Brake rotor size redux

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Brake rotor size redux

Postby redwagon » 20 Sep 2006, 12:33

This thread prompted me to forward a few ideas about evaluating motorcycle or scooter disk brakes. A lot of people automatically equate brake disk diameter with performance, but there are a few other factors that are equally, if not more relevant.

Skip to the Cliff Notes at the end if tech stuff bores you.

First up I’ll start by saying that brake performance should be measured in two ways, brake torque and heat capacity. The first refers to how much negative torque the brake system can apply to the wheel, in other words how much rotational force the brake can counteract. The second refers to how long the brake can keep absorbing rotational force before overheating, since what a brake does is to convert kinetic energy into heat energy. These two aspects of performance do not always go hand in hand.
You’ll notice that I don’t use any terms like ‘stopping power’. Tires stop vehicles, not brakes. Without the friction between the road and the tire the best brakes in the world would not slow you even 0.1kph.

There are five main factors which determine maximum brake torque:

1. The average radius of the brake disk.
2. The surface area of the brake pads.
3. The pressure applied to the brake pads.
4. The coefficient of friction between the pad material and the brake disk.
5. The percentage of the average brake disk diameter to the rolling radius of the tire.

Some of these parameters are fixed, some of them change with wear and temperature.

The average radius of the brake disk is the point at which the average friction force is applied to the wheel, measured from the axle center. In the same way that using a long breaker bar helps to loosen a stubborn nut, increasing this radius results in more brake torque without changing any other inputs. The downside is that for any given road speed the surface speed of the braking area past the pad will increase, and more heat will be generated.

The surface area of the brake pad is easy to understand, and it’s obvious that the larger the pad area the more friction would be generated, as long as the pressure stayed the same. In real life however it’s not easy to increase the pad area without increasing the size of the caliper along with it, and that brings other headaches. Of course one trick is to double up on the disks.

The pressure applied to the pads is (on a motorcycle without a brake booster anyway) limited by the ratio between the sizes of the pistons in the hydraulic cylinders at either end of the system and the levers used to apply pressure from the hand or foot to the master cylinder. It’s possible to increase pad pressure by fitting calipers with larger pistons or more pistons. In practice there are limits to line pressure that normal hydraulic seals and hoses can put up with, so ultimate pressure at the pad does have finite limits.

Motorcycle brake disks these days are usually made from stainless steel because owners don’t like looking at rusty bike parts. Cast iron disks are superior in many ways, but stainless is price of fashion. Of course race bikes may have plasma coated alloy disks or even ones made from carbon, but let’s stick to what’s streetable eh?
Most pads meant for street use are a compromise between friction, wear, heat range and dusting. The wear and dusting considerations are somewhat dominant in the compromise and most street pads have similar performance.

The relationship between the average disk radius and the rolling radius of the tire is very important and often overlooked. Applying the brake torque very close to the wheel rim is much more efficient than applying that force close to the hub, or center or rotation. For one, the higher surface speed makes it easier to develop friction without needing pads of extremely high friction coefficients or using excessively high line pressures.


CN:

So, when eyeing up the potential brake torque you should first consider how close in diameter the brake disk is to the wheel rim rather than it’s overall diameter. In other words, an 8” disk on a 10” wheel has better potential brake torque than a 9” disk on a 12” wheel, all other things being equal. The mitigating factor here is the tire contact patch. A 12” tire will have a much bigger contact patch through which it can apply all your brake torque to the road than a 10” tire.


The other side of brake performance is heat capacity. A brake is just a means of converting kinetic energy into heat energy via friction. More brake performance naturally means more heat. You may have seen tests on cars where they see how much distance it takes to stop from (say) 100kph once, and then on repeated stops afterwards. It’s a good measure of how much heat capacity the brakes have, how the stopping distance changes with repeated tests. On a motorcycle you usually only surpass the brake system’s heat capacity at the race track with repeated hard braking corner after corner, lap after lap. On a scooter however, having no engine braking to speak of, long mountain descents can be an issue. In this case you may want to buy the scooter with the biggest disk brake available so it staves off brake fade for as long as possible.
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Postby plasmatron » 20 Sep 2006, 12:52

:bravo: :bravo: :bravo:

I'm going to go back and find every technical post redwagon ever made, cut and paste them into book form, and publish it... I'm going to be rich dammit, rich!.... :yay:

hey... you guys better not copy my idea now... especially redwagon... I won't stand for intellectual copyright infringements... :no-no:
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Postby Lord Lucan » 20 Sep 2006, 13:49

What we need are rubber roads. That would certainly increase the coefficient of friction between tyre and road, which is the problem I usually face.
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Postby redwagon » 20 Sep 2006, 14:40

Lord Lucan wrote:What we need are rubber roads. That would certainly increase the coefficient of friction between tyre and road, which is the problem I usually face.

It would be much nicer when you fall off your bike... :ponder:
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Re: Brake rotor size redux

Postby Ducked » 20 Sep 2006, 22:38

redwagon wrote:

Motorcycle brake disks these days are usually made from stainless steel because owners don’t like looking at rusty bike parts. Cast iron disks are superior in many ways, but stainless is price of fashion.


UK MOT (Ministry of Transport Vehicle Safety Inspection) testers don't like looking at rusty disks either. My UK cars (dunno about bikes) have always passed their brake function test (rolling road type thing) but failed for "visible rust" every bloody time. That'll be new disks and pads, Sir. Kerchinnng!

Never heard a justification for this other than "because we say so".

Pissed me off so I took to keeping a "shiny" set for testing, and replacing the failed set after inspection. Hassle of course and only really worth it if you keep a car for a few years, which didn't happen with my bangers very often.

Taiwan inspection seems a good bit more casual/less anal. They don't poke about much at all, and AFAIK dont visually inspect anything. Fine by me, but the six-monthly interval is a pain.
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Re: Brake rotor size redux

Postby MJB » 20 Sep 2006, 23:11

redwagon wrote:This thread prompted me to forward a few ideas about evaluating motorcycle or scooter disk brakes. A lot of people automatically equate brake disk diameter with performance, but there are a few other factors that are equally, if not more relevant.

Skip to the Cliff Notes at the end if tech stuff bores you.

First up I’ll start by saying that brake performance should be measured in two ways, brake torque and heat capacity. The first refers to how much negative torque the brake system can apply to the wheel, in other words how much rotational force the brake can counteract. The second refers to how long the brake can keep absorbing rotational force before overheating, since what a brake does is to convert kinetic energy into heat energy. These two aspects of performance do not always go hand in hand.
You’ll notice that I don’t use any terms like ‘stopping power’. Tires stop vehicles, not brakes. Without the friction between the road and the tire the best brakes in the world would not slow you even 0.1kph.

There are five main factors which determine maximum brake torque:

1. The average radius of the brake disk.
2. The surface area of the brake pads.
3. The pressure applied to the brake pads.
4. The coefficient of friction between the pad material and the brake disk.
5. The percentage of the average brake disk diameter to the rolling radius of the tire.

Some of these parameters are fixed, some of them change with wear and temperature.

The average radius of the brake disk is the point at which the average friction force is applied to the wheel, measured from the axle center. In the same way that using a long breaker bar helps to loosen a stubborn nut, increasing this radius results in more brake torque without changing any other inputs. The downside is that for any given road speed the surface speed of the braking area past the pad will increase, and more heat will be generated.

The surface area of the brake pad is easy to understand, and it’s obvious that the larger the pad area the more friction would be generated, as long as the pressure stayed the same. In real life however it’s not easy to increase the pad area without increasing the size of the caliper along with it, and that brings other headaches. Of course one trick is to double up on the disks.

The pressure applied to the pads is (on a motorcycle without a brake booster anyway) limited by the ratio between the sizes of the pistons in the hydraulic cylinders at either end of the system and the levers used to apply pressure from the hand or foot to the master cylinder. It’s possible to increase pad pressure by fitting calipers with larger pistons or more pistons. In practice there are limits to line pressure that normal hydraulic seals and hoses can put up with, so ultimate pressure at the pad does have finite limits.

Motorcycle brake disks these days are usually made from stainless steel because owners don’t like looking at rusty bike parts. Cast iron disks are superior in many ways, but stainless is price of fashion. Of course race bikes may have plasma coated alloy disks or even ones made from carbon, but let’s stick to what’s streetable eh?
Most pads meant for street use are a compromise between friction, wear, heat range and dusting. The wear and dusting considerations are somewhat dominant in the compromise and most street pads have similar performance.

The relationship between the average disk radius and the rolling radius of the tire is very important and often overlooked. Applying the brake torque very close to the wheel rim is much more efficient than applying that force close to the hub, or center or rotation. For one, the higher surface speed makes it easier to develop friction without needing pads of extremely high friction coefficients or using excessively high line pressures.


CN:

So, when eyeing up the potential brake torque you should first consider how close in diameter the brake disk is to the wheel rim rather than it’s overall diameter. In other words, an 8” disk on a 10” wheel has better potential brake torque than a 9” disk on a 12” wheel, all other things being equal. The mitigating factor here is the tire contact patch. A 12” tire will have a much bigger contact patch through which it can apply all your brake torque to the road than a 10” tire.


The other side of brake performance is heat capacity. A brake is just a means of converting kinetic energy into heat energy via friction. More brake performance naturally means more heat. You may have seen tests on cars where they see how much distance it takes to stop from (say) 100kph once, and then on repeated stops afterwards. It’s a good measure of how much heat capacity the brakes have, how the stopping distance changes with repeated tests. On a motorcycle you usually only surpass the brake system’s heat capacity at the race track with repeated hard braking corner after corner, lap after lap. On a scooter however, having no engine braking to speak of, long mountain descents can be an issue. In this case you may want to buy the scooter with the biggest disk brake available so it staves off brake fade for as long as possible.


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Postby Bubba 2 Guns » 21 Sep 2006, 03:42

redwagon wrote:
Lord Lucan wrote:What we need are rubber roads. That would certainly increase the coefficient of friction between tyre and road, which is the problem I usually face.

It would be much nicer when you fall off your bike... :ponder:


They should make the cars out of rubber and blue trucks out of Mo AU JI and jello.

I've noticed a huge increase in the size of disc brakes on sport bikes in the past 5 years so you're saying the difference is made from the diameter of the brake disk to wheel rim and their ratio- so I need smaller rims and giant discs?

They use asbestos in brake pads don't they? Is this to absorb heat? Isn't there something nicer they could use to make pads? (I'm sure it ain't cheap)

Has anyone put in steel braided brake lines on their bikes and noticed a difference?
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Postby x08 » 21 Sep 2006, 07:59

braided lines supposedly offer a lot more feeling from your brakes.. but I've never had issues with standard rubber lines... so i see no point in me upgrading apart from the bling factor

redwagon... what you forgot to talk about is the wave rotors etc. that they use on bikes now.. I would presume that it would have a similar affect as slotted rotors Vs. standard rotors... not to mention the bilng bling baby
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Postby redwagon » 21 Sep 2006, 09:10

Bubba 2 Guns wrote:They should make the cars out of rubber and blue trucks out of Mo AU JI and jello.
:lol: You print the banners and I'll organize the bullhorns. :wink:
Bubba 2 Guns wrote:I've noticed a huge increase in the size of disc brakes on sport bikes in the past 5 years so you're saying the difference is made from the diameter of the brake disk to wheel rim and their ratio- so I need smaller rims and giant discs?
Well, need is a big word. Remember that a lot of equipment that gets fitted to sportbikes may only be useful or necessary on the track. If you're not racing the bike you could probably get away with less brake than you have now. A lot of it is for show.
But, yes, you have theory right. Increase in diameter (alone) gets an increase in brake torque. Keeping the same disk diameter and reducing the rim diameter also gets you an increase in brake torque. On the flip side, increasing the diameter of anything rotating increases it's mass moment of inertia even if it's mass doesn't increase. More gyroscopic effect, more inertia to overcome either accelerating or decelerating...
Have you ever seen an inverted disk setup, with the disk attached to the rim and the caliper being mounted from the inside? It's expensive, heavy, problematic in certain ways and it transfers all the brake heat into the tire, but it's probably the wave of the future for sportbikes.
Bubba 2 Guns wrote:They use asbestos in brake pads don't they? Is this to absorb heat? Isn't there something nicer they could use to make pads? (I'm sure it ain't cheap)
They haven't used asbestos in brake or clutch linings in many years. They stopped for safety reasons but in the search for replacement materials found far better ones. Carbon, sintered metal, ceramics, kevlar and so on all offer better performance than asbestos and avoid the health problems. Modern brake materials work totally differently from asbestos. New pads transfer material from the pad to the disk and the friction between the two similar surfaces is far higher than between pad and metal. The drawback is increased dusting, the aggressive nature of the dust itself and the tendency for very high performance materials to transfer material in an uncontrolled or uneven fashion if used improperly. For example, a pure race pad will create dust that sticks to the rims like crazy and eats into the surface. If you brake to a standstill with them while at high temperature like after a canyon descent or hot lap, they will transfer so much material to the disk you will think your disk is warped when you take off again. Hence the cool-off laps at the track. Pretty hard to be this disciplined on the street, which is why you see the warnings again and again not to use race pads on the street. I bet that you see a lot of this kind of complaint on sportbike forums: "I put brand x race pads in my bike and now my rotors are warped!"

Many high temperature pads will hardly work at all when cold, so when some clod brake checks you on the freeway you'll ram him before your brakes start working... which is one of the reasons they have warm up laps at races. The worst is with graphite disks (pretty rare) that don't work at all until warm. :eek:
Myself, I like ceramic pads but everyone has their favorites.

The change in pad material is also why you just don't see cast iron disks anymore. You used to see those in real performance brakes (think older Brembo) when the reliance was on pad to metal friction. Now they are relying on friction between the pad and a disk coated with pad material, stainless works fine if the surface quality is at about Ra0.8u (or whatever is enough to allow the pad material to adhere to), even when wet. Of course, cast iron still rejects heat better than stainless, but it's less of an issue on a bike. On a car it's more critical so they stick with cast iron outside of the real exotica.
Bubba 2 Guns wrote:Has anyone put in steel braided brake lines on their bikes and noticed a difference?
I have a stainless line on my enduro bike because I couldn't get a stock rubber hose. Unless the stocker was worn out I don't think you'll notice a difference. The nice thing is that you can get a stainless hose made almost anywhere in this country for less cost than a factory one, especially if the bike's an import.

x08 wrote:redwagon... what you forgot to talk about is the wave rotors etc. that they use on bikes now.. I would presume that it would have a similar affect as slotted rotors Vs. standard rotors... not to mention the bilng bling baby.

:lol: Forgot? My fingers are still sore from all the crap I typed yesterday. What do you want, blood? :wink:

Wave rotors are (IMO) a carryover from the mountain bike world. Weight is everything to a cyclist so they use every trick in the book to reduce weight and mass moment of inertia. Drilling or slotting disks:

1. Is pretty.
2. Saves weight.
3. Increases surface area and thus cooling (by a tiny amount).
4. Allows somewhere for water to go on initial brake application.
5. Weakens the structural integrity of the disk.
6. Increases pad wear by shaving.
7. Makes noises some will feel is cool.
8. Is pretty.

Did I mention drilling also looks pretty? :wink:

You never see drilled or slotted disks on race cars because they get so hot the stress risers at holes or slots allows cracks to form as the disks expand and contract. Motorcycle brakes rarely get quite that hot so it's less of an issue. The wave design saves weight and helps with water dispersal at the expense of reducing slightly the surface area. It also makes less noise than drilling or slotting. Mind you, with modern pad materials water is not a problem, though it's really useful in muddy or very gritty conditions. That's why MX bikes will have almost as much slot as surface area. IMO, wave/slot disks are used because they look cool and they save weight. Other concerns are secondary or imaginary. :)

Want to talk about floating vs. solid disks next? :lol:
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Postby Joesox » 21 Sep 2006, 18:31

Bubba 2 Guns wrote:Has anyone put in steel braided brake lines on their bikes and noticed a difference?
Yes. I noticed big differences. And I'm not the only one. However, this was on little bikes and scooters, so I'm not sure whether the differences were simply due to the poor quality of the stock brakelines.
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