Rotor size/lever leverage/stopping distance discussion

[Alexbn921]

Logically even, if both brakes can lock the wheels then how can adding more power to the brakes slow you any quicker?! Bigger brakes can't increase the traction at the tyres, so it's just physically impossible. Adding power will just make it easier to lock the wheels and thus less easy to modulate the brakes.

This not always the case. If you took a pen that weighted 10lb and tried to write with it you would have very little control. It's very easy to exceed the most sensitive range even if you have very strong hands.

Not all of the energy is in being dissipated by the tires traction limit. Locking the wheel is not the goal.

Finding the absolute threshold of traction repeatable and in the shortest amount of time is. Having a braking system that can do this makes you faster.

 

[RJUK]

There is an optimum range of finger control. Pick a power level that lets you stay in that range.

Wheels and tires have a much bigger impact on how a brake feels then most people think. The flywheel effect of heavy tires, wheels and Cushcore significantly increase the braking requirements with zero effect on the traction limit.

Brakes that activate quicker stop you faster. More powerful brakes with bigger rotors usually have a faster ramp.

Steady state braking on very steep terrain requires lots of finger pressure and high thermal mass in the rotors. Optimizing this leads to less hand fatigue and more control.

Not all brakes can lock the wheel. This is especially true if you are already braking. Pads off-gas and the friction of the pads varies with temperature. The temperature at the point of pad/rotor contact is much much higher then the system overall temperature.

Leverage ratios can be variable and where you are in the leverage ratio effects modulation. This mostly applies to Shimano systems where having very little free stroke makes them feel very on/off.

If you need more braking like on a DH bike, then Bigger rotors will stop you faster and more consistently.
If you are on an XC bike less power will be faster as you have better control of the reduce energy requirements.

For me on my Kenevo, Sram brakes with 160mm rotors would NOT be able to lock the wheel and would overheat after a couple seconds of braking.

Eh? If a brake can't lock the wheel then it's faulty and dangerous. Being able to lock the wheel is a pretty basic requirement. Also, any correctly functioning brake with a 160mm rotor would be able to lock the wheels on your Kenevo. How weak are you thinking 160mm brakes are?!

And no, bigger brakes on a DH rig will not stop you faster. That's the whole point. They will resist fade better and require less pressure to be applied at the lever, but they won't stop you faster.

 

[RJUK]

This not always the case. If you took a pen that weighted 10lb and tried to write with it you would have very little control. It's very easy to exceed the most sensitive range even if you have very strong hands.

Not all of the energy is in being dissipated by the tires traction limit. Locking the wheel is not the goal.

Finding the absolute threshold of traction repeatable and in the shortest amount of time is. Having a braking system that can do this makes you faster

That's not really the point that's being debated though. I don't disagree that larger discs manage heat better, which allows for more consistency that allows you to "be faster".

The point is that a larger disc will not stop a vehicle faster than an otherwise identical vehicle with a smaller disc. Not until brake fade comes into it, which is exactly what I've always said the benefit of the larger disc is.

Arguing about modulation is irrelevant to the point being argued.

 

[RJUK]

In addition though, as it seems to be your assertion - if a brake was indeed so weak that it didn't even have the necessary power to lock the wheel, then yes, a larger rotor would stop you faster due to the additional leverage provided by the larger disc.

However, if a bike is fitted with brakes so weak that they cannot even lock the wheel then they are not fit for purpose, which is a whole other conversation. Also, in that scenario, adding a larger disc might still not give them enough power to lock the wheel, so even with the larger rotor they would likely still be unsafe.

 

[RJUK]

Jason, over to you...


Edit: Yes I know it's with reference to cars, but it's still the same thing, except the part about affecting the braking bias, as bike brakes are independently controlled.

 

[Mars2]

I have also been long time on car scene and love to tune and work on my engine, Suspension......

I build up my VW transporter to 416 Bhp by myself and all guy's come to me and say's OH!you still on OEM brake's. You know you should upgrade brake before engine bla bla bla... Always this same bullshit about big brake.

I drive very very fast down and up in the alps. At the beginning I experience some fading after about 10 minutes of spirited drive down hill. Then I upgrade the pads to some that work better at high temp and specially brake fluid to MOTUL RBF 700 Dot4 ( boiling point at 336°C or 637°F ) and no more brake fading at my level of driving. No need big brake kit.

I think 90% of the MTB rider also don't need big brake on their bike specialy when I see at what speed they ride.

 

[p3eps]

I was looking at Magura rotors last night, and here's what they say...

ROTORS​

The bigger and more massive, the better? Not always! Choosing the right rotor – and its diameter – is crucial, because the wrong choice will significantly reduce your braking performance – and that's why MAGURA offers four different rotor designs, allowing you to tailor your setup to your usage scenario in the best possible way.

Assuming the same manual force, a rotor with a diameter of 203 mm generates about 10 % more braking force than one with a diameter of 180 mm, and a rotor´s force increase by 20 %. By selecting the right diameter you can reduce the demand on arm strength, prevent brake fading and gain added safety.

But keep in mind: Bigger is not always better. You will only reach the sweet spot of performance, if you’re able to bring your pads and rotors up to temperature. Too big of a rotor combined with too low demands on the system or lower rider weights will lead to an underchallenged system. It will also decrease your ability to properly feather the braking power. The brake will act more abruptly, the wheel will lock up more often, leading to a decrease in controllability.

 

[hansfrans]

Thanks Steve! The thing is - I've been witness to this exact argument before on car forums. The boy racers all come in thinking bigger brakes = shorter stopping distances and the old boys quickly put them in their place and point out that basic physics shows that's not the case.

But some people just don't want to listen and feel the need to justify their shiny new thing, even if it contradicts physics.

Logically even, if both brakes can lock the wheels then how can adding more power to the brakes slow you any quicker?! Bigger brakes can't increase the traction at the tyres, so it's just physically impossible. Adding power will just make it easier to lock the wheels and thus less easy to modulate the brakes.

Having made this argument repeatedly here, I'm not saying that larger brakes are inferior, just that you need to understand what you're actually getting them for. Don't buy them thinking they will stop you quicker. Buy them because you want better resistance to heat fade or so that you have to put in less effort at the lever (possibly at the expense of ultimate modulation).

Anyway, it's all not worth falling out over. If anyone wants bigger brakes, buy them - it makes no odds to me.

You are just wrong because you only look at what happens once the wheels are locked and nobody doubts you can't reach that point with a smaller, weaker brake, too.

"Also, with a larger diameter rotor there's more of a levering force, so you probably get a bit more power, which doesn't really translate to stopping you faster, but can make it more likely that you'll lock the rear wheel when you didn't intend to. "Snapper" brakes, I guess."
Quoted from an earlier post of yours. So what are you saying? Sometimes you say smaller brakes are better in some situations (usually you ad a note that you don't want to say smaller brakes are better, but then in fact you say exactly that).
In that quote you admit that there is more levering force resulting in more power. For my understanding, that is what the discussion was about. And if we'd go into milliseconds, then the less power at the lever will also result in quicker locking the wheel and that will result in quicker stopping, so the larger disc makes a stronger brake, but again: that's not what I wanted to say and not even worth the discussion. The point is that you compare the power of brakes when the wheel is already locked, and that makes no sense.
Of course you can apply more power to the lever to make a smaller brake also lock the wheel (you need more power at the lever because the smaller brake/disc is weaker) but when going seriously downhill that will require more manual power which will lead to less control for steering the bike. Also modulation is more difficult than with a larger disc.
At the same time, modulation depends on the design of the lever unit, so a smaller disc doesn't always mean better modulation and a larger disc dos not automatically result in a "snapper brake".
And again: going downhill one will want a larger brake that gets the wheels close to locking with little manual power necessary at the lever. Stronger brakes.
It is not all about friction, that is a far to theoretical approach.
But even in theory a larger disc results in more braking power.
At some point you came up with rim brakes to show that larger diameter does not result in more braking power. But as you hopefully know, you cannot compare rim brakes to modern disc brakes. The friction is much lower on rim brakes. But if you would use a smaller rim of course the braking power would also be less then with a larger rim.

ONLY when the wheel is locked it no longer comes down to the power of the brakes (better steering control with large discs aside) but to the tires. Anyway, usually one does not brake to lock the wheels.
At least not downhill where you sometimes need to get to close to zero speed quickly and for that you need leverage resulting in more braking power. You don't want to lock the brakes
Larger disc, more power. More power, better control and faster stopping times.

 

[RJUK]

You are just wrong because you only look at what happens once the wheels are locked and nobody doubts you can't reach that point with a smaller, weaker brake, too.

"Also, with a larger diameter rotor there's more of a levering force, so you probably get a bit more power, which doesn't really translate to stopping you faster, but can make it more likely that you'll lock the rear wheel when you didn't intend to. "Snapper" brakes, I guess."
Quoted from an earlier post of yours. So what are you saying? Sometimes you say smaller brakes are better in some situations (usually you ad a note that you don't want to say smaller brakes are better, but then in fact you say exactly that).
In that quote you admit that there is more levering force resulting in more power. For my understanding, that is what the discussion was about. And if we'd go into milliseconds, then the less power at the lever will also result in quicker locking the wheel and that will result in quicker stopping, so the larger disc makes a stronger brake, but again: that's not what I wanted to say and not even worth the discussion. The point is that you compare the power of brakes when the wheel is already locked, and that makes no sense.
Of course you can apply more power to the lever to make a smaller brake also lock the wheel (you need more power at the lever because the smaller brake/disc is weaker) but when going seriously downhill that will require more manual power which will lead to less control for steering the bike. Also modulation is more difficult than with a larger disc.
At the same time, modulation depends on the design of the lever unit, so a smaller disc doesn't always mean better modulation and a larger disc dos not automatically result in a "snapper brake".
And again: going downhill one will want a larger brake that gets the wheels close to locking with little manual power necessary at the lever. Stronger brakes.
It is not all about friction, that is a far to theoretical approach.
But even in theory a larger disc results in more braking power.
At some point you came up with rim brakes to show that larger diameter does not result in more braking power. But as you hopefully know, you cannot compare rim brakes to modern disc brakes. The friction is much lower on rim brakes. But if you would use a smaller rim of course the braking power would also be less then with a larger rim.

ONLY when the wheel is locked it no longer comes down to the power of the brakes (better steering control with large discs aside) but to the tires. Anyway, usually one does not brake to lock the wheels.
At least not downhill where you sometimes need to get to close to zero speed quickly and for that you need leverage resulting in more braking power. You don't want to lock the brakes
Larger disc, more power. More power, better control and faster stopping times.

Yeah, I'm not reading all that nonsense. Just Google it. Believe what you want, but larger rotors don't stop you faster. It's a common misconception because everyone thinks bigger=better and can't be bothered to look into the actual physics of it.

If you still don't get that after all I've said already then you're never gonna get it.

FWIW, I've never said that larger rotors are inferior and I have large brakes on both my car and bike. But I bought them knowing why - that they offer better heat dissipation under repeated braking, not because I think bigger brakes will stop me in a shorter distance, because I know that's not physically possible.

 

[Planemo]

If you still don't get that after all I've said already then you're never gonna get it.

You're right.

I've spent a fair amount of time around brakes, cars in particular. Sourcing customer solutions for road, rally and race. Using Brembo, AP, Willwood etc. I must have sold 100's of each.

The only time you need a bigger brake is 1. if they are overheating and different pads won't sort it or 2. they can't lock the wheels with a reasonable pedal/lever force. The only other variable is modulation but a driver/rider can generally work around that, it's not a necessity to go straight to a bigger brake when it can generally be solved with a change of master cylinder (ratio) and/or leverage point.

I will be the first to admit I don't have anything like the necessary kahunas to need my 220/220 MT5 setup. It's purely because I think they look the danglies and yes I am a tart. Much like most other riders/drivers who fit big brakes...

 

[RiderOnTheStorm]

Believe what you want, but larger rotors don't stop you faster.

I think this statement is ambiguous (because it's incomplete) and therefore causing some disagreement here as far as I can see.

My take is that larger rotors will stop you faster assuming the same manual force is applied to the brake levers since larger rotors would generate more braking force than smaller rotors.

 

[Mars2]

Every second month I have to block myself from buying an AP kit for my fast van. Just for the look and shaving a bit of weight ( aluminium center bowl and calipers).
My bike already came from factory with 220 and I have a hard time to swap for 200mm even though I know it would be right. so also in the tart Club.

But I know I'm tart.

 

[Planemo]

I think this statement is ambiguous (because it's incomplete) and therefore causing some disagreement here as far as I can see.

You're right, as is your full (non ambiguous) statement. I'm not sure that is whats causing the disagreement though, I believe most understand what RJUK is trying to say, semantics or otherwise.

 

[RJUK]

I think this statement is ambiguous (because it's incomplete) and therefore causing some disagreement here as far as I can see.

My take is that larger rotors will stop you faster assuming the same manual force is applied to the brake levers since larger rotors would generate more braking force than smaller rotors.

Yes, but that's never been the argument, because unless you have arthritic hands you'll always be able to apply enough power to lock the wheels. And if you had arthritic hands you probably wouldn't be mountain biking!

The argument always has been that larger rotors don't stop you in less distance. People just seem to want to throw a plethora of mitigating circumstances at the debate to refute that fact. "For the same lever force they do", "if the smaller brakes overheat they would" etc etc.

That's never been the argument.

If you threshold brake with two otherwise identical bikes, then they would both stop in the same distance. Ergo, the rotor size does not determine the stopping distance.

To stop quicker/in a shorter distance you need to increase grip, either with better tyres/suspension or by setting the tyres/suspension up to provide more grip.

The purpose of a larger rotor is improved heat dissipation. A side aspect is that a larger rotor will provide more leverage over the hub, which means that less force at the lever equals the same amount of leverage at the hub, so if you experience fatigue in your hands from braking, then a larger rotor can be beneficial.

I've never disputed the actual benefits of a larger rotor. But some just seem to equate my saying they don't stop you faster for "they're rubbish and you should buy smaller brakes", even though I've said several times that I have large brakes.

 

[RJUK]

You're right, as is your full (non ambiguous) statement. I'm not sure that is whats causing the disagreement though, I believe most understand what RJUK is trying to say, semantics or otherwise.

Thank you!

 

[steve_sordy]

......... And if you had arthritic hands you probably wouldn't be mountain biking!

.................

I have arthritic hands and they are not normally a problem in my life. They are not fully crippled like I have seen on others, but bad enough that I have to hold a pint glass with both hands! I am now wary of shaking hands with younger blokes who have apparently been taught that bone-crushing handshakes are manly. Fair enough, I used to think the same. So now I warn them before handshaking.

I am grateful for the 200mm discs at each end of my emtb that allow me to brake hard without my hands hurting.

In fact, it is mostly not a matter of avoiding the pain, because sometimes my fingers just won't work! It's as though there is a pressure relief reflex that cuts in whenever too much pressure is exerted.

 

[RJUK]

I have arthritic hands and they are not normally a problem in my life. They are not fully crippled like I have seen on others, but bad enough that I have to hold a pint glass with both hands! I am now wary of shaking hands with younger blokes who have apparently been taught that bone-crushing handshakes are manly. Fair enough, I used to think the same. So now I warn them before handshaking.

I am grateful for the 200mm discs at each end of my emtb that allow me to brake hard without my hands hurting.

In fact, it is mostly not a matter of avoiding the pain, because sometimes my fingers just won't work! It's as though there is a pressure relief reflex that cuts in whenever too much pressure is exerted.

Oof, that sucks, but it's good that you're still out riding and haven't let it stop you.

Have you tried more compliant handlebars or grips to see if that helps? I guess the vibration exacerbates things?

 

[irie]

Oof, that sucks, but it's good that you're still out riding and haven't let it stop you.

Have you tried more compliant handlebars or grips to see if that helps? I guess the vibration exacerbates things?

I also have some arthritis in my hands which is why I fitted SQlab 30X carbon handlebars (to minimise vibration). Also have Sensus Lite grips because small diameter grips work better for me than thicker ones, several of which (such as Ergon) I have tried.

 

[irie]

I defenetly need to go back to 200mm as my 220 don’t have any color change compare to my ex 200 mm. Guideline from hope “ Disc size is important. A bigger size rotor will increase braking power but make sure you are generating enough momentum for the whole system to work at the optimal temperature. If not enough braking intensity, frequency and duration the system could remain too cool, and you won’t get the most out the brake pad, in this case a smaller rotor might produce more power. A good pointer is to look at the color of the disc just below the braking surface. Ideally it will be a light brown. Darker or showing a rainbow effect and you could benefit from a bigger size disc. No color means your rotor is too large and not reaching the optimum temperature.“

"No color" does not necessarily mean that the rotor is "too large".

 

[Mars2]

So you know better than HOPE engineers? I just copy that from what they published.

Do You Have Enough Stopping Power?

Hope Technology manufacture a wide range of high quality disc brake rotors, perfectly suited to our brakes or an excellent upgrade for any system. Rotors are a critical part of the braking system and subtle differences can completely change how a brake system performs. With over 30 years...

www.vitalmtb.com

 

[p3eps]

I didn’t realise how important bedding in the lads was…

I bought a new Magura Mt7 last week after having various issues with the lever, and needing a longer hose… it seemed almost cost economical to get a whole one since I’d get a new set of pads too… and could potentially sell my old caliper to someone wanting a Shigura!

Magura recommend doing 30x stops to 30kph to zero to bed them in. I have a hill outside my house, so spent 20 mins on Friday night accelerating down the hill and then braking suddenly.

The first stop I was thinking ‘are these actually going to stop?’, and each shot, my stopping distance improved.
Perhaps 30x was overkill, as I don’t really think it improved much after about 10… but they’re night and day from out of the box now!

 

[Bummers]

I can't believe some people refuse to accept the truth, the mental gymnastics to avoid admitting they are wrong is staggering

 

[Mars2]

So according to HOPE the 220mm that my Strive : on CFR came with is too big for me.
Here are picture of some of my old rotor's next to my Strive 220mm one's
The 180 are too small and the 200 are spot on for me


Ps: the 220mm rotor have been used for 200km

 

[Bummers]

So according to HOPE the 220mm that my Strive : on CFR came with is too big for me.
Here are picture of some of my old rotor's next to my Strive 220mm one's View attachment 120297

Looks like 200 were the sweet spot.

 

[Mars2]

Yes I might swap the 200 rear and 220 front. Maybe the 220 at te back will be better for me. I like to drift the bike so I use those back brake.

 

[RiderOnTheStorm]

That's never been the argument.

@RJUK I think your detailed explanations from an absolute standpoint (on stopping distance strictly in relation to larger rotors) are spot on and make perfect sense to me.

What is the core argument though? From reading through this thread, it seems the questions are more about braking effectiveness and performance when it comes to using larger rotors. In a practical sense, when should I use larger rotor and why?

• Heavier riders and/or ebikes may benefits from larger rotors e.g due to increased braking power.
• Faster, more demanding DH/Enduro riders may benefit from larger rotors e.g due to better heat dissipation.
• Riders with physical limitations who cannot squeeze brake levers as hard may benefit from larger rotors e.g due to higher leverage.

In other words, some guidance on when should I go with large/larger rotors (and how large should they be) if I check any of the above boxes?

 

[Mars2]

I would just change
"Heavier riders and/or ebikes may benefits from larger rotors e.g due to increased braking power."
To
"Heavier riders and/or ebikes may benefits from larger rotors e.g due to better heat dissipation"

Because As some say's since the beginning even though larger rotor add braking power they don't increase grip of tire so stoping distance on will not decrease. So you fit bigger rotor to control heat dissipation.

Think some people mix Braking Power and stoping distance. those two thing are not link together.
their are many other parameter than come in to stoping distance than just braking power the first one being the tire that is the link of your bike to the ground. the second one will be body position on the bike to have equal pressure on both tire, Then rotor at the right temp for the pads temp range..............

Why is Porsche one off of the best braking car? because the engine is at the back so getting better efficiency of the 4 tire when braking.

Did you ever try a car with big brake and racing pads compound. when cold the first time you brake you will think fuck this car got no brake.

Braking power will help compensate slowing down more rotational mass like heavy tire, insert, bigger wheel 29.....

 

[RJUK]

@RJUK I think your detailed explanations from an absolute standpoint (on stopping distance strictly in relation to larger rotors) are spot on and make perfect sense to me.

What is the core argument though? From reading through this thread, it seems the questions are more about braking effectiveness and performance when it comes to using larger rotors. In a practical sense, when should I use larger rotor and why?

• Heavier riders and/or ebikes may benefits from larger rotors e.g due to increased braking power.
• Faster, more demanding DH/Enduro riders may benefit from larger rotors e.g due to better heat dissipation.
• Riders with physical limitations who cannot squeeze brake levers as hard may benefit from larger rotors e.g due to higher leverage.

In other words, some guidance on when should I go with large/larger rotors (and how large should they be) if I check any of the above boxes?

Yeah, throughout I have said the benefits of larger rotors (repeatedly, actually) but people just seemed to assume that because I was saying they won't stop you any shorter that I was saying larger brakes are bad.

That's absolutely not the case. High performance bikes and cars have larger brakes for a reason... It's just that the reason isn't shorter stopping distances!

The reasons you've given above are spot on though. I'd say to use the bike as you buy it to start with and make adjustments as needed.

In the case of downsizing.... I'd probably not bother. IMO, it's better to be over-braked than under-braked. Whilst you might not be using your brakes to the fullest on your normal trails, if you go to a bike park or away on a biking holiday you don't want to find yourself cooking the brakes and in a world of bother half way down a long descent.

As for up sizing them, I'd probably ride the bike and look to replace pads and maybe discs if I found myself overheating them.

This discussion also started on the Strive:On thread, which comes with 220/203 rotors, so it has pretty beefy brakes to begin with.

 

[RJUK]

I would just change
"Heavier riders and/or ebikes may benefits from larger rotors e.g due to increased braking power."
To
"Heavier riders and/or ebikes may benefits from larger rotors e.g due to better heat dissipation"

Because As some say's since the beginning even though larger rotor add braking power they don't increase grip of tire so stoping distance on will not decrease. So you fit bigger rotor to control heat dissipation.

Think some people mix Braking Power and stoping distance. those two thing are not link together.
their are many other parameter than come in to stoping distance than just braking power the first one being the tire that is the link of your bike to the ground. the second one will be body position on the bike to have equal pressure on both tire, Then rotor at the right temp for the pads temp range..............

Why is Porsche one off of the best braking car? because the engine is at the back so getting better efficiency of the 4 tire when braking.

Did you ever try a car with big brake and racing pads compound. when cold the first time you brake you will think fuck this car got no brake.

Braking power will help compensate slowing down more rotational mass like heavy tire, insert, bigger wheel 29.....

Good post. Spot on.

And yes, having the engine hanging off the back of the car makes Porsches brake great. Weight transfers forwards when you brake, so having the engine right at the back means you can fit much beefier brakes at the rear (because the rear isn't unweighted as much as in other cars and thus has better grip available). It also helps with traction for acceleration, because again that weight is at the back of the car where the driven wheels are and where the weight transfers to under acceleration.

It becomes problematic though when you lose traction at the rear end, as all that weight gives it a lot of inertia and makes it more difficult to save the slide.

Everything is a compromise.

 

[Mars2]

Going to spend a few days in the alps bike park from next Friday. I will see if I can give some color to this 220 mm disc . The 200km until now as been done 100% on the bike park next to my house. But it’s short 200 meter D-