[Closed] Disc rotor size. Does bigger really give better stopping power ?

They do. Whether you need more stopping power can only be answered by you. Swapping to SLX with the same size rotors will make bugger all difference.

Yes, I went from 165 to 185mm rotors on BB7s and the difference is very noticeable.

They definitely do, there will be others along shortly to say ‘buy (insert brand here), they’re awesome’
But ime, the easiest thing to increase power is to increase rotor size.
You may get a small increase by changing pads, but it’s the rotor size that matters most.
I have bikes fitted with 180/160, 180/180, and 200/180.
The one fitted with the 200mm rotor has the most power.

I get the physics, however isn’t the weakest link going to dictate stopping power, namely tyre traction.

If you squeeze the lever hard enough surely you can lock up a 160mm as well as a 200mm disc. So can the same stopping effect not be achieved with both.

It’s just easier with the larger rotor and it also offers better heat management - being a greater surface area so more to heat up and cools faster (long descents is when this will be apparent)

If you squeeze the lever hard enough surely you can lock up a 160mm as well as a 200mm disc. So can the same stopping effect not be achieved with both.

You don't have to squeeze as hard to get the same effect with 200mm. Or more power for the same effort.

Yes.

See Archimedes.

He was a late adopter though - see the Egyptians - pyramids etc.

If you squeeze the lever hard enough surely you can lock up a 160mm as well as a 200mm disc. So can the same stopping effect not be achieved with both.

Never underestimate the benefit of not having to pull the lever as hard, it makes a huge difference.

If you squeeze the lever hard enough surely you can lock up a 160mm as well as a 200mm disc.

I could lock wheels with 1992 Deore DX cantilevers. Doesn’t mean they’re not crap compared to a 180mm disc brake, though.

Never thought about the why before. At first I thought this was a heat thing... but a couple
of beers and a bag of crisps have helped with enlightenment...

It appears to be a moment thing... try to stop something from pivoting is easier if you’re further away from the pivot (axle).
So changing a 180 to a 200 rotor gives an 11% increase in stopping power assuming all else remains the same.

There's also modulation to consider, specially at lower speeds, it can be. Bit sketchy if don't do so much as breath on the levers and you lock up.

More brake power is great but you don't want an on/off scenario.

Bit like fork travel I suppose.

If you squeeze the lever hard enough surely you can lock up a 160mm as well as a 200mm disc. So can the same stopping effect not be achieved with both.

Can you actually lock up the wheel in any given situation? Actually getting either wheel to lock up when you've got the weight back is a bit of effort.

Or flip that the other way around, if you can lock your wheels up you need grippier tyres not weaker brakes.

One slightly dissenting voice - well almost.
Bigger rotors do give you more power, but with some (all?) brakes you get better performance long term if you use the power.
Or to put it another way work a set of 160s hard and they will continue to have their “peak” power.
If you use very little of the power of 180s then the power will fade a little over time.
Same reason you bed brake pads in.

There’s still more power in the 180s if you use them hard though.

Well that’s my impression anyway.

Does bigger really give better stopping power ?

Yes, definitely.

There’s also modulation to consider, specially at lower speeds, it can be. Bit sketchy if don’t do so much as breath on the levers and you lock up.

There's a trade off between low-speed modulation and high-speed power. My commuter bikes have 6" rotors for this reason. All my MTBs have 8" front and 6" rear rotors.

but with some (all?) brakes you get better performance long term if you use the power.
Or to put it another way work a set of 160s hard and they will continue to have their “peak” power.
If you use very little of the power of 180s then the power will fade a little over time.
Same reason you bed brake pads in.

You would have to barely use the brakes to have a problem with this. Even fairly modest braking will get the contact patch of the pads and disk extremely hot and burn pad material onto the disk. You may have less pressure with a larger rotor, but the rotor is turning faster, so the braking power will be the same and the contact patch between the pads and rotor will get just as hot.

Bigger rotor = more likely to take a hit and need straightening = pain in the...

180 seems to be the sweet spot for me. Had 200s that have been nothing but a nuisance.

I Went from 200 to 160 on the front on a new build as thats what I had lying around and I thought I'd notice the weight difference. Same brakes, same pads.

After the first fast corner where I felt like I was going to crash I went back to 200mm.

200mm on the front for life 🙂

I've often wondered though whether the amount of braking surface area (actual metal) that the pads grip, makes a difference; I'd assumed that it does.

For example:

[img] [/img]

Vs. where the pads would appear to be grabbing 60% fresh air.

[img] [/img]

I'd assume that the first, heavier rotor would offer more stopping power; it'd be interesting to see how a solid 160mm rotor might fair against an equally weighted, more drilled 180mm.

I have SRAM Level T on two bikes, one with 200 mm rotor and one with 180.

200mm is miles better.

Not very scientific but tells me what I need to know!

I went from Sram Gyude R's to RS and added a 203mm up front and kept the 180mm rear. The stopping power is pretty damn good TBH!

I’ve often wondered though whether the amount of braking surface area (actual metal) that the pads grip, makes a difference; I’d assumed that it does.

Broadly speaking, no, it doesn't. For theoretical hard surfaces, friction is independent of contact area. The softer and stickier the friction material, the less this is true: so a soft racing car tyre will stick itself to the rough profile of a road surface and provide more grip as the contact area increases. But essentially disc brake rotors and pads are hard; pads do compress a little but if anything this should make highly open rotor surfaces more effective, since the pad will expand very slightly into the holes and then the very top of the pad will be hit side-on by the approaching metal.

So…

it’d be interesting to see how a solid 160mm rotor might fair against an equally weighted, more drilled 180mm.

The 180 will win, no doubt whatsoever.

Broadly speaking, no, it doesn’t. For theoretical hard surfaces, friction is independent of contact area..........The 180 will win, no doubt whatsoever

A-level physics says you're right. Practical experience says you're wrong.

I went through a phase of using those alligator rotors (Aries, sawtooth and another one). They all lacked power, easily as much as one rotor size, maybe even two!

Some cutouts or drilling helps by letting water escape, and the gasses produced by burning resin, the pad does get gripped slightly by the edges, and the edges clean (abrade) the surface of the pad which stops ferrous material from the rotor contaminating the pad.

Once you've got some though, more doesn't seem to help all that much and the friction definitely decreases.

More edges to the braking surface actually give a higher friction coefficient don’t they? So highly drilled discs can be grabbier but if you go to far they lose their thermal performance and fade more easily.

I’ve read somewhere that bigger disks actually give better modulation as well. Not sure how true that is.

Wont the solid disc heat up too much?

Much like car brakes - high performance ones are drilled and grooved for better performance?

Of course.

But rotors like Ashimas take it too far and lose the thermal performance. There comes a point where drilling the disk can’t overcome the loss of material for the heat to get dumped into, hence why a lot of lightweight rotors end up melting on long descents.

I get the physics, however isn’t the weakest link going to dictate stopping power, namely tyre traction.

There's more to it than that. For starters, tyre traction is enough except on loose surfaces to put you over the bars before it locks. But think about when you really need max braking power - it's when you are on something steep and/or going very fast. When it's steep, you put far more of your weight on the front wheel and your braking drives the front wheel into the floor even more. When you are going fast though, ultimate power isn't what you need, it's being able to shift the generated heat. Doing 5mph on steep tech you aren't putting that much energy into the brakes, whereas slamming on at 35mph on tarmac you are.

At high speed you can in theory slow down as fast with smaller rotors as bigger ones, by squeezing harder, in which situation you would be putting the same amount of power into the disk. But with a bigger disc it has more time to shed the heat to the environment because each bit of disc takes longer to come around to the pad again. And being a larger piece of metal overall, including the inboard bits, it's like a bigger heatsink.

I've got 203mm Hope V2s on one bike. When doing slow stuff they are a bit better, for sure. But the most remarkable difference is when going faster - they still give you a good solid shove in the chest regardless of how fast you are going, for the same lever pressure; whereas with my XC type brakes (Mono Minis, XT) they feel less solid and need squeezing harder at speed, and stop me less.

I think that the limiting factor is the pad/disc interface, as it starts to burn and produce gasses, which means instantaneous heat dissipation from the pad/rotor interface is key, not just longer term heating of the caliper itself.

More edges to the braking surface actually give a higher friction coefficient don’t they? So highly drilled discs can be grabbier but if you go to far they lose their thermal performance and fade more easily.

Fade at high temperatures is the resin in the pad compound burning off, the drilling and grooves in high performance disks give that gas so where to go. Without them the pad would float on the surface of the disk.

There's some good graphs on the brembo site showing the difference in braking g force with heat. The grooved and drilled disks are much of a muchness but do have more friction even from cold (and there's an argument that it will be even greater in the real world with water and as pads wear), but past a point the gap between the basic solid disk and the modified one widens considerably, that's what's meant by fade.

My experience of the ashima/alligator/Aries rotors was they ran hot all the time, even an xc ride would turn them blue. The power was a crap but never faded. The advantage of hot brakes was they remained bedded in all winter, managed months more of use without wearing out.

On balance, high friction pads and mostly solid rotors definitely offer the best braking performance.

I'd be interested to see some proper cast iron brake disks on a mountain bike, they'd cost a fortune but could have much higher friction than the current stainless ones.

I went through a phase of using those alligator rotors

I had a set of these rotors on my SS. I swapped out a set of Magura rotors with them thinking they would suit the minimal look of the bike.
Well the braking performance was also very minimal, shocking discs.
The dentist suggested I didn't use them again 🙂

It helps to understand mechanical and adhesive friction in braking systems, and how adhesive friction is created and maintained.

Are you going to help us out then Scienceofficer?

thisisnotaspoon, gassing is a function of the heat in the brake system, the more heat the more the pad cooks Off.

https://en.m.wikipedia.org/wiki/Brake_fade

My experience is that lightweight rotors fade.

Lightweight rotors fade more easily because the extra venting cannot remove the gas quick enough and the heat from the brake system to compensate for the lack of material in the disk.

You're not getting not "More power" from a bigger rotor it's simply providing mechanical advantage, assuming that all other factors are equal (i.e. same pads, same contact area same hydraulic system actuating) then for a given pressure at the lever you *should* see a greater effect at the wheel.

Or put another way, to achieve the same effect at the wheel with a smaller rotor you'll have to put more energy into the braking system, i.e. pull harder on the lever to generate greater pressure on the pad and correspondingly more friction and heat at the rotor...

But it's seldom stopping "power" that people really have an issue with it's either heat buildup leading to fade or more commonly a lack of "Modulation" or sensitivity leading to less control, both of which can be affected to some extent by an increase/decrease in either rotor's diameter.

It's worth noting that a push bike is different to many other forms of vehicle in that the front and rear brakes are independently controlled by the rider, and hence can be used for slightly different things. It's not just about "Stopping Power" you might use the rear on it's own just to scrub a little speed, or even deliberately lock up (skid), you might combine it with the front to effect a rapid deceleration (say tanking it from a fast straight section into a slower techy corner) you might even be looking to hold the whole bike up on the front brake to un-weight the rear and allow some more complex maneuvers (i.e. a stoppie)...

So outright mechanical advantage ("power" to most people), sensitivity (modulation) and of course the brakes ability to manage heat and therefore remain consistent and operable affect them in different ways.

Personally I find a larger front rotor and slightly smaller rear suits me for that mixture of requirements, but that's just me...

Are you going to help us out then Scienceofficer?

Not right now, no. I'm a bit busy today, but it's all there on t'internet, most usefully in the Motorsport sphere.

EDIT

Actually, I found this I wrote 8 years ago elsewhere here. Remarkably the search function found it!

Posted 8 years ago REPORT
Scienceofficer

Member
I think TJ’s theory is supported by reading I’ve done on this topic where any (and there is little) research has been done – Predominantly in automotive circles, but I can’t see any fundamental differences between the processes in auto brakes and mtb brakes (other than disc material, which is really a kind of substrate for deposition of pad material in a ‘correctly working’ brake.

The idea is to cure the pads initially under pressure and heat to allow them to deposit a micons thick layer of pad material on the disc and allow the pads to mate fully with the disc surface. This facilitates adhesive friction between the two layers of pad material

After that, during continued use the pads need to get adequately hot to renew/topup the layer of pad material on the disc. Otherwise the layer of material on the disc gets removed and you’re relying on abraisive friction – generally a weaker friction force and much more erosive on materials (both disc and pads).

This is why bedded in pads can feel great to start with and get worse with use if they never get hot enough again.

Sound familiar? Rear disc brakes in the dead of winter any one? Pads disappearing in one ride because they’re not getting hot enough to get past the abraisive wear/friction stage?

The inference is that disc brakes need to be run hot hot enough to continue the pad material transfer process, but not so hot that they boil and fade.

This would suggest that many of us are running discs too large that don’t get hot enough, often enough after our initial efforts at bedding pads in.

Glazed pads (if they are actually glazed) are a function of this, but are effectivly being polished shiny due to the inadequate force and heat being put into them.

They’re either not getting hot enough for long enough, or are contaminated.

RE lightweight 180s vs more solid 160s. Brakes slow you down by turning your kinetic energy into heat, right? A bigger rotor means more material passing under the pad for a given speed, greater angular velocity, the bigger rotor brakes better. Does a very lightweight holed-out rotor convert friction to heat and dissipate it as fast as a more solid rotor? It may lack brake surface area but it should be able to cool fast if it has more holes and air flow.
What I don't know is how the variation due to rotor design compares to the basic difference between 160 and 180mm dia of the same design. I'd guess that dia makes the bigger difference from the rotors I've used. The lightweight ones didn't stay on the bike long though, daft place to save weight and many of them are badly designed/shaped - the brake surface area varies too much as it passes under the pads, sets up vibrations.

I was going to allude to what scienceofficer said up there.

Basically as long as it's getting up to temp from time to time, a larger brake will give more power (both in terms of sensitivity and heat dissipation). Other options are available for both (pad material and finned pads/ice tec rotors or even the Hope vented discs if they still do them) - and a "better" lever/caliper assembly with lower friction pivots/seals.

A new lever/caliper might give better performance, if it's an upgrade it may have lower friction bushings or bearings on the pivot points, or perhaps a different master cylinder to slave cylinder and brake lever leverage ratio. This can certainly make more difference than going up to a bigger rotor in some cases.

So I guess do what you like, the cheapo 4 pots with a set of finned pads would give more power in both senses, but a larger rotor equally would help.

If you aren't getting adequate power for general riding though, I'd suggest there might be contamination and perhaps your best bet would be to put new pads and either clean up the discs a lot or put new discs on too.

It's the trade off between a larger qty of material, greater thermal mass and having more surface area to radiate heat away (and of course saving some weight)...

In most cases it's probably marginal for heat build up until you are doing long duration, steep technical descents, requiring frequent, hard braking and giving minimal time to dissipate heat...
How often do people actually boil their brakes these days?

Anyone who ever used old shimano lx disc brakes knows the only way to get any sort of stopping power was to fit 200mm rotors.

Precisely. Like I was alluding to, rotor design is a trade off. Drill away too much material for venting and weight and you get a rotor that heats up to easily, modulates poorly and eats pads. Don’t vent it enough and the gases can’t escape and it doesn’t dump heat quickly enough into the environment.

There’s a reason why every major manufacturer has moved away from highly cut out wavey razer like discs back to good old round drilled discs, the former were shit.

You’re not getting not “More power” from a bigger rotor it’s simply providing mechanical advantage, assuming that all other factors are equal (i.e. same pads, same contact area same hydraulic system actuating) then for a given pressure at the lever you *should* see a greater effect at the wheel.

The "power" of a brake relates to how quickly it can dissipate kinetic energy as heat (the opposite of engine power, which is how fast it can convert chemical energy into kinetic energy). A larger rotor will give greater mechanical advantage, so for the same clamping force, it will dissipate heat more quickly. Obviously, this is limited by grip and stability - the point at which the brake is powerful enough to lock a wheel or to throw the rider over the bars defines the upper limit on its power. Obviously, a rider with strong hands can squeeze the lever harder than a rider with weak hands, but our wrists get tired very quickly on long descents, so larger rotors provide more braking power with less rider fatigue. Also, heat build up is a limiting factor on long descents. The brake has to dissipate the heat to the atmosphere. A larger rotor provides more metal to absorb the heat and a greater surface area to dissipate it. Hence they can sustain higher braking power for longer. So, yes, larger rotors do provide more braking power. This is why high performance cars use enormous brakes, but little shopping cars have tiny little brakes.

Or put another way, to achieve the same effect at the wheel with a smaller rotor you’ll have to put more energy into the braking system, i.e. pull harder on the lever to generate greater pressure on the pad and correspondingly more friction and heat at the rotor…

Energy = force x distance. A properly bled brake has a nearly solid lever feel, so the lever moves very little once the pads contact the rotor. What movement there is comes from compression of the fluid (especially if it hasn't been bled properly), and flex in the lever, hoses, and other parts. These basically act as a spring and absorb energy when you squeeze the lever, then return it when you release the lever. The more solid the lever feel, the less energy is absorbed. A theoretically limiting case of a system with zero flex would absorb zero energy (i.e. no springiness). Once you've actually clamped the brakes on and settled into a stable level of braking, your hands no longer contribute any energy to the system (because the lever has stopped moving). Thus, braking force is not really dependent on your hands contributing energy. The do have to exert force, but force and energy are two different things.

This would suggest that many of us are running discs too large that don’t get hot enough, often enough after our initial efforts at bedding pads in.

Glazed pads (if they are actually glazed) are a function of this, but are effectivly being polished shiny due to the inadequate force and heat being put into them.

They’re either not getting hot enough for long enough, or are contaminated.

I second this, especially on my commute where I rarely have to touch the brakes and the pads get glazed very easily despite me doing a proper bed-in every time.

Cheers guys, really appreciate your input. Overall then I’m going for 180’s - I think 🙃 I’ll repost when I’ve done it.