Why do bigger rotors stop you quicker? (physics)

nozzac

Gizmokev wrote:

So yes a larger disc should in theory give a larger braking force but as I have been trying to explain all along there are other factors in this which reduce the argument on the effectiveness of the larger disc increasing braking force, the 2 from above being temp and speed because as we have established the pressure will remain constant.

It's not "in theory". It does give a larger braking force as is self-evident to anyone who has changed rotors.

Please prove, with equations, that the coefficient of friction drops at such a rate as to totally negate the increase in mechanical advantage. The clamping force equation is pretty simple as you know: C= T/(r x u x n) where C is the clamping load, T is the brake torque, u the coeff of friction and n the number of braking surfaces. We can eliminate n here I think. So all you need do is show that u drops at the same or greater rate as r rises and you'll have proven your theory. Otherwise I think it would be wise to accept that the larger braking force is a reality.

supersonic

Infact, it seems Hayes reinforce my views if we are looking at those latter statements. For a given decelleration, you need need more lever effort with a smaller rotor to provide that torque, as you agree. More lever effort = more clamping force = lower coeff of friction.

Same rate of energy is converted for a given decelleration . So the smaller rotor heats up quicker as does not dissipate heat as quick = reduced coeff of friction.

Given this, you need to pull even harder with a smaller rotor than just the proportions in radius!

supersonic

Anyway, I am going to the pub lol. 160mm at each end, daren't use any more when drunk!

Anonymous

Gizmokev wrote:

yeehaamcgee wrote:

Gizmokev, now you've crossed the line into "being a prick" territory. Well done, good job.

FFS lighten up kid this is only a forum...surely when we left the playground at 16 we stopped name calling.

You reached for the insults with your "troll" remark. If you're trying to push the right buttons to get a reaction, then I'll give you a damned reaction.

Gizmokev

Ok in a static form a larger disc will give a larger torque...that is true.

Torque = F1 x r (where F1 is the frictional force and r the radius from the axis of rotation)

However what is in dispute and I have no reason as to why you are all fighting it so vehemently is the fact that CoF is not a constant value and that this has a huge impact on the braking power. CoF is determined and affected by so many variables that it is impossible to pin it down to one figure at any one particular time. I am sure that most people when changing up a rotor size see an increase in braking power for a given lever input but I am not so sure that it is down to the increase in radius from axis of rotation. I think it has more to do with the heat dissipation.

Looking at the equation above it can be illustrated that F1 = F2 x u (where F2 is the lever input and u is the CoF)

So Torque = F2 x u x r (remember F2 stays constant in our case)

CoF in not calculable so how am I meant to prove it with equations. It is obvious that CoF and r are directly related and one has a direct affect on the other as to whether or not they are inversely proportional I dont know and how much CoF is affected by tangential speed again I dont know but you cannot seriously just say that I am talking nonsense.

Gizmokev

yeehaamcgee wrote:

You reached for the insults with your "troll" remark. If you're trying to push the right buttons to get a reaction, then I'll give you a damned reaction.

Whats wrong didnt you like "My" reaction to your post????

Grow up and move on

cooldad

cooldad wrote:

Gizmokev wrote:

@ Cooldad

WTF....I still stand by what I have wrote re HHO. I chose to apologise for spamming my company in the thread. Now I suggest you move on down the bus.

And I still maintain it is garbage. 100% of the worlds automotive engineers agree with me.
If I'm wrong find a manufacturer who is using the 'technology' to give their vehicles massive and cheap fuel saving.

Now that SS has taken the banstick to the pub, I would still like an answer to this, seeing as the brake question has been well and truly mothered.

mossychops

NozzaC wrote:

Do you also go around denying that it's harder to open a door by pushing it near the hinge?!

The door/lever thing is kind of irrelevant in this thread. It is easier to open the door further from the hinge, but you have to push the door much further so the energy taken to open a door is the same.

If you open a door at the hinge you only have to move your hand a couple of inches, if you open it from the handle, you have to move your hand a few feet, this is why it is easier because you are spreading the force over a greater distance which is quite simply a lever.

Gizmokev

cooldad wrote:

And I still maintain it is garbage. 100% of the worlds automotive engineers agree with me.
If I'm wrong find a manufacturer who is using the 'technology' to give their vehicles massive and cheap fuel saving.

I would suggest you start another thread where it can be discussed.

bluechair84

When out to the pub, you should always find an attractive cocktail waitress who knows how to handle a lime, and has a large head. This way, it's easier to put it on backwards when leaving her in a ditch.

[/catcher response]

nozzac

mossychops wrote:

NozzaC wrote:

Do you also go around denying that it's harder to open a door by pushing it near the hinge?!

The door/lever thing is kind of irrelevant in this thread. It is easier to open the door further from the hinge, but you have to push the door much further so the energy taken to open a door is the same.

If you open a door at the hinge you only have to move your hand a couple of inches, if you open it from the handle, you have to move your hand a few feet, this is why it is easier because you are spreading the force over a greater distance which is quite simply a lever.

Of course it's relevant. it's an example of moment increasing as radius increases which is entirely pertinent to clamping force

nozzac

Gizmokev wrote:

Ok in a static form a larger disc will give a larger torque...that is true.

Torque = F1 x r (where F1 is the frictional force and r the radius from the axis of rotation)

However what is in dispute and I have no reason as to why you are all fighting it so vehemently is the fact that CoF is not a constant value and that this has a huge impact on the braking power. CoF is determined and affected by so many variables that it is impossible to pin it down to one figure at any one particular time. I am sure that most people when changing up a rotor size see an increase in braking power for a given lever input but I am not so sure that it is down to the increase in radius from axis of rotation. I think it has more to do with the heat dissipation.

Looking at the equation above it can be illustrated that F1 = F2 x u (where F2 is the lever input and u is the CoF)


So Torque = F2 x u x r (remember F2 stays constant in our case)

CoF in not calculable so how am I meant to prove it with equations. It is obvious that CoF and r are directly related and one has a direct affect on the other as to whether or not they are inversely proportional I dont know and how much CoF is affected by tangential speed again I dont know but you cannot seriously just say that I am talking nonsense.

No the equation is not designed only for a static disk. it is the one used by engineers in their design of braking systems.

You can't just get out of it by saying that something MIGHT affect CoF in some way but you dont know what that is or how exactly.. You're making the claim that goes against experience and what the engineers and brake manufacturers say, so you need to prove it. All sorts of things MIGHT be true. The CoF MIGHT move in the opposite direction for certain materials at temperatures for all you know. But where is the evidence for this too? Your unsubstantiated feelings are not enough.

You are saying you think it's more to do with braking dissipation than the proven increase in braking force so provide evidence to back that up. Give us the equations and numbers or failing that at least some concrete examples.

Heat build up isn't even going to come into sudden braking at low speeds since the rotor does not warm perceivably, yet this is noticeably better with a larger rotor with all other factors unchanged. How do you explain that if it is "more to do with the heat dissipation"

The Rookie

supersonic wrote:

It is relevant to kinetic energy being converted into heat though.

No its not, the calculation of friction is time based, the disc can be going through the pad as fast or slow as you like but friction and therefore energy converted to heat remains the same. Brakes don't loose efficiency as you go slower do they they keep slowing you at the same rate!

Simon

albo

Gizmokev wrote:

yeehaamcgee wrote:

You reached for the insults with your "troll" remark. If you're trying to push the right buttons to get a reaction, then I'll give you a damned reaction.

Whats wrong didnt you like "My" reaction to your post????

Grow up and move on

:shock: Don't f*ck with McGee... :shock:

supersonic

The Beginner wrote:

supersonic wrote:

It is relevant to kinetic energy being converted into heat though.

No its not, the calculation of friction is time based, the disc can be going through the pad as fast or slow as you like but friction and therefore energy converted to heat remains the same. Brakes don't loose efficiency as you go slower do they they keep slowing you at the same rate!

Simon

I'll rephrase that - it is relevant to the rate of energy change. Power. Larger rotor, more power for given pad force. Total energy change from v1 to v2 is the same. I am not saying the force from friction is changing, but the rate of energy change has to be higher for a larger rotor, same pad force f and therefore higher rate of speed loss.

Gwaredd

yeehaamcgee wrote:

Gwaredd, there is no reason to use the rear brake more than the front in motocross. Same principles apply.

Wow. Me, my mates & every other MX & supermoto rider I know must have been doing it wrong for years then!

That's the great thing about the internet, actual life experience of something counts for jack

FWIW, it works for me & I'm pretty quick down a hill using this method. If it upsets bike purists, all the better

shoddy

1st class thread this.
I know its theoretical but what about the increased unsprung mass of a larger rotor?

I'll get my coat......................

nozzac

shoddy wrote:

1st class thread this.
I know its theoretical but what about the increased unsprung mass of a larger rotor?

I'll get my coat......................

Yeah you're right. Thinking about it, the increase in rotational mass almost certainly explains the improved performance. Well that and heat loss of course

Anonymous

mossychops wrote:

The door/lever thing is kind of irrelevant in this thread. It is easier to open the door further from the hinge, but you have to push the door much further so the energy taken to open a door is the same.

Right, so the energy required is the same, but one makes it easier to expel the energy than the other. Like a car jack. Lifting a car requires a certain amount of energy. Whether you do it all in one go, or over a long length of time, using a hand jack, the theoretical energy usage (ignoring system losses due to inefficiency) is the same.

So a bike travelling at 30mph of a certain weight needs to loose a set amount of kinetic energy to come to a halt whether it's got 160 or 205mm discs. That kinetic energy needs to be turned into heat.
The larger rotors not only dissipate heat better, but they also generate the heat quicker in the first place.
Again, this is a demonstrative fact, that should not need any maths to highlight (to any reasonably sane or intelligent person). Sonic's already mentioned it with the string being pulled through your fingers.

We can safely handle ropes all day. In a day's climbing, for example, several hundred metres of rope may pass through our hands. And it's fine, no burning.
If you were to take that same length of rope, and pull it through our hands at some pace, then we'd get severe rope burns. Same friction. Same hands (brake pads) same rotor (rope) The faster the rope (rotor) goes, the more heat is generated.
Similarly, the faster the braking surface is travelling through the calliper, the more efficiently it turns kinetic energy into heat.

The Beginner wrote:

Brakes don't loose efficiency as you go slower do they they keep slowing you at the same rate!

At a lower speed, brakes do not need to turn as much kinetic energy into heat, since there is less kinetic energy to start with.

Gizmokev

NozzaC wrote:

You can't just get out of it by saying that something MIGHT affect CoF in some way but you dont know what that is or how exactly.

Have a look at the friction-temperature curves. I am not saying something MIGHT affect it....I am saying it DOES affect it.

NozzaC wrote:

You're making the claim that goes against experience and what the engineers and brake manufacturers say, so you need to prove it. All sorts of things MIGHT be true.

Really...then why do they all clearly state that CoF is affected by temperature and velocity. Remember that temp and velocity or momentum to be more precise are directly related to each other.

NozzaC wrote:

Heat build up isn't even going to come into sudden braking at low speeds since the rotor does not warm perceivably, yet this is noticeably better with a larger rotor with all other factors unchanged. How do you explain that if it is "more to do with the heat dissipation"

Heat build up does occur even at low speeds. That is how braking works...kinetic energy is turned into thermal energy. Less kinetic = less thermal transfer....more kinetic = more thermal transfer.

I will change my statement to this one though after having given it some thought. (I am not afraid to listen to others and change my hypothesis)

below a certain speed (which can be determined by plotting the friction-temp curve against the torque-radius line) the extra torque will provide better braking. However beyond that speed it is the increased cooling that gives the extra braking efficiency and not the torque alone.

rhext

Gizmokev wrote:

Have a look at the friction-temperature curves. I am not saying something MIGHT affect it....I am saying it DOES affect it.

Really...then why do they all clearly state that CoF is affected by temperature and velocity. Remember that temp and velocity or momentum to be more precise are directly related to each other.

Heat build up does occur even at low speeds. That is how braking works...kinetic energy is turned into thermal energy. Less kinetic = less thermal transfer....more kinetic = more thermal transfer.

I will change my statement to this one though after having given it some thought. (I am not afraid to listen to others and change my hypothesis)

below a certain speed (which can be determined by plotting the friction-temp curve against the torque-radius line) the extra torque will provide better braking. However beyond that speed it is the increased cooling that gives the extra braking efficiency and not the torque alone.

Geez, you know just enough to be dangerous: you talk like you understand this, but you don't seem to have the basic grounding.

Its really simple: bigger rotors = more torque = greater deceleration for a given input.

Yes, coefficient of friction may vary with temperature, but it's a second-order effect at most. You'd have to be a pretty crap brake designer to use pad materials which significantly changed their coefficient of friction as temperature increased. The reason brake performance changes dramatically with heat is much simpler than that: the brake fluid boils, and when it boils it becomes compressible so when you pull on the levers it dosnt transfer the force to the pads. And until that happens, you're stuck with bigger rotors = more torque = greater deceleration for a given input.

You seem fascinated with science, why don't you go on a course and get some basic principles and facts under your belt? This stuff is A level at best.

cooldad

It's no use. The words banging, head, brick wall come to mind.

lemoncurd

Get a record player.

Put it on at 33 rpm.

1. Stop the turntable rotating by placing your finger near the centre of the turntable, this is difficult.

2. Stop the turntable by placing your finger near the outer edge of the turntable, this is easy.

Therefore, Bigger Rotors == Better

cooldad

Record player? Some of these kiddies are to young to even remember CDs.
And it will only work until your finger gets so hot it bursts into flame. Unless you have really fat fingers that dissipate heat quicker.
I think.

lemoncurd

cooldad wrote:

Record player? Some of these kiddies are to young to even remember CDs.
And it will only work until your finger gets so hot it bursts into flame. Unless you have really fat fingers that dissipate heat quicker.
I think.

OK - Record players aren't that popular, but the only other example I can think of is disk brake rotors

Anonymous

lemoncurd wrote:

cooldad wrote:

Record player? Some of these kiddies are to young to even remember CDs.
And it will only work until your finger gets so hot it bursts into flame. Unless you have really fat fingers that dissipate heat quicker.
I think.

OK - Record players aren't that popular, but the only other example I can think of is disk brake rotors

Bike wheels and spokes? He claims to have already treid that though, which suggests he's either lying or living in a parallell universe.

nicklouse

you spin me round like a.....

topic locked.