whatever force is applied will translate to more braking force. Heat and heat dissipation is a byproduct of and solution to applying friction.


Yes, it sure is. It's amazing how you're constantly wrong about these things. I am not saying you don't need to dissipate heat, but that's not the reason for going to bigger rotors and more powerful brakes.

It is amazing how you seem to completely "forget" that a larger rotor provides more stopping power - regardless of the brakes being hot or not. That should be basic knowledge - especially for you who likes to pretend that noone knows anything while demonstrating your own ignorance time and time again.



LOL; i'm the one "confused". What a frigging joke.
You quoting that sentence while completely fail to understand the basics is exactly why I can say that once again project your own ignorance of the subject at hand.

Here, let me spoon feed you the info: If it was all about heat dissipation they could go that route instead of making the rotors bigger.. If it was about heat dissipation, with the same surface area and mass as the bigger rotor, it would have exactly the same braking force. Or put more fins on smaller rotors

However, that is not the case when it comes to brakes. They don't do that. You get a bigger rotor for better stopping power. The bigger rotor simply stops you better because the force lever is longer.

What is so frigging hard to understand here? Or is that you simply are so eager to show off your ignorance on these matters too?

tomato coupe

Bigger rotors can shorten the stopping distance if the braking system is limited by how much force you can apply to the rotors. But, that isn't the limiting factor – tire / road friction is.

CargoDane

Tire/road friction is always in play. That's how the world works. You can say the same with Bentley's brakes. They too are limited by the tyre/road interface.
You both are saying that if that somehow shows that rotor size is not important for braking power. Makes you wonder why Bentley doesn't use disc brakes from a mini and only adding fins.

Someone: Leather brake pads on stainless steel rims aren't good, rubber is better
You two not knowing anything about physics: But, that isn't the limiting factor – tire / road friction is.

Someone: Disk brakes on cars, motorcycles, and bicycles are better than drums and rim brakes because the diameter can be bigger.

You two not knowing anything about physics: But, that isn't the limiting factor – tire / road friction is

Someone: We need more powerful brakes for downhill and heavy bikes.

You two not knowing anything about physics: But, that isn't the limiting factor – tire / road friction is

Someone: We need big brakes on the new Land Rover Defender, so it can tow heavy stuff safely. Hence the minimum size wheels will be 18" so the brake rotors will fit.

You two not knowing anything about physics: But, that isn't the limiting factor – tire / road friction is.

I get it. You two are completely clueless about physics. Let me ask you this: Why would you want bigger rotors if all you wanted was more mass and surface area? Bigger rotors are more exposed and because they are bigger (see "lever arm"), they will also be more prone to bending than a smaller diameter rotor.

Neither of you have grasped even the very basic concept of force lever arms. Unbelievable.

No one is saying that powerful brakes are useful on all surfaces and in all situations. That is a ridiculous strawman that both of you employ constantly. It's called MODULATION!

Frigging pathetic, both of you.

tomato coupe

We've been through this before. I guarantee my physics training greatly exceeds yours.

CargoDane

LOL; apparently not since you don't understand torque at all. Least of all how it pertains to rotor size and brakes in general.

Here's a very simple primer:




Torque and Equilibrium

Rotor size work the exact same way. Something you argue against, yet claim to have "physics training". You have absolutely no idea of the concept of levers and how it relates to torque. So much for your "guarantee".

tomato coupe

Clearly you have some reading comprehension problems. My statement was very simple:

As stated, however, a bicycle's braking system is not limited by how much force can be applied to the rotors, therefore bigger rotors do not lead to shorter stopping distances.

CargoDane

But it does. Of course the substrate matters. Again, with your argument, we might as well use leather pads on polished steel rims.
What you are saying is so ridiculous and obvious that it can be used as an argument against any improvement in braking tech as I have shown with my examples, all the while ignoring the fact that better brakes actually works better than brakes than can only just make your rear wheel skid on loose gravel over tarmac.

Of course, in those scenarios where better brakes does not help, they obviously do not help. That's why you modulate your brakes according to the surface.
There's a reason heavy or fast things don't use weak bicycle rim brakes. They don't because it is isn't good enough. It may be good enough on loose gravel over tarmac or on ice, but for everything else it won't be.

You don't size car brakes to only work good enough over that gravel or on ice. You size them so they can stop the car as fast as possible in good grip situations. You can modulate your brakes for the gravel or ice.

It's as if neither of you have ever been able to brake hard without falling. Or you only use your rear brake or whatever it could be. In that latter case, yeah you can probably use those small road bike rim brakes and be good.

tomato coupe

My statement is about disc brakes on bicycles, which is the topic under discussion in this thread. It is not about hypothetical bicycles with leather rim brakes, Land Rovers, or whatever other side issues you want to inject into the conversation.

cxwrench

Yes, that's why I said that it's a 'part' of it.

subgrade

Yes, the tire/road traction is the limiting factor of how fast you can stop. That does not mean bigger rotors don't deliver more braking force than smaller ones. If you brake hard enough to lock up your wheel, at that point there is no difference anymore, however, since less power at the lever is needed to reach that point with bigger rotors, it's easier to modulate the braking force so as to not overshoot that point, even if it takes some finesse and getting used to.