Rotor size/lever leverage/stopping distance discussion

[Stoneilove]

Interesting thread, i use larger rotors not to stop faster but to manage heat dissipation which in turn allows the brakes to work harder for longer.
Hot brakes are inconsistent brakes, lever feel and overall stopping power is effected, the quicker the brakes can cool between hard braking the better the overall perfomance will be, increasing the surface area is by far the easiest and most effective way of managing heat.

 

[RJUK]

Head, wall and bang. I'm out.

It's like pulling teeth, isn't it!

 

[RJUK]

Interesting thread, i use larger rotors not to stop faster but to manage heat dissipation which in turn allows the brakes to work harder for longer.
Hot brakes are inconsistent brakes, lever feel and overall stopping power is effected, the quicker the brakes can cool between hard braking the better the overall perfomance will be, increasing the surface area is by far the easiest and most effective way of managing heat.

Agreed. Larger brakes have definite benefits.

 

[RJUK]

I haven’t read every post so please excuse me if I missed some of your points. I think people here agree with you that if you can lock up your brakes than putting bigger rotors on will not help you stop any faster.

The problem is, while your statement is true under most regular braking, but will fall short under extreme repeated braking of any decent descent. Those small rotors will begin to fade, requiring more hand pressure and longer stopping distance until the brake system cools off. If you are a lighter rider, or not aggressive rider, you may not experience this fading concern.. I think this is what people are disagreeing with you about.

Yeah, this has been extensively covered. I've made it very clear throughout the whole thread that the main purpose of larger discs is heat dissipation. That is THE benefit of larger rotors and the correct reason to fit them.

 

[RJUK]

My own (worthless) opinion. It's about effort.

Bigger disc=less effort
Less effort=more control/less fatigue

That's what matters to me anyway on long and steep descents.

Less effort does not equal more control though, it's actually the opposite.

Adding power makes it easier to accidentally lock the wheel, I.E. less control.

Less fatigue is right though - the additional leverage of a larger disc means less pressure is required at the lever to slow the same amount.

However, I'm quite a feeble rider with zero fitness or strength and even I don't find that I have issues during a ride with standard fit brakes, so I think fatigue is more of a perceived concern than an actual one. (Excluding people with medical issues or perhaps those going on really long, tiring rides.)

 

[Alexbn921]

No it's not. The issue being argued is solely about braking distances. Not control, nor heat dissipation, or anything else.

Actually, my argument is that having the optimum control window will lead to the best braking in the shortest distance. Also single event, flat ground braking is not a good indication of overall performance of a braking system. The majority of my braking is at very high speed fall lines with a wide range of brake temperature. Consistancy is key.

This isn't theoretical. Also brakes ramp friction at different rates and heat ranges. There is a reason that the Enduro brake test has significantly different times to dissipate a similar amount of energy. This was with in a lab test.
NOT all brakes are the same and even if they can lock the wheel they will yield different stopping distances. Since the rider has to control this. Confidence, ramp up and feel are massively important.

On my XC bike 180/160mm rotors with XTR race gives the greatest consistency and confidance.

On my DH eBike 220/200mm with saints is just barely enough to get optimum control over the temperature range and massive amount of grip super soft tires have. My buddy has Trickstuff and they have even better feel with more power.

Cars with good computer controls are limited by traction only for a single braking event.

 

[Alexbn921]

Less effort does not equal more control though, it's actually the opposite.

Again there is a range of control. If you have to drive a nail with a 50lb hammer it's hard to control. if you have a .1lb you have to swing it way to hard. There is always a sweet spot. Most times if you feel you need more power then it = more control.

 

[RJUK]

Actually, my argument is that having the optimum control window will lead to the best braking in the shortest distance. Also single event, flat ground braking is not a good indication of overall performance of a braking system. The majority of my braking is at very high speed fall lines with a wide range of brake temperature. Consistancy is key.

This isn't theoretical. Also brakes ramp friction at different rates and heat ranges. There is a reason that the Enduro brake test has significantly different times to dissipate a similar amount of energy. This was with in a lab test.
NOT all brakes are the same and even if they can lock the wheel they will yield different stopping distances. Since the rider has to control this. Confidence, ramp up and feel are massively important.

On my XC bike 180/160mm rotors with XTR race gives the greatest consistency and confidance.

On my DH eBike 220/200mm with saints is just barely enough to get optimum control over the temperature range and massive amount of grip super soft tires have. My buddy has Trickstuff and they have even better feel with more power.

Cars with good computer controls are limited by traction only for a single braking event.

View attachment 120770

Great, lovely argument, but again completely irrelevant to the argument being made here.

Saying you're having your own argument has no relevance to the point I'm talking about.

Also, your last point is incorrect. Cars would be crashing all over the place if they heat soaked after a single braking event to the point where they couldn't even lock the wheels anymore.

 

[irie]

Great, lovely argument, but again completely irrelevant to the argument being made here.

Everyone who says something with which you disagree is summarily dismissed as being "irrelevant to the argument being made here".

 

[RJUK]

Again there is a range of control. If you have to drive a nail with a 50lb hammer it's hard to control. if you have a .1lb you have to swing it way to hard. There is always a sweet spot. Most times if you feel you need more power then it = more control.

A lot of feelings and opinions going on here, not a lot of facts though...

 

[RJUK]

@irie That's the point though. I don't necessarily disagree with what points people are making. Yes, larger rotors resist heat fade better. But people are confusing that with meaning that the brakes stop you in a shorter distance. You can't argue my point with a completely different point.

#facepalm

 

[irie]

@irie That's the point though. I don't necessarily disagree with what points people are making. Yes, larger rotors resist heat fade better. But people are confusing that with meaning that the brakes stop you in a shorter distance. You can't argue my point with a completely different point.

#facepalm

 

[RJUK]

Yeah, now you're finally getting it. The argument seems to be over your head.

 

[RiderOnTheStorm]

Is this thread becoming the place to throw punches at each other after a long and frustrating day at work??? We're all entitled to our opinions right?

 

[RJUK]

Seems so. I think it has run it's course.

Opinions don't really come into it, as it's a straight facts/physics-based argument, but I get that some people aren't prepared to change their mind from their preconceived misconceptions, so let them believe whatever they want. You can lead a horse to water...

Have a good weekend all!

 

[Alexbn921]

It's not just a physics argument.
You are trying to extract maximum stopping distance in an analog system with consequences.
If was as simple as can the brake produce more friction than the tire over the stopping distance, we would be done.
The answer is NO, adding additional brake power doesn't shorten the distance.

In the real world there are a lot of additional factors AND the controller in charge of those variables needs training to maximize the outcome.

I wish we had carbon ceramic rotors. I rode them for about 6 months as I was a Kickstarter backer of sicc rotors and they where truly game changing. Nothing else comes close to how good they where. Unfortunately sicc went bankrupt.

 

[irie]

It's not just a physics argument.
You are trying to extract maximum stopping distance in an analog system with consequences.
If was as simple as can the brake produce more friction than the tire over the stopping distance, we would be done.
The answer is NO, adding additional brake power doesn't shorten the distance.

In the real world there are a lot of additional factors AND the controller in charge of those variables needs training to maximize the outcome.

I wish we had carbon ceramic rotors. I rode them for about 6 months as I was a Kickstarter backer of sicc rotors and they where truly game changing. Nothing else comes close to how good they where. Unfortunately sicc went bankrupt.

I've raced with carbon ceramic rotors. Fantastic stoppers when there's enough heat in them but if they get too cold it's scary and crash time ...

 

[RJUK]

The answer is NO, adding additional brake power doesn't shorten the distance.

Finally, you get it!

And yes, of course it's physics.

The carbon ceramic brakes might be good... Still wouldn't stop you any quicker though...

 

[jimbob]

Finally, you get it!

And yes, of course it's physics.

The carbon ceramic brakes might be good... Still wouldn't stop you any quicker though...

I've read through the whole thread, and feel the pain.

The only pedantic point would make, is that larger (or more powerful) brakes will reduce your stopping distance if the original ones can't lock the wheels. The limiting factor is the lesser of friction between tyres and ground and the brakes. But, as said, ost braking systems can do that now, hence not reducing the stopping distance by increasing rotor size.

 

[RJUK]

I've read through the whole thread, and feel the pain.

The only pedantic point would make, is that larger (or more powerful) brakes will reduce your stopping distance if the original ones can't lock the wheels. The limiting factor is the lesser of friction between tyres and ground and the brakes. But, as said, ost braking systems can do that now, hence not reducing the stopping distance by increasing rotor size.

Agreed 100%. (I'm sure I've even made this point myself at some point in the thread.)

If the default brakes are incapable of locking the wheels, then sure, more powerful ones will stop you in a shorter distance, so the additional leverage from a larger rotor would achieve this.

However, if your brakes can't even lock the wheels then they're not fit for purpose, so it would be unwise to just increase the rotor size. (Doing so might still not provide enough power to lock the wheels and it's likely the brakes would still be really underpowered for the vehicle). You'd be better off replacing the whole system, or having it fully rebuilt if the lack of power is due to degradation.

But yes, in that particular scenario, a larger rotor would reduce the stopping distance, as the brakes aren't capable of exceeding the tyre's traction. Also, in this scenario, increasing the tyre's traction would not yield shorter stopping distances, as traction isn't the limiting factor in this case.

 

[Alexbn921]

Stopping distance is not just a matter of crushing the level and locking the wheel.

There is a ramp in power from the hydraulic system.
There is a ramp in power from the temperature rise of the pads and rotor.
There is a decrease in power as gas is expelled from the pads and forms a boundary layer.
There is a decrease in power from the pads and surface of the rotor at the contact point from non transferred heat.

Static friction has a huge jump when the wheel stops. it's easy to keep a lock wheel locked. It's also easy to lock a wheel by stabbing the brakes.

The skill of keeping a brake at absolute threshold without locking is very hard. It requires a significant increase in steady state friction over just being able to lock the wheel. I have found the limit of smaller rotors (203mm) during steady state threshold braking that are not heat soak related. Pulling the lever harder did nothing to slow me faster and while i could have shocked the system into locking that is not desirable. This on a DH ebike with gummy tires.

 

[RJUK]

Stopping distance is not just a matter of crushing the level and locking the wheel.

There is a ramp in power from the hydraulic system.
There is a ramp in power from the temperature rise of the pads and rotor.
There is a decrease in power as gas is expelled from the pads and forms a boundary layer.
There is a decrease in power from the pads and surface of the rotor at the contact point from non transferred heat.

Static friction has a huge jump when the wheel stops. it's easy to keep a lock wheel locked. It's also easy to lock a wheel by stabbing the brakes.

The skill of keeping a brake at absolute threshold without locking is very hard. It requires a significant increase in steady state friction over just being able to lock the wheel. I have found the limit of smaller rotors (203mm) during steady state threshold braking that are not heat soak related. Pulling the lever harder did nothing to slow me faster and while i could have shocked the system into locking that is not desirable. This on a DH ebike with gummy tires.

Well obviously. Locking the wheel is not the objective.

The rest isn't really relevant and would apply to either brake, and the last bit I have no idea what you're on about. The brake just lost all power and stopped working, but wasn't heat soak? So what do you suppose it was that was suddenly preventing your brake from having the power to lock the wheel? This all sounds a lot like your opinion about your experience... Which doesn't hold any relevance against facts. Watch the friggin YouTube vids if you need someone else to explain it, because clearly you aren't gonna listen.

 

[Alexbn921]

During prolonged threshold braking the interface of the pad and rotor reach a temperature range that starts to decrease the friction coefficient. At this point you need to pull harder to keep the same breaking rate. Adding more pressure doesn't add a linear amount of stopping power. Afterwards neither the rotor or pads are heat soaked.

Braking is a much more complicated system then you are laying it out to be.


From a white paper.
The characteristics of the contact between the brake disc and pad is generally measured through dynamometer tests using the methods in references [7,8], and then the obtained average friction coefficient is used for the brake control and the brake performance calculations. However, the measured instantaneous friction coefficient changes within the specified range during a braking period. This variation of the friction coefficient affects the short-term brake behaviors, such as the jerk of the vehicle or the slip-slides of the wheel, even though the average friction coefficient is sufficient to estimate the brake performance under normal running conditions.

The tribological phenomena for a disc-pad pair includes the heat transfer, which is complex and time-consuming to compute accurately using a theoretical mathematical model, so it is not appropriate to include a heat transfer model in real-time simulations when using the HILS system. In this study, alternately, we focused on how to represent the instantaneous friction coefficients between the disc and the pad in an equation form that included the thermal effects in the HILS system of the railway vehicle.

The friction coefficient is normally expected to primarily depend on the temperature of the surfaces in contact and the relative speed of the contacting surfaces. The pattern of the instantaneous friction coefficient at the beginning of braking or at low speed is observed to be relatively high compared with the steady-state value. With this phenomenal understanding based on observations, Eq (2) is introduced in this paper to represent the characteristics of varying friction between the brake disc and the pad.

μd(t)=μd0 (nv e−m vvd(t)+1) (nT e−m TTd(t)+1)
(2)
where μ d0 is the steady-state friction coefficient between the disc and the pad, n v is a multiplication factor that is caused by the friction speed, m v is a parametric coefficient of the exponential function of the friction speed, n T is a multiplication factor caused by the increase in temperature, m T is a parametric coefficient of the exponential function of the increase in the temperature, c is a correction constant. The friction speed v d is the relative speed between the disc and the pad in m/s, and T d is the increase in temperature of the disc surface in Celsius. In Eq (2), the variable friction coefficient μ d is a function of the two variables v d and T d, and the five parameters μ d0, n v, n T, m v, and m T.

The proposed equation (Eq (2)) for a variable friction coefficient considers the effect of the increase in temperature of the friction materials and the friction speed. More specifically, Eq (2) is composed of the multiplication of three parts: the first part is the steady-state value of the friction coefficient, the second part is a correction due to the friction speed, and the third part is a correction due to the increase in temperature of the friction materials.

Eq (2) is derived based on the dynamometer test results for a specific disc and pad. Fig 2 shows the dynamometer test results for variable friction coefficients between the disc and the pad due to the increase in temperature and friction speed. The asterisk points imply measured values, and the solid lines imply curve-fitted values using Eq (2) and the parameter values in Table 1. Fig 2(a) shows that the friction coefficient between the disc and the pad decreases as the surface temperature of the disc and the pad increases at a friction speed of 9.6 m/s. The friction speed is the tangential speed of the contact point of the disc and the pad. Fig 2(b) shows that the friction coefficient between the disc and the pad decreases as the friction speed increases at a surface temperature of 50°C. By comparing the measured and curve-fitted values in Fig 2, we see that the proposed equation (Eq (2)) validly represents the varying characteristics of the brake friction behaviors. Friction speed 20 m/s in Fig 2 corresponds to 113 km/h vehicle speed for the present HILS system since the wheel diameter is 0.86 m and the wheel disc diameter is 0.55 m for the present HILS system.

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Fig 2
Measured and curve-fitted friction coefficients between the brake disc and the pad.
The left portion of the figure shows the variation of the friction coefficient due to the temperature change at a friction speed of 9.6 m/s, and the right portion of the figure shows the variation of the friction coefficient due to the friction speed change at a temperature of 50°C.

 

[RJUK]

Jesus! Don't throw that at me on a Saturday!

Yes, you can get very into detail with a million different variables and it's all very complex. I'm only simplifying it because my argument is simple. I'm not arguing that larger rotors are inferior or that heat fade doesn't exist or that practically, in a real life scenario having brakes that are easier to control isn't more important than outright power. My argument is literally the very simple one that a larger rotor doesn't stop you in a shorter distance. With the caveats that we're comparing like-for-like brakes and bikes otherwise, and we're not factoring in heat fade (which of course) would favour the larger rotor.

When you do repeated braking and get the brakes hot, or a long descent, then sure, that build up of heat begins to favour the larger rotors. I'll point out again that I have large brakes on my car and bike. Pointing out a common misconception about brakes does not mean I'm against them or don't understand the benefits of them.

I suggest we just leave it there, as we seem to be going round in circles, yet we seem to agree on pretty much everything except that one point.

 

[mtbbiker]

Yeah, this has been extensively covered. I've made it very clear throughout the whole thread that the main purpose of larger discs is heat dissipation. That is THE benefit of larger rotors and the correct reason to fit them.

I think we all find a preferred sized rotor and really don’t think twice about it. I used to race DH in my twenty’s. Close to 54yrs old now. I go down steeper and crazier trails now than when I raced. Todays bikes are so dialed in.

I have 2021 Kenevo with custom tuned suspension front and back. I change out my front fork to either an older Dorado 200mm or custom tune Fox 38 180mm. The older Dorado has max 200mm rotor size and the 38 can go 220mm. I honestly can’t tell the difference in braking power between the 2 rotor sizes. I take both forks down steep ass trails, where the brakes only stop you from speeding up. I run Hayes Dominion A4 brakes and can’t say I’ve experienced and brake fade with either sized rotors. But I can’t remember the last time I had any rotors smaller than 200mm on my bike. Interesting topic, pick a rotor size best suited to your needs.

 

[RiderOnTheStorm]

Personally I'm more interested in what happens before locking the wheels.

For me the main question is, do larger rotors help me decelerate quicker, in other words, slow down the bike (not stopping it) in a shorter distance than with smaller rotors?

With my riding style/long descent, full weight, ambient temperature, similar force applied to levers, pad quality, my experience with 220mm rotors has been that, all things considered, larger rotors slow me down quicker than 203mm or 180mm rotors. YMMV of course!

 

[RJUK]

Personally I'm more interested in what happens before locking the wheels.

For me the main question is, do larger rotors help me decelerate quicker, in other words, slow down the bike (not stopping it) in a shorter distance than with smaller rotors?

With my riding style/long descent, full weight, ambient temperature, similar force applied to levers, pad quality, my experience with 220mm rotors has been that, all things considered, larger rotors slow me down quicker than 203mm or 180mm rotors. YMMV of course!

No, that's the point, they don't. (At least, not until brake fade becomes a factor.) You're misundersranding what I mean by "if the brakes have the power to lock the wheels". I'm not saying that's how you'd be braking in the comparison, nor that locking the wheels is desirable. The point is that if the brake has the power to lock the wheel, it has the power to brake below that point. If locking the wheel is 100% braking power, then it can do, 99% and 98% etc as well. As such, both rotors can stop the bike in an equal distance. Only if the brakes are unable to fully lock the wheel is adding "power" going to slow the bike faster.

Adding larger rotors/new brakes can often feel like you're stopping faster, but in reality that's not the case, as the brakes can't increase traction. If you're experiencing fade with the smaller rotors and then the larger rotor eliminates that fade, then yes you may stop faster. I suspect it's more likely to be placebo though.

 

[irie]

Personally I'm more interested in what happens before locking the wheels.

For me the main question is, do larger rotors help me decelerate quicker, in other words, slow down the bike (not stopping it) in a shorter distance than with smaller rotors?

With my riding style/long descent, full weight, ambient temperature, similar force applied to levers, pad quality, my experience with 220mm rotors has been that, all things considered, larger rotors slow me down quicker than 203mm or 180mm rotors. YMMV of course!

Agreed. In spite of being able to lock brakes using smaller rotors, larger rotors have enabled me to more reliably brake closer to the point of locking brakes than with smaller rotors.

 

[RJUK]

Agreed. In spite of being able to lock brakes using smaller rotors, larger rotors have enabled me to more reliably brake closer to the point of locking brakes than with smaller rotors.

Cool. Good for you.

 

[Alexbn921]

For my bike setup 180 lack the power to stop me, 200 work but requires too much hand force and I can feel them overload on 1 section of trail. 220 give me the best feel and more consistent braking. Better consistency leads to more confidence in where the limit is.
I have experienced this on my race car. It uses relatively small rotor and even with race pads it's steady state deceleration caps out. Of course it can full lock the wheels with ease, but had a steady state limit right around the tires limit.