Why do bigger rotors stop you quicker? (physics)

getonyourbike

I'm having a debate with someone I know about bigger rotors of MTBs. I'm saying that bigger rotors stop you quicker and it's all to do with leverage, while he says the only way to stop you quicker is use bigger pistons and a bigger surface area. He says the only advantage of bigger rotors is that heat is dispersed faster which is true but only one advantage of them. Oh, and he thinks that everyone on bike forum is a dimwit with no grasp of physics or a bike shop or bike company owner.

Can anyone explain the physics of why bigger rotors stop you quciker scientifically? I know that there are quite a few people on here with a firm graps of physics and with quite a few qualifications, so could you explain, please. He's doing my head in because he's so stubborn.

Ta muchly

supersonic

Your mate is wrong. Well, you can alter the master and slave pistons and leverage of the actual brake lever. But larger rotors have a larger sweep which converts energy into heat at a faster rate for a given lever effort. Think about it - a full rotor rotation of a larger rotor sees much more surface pass through the pads than a smaller rotor, and at a faster rate. But it takes the same time for one rotation of either.

Highest decelleration is usually governed by your tyre grip.

supersonic

To add to that, farting about with piston sizes too much can end up with odd lever feel. Sort of like when you use pre v brake levers with v brakes - loads of leverage, but very mushy feel.

Daz555

Get your mate to spin his wheel. Tell him to stop it by grabbing the tyre - easy. Now tell him to do it by grabbing the spokes 2" out from the rim......

Anonymous

Supersonic is correct, but Daz is also not wrong. For holding a wheel still, a larger disc does have a greater mechanical advantage than a smaller disc.

getonyourbike

Unfortunately, he's my dad and he can't admit to being wrong and being proven wrong by his son. :roll: I tried to show him this thread and all he said was to look on a science website WTF?! How would that help when all the informaton has already been passed before his eyes? It's even more stupid that he thinks you're stupid with no grasp of physics when you're all more highly qualified than him and know more about it. :roll:

I don't think there's anyway to get throguh to him, unfortunately. It's pissing me off

Anonymous

Don't let it pi* you off. Just know that you know something better than your dad.
My sister believes in all kinds of trippy hippy shite, so we just ignore her when she prattles on.

TuckerUK

You should point him in the direction of those car manufactures who fit larger discs but the same calipers to higher performance models in the range. I can think of two examples straight away, the old Mazda MX-5 NA and NB 1.8 brake upgrade from 1.6, and the rear discs on Ford Focus 2.0 vs. the larger disc ST170.

al2098

Saw this from another forum:
"im a highschooler in physics, and none of you could answer this... wow. its a matter of torque. the diameter of the rotor provides more neutons of torque than a smaller rotor. the formula is t=Rf
t=torque,R=radius,f=force applied. then you can get more complicated with all the angle of atack and lever arm crap, but screw that. lets say your *snicker* bb7's apply aprox 150 neutons of force. torque for a 160mm rotor would = 12,000 neutons. 185mm rotor = 13875 neutons, and a 203mm rotor = 15,225.
whatd u guys fail physics? jeez..."
His spelling is poor but his science is spot on.
larger rotor = more stopping power?

lemoncurd

The cassette is a perfect example of the mechanical advantage provided by larger rotors.

The bigger the sprocket, the greater the mechanical advantage, the easier it is to pedal.

nozzac

You might want to point out that the rotational kinetic energy of the wheel and rotor are only converted into heat during partial breaking. If you apply the brake very suddenly and strongly on a relatively slow moving wheel then the wheel simply stops with little or no heat being produced by friction. In this case the heat dispersal is not relevant at all. Will he then still claim that a smaller rotor is just as effective under these circumstances? If so ask him to demonstrate by stopping a wheel by pinching the disk with his fingers. Once you've remove the bits of skin, nail and blood, you can then demonstrate how easy it is if you instead pinch the rim.

al2098

NozzaC wrote:

You might want to point out that the rotational kinetic energy of the wheel and rotor are only converted into heat during partial breaking. .

And what is your point?
I thought the guy I quoted was waffling. This is another level..

supersonic

Which is why I said when the wheel locks... ultimate decelleration is governed by tyre grip. When the wheel locks, the tyre is providing the friction.

tri-sexual

show him this

www.hayesdiscbrake.com/hayes-u/rotor/

hobbitsharley

there is one other way to look at this - as most of the braking force is supplied at the TIRE - yes the tire if you could brake at the tire your stopping power would be immense.
Some motorcycle manufactuers have even applied this theory as best they can - Erik Buell being one of them with his ZTL (Zero Torsional Load) braking system which has proposionally more stopping power than a twin disc set up running 6 pots in each caliper and 320mm discs - the ZTL system uses a single 6pot caliper mounted normally on the fork but the biggy is the brakes disc its attached to the RIM and after owning and riding one of his bikes nothing and i mean nothing compares to the front brake on a Buell motorcycle - it has near perfect modulation and the progressive power is unreal you could out brake most vehicles and balance the bike on the front wheel from most speeds but the biggy is from flat out it has that much controllable power you do not flip the bike but can squeeze the front end into the tarmac
its all to do with torque and rotational mass the further the braking surface from the fulcrum the better the braking power
http://en.wikipedia.org/wiki/Buell_Motorcycle_Company

getonyourbike

Supersonic- For the purposes of this we're not getting into tyre grip even though it is as you say, ultimate decelleration is governed by tyre grip.

NozzaC- I've already told him that, like for trials riders. They don't really drag their brakes so aren't affected as much by brake fade. So why don't they use the tiniest little rotors to save weight? Nope, won't accept it.

Tried to explain to him the idea of the wheel near the hub and then the rim, won't accept it.

I think I've got to accept that there's no getting through to him, he's always right. :roll:

nozzac

al2098 wrote:

NozzaC wrote:

You might want to point out that the rotational kinetic energy of the wheel and rotor are only converted into heat during partial breaking. .

And what is your point?
I thought the guy I quoted was waffling. This is another level..

Just because you don't understand something doesn't make it waffle.

It's entirely pertinent to the OP. The guys dad is claiming that the large rotor's only advantage is heat dispersal. That can only matter if the braking is due to friction converting kinetic energy into heat.

Anonymous

This has all been sorted in the first four responses.

Anonymous

yeehaamcgee wrote:

This has all been sorted in the first four responses.

This is Bike Radar. This is no reason we can't keep going round in circles for another 12 pages. :roll:

Also, OP - Your not by any chance trying to persuade your Dad to buy you some bigger rotors are you? This could be his reason for failing to agree with you.

getonyourbike

RichardSwt wrote:

yeehaamcgee wrote:

This has all been sorted in the first four responses.

This is Bike Radar. This is no reason we can't keep going round in circles for another 12 pages. :roll:

Also, OP - Your not by any chance trying to persuade your Dad to buy you some bigger rotors are you? This could be his reason for failing to agree with you.

No, it's all my money going into the bike. He's just stubborn and 'can't' be proved wrong.

Jay dubbleU

Basic laws of physics state that energy cannot be lost or destroyed - what disc brakes do is to convert kinetic energy into heat energy - the larger the rotor the more efficient it is at dissipating heat.

Anonymous

O dear gods!

getonyourbike

Jay dubbleU wrote:

Basic laws of physics state that energy cannot be lost or destroyed - what disc brakes do is to convert kinetic energy into heat energy - the larger the rotor the more efficient it is at dissipating heat.

we already established that in the OP. I'm after evidence that proves that larger rotors also stop you more effectively. We already know from experience that larger rotors do stop you quicker (except my dad).

nicklouse

getonyourbike wrote:

I'm after evidence that proves that larger rotors also stop you more effectively.

define effectively.


lees hand force, as covered earlier, is an easy way to define it.

It should also be mentioned that when he says

while he says the only way to stop you quicker is use bigger pistons and a bigger surface area.

he is not wrong. But only if changing the rotor size is not possible. well he is partly right.

braking systems have various forces acting on them. the ratio between the pistons. the swept area and the leverage ratio from the center of the contact area to the center of rotation.

Anonymous

Ahhh, I had a dad like that when growing up, would never be proved or admit he was wrong. But, he had a degree in physics and was a quite high up in a oil company, so he hardly ever was wrong LOL.

getonyourbike

nicklouse wrote:

It should also be mentioned that when he says

while he says the only way to stop you quicker is use bigger pistons and a bigger surface area.

he is not wrong. But only if changing the rotor size is not possible. well he is partly right.

braking systems have various forces acting on them. the ratio between the pistons. the swept area and the leverage ratio from the center of the contact area to the center of rotation.

Yes, he is right about the larger surface area and bigger pistons but he refuses to admit that a larger rotor will give you more power.

By effectively, I mean quicker, so more power. I know the tyres are the limit of how fast you can stop so just forget aobut the tyres

leaflite

Ask him why many people run a smaller rear rotor. As both get a similar amount of use- and therefore give off a similar amount of heat-most people would use rotors of the same size if heat dissipation was the only reason for the larger size.

Anonymous

leaflite, you're actually making so little sense, that my toes hurt.

cooldad

Similar amount of use? I have same size rotors F & R and my rear pads last roughly twice as long as the front. And I'm sure that's fairly normal.

red040281

basic physics = bigger discs or brakes = quicker stopping thats why a ferraris disc brake is the size of a clios wheel

basic dads + dads opinionis = wright even if he is wrong lol

mossychops

My tuppence:

The size of the disc/surface area will not improve stopping efficiency- it can however stop locking up easily as the more heat dissipated away from the rotor and pad the higher the coefficient of friction (drilling holes in the rotor also increases the surface area)

The wheels only lock up under breaking once the coefficient of friction of the brake pads on brake disk exceed the tyres on the ground (ie if its easier for the wheels to skid than the brakes to turn you will lock the wheel). Higher pressure (squeeze harder or bigger pistons) and you will increase the friction, but once the heat is no longer to be able to dissapated your coefficient of friction goes up until it exceeds that of dragging the wheel across the ground (or throwing you over the handlebars which ever is easier). A bigger rotor will help with heat dissipation and is why racing cars have huge vented discs. similarly poor tyres or a wet road decrease the coefficient of friction between the tyre and the road so you have to "brake less" for the wheel to lock up than with good traction.

The size of the rotor will also count toward how fast the rotor moves (the larger the circumference the slower in cm/s the edge of the rotor will travel for the same RPM) but the amount of force required to stop will still be the same so the same heat generated so no advantage.

In short I think that bigger rotors do not improve braking efficiency, but they will let you brake harder before the wheel locks up.