Brake Rotor Size

thefartingcockeral

HI, can anyone explain why it is the preferred method to put a larger disc rotor on the front of an MTB IE 180/160 F+R.
Cheers

dhxcme

9" F / 8" R

or

8" / 8"

thefartingcockeral

Does it serve any purpose to put a larger rotor up front?

Chris`I

Its much easier to lock the rear wheels up than the front. Therefore you can put more braking force up front over the rear and to achieve this you run a larger disc.

Anonymous

Front brake is the one that really slows you down, the back brake basically just locks up the rear.
So, bigger brakes on front, smaller at the back.

x-isle

You use your front brake! :shock:

Anonymous

Yes.

Splasher

It's actually got nothing to do with brake force since a decent brake will lock either wheel with a 6" rotor. It's to do with conservation of energy and as eluded to above, your front brake can convert more kinetic energy to heat (the job of a brake) than the rear. That heat needs to be dissipated and the larger the surface area, the faster the dissipation. Failure to dissipate the heat can lead to overheating and brake fade.

thefartingcockeral

Thanks for the advice. makes sense

Anonymous

Actually Splasher, a larger brake disc does increase the mechanical force applied by the brake, as it increases the amount of leverage acting to counteract the rotation.

Splasher

yeehaamcgee wrote:

Actually Splasher, a larger brake disc does increase the mechanical force applied by the brake, as it increases the amount of leverage acting to counteract the rotation.

I didn't say it doesn't increase the brake force, I said that's not the reason for doing it. Don't make me give you another engineering lecture!

Anonymous

Fair enough.

CraigXXL

As Splasher said the; the larger the disc quicker it can dissipate the heat. As you brake your momentum sends your weight forward so that the front brake is working harder creating more heat. The rear on the other hand has gone light hence why you are able to lock the rear with ease with the disc clamped between the pads producing no more heat so a larger disc not required there.

Some riders use there brakes to not only control speed but their balance too at speed this creates massive heat hence the need for large discs to dissipate the heat before too much heat builds up and brake fade starts.

dhxcme

Splasher, what Yeehaamcgee said about increased breaking force is true also. The reason for increased size is dual purpose with better heat disapation and more leaverage when it comes to stopping and realistically the more power is the greater factor. To increase heat disapation they would make the rotors solid with masses of holes to increase surface area.

Splasher

dhxcme wrote:

Splasher, what Yeehaamcgee said about increased breaking force is true also. The reason for increased size is dual purpose with better heat disapation and more leaverage when it comes to stopping and realistically the more power is the greater factor. To increase heat disapation they would make the rotors solid with masses of holes to increase surface area.

For the second and last time, I didn't say larger discs can't produce larger braking forces of course they can. What I said was that was not the over-riding reason to fit larger rotors since a 6" rotor can produce more braking force than a tyre can handle. The maximum force you can ever use at the brake is the limit of adhesion regardless of the rotor size. What is more important is that on a big descent, as fast as your brakes are turning kinetic energy into heat, your bike is turning potential energy into kinetic. And you brakes get hotter and hotter.

So when you say, "more power is the greater factor", do you know what the power of a brake is? The reason I ask is that you seem to think it is brake force. How fast your rotor gets hot is in fact the power of your brake? In other words how fast it can turn kinetic energy into heat energy.

As for holes, what do you think has a greater surface area, the inside of the hole or the piece of metal you removed which had surface area on both sides? Drilled discs are for weight reduction and to help clear water. Trains have massive discs, but no holes.

So you all understand that larger rotors make for more powerful brakes. But since you confuse power and force, you think the reason for fitting more powerful brakes is to increase brake force. This it can do of course, but the extra force isn't useable since it is beyond the adhesion limit, and hence the real reason for fitting more powerful brakes is in fact it because they handle more power!

The more useful side effect of the increased brake force, is not to create more brake force at the wheel since this is limited by adhesion anyway. But rather it is to reduce the master-cylinder/caliper force ratio. In other words you get the the same brake force with less finger effort.

dhxcme

The limit of adhesion is fixed but by moving if away from the centre of rotation the you require less force to stop and so generate less heat.

Trains don't use disk brakes the use rim brakes and so have a large diameter to work off.

Bigger disc less braking force to generate equal stopping force.

supersonic

Splasher wrote:

The more useful side effect of the increased brake force, is not to create more brake force at the wheel since this is limited by adhesion anyway. But rather it is to reduce the master-cylinder/caliper force ratio. In other words you get the the same brake force with less finger effort.

This generally is the reason why trail riders up from 160 to 180 or so - not because they are frying the brakes with 160!

Splasher

dhxcme wrote:

The limit of adhesion is fixed but by moving if away from the centre of rotation the you require less force to stop and so generate less heat.

Trains don't use disk brakes the use rim brakes and so have a large diameter to work off.

Bigger disc less braking force to generate equal stopping force.

Look, no disrespect, but you really don't understand. You've confused force, energy and power and now you're confusing heat and temperature. To bring a mass at a given speed to a standstill, you have to generate exactly the same amount of heat - equal to half MV squared. A small rotor has a smaller heat capacity and a smaller radiating surface and so that fixed amount of heat results in a high temperature. A big rotor has a higher heat capacity and a larger radiating area and so that same amount of heat results in a lower temperature.

As for trains, trust me, every train built in the last 20 year has disc brakes. Tread brakes are a thing of the past. Eurostar has 4 discs per axle and sintered pads to make sure it can stop from 186mph in a minute. A fully laden set weighs just over 800t and so the brake power is 94 million Watts or 1 1/2 million lightbulbs.

dhxcme

You also have to look at levers as well. If you have a 4" radi then you get 25% more leverage than a 3" radi. A larger disc gives more leverage which results in a need for less friction to be generated by the pads to stop a mass traveling with the same velocity.

Hence why you can undo tighter bolts with a ratchet and scaff tube than with a scaff tube alone.

Splasher

You are correct that in the case of a larger rotor, the friction force required to create the same deceleration is less. What you are not recognising is that the swept area of the rotor is travelling through the pads at a correspondingly faster linear speed as a result of doing the same rotational speed but having a larger circumference. Hence exactly the same amount of kinetic energy is converted into exactly the same amount of heat whether with a 160, 180 or 203 rotor.

I'm trying to concvince you of a scientific principle known as Conservation of Energy. Trust me when I say it is a fact and not something that can be argued with.

dhxcme

Bigger discs = more stopping power. The simple reason why people go big. Another fact that can't be argued with.

supersonic

dhxcme wrote:

Bigger discs = more stopping power. The simple reason why people go big. Another fact that can't be argued with.

MTBers relate 'power' of the brake to the effort required at the lever to produce a set decceleration. Under these terms, a larger rotor does give the sensation of more 'power'. But in physics terms, the energy displaced for a given decceleration is the same. (power = energy/time).

Anonymous

am I correct in stating that for a larger disc, Less brake pad pressure is required to produce the same heat, because not only does it have a mechanical advantage of a larger leverage, but the linear velocity of the disc realative to the pad is also higher?

I'm not disagreeing with you splasher, you just explain things in the most complex way possible. You seem to start at the complex end, then work your way towards first principles :shock:

Splasher

yeehaamcgee wrote:

am I correct in stating that for a larger disc, Less brake pad pressure is required to produce the same heat, because not only does it have a mechanical advantage of a larger leverage, but the linear velocity of the disc realative to the pad is also higher?

I'm not disagreeing with you splasher, you just explain things in the most complex way possible. You seem to start at the complex end, then work your way towards first principles :shock:

Here's an attempt to simplify what I've said:

first post: that the engineering imperative for rotor size is one of heat management, not brake force as most people had stated.

second post: that I didn't say it doesn't increase the brake force, simply that that is not the imperative.

third post: that I didn't say it doesn't increase the brake force, simply that that is not the imperative. Given that the second post was obviously not clear, I went on to explain why this was the case and why holes create less surface area, not more. I also added, for that larger rotors also help reduce finger force.

fourth post: I had to explain the difference between temperature and heat and correct an assertion that trains don't have disc brakes.

fifth post: I tried to explain why the perceived reduced friction in a larger rotor still ends up producing the same amount ouf heat - because the rotor linear velocity is greater.

Splasher

dhxcme wrote:

Bigger discs = more stopping power. The simple reason why people go big. Another fact that can't be argued with.

And as you now know, "power" is the ability of the brake to cope with heat.

Anonymous

Splasher, I still think that in terms of a mountain bike brake, the heat dissipation is a secondary factor when specifying a larger rotor.

Very rarely will a rider bring a 160mm rotor to the point where excessive heat build up will cause brake fade. However, that rider could still benefit from a larger rotor, simply because the larger size makes it more effective.

Splasher

It depends on what the OP meant in the question by "the preferred method".

When you specify your own bike, you know exactly where you're going to ride, how much you'll be on the brakes and for roughly how long, how strong your grip is etc etc. and you make a judgement based on those factors. Most riders in the UK don't ever get decents long enough for brake fade to be an issue because of the size of our hills and so lever force is a major consideration.

However, when a manufacturer specifies a bike with a larger rotor (like Giant, Specialized etc) it is because they sell the bikes all over the world and so for safety reasons, they have to be able to cope with riding the kind of terrain they were designed for without endangering the rider. For example, it is perfectly reasonable to expect someone buying a Reign in France to ride Super Morzine and if the manufacturers specced 160 rotors, they would end up in litigation.

Equally, they could just play safe and put 203 rotors on but the first thing the magazines compare in a test is weight. This is also why a lot of OE tyres are just not up to the job - flimsy sidewalls take a pound off the weight of a bike.

Sorry if the explanations were over technical, but I used to work as a brakes engineer.

soptom

No wonder I stopped posting in these forums, they've gone from a place to learn and have a laugh doing so, to a place full of pedantic know-it-alls who seem desperate to show off their knowledge in a way that belittles others.
The simple answer to why there are bigger discs on the front is because they work best and have similar lever feel that way, try it the other way round and you'll just be locking your rear and white knuckling your front. :roll:
Unless of course you wanted to know how many millions of watts was needed to stop the Eurostar, in which case, either see above, or get a life.

Anonymous

Soptom, I agree. As well educated as Splasher obviously is, he does seem a little misguided at times .

Splasher

soptom wrote:

No wonder I stopped posting in these forums, they've gone from a place to learn and have a laugh doing so, to a place full of pedantic know-it-alls who seem desperate to show off their knowledge in a way that belittles others.
The simple answer to why there are bigger discs on the front is because they work best and have similar lever feel that way, try it the other way round and you'll just be locking your rear and white knuckling your front. :roll:
Unless of course you wanted to know how many millions of watts was needed to stop the Eurostar, in which case, either see above, or get a life.

WTF.