Grooves in discs (rotors)

[phoenix]

Originally Posted by falcemob

phoenix, are you certain that Porsche discs are completely trouble free?

No, I am not at all sure they are trouble free; however, I am sure this would not stop Porsche using them if they had evidence there was a performance advantage, which it might be worth paying for by the cost of having to replace rotors more often.

I guess I will have to ask Porsche!

[phoenix]

Originally Posted by AU N EGL

The do not put them on their race cars.

I am pretty certain they do.

[AU N EGL]

Originally Posted by phoenix

Is that speculation or fact?

Fact. Not everything is done for engineering purposes. Marketing and sales has a lot to do with HOW a car is equipped.

[phoenix]

Originally Posted by Notso Swift

Are there Ceramic disks on the road based GT2 and GT3?
I know Ceramic are fitted to some models

I believe they are an option.

[KAM]

when you are ultimately trying to make a car as light as possible drilling may be a valid option if the risk of a new disk cracking is minimal. If you are changing everything between races then theres only an advantage to be had..

[AU N EGL]

Originally Posted by KAM Racing

when you are ultimately trying to make a car as light as possible drilling may be a valid option if the risk of a new disk cracking is minimal. If you are changing everything between races then theres only an advantage to be had..

Maybe but not likely.

Most classes have minium wts. Yes removing wt from some areas, especially rotational wt and putting the balast near the CoG would help.

When we look at the mathmaics of braking, holes in rotors hinder braking more then they help. What is the friction co-effecnt ( u ) of a hole? ZERO
Outgassing no longer happens with the organic brake pads. Heat disipation comes out the top of the rotors. Holes only cause very uneven heating and cooling of the rotor thus causing cracks 3x faster then a solid sided rotor.

Slots, groves or j-hooks help clean pads of debris and glazing from overheating ( poor brake technique) Yet still are starting points for cracks.

Then what is your budget? can a team or indvidual. afford to replace rotors every race? That is up to the individual, Most can not.

[GordonG]

How is brake overheating down to poor braking technique? in simple terms, the temperature of the brakes is a result of the energy to be dissipated over the time of the brake application. The harder you brake (in simple terms) the hotter they get, right?

If you brake too hard (poor technique) the brakes run cooler (the tyres do the energy dissipation and go square). If you brake too early/gently (poor technique) they run cooler as the energy is being converted over a longer time, during which the disk et al radiates some of the energy, reducing peak temp. The only poor technique I can think of that would increase temps is riding the brake with your left foot.

Am I wrong?

G

[Dave Brand]

Originally Posted by AU N EGL

When we look at the mathmaics of braking, holes in rotors hinder braking more then they help. What is the friction co-effecnt ( u ) of a hole? ZERO

Show me a formula for braking effort which takes surface area into account.

Outgassing no longer happens with the organic brake pads.

Same resins, same propensity to form gases. Maybe you meant inorganic pads, e.g. sintered?

[AU N EGL]

Originally Posted by Dave Brand

Show me a formula for braking effort which takes surface area into account.

Try this little calculator that lets you know the stress concentration factors of a plate with holes. Put in the radius of the hole, the distance to the hole from another hole or edge and see how much faster the holes will make your rotors fail. We all know all rotors will fail, it is a matter of when and this will give you a factor of 2-4x less life before cracking. BTW, slots do a similar concentration factor. This is especially true in a brittle material like cast iron. Use the case of an infinite plate with 2 circular holes. For the case of 2mm radius holes 25mm apart, the stress is 2.91 times the "usual" rotor stress. Or to put it another way, you willl crack 2.9 times sooner.
http://www.fatiguecalculator.com/finders/findkt.htm

Same resins, same propensity to form gases. Maybe you meant inorganic pads, e.g. sintered?

Yes InOrgainic - still no outgassing

No need for holes in rotors.

GordonG How is brake overheating down to poor braking technique? in simple terms, the temperature of the brakes is a result of the energy to be dissipated over the time of the brake application. The harder you brake (in simple terms) the hotter they get, right?

You are correct

If you brake too hard (poor technique) the brakes run cooler (the tyres do the energy dissipation and go square). If you brake too early/gently (poor technique) they run cooler as the energy is being converted over a longer time, during which the disk et al radiates some of the energy, reducing peak temp. The only poor technique I can think of that would increase temps is riding the brake with your left foot.

Am I wrong?

Too much, too late, too often will over heat the brake pads and rotors, if you dont lock up your tires. Going consistanlty too deep into braking zones. This is where, as you know, the choice of brake pad and its different temperature.

Unforutnilty many ppl as you have mentioned brake too soon and too little and the other I can think of, drivers braking where no braking is needed, but a slight lift of the throttle will do.

Some ppl will continue to use rotors with holes in, drilled, milled or cast in, and that is their choice, or there brake sponsors choice. Some of us found that solid side rotors better for braking and last longer.

I dont have a brake sponsor and I buy my own rotors.

Cheers

[phoenix]

So grooved discs are a no-no too.

I am so glad I spoke to Porsche. Longevity of rotor life is not expected from grooving or cross drilling of discs, but improved braking (and therefore better lap times) are.

So, if you want 35,000 miles out of your discs, make sure they are not machined in any way. If you want quicker lap times, machining becomes either a budget consideration or you make your discs out of material and with dimensions that are able to cope with the increased stress. Or better still, a combination of both!

p.s. anyone interested in the reason Porsche still use cross drilled discs can PM me - this open forum argument stuff is not for the faint hearted like me.

[phoenix]

Originally Posted by GordonG

If you brake too hard (poor technique)

What is 'too hard'?

Braking too hard is when you exceed the grip of the tires - i.e. one or more (loaded) wheels lock.

When a loaded tyre is close to the point of losing grip, but hasn't, is the most effective use of the grip available - just as in cornering (ignoring slip angles for the smart bottoms around here).

So the brakes should be able to create the level of retardation (a function of friction and time) that will match the tyre grip, plus a bit, regularly, every corner, for one race/one season/a lifetime (budget dependent) WITHOUT exceeding the temperature (or heat cycles) a) at which the pad loses it's effectiveness, or becomes irreversibly damaged b) that will damage the disc.

The killer of brakes is heat above the design parameters of the braking system - of what the pad and disc materials will tolerate, whatever their surface area, construction or design.

For those prepared to think a bit, here is an brain exercise:

If you take a disc of metal 5 mm thick and 20 mm in diameter and drill a hole in it, 6mm in diameter, what property or propeties of the disc will change, and by how much?

Now change the disk to 28 mm thick. What property or properties of the disc are changed, and by how much, and can you see a benefit in either drilling or not in drilling the hole? What is/are the benefit(s)?

Answers on a postcard to: Porsche AG, Stuttgart, Germany. (I can't promise a prize for the winning answer)

A clue: if the disc of metal is only 3mm thick there is no benefit.......

[phoenix]

Originally Posted by AU N EGL

Try this little calculator that lets you know the stress concentration factors of a plate with holes. Put in the radius of the hole, the distance to the hole from another hole or edge and see how much faster the holes will make your rotors fail. We all know all rotors will fail, it is a matter of when and this will give you a factor of 2-4x less life before cracking. BTW, slots do a similar concentration factor. This is especially true in a brittle material like cast iron. Use the case of an infinite plate with 2 circular holes. For the case of 2mm radius holes 25mm apart, the stress is 2.91 times the "usual" rotor stress. Or to put it another way, you willl crack 2.9 times sooner.

Did you notice, if you play with this calculator thing, that if you make the 2mm radius holes bigger, the stress gets lower? Stress is reduced to 2.82 from 2.92 when you go to 5mm radius holes. Weird, huh?

Bye the way, are our rotors in parallel or perpendicular tension - and is it one of these rather than the other that makes the cracks appear?

[Dave Brand]

Originally Posted by AU N EGL

Try this little calculator that lets you know the stress concentration factors of a plate with holes. Put in the radius of the hole, the distance to the hole from another hole or edge and see how much faster the holes will make your rotors fail. We all know all rotors will fail, it is a matter of when and this will give you a factor of 2-4x less life before cracking. BTW, slots do a similar concentration factor. This is especially true in a brittle material like cast iron. Use the case of an infinite plate with 2 circular holes. For the case of 2mm radius holes 25mm apart, the stress is 2.91 times the "usual" rotor stress. Or to put it another way, you willl crack 2.9 times sooner.
http://www.fatiguecalculator.com/finders/findkt.htm

We all know all rotors will fail? BS!

Now answer my question:

Show me a formula for braking effort which takes surface area into account.

[boyracer]

Best explantion I've heard is 'Transient Molecular Adhesion' at least it sounds techy

Why J-Hooks in a disc, what's the advantage over convential slots/cross drillings ? It's a more complex profile to machine (though I guess CAD?CAM?CNC probably makes that argument null and void), so there's got to be something in it.

[dtype38]

Originally Posted by Dave Brand

Now answer my question:

Show me a formula for braking effort which takes surface area into account.

Sorry if you've posted this already, but from the tone of your question it sound like you are pretty convinced that "braking effort" is completely unrelated to "surface area".

Just so as the less knowledgeable of us can follow this properly, could you just confirm your definition of "braking effort" and "surface area". I ask because my brake supplier seems convinced that my pad area has a lot to do with how quickly I'll be able to stop. The thing is, that could be for all sorts of reasons not directly related to any formula for force vs area, such as thermal and wear considerations.

Thanks.

[dtype38]

Originally Posted by AU N EGL

Quote:
GordonG How is brake overheating down to poor braking technique? in simple terms, the temperature of the brakes is a result of the energy to be dissipated over the time of the brake application. The harder you brake (in simple terms) the hotter they get, right?

You are correct

I know I'm going to get into a shedfull of trouble for this, but from personal experience, both of these are wrong.

I spent nearly a year trying to stop the brakes on a big Jaguar from overheating when I was racing. I tried all sorts of high temperature pads and fluids and various ducting and nothing helped... until one day I read that braking too gently could be the cause. So I went out and instead of gently progressing the brakes on, modulating them over bumps and and surface changes, then rolling gently off them, I tried nailing them as hard and short as I could without locking the tyres. I then had all sorts of suspension instability to deal with, but one thing was for sure, the short hard braking completely sorted my pad/fluid/disc overheating problems. I then even managed to change to a softer pad and get better feel and still didn't overheat them.

I belive the explanation for all of this is quite simple. Time may not be a factor in the equations which relate the total energy disappated to the total retardation, but it is certainly a big factor when it comes to transmitting heat through materials, and for disappating heat from surfaces. Put simply, consider these two things... First, the more of a lap you spend on the brakes, the less of it is spent cooling the discs down, and the latter is one of the main controlling factors in how hot the brakes run. Secondly, I'm pretty sure (please don't make me go get my old thermodynamic books out on this) that exposing the surface of an object to a very high temperature for a very short time transmits less heat into its interior that a lower temperature for a longer time. So short hard braking doesn't heat the interior of the calipers and discs up as much as longer braking.

Ducks and waits for verbal lashing

[Notso Swift]

Well dtype, you have put your left one out there with that post Kudos to you for asking the question, and then backing it up with your personal experiences.
I have no idea about the answers, by the way, but I hope you don’t get flamed because I would like more of this type of constructive, practical input into any of our Tech discussions.
Personally I know that I slam the brakes on as hard as I can when I am driving, I then often find that I am backing off during the process to modulate the brakes as you slow down, though I am sure that part of this is due to my compounds. Generally, I work on the basis that you are either trying to go faster or slower, there are very few times where you want to be in between.

[asym]

Usually only lurking, but thought I'd comment on dtype's experience (not flaming, just discussing!): Braking energy is absorbed by track surface, tyre, rotor, and pad. By braking harder you are probably shifting energy absorbtion to the tyre. E.g. because it is deflecting more. I don't think time is such a big factor, if it was it would be the other way round.

[tristancliffe]

Originally Posted by Dave Brand

We all know all rotors will fail? BS!

Now answer my question:

Show me a formula for braking effort which takes surface area into account.

F = mu.R

Where F is the friction force
mu is the coefficient of friction between the two mediums and objects
R is the normal force acting on that contact patch

You might say that I haven't answered your questions of "Show me a formula for braking effort which takes surface area into account.", but consider that in the real world, mu varies with heat, surface area, surface roughness etc (either averaged over the area of contact, or even more accurately considered in small areas within that area).

It's far too complex a science to have 'an equation' given to it (just like there isn't an equation for "how much fuel does it take to get from England to America), so you have to give some thought to at least as many things as possible.

It's the reason wide tyres give more grip than narrow tyres (generally speaking).

[phoenix]

Originally Posted by tristancliffe

It's the reason wide tyres give more grip than narrow tyres (generally speaking).

No, it isn't. Wider tyres do not give more grip because of more tyre area in contact with the road. In the equation F=mu.R R decreases linearly as the contact patch of the tyre increases, so F remains the same as long a mu is constant.

[Dave Brand]

Originally Posted by dtype38

Just so as the less knowledgeable of us can follow this properly, could you just confirm your definition of "braking effort" and "surface area". I ask because my brake supplier seems convinced that my pad area has a lot to do with how quickly I'll be able to stop. The thing is, that could be for all sorts of reasons not directly related to any formula for force vs area, such as thermal and wear considerations.

When you press the brake pedal you apply a force to the brake pads. The resultant force, the force that produces the braking effort on the disc, is that force multiplied by the coefficient of friction. Pad area has no significant real-world effect on the braking force.

Reduced to basics, a brake is a device to convert kinetic energy into heat. Heat is generated at the pad-disc interface. If the pad is very small the heat will be concentrated into a very small area, thus for a given rate of energy dissipation the temperarture rise will be much higher than it woould be for a large pad.

Pad wear, measured in volume of material per unit of energy dissipated, is fairly constant. It follows, therefore, that the bigger the pad area the less the reduction in thickness & therefore the longer the life of the pad.

A lot of road car pads have large chamfers to reduce the tendency to squeal. As a result of this, the effective pad area can vary by probably as much as 15% over the life of the pad, with no discernable variation in braking effort.

Originally Posted by tristancliffe

It's far too complex a science to have 'an equation' given to it (just like there isn't an equation for "how much fuel does it take to get from England to America), so you have to give some thought to at least as many things as possible.

Friction materials are as much art (some some say Black Magic!) as science!

It's the reason wide tyres give more grip than narrow tyres (generally speaking).

I would suggest that brakes & tyres are two different cases. In the case of brakes you're dealing with kinetic friction because there is relative movement between the two components of the friction couple; with tyres I would suspect that tread deformation & conformity with the road surface is the most important factor in determining grip.

[tristancliffe]

Originally Posted by phoenix

No, it isn't. Wider tyres do not give more grip because of more tyre area in contact with the road. In the equation F=mu.R R decreases linearly as the contact patch of the tyre increases, so F remains the same as long a mu is constant.

So you are saying you don't get more grip from wider tyres? That an F1 car would still be able to corner at 4g even if it has 100mm wide tyres? Well, that's just silly.

Surface area has no effect in smooth body problems - but it does in real life problems. Whilst drilling and grooving are unlikely to have a noticable effect on either surface area or braking torque (indeed, I'd imagine it's so negligable that this discussion is a bit silly, but it was a question and this is what happens), it's still an effect.

Of course tyres deform more than brake pads (we hope), and of course the problem is slightly different, but the comparison is still valid - surface area changes the force due to friction. A tiny brake pad (say 1mm^2) will give you less friction (and a hell of a lot more wear) than a pad with 500mm^2 area (which will wear less). That's before you even consider the thermal issues of the pad.

[dtype38]

First, although the comparison with tyres might be interesting, I think it might be clouding the issue a bit. The original question was about grooves in brake rotors which expanded to a discussion of putting grooves or holes in rotors and their theoretical and practical effects, which has then moved onto pads and brake performance in general. It's producing some interesting discussion, so can we keep it focussed there please chaps (and chapesses if there are any reading), thanks....

Takes "mod" hat off and puts "techy" hat on....

Originally Posted by Dave Brand

When you press the brake pedal you apply a force to the brake pads. The resultant force, the force that produces the braking effort on the disc, is that force multiplied by the coefficient of friction. Pad area has no significant real-world effect on the braking force.

Reduced to basics, a brake is a device to convert kinetic energy into heat. Heat is generated at the pad-disc interface. If the pad is very small the heat will be concentrated into a very small area, thus for a given rate of energy dissipation the temperarture rise will be much higher than it woould be for a large pad.

Pad wear, measured in volume of material per unit of energy dissipated, is fairly constant. It follows, therefore, that the bigger the pad area the less the reduction in thickness & therefore the longer the life of the pad.

A lot of road car pads have large chamfers to reduce the tendency to squeal. As a result of this, the effective pad area can vary by probably as much as 15% over the life of the pad, with no discernable variation in braking effort.

Thanks for that Dave. I understand your logic and can't fault it, so that leaves me with trying to reconcile it with my practical experience. That leads me onto something my brake supplier said about brake pads and selecting the right compound. He said essentially that pad wear is only directly proportional to area when working well within the temperature limit of the pad. But as the limit is approached the wear rate increases dramatically. He says that once the pad reaches a certain temperature it will wear very rapidy indeed and be next to useless.

This would suggest that the argument about braking force vs pad area only holds true while well within the operating parameters of the pads. This obviously falls down when comparisons are made between 1mm sq pads and 500mm sq pads (not getting at you trist) so means that this sort of discussion can't be made by considering extremes.

Anyway, it seems that to get a nice linear relationship of area to braking force we'd need to pick a pad with temperature resistance well above where we want to use it... but (as I'm told, and from my own experience) high temperature resistance = very hard and that I'd get no "feel" from them (an issue we haven't covered yet). My brake supplier says that I should aim for the softest pad I can without reaching the critial high wear temperature. This is supposed to give me the best "feel" (NOT braking force) even though I'll probably get through pads quite quickly. That suggests that I could quite happily buy some really hard pads that would last me all season, but my braking wouldn't be as effective. (Hmm, seems I've been told something similar about tyres, but lets not go there please )

So, I think what I'm trying to say is that our two arguments aren't actually contradictory, but apply to different operating temperatures. Ie, if you have very hard pads, then grooves, holes etc make no difference, if you're right on the temparture limit of the pads, then they can have beneficial effects, but if you're right on the temperature (or structural) limit of the rotors then they could be bad news. Am I making any sense at all or should I go and hide my head now?

[Dave Brand]

Originally Posted by dtype38

That suggests that I could quite happily buy some really hard pads that would last me all season, but my braking wouldn't be as effective.

Any friction material has an optimum temperature range - generally speaking a 'soft', high friction pad will give good braking at lower temperatures, but fade when it gets hot, while a 'hard', low friction pad will not fade at high temperatures but will be ineffective from cold. Modern materials are closing the gap, but as with any engineering material all friction materials are a collection of compromises.

I once bought a Cortina GT which had been fitted with DS11 pads & had no brake servo......practically needed both feet on the brake pedal to stop it first thing in the morning!

[phoenix]

Originally Posted by tristancliffe

So you are saying you don't get more grip from wider tyres? That an F1 car would still be able to corner at 4g even if it has 100mm wide tyres? Well, that's just silly.

F1 tyres have a high mu (due to the rubber mainly) and a large R (due to downforce) which gives the extra F. Nothing to do with tyre width. Honest.