Front disc replacement...drilled, slots or std - Alfa Romeo Forum
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Front disc replacement...drilled, slots or std

Hi, I’m almost due to have the front discs replaced on my 147 JTD 16v.. I’ve seen some MTECH drilled/slotted discs that do look damn good & not a lot different in price to std discs. Does anyone have any experience with these? How do they perform compared to std discs. Thanks for any info
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For day to day use, I would just get regular discs. Drilled ones have a habit of cracking, and slotted ones will increase noise and wear out your pads faster.
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I put MTEC dimpled, grooved and painted on some years ago and have, this year, done it again.

Never had an issue with noise, look cool too.

All the best, Pub.
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Yes, dimpled or blind-drilled (not drilled all the way through) do not have the same tendency to crack.

Aside from looks, the reasons for fitting are for better brake cooling. The reduced area of friction surfaces contacting should mean greater pedal pressure (probably slight). Narrow square grooves work differently to wider rounded grooves. Narrow grooves de-glaze pads better. Wide grooves seem better for wet conditions are are quieter than narrow square grooves.

As my front pads always last over 50k and it is disc condition which drives the change, I've stuck with standard but I have considered a change in the past. I decided to stay standard to try to keep heat in the brakes in winter conditions.
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I had MTECH drilled with Ferodo Performance pads on my 156 and it transformed the braking (along with Goodridge hoses and Castrol Response fluid which had already made things 100% better than stock with Mintex discs and pads).

The drilled discs were not noisier and had hardly marked the pads.I've read Grooved discs can be noisy.
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The reduced area of friction surfaces contacting should mean greater pedal pressure (probably slight).
No it won't!
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That statement contradicts advice on EBC website. As I pointed out, any change would probably be slight but given all the variables, better discs, possibly higher friction pads, perhaps no change is likely.
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Just get stock discs. Slotted, drilled discs will make no difference for road use. All they do is rust and corrode quicker. I’ve used them on Peugeot 306 gti6, Subaru wrx and Alfa 156 v6 I’ve owned the past 20 years and noticed no difference in braking performance on the the road apart from them corroding more quickly.
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ive just ordered a fresh set of brembo discs and padds all around, honestly ive never had issues with stock brakes on any modern car, braking to the limit of your discs/pads is always so harsh that i try avoiding it anyway, and even if you brazenly ignore the law, trying to keep your actions somewhat safe-ish means you cant go max-attack with most cars anyway.

I could have gotten groved or drilled brembos, but they cost a good bit more, without a clear need for more performance..

If it is a looks thing, id invest the money saved in some caliper paint
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My understanding:

I can't see how it could be correct that grooves or holes will affect pedal feel in any significant way.

Grooves and holes don't significantly reduce heat. To a very minor degree thay can help the disc get rid of heat because both grooves and holes can increase the surface area of the disc. But, with holes this can go the other way, i.e. if the holes are not large enough relative to the depth of the metal through which the hole is drilled, then the surface area of the disc can actually be slightly reduced. This would have the opposite effect, i.e. cause the disc to retain more heat (though either way the effect will be very slight).

The primary purpose of holes and grooves is to fascillitate the evacuation of gasses which are a product of the pads overheating (or at least getting very hot). When the pads get hot enough they 'outgas', i.e. produce gasses which are vented from the pad faces against the disc faces. When the gas quantity is low the gasses can escape quickly enough that it has little affect on braking performance, but when a lot of gas is being produced it cannot escape quickly enough (from between the working surfaces of the pads and disc). When this occurs, the pads are lifted off the disc faces on a microscopically thin film of highly pressurised gas. Since gas has an extremely low coefficient of friction, the result is a dramatic loss of braking effect, often very sudden. This effect is known as 'pad fade'. It's often visible as 'smoking' brake pads, but you may have problems before smoke is actually seen.

If the disc has holes drilled from the working faces through to the internal vane voids, this provides an escape route for the gasses. If the disc has grooves machined into the face surfaces, then gasses can escape along the grooves to where the pads are not covering the grooves, and vent more easily to atmosphere. Either way the gasses will vent more readily, and so not as easily prevent the pads from properly contacting the disc faces (for friction to occur, the pads must be contacting the metal of the disc faces, not riding on a film of pressurised gas). This is why even stock brake pads very often (usually) have a groove machined into the pad material, i.e. to help vent gasses from between the pad and disc faces.

Disc grooves can help to remove glazed pad material from the pads, but in doing so can contribute to accelerated pad wear. For this to happen, the edges of the grooves must be fairly 'sharp', so that the groove edge can very lightly 'machine' the pads every time the brake is used. If the edges of the grooves are 'rounded', then the machining affect won't occur (or at least much less so).

Disc holes / grooves may well be useful if the brakes are used hard enough that the pads 'outgas' significantly and cause pad fade. If not then they probably only increase pad wear. If the car never experiences sudden brake loss (due to pad fade) when using stock ungrooved / undrilled discs, then it doesn't need grooved / drilled discs. If it does experience pad fade, then it may benefit from drilled / grooved discs, but equally, 'harder' pads may work just as well. Using grooved / holed discs, it may be possible to use softer pads.

Grooves / holes will reduce unsprung and rotational mass, but only to an insignificant degree.

Grooves can be stress risers, and so can initiate cracks. This is why the grooves should never extend right to the edge of the disc, i.e. this makes cracks more likely to form. It is also why some disc grooves are kind of 'J' shaped, i.e. the recurved end of the groove lessens the stress raising characteristic of the groove.

Regards,
John.
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I can’t see how drilled/grooves disks can corrode more quickly, that is determined by the material used in manufacturing and not if it has grooves/holes put in.

I have recently fitted grooved/dimpled discs to my 5 series and not found them to be noisy at all. Not done many miles on them so can’t comment on pad wear as yet but for the price I paid it was a no brainier compared to bmw price for 2 piece discs even on their value range.
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I canít see how drilled/grooves disks can corrode more quickly, that is determined by the material used in manufacturing and not if it has grooves/holes put in.

I have recently fitted grooved/dimpled discs to my 5 series and not found them to be noisy at all. Not done many miles on them so canít comment on pad wear as yet but for the price I paid it was a no brainier compared to bmw price for 2 piece discs even on their value range.
Theoretically, if the discs arent coated with some asti-rust agent, the surface inside the grooves/holes is free to develop corrosion without getting scrubbed clean by the brakepads. I have no idea if this would be of any significance though, it might impact the structural integrity to the point where cracks develop easier, but again, i simply dont know.
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I think the conditions in which the car operates in will have a major effect (think wet and salt).

I agree with John but I don't think discs or pads have to be hot. Some of the fade he describes is what is termed 'speed sensitivity'. Even discs and pads at ambient temperature will gas at speed when hard braking is called for. If a car does not have a sports brake set up, it is usually impossible to lock the wheels at high speed (say 80mph or above) yet the same pedal pressure can easily lock the wheels at speeds below 50mph. The point is pads can gas even when quite cool and this speed sensitivity is something that machined discs and more sporting pad compounds attempt to reduce.

In short, I think machined discs could improve high speed braking even without using a sport pad (which tends to have trade-offs in cold response, noise and disc life). I'm sure different drivers will have different experiences based on variables of choices, their preferences and operating conditions. It's a bit like tyres in that respect but I don't question an individual's experiences or the value of such.
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I've not heard great things about MTEC - not used them myself but a lot of reviews have mentioned warping, vibrations etc - I have to question why they're as cheap as they are?

I last used a set of standard Pagid disks, which had the anodized/anti-rust coating on that I was looking for, matched them with performance friction pads, new hoses and performance fluid and found that the braking was improved terrifically on my 147.
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Originally Posted by Fruity View Post
I think the conditions in which the car operates in will have a major effect (think wet and salt).
As I see this;

Wet conditions will tend reduce the heat produced by the brake simply because speeds will tend to be somewhat lower and braking distances longer. A wet disc will momentarily be lubricated with water, then very quickly turn that water to steam which will then behave similarly to the gas given off by overheated pads. But these affects will be very short lived and probably near non existstant with already warmed up brakes. Note that rotational speed will tend to throw water off the disc.

I can't think what affect salt might have...

Quote:
Originally Posted by Fruity View Post
I agree with John but I don't think discs or pads have to be hot. Some of the fade he describes is what is termed 'speed sensitivity'. Even discs and pads at ambient temperature will gas at speed when hard braking is called for. If a car does not have a sports brake set up, it is usually impossible to lock the wheels at high speed (say 80mph or above) yet the same pedal pressure can easily lock the wheels at speeds below 50mph. The point is pads can gas even when quite cool and this speed sensitivity is something that machined discs and more sporting pad compounds attempt to reduce.
'Outgassing' and the consequent 'pad fade' is a product of the temperature of the pad material, not of anything else. The pads will 'outgas' (smoke) only when they get too hot, that temperature being dependant on the pad composition. Pads get very hot very quickly when braking heavily from very high speed, so 'soft' pads can go from cold to overheated in just a few seconds if the braking is hard enough and the speed high enough.

'Soft' pads will 'outgas' at lower temperature than 'hard' pads, with 'soft' and 'hard' being references to the temperature at which the pads start excessively 'outgassing' rather than the actual hardness of the pad material (though there may be some general association between pad material hardness and outgassing temperature). 'Soft' pads (typical road pads) generally have a better coefficient of friction at lower temperatures, but 'outgas' more at relatively low temperature. 'Hard' pads (typical racing and 'sports' pads) generally have a relatively poor coefficient of friction at low temperature, increasing as the temperature rises, and outgas excessively only at a relatively higher temperature.

The "speed sensitivty" that Fruity is talking about (I think) is something quite different to 'pad fade'. As rotational speed rises the wheel gains an increasingly large amount of embodied kinetic energy, which becomes quite considerable at higher road speeds. The brake not only has to absorb the kinetic energy of the whole vehicle (from high to low speed), it also has to slow the rotational speed of all the rotating masses, and absorb that kinetic energy too. The rotating mass is substantial; the drive shaft, the hub, the brake rotor, the wheel, and the tyre (in the order of mass distribution away from the rotational axis, with the tyre being the most influential due to its' substantial mass and distance from rotational centre), so it takes a very significant %age of the brakes' performance to slow it's rotational speed.

At high road speeds a lot of the brakes' performance is taken up with absorbing the kinetic energy of the rotating masses, more so the faster the wheel etc is rotating. This leaves less of the brakes' performance available for slowing the vehicle mass, and so less available for 'locking up' the brake (i.e. overwhelming tyre grip). This is much more obvious at high speed than at low speed, because the rotational energy is far greater at higher speeds than at lower speeds. I think it is correct that the kinetic energy embodied in a rotating mass increases with the cube of the rotational speed (though it might be with the square of rotational speed).

This is why the pads get so very hot so very quickly when braking hard from very high speeds, i.e. it's not just the energy being absorbed (converted to heat) by the brake to reduce the vehicle road speed, it's also the absorbtion of the quite substantial rotational energy of the rotating masses. This is (IMO, and ABS notwithstanding) also why it is so much harder to lock up the brakes at higher speed relative to at lower speed, i.e. at high speeds so much of the brakes' performance is already being used to slow the rotating masses and so is just not available to overwhelm tyre grip. At low speeds much more of the brake performance is available to overwhelm tyre grip, so locking up is much easier to 'achieve'...

Keep in mind that after braking hard from high speed the pads are now very hot, and may well be 'outgassing' like crazy, so even low speed brake performance might now be seriously compromised (until the pads cool off).

This is another reason why reducing wheel, tyre, and brake mass is a 'good thing' in and of itself (i.e. not considering the compromises this might create). A reduction in rotational mass (and therefore some of the unsprung mass) not only improves handling and acceleration, but will also assist braking peformance, especially from higher speed (significantly more so than the same amount of mass reduction that is not rotational mass). On the other hand, increasing disc diameter will increase braking effect, so bigger and probably heavier brakes will have more braking affect at higher speeds, so the speed at which it is possible for the brakes to overwhem tyre grip should increase with larger brakes (especially if the disc mass isn't increased excessively as diameter is increased...).

Quote:
Originally Posted by Fruity View Post
In short, I think machined discs could improve high speed braking even without using a sport pad (which tends to have trade-offs in cold response, noise and disc life).
Grooved or drilled discs will improve high speed braking performance, if the pads get hot enough to 'outgas', but not otherwise (i.e. if the brakes never suffer from 'pad fade' then disc grooves or holes are not going to make them better brakes). Such discs are more likely to be useful if the pads are too soft (as you infer), or with brakes that are subject to extreme usage and not even hard pads are fade resistant enough. There may be an advantage in fitting grooved / drilled discs insofar as their use may mean it is possible to use softer pads than may otherwise have been required, or lighter brakes than may have otherwise been needed.

I can't see much point in 'dimpled' discs, other than a very small increase in the disc surface area. In theory this will enhance the rate at which the disc can cool off (and disc temperature has a large affect on pad temperature as the pads 'lose' a lot of heat into the disc), though without data I tend to think the effect is likely to be very very small. I suspect that dimples are useful mostly because they look a bit bling, which helps to sell product...

Regards,
John.

Last edited by johnlear; 14-06-19 at 01:44.
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Salt is a lubricant. As we all know it corrodes ferrous metals. Rust or blackened metal of a grey iron brake disc has a lower coefficient of friction than good shiny metal. That's why discs in good condition are so (and so massively under-appreciated) much more efficient at braking quickly. The MOT brake roller test is such a minimal standard for brakes and doesn't take into account the effect disc degradation has in a dynamic, high speed hard brake application. Not only is initial brake response delayed but so too is the maximum braking effect and the speed the brake reaches maximum retardation (which then needs ABS intervention) can be very much lower, or may simply not be reached at all.

If anyone cares to measure braking distance of corroded brake discs from highway speeds against that of bedded in new brakes, the result will evidence my point.
Unfortunately it is that same reason why many people often think the new brakes just fitted are an upgrade. Even rubbish quality new brakes will be far better than expensive old brakes which were in poor condition.

In short, don't necessarily go for expensive brakes. Being able to afford more regular disc changes will probably give more real life benefits. That said, more exotic brakes should give favourable results at high speed but lower speed stopping distances won't be much different.

That last point is something no one discusses and certainly not something a performance brake manufacturer or supplier of upgrade brakes which cost many times that of standard replacements will ever disclose. Finally, the main reason for a more expensive standard pad over a cheaper one is in the single pressing manufacture rather than batch manufacture. A pad formed in an individual pressing tends to be a much more consistent pad which has less performance variation. Obviously more expensive materials also play a significant part.

Last edited by Fruity; 14-06-19 at 02:33.
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Quote:
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Quote:
Originally Posted by Fruity View Post
I think the conditions in which the car operates in will have a major effect (think wet and salt).
As I see this;

Wet conditions will tend reduce the heat produced by the brake simply because speeds will tend to be somewhat lower and braking distances longer. A wet disc will momentarily be lubricated with water, then very quickly turn that water to steam which will then behave similarly to the gas given off by overheated pads. But these affects will be very short lived and probably near non existstant with already warmed up brakes. Note that rotational speed will tend to throw water off the disc.

I can't think what affect salt might have...

Quote:
Originally Posted by Fruity View Post
I agree with John but I don't think discs or pads have to be hot. Some of the fade he describes is what is termed 'speed sensitivity'. Even discs and pads at ambient temperature will gas at speed when hard braking is called for. If a car does not have a sports brake set up, it is usually impossible to lock the wheels at high speed (say 80mph or above) yet the same pedal pressure can easily lock the wheels at speeds below 50mph. The point is pads can gas even when quite cool and this speed sensitivity is something that machined discs and more sporting pad compounds attempt to reduce.
'Outgassing' and the consequent 'pad fade' is a product of the temperature of the pad material, not of anything else. The pads will 'outgas' (smoke) only when they get too hot, that temperature being dependant on the pad composition. Pads get very hot very quickly when braking heavily from very high speed, so 'soft' pads can go from cold to overheated in just a few seconds if the braking is hard enough and the speed high enough.

'Soft' pads will 'outgas' at lower temperature than 'hard' pads, with 'soft' and 'hard' being references to the temperature at which the pads start excessively 'outgassing' rather than the actual hardness of the pad material (though there may be some general association between pad material hardness and outgassing temperature). 'Soft' pads (typical road pads) generally have a better coefficient of friction at lower temperatures, but 'outgas' more at relatively low temperature. 'Hard' pads (typical racing and 'sports' pads) generally have a relatively poor coefficient of friction at low temperature, increasing as the temperature rises, and outgas excessively only at a relatively higher temperature.

The "speed sensitivty" that Fruity is talking about (I think) is something quite different to 'pad fade'. As rotational speed rises the wheel gains an increasingly large amount of embodied kinetic energy, which becomes quite considerable at higher road speeds. The brake not only has to absorb the kinetic energy of the whole vehicle (from high to low speed), it also has to slow the rotational speed of all the rotating masses, and absorb that kinetic energy too. The rotating mass is substantial; the drive shaft, the hub, the brake rotor, the wheel, and the tyre (in the order of mass distribution away from the rotational axis, with the tyre being the most influential due to its' substantial mass and distance from rotational centre), so it takes a very significant %age of the brakes' performance to slow it's rotational speed.

At high road speeds a lot of the brakes' performance is taken up with absorbing the kinetic energy of the rotating masses, more so the faster the wheel etc is rotating. This leaves less of the brakes' performance available for slowing the vehicle mass, and so less available for 'locking up' the brake (i.e. overwhelming tyre grip). This is much more obvious at high speed than at low speed, because the rotational energy is far greater at higher speeds than at lower speeds. I think it is correct that the kinetic energy embodied in a rotating mass increases with the cube of the rotational speed (though it might be with the square of rotational speed).

This is why the pads get so very hot so very quickly when braking hard from very high speeds, i.e. it's not just the energy being absorbed (converted to heat) by the brake to reduce the vehicle road speed, it's also the absorbtion of the quite substantial rotational energy of the rotating masses. This is (IMO, and ABS notwithstanding) also why it is so much harder to lock up the brakes at higher speed relative to at lower speed, i.e. at high speeds so much of the brakes' performance is already being used to slow the rotating masses and so is just not available to overwhelm tyre grip. At low speeds much more of the brake performance is available to overwhelm tyre grip, so locking up is much easier to 'achieve'...

Keep in mind that after braking hard from high speed the pads are now very hot, and may well be 'outgassing' like crazy, so even low speed brake performance might now be seriously compromised (until the pads cool off).

This is another reason why reducing wheel, tyre, and brake mass is a 'good thing' in and of itself (i.e. not considering the compromises this might create). A reduction in rotational mass (and therefore some of the unsprung mass) not only improves handling and acceleration, but will also assist braking peformance, especially from higher speed (significantly more so than the same amount of mass reduction that is not rotational mass). On the other hand, increasing disc diameter will increase braking effect, so bigger and probably heavier brakes will have more braking affect at higher speeds, so the speed at which it is possible for the brakes to overwhem tyre grip should increase with larger brakes (especially if the disc mass isn't increased excessively as diameter is increased...).

Quote:
Originally Posted by Fruity View Post
In short, I think machined discs could improve high speed braking even without using a sport pad (which tends to have trade-offs in cold response, noise and disc life).
Grooved or drilled discs will improve high speed braking performance, if the pads get hot enough to 'outgas', but not otherwise (i.e. if the brakes never suffer from 'pad fade' then disc grooves or holes are not going to make them better brakes). Such discs are more likely to be useful if the pads are too soft (as you infer), or with brakes that are subject to extreme usage and not even hard pads are fade resistant enough. There may be an advantage in fitting grooved / drilled discs insofar as their use may mean it is possible to use softer pads than may otherwise have been required, or lighter brakes than may have otherwise been needed.

I can't see much point in 'dimpled' discs, other than a very small increase in the disc surface area. In theory this will enhance the rate at which the disc can cool off (and disc temperature has a large affect on pad temperature as the pads 'lose' a lot of heat into the disc), though without data I tend to think the effect is likely to be very very small. I suspect that dimples are useful mostly because they look a bit bling, which helps to sell product...

Regards,
John.

Not read complete post as it’s far too long and I couldn’t possibly concentrate for that long. I had a quick scan and saw dimpled discs mentioned.

The reason for dimpled as I understand it is they are syringed than drilled but also help to reduce gasses and dust build up on surface during braking. One would expect mainly heavy braking from speed etc and not road use.
Whatever the pros and cons I opted for these due to price and not bling factor.
As said above somewhere it’s like tyres some will like some will not, personal preference and all that.
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If anyone cares to measure braking distance of corroded brake discs from highway speeds against that of bedded in new brakes, the result will evidence my point.
Unfortunately it is that same reason why many people often think the new brakes just fitted are an upgrade. Even rubbish quality new brakes will be far better than expensive old brakes which were in poor condition.

In short, don't necessarily go for expensive brakes. Being able to afford more regular disc changes will probably give more real life benefits. That said, more exotic brakes should give favourable results at high speed but lower speed stopping distances won't be much different.
.
Fruity, just curious, how would any disc reach a point where there is sufficient rust etc.. on it that it wouldnt be cleared with the first few uses on a drive? My current discs are down to minimum thickness, so about as far gone as any would be, but since the car never sits unused more then a few days, any surface rust easily gets taken off on the next drive.

The only way i would see a disc degrade to the point where the surface is permanently deteriorated that new brakes are a significant upgrade, is if a car is left sitting for months/years in a humid environment. Not really a concern for the average user.
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I do not get more than 2 years out of brake discs even when doing 28k per year. I could push it to 3 but I noticed the degradation in brake performance. My discs tend to suffer from blackening even when the car is used 6 or 7 days per week throughout the year.

I know I am worse than most and I've worked out the reason for that. It is because I use the brakes less than most and don't use light check braking for extended periods which seems to be the norm these days. Because I don't use the brakes for extended periods, the discs don't have the same cleaning action which causes corrosion to be able to start to build up in the form of black pitting.

As said, this only becomes noticeable when braking at higher speeds and I don't let my cars' braking performance degrade to the point that most people do. Years ago, I nearly witnessed another car take out a group of people which was obviously down to poor braking performance. It's just one of these things which has stayed with me.

I point this out because the thread is really about braking performance and people who look at upgrade brakes tend not to have such an ambivalent/uneducated attitude to car and brake maintenance as the majority.
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Originally Posted by johnlear View Post
As I see this;
Good post, John!

Just a couple of comments.

The differentiation between "hard" & "soft" pads is much less marked with modern friction materials. A lot of work has gone into, & continues, in developing pads which have acceptably high friction levels over a very wide temperature range.

Friction level is dependent on rubbing speed; again, modern pads have reduced this effect to almost insignificant levels.

Developing friction materials isn't easy - it's still as much a black art as it is a science!


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Finally, the main reason for a more expensive standard pad over a cheaper one is in the single pressing manufacture rather than batch manufacture. A pad formed in an individual pressing tends to be a much more consistent pad which has less performance variation.
How so? Do you mean single-cavity versus multi-cavity dies, are we talking press-cured, open-cured or pack-cured pads? Define "performance variation" in this context.

A big advantage of mass-production methods is rigid process control which helps reduce differences in the products within a batch & from batch to batch compared with manufacturing a series of one-offs.

The plural of "anecdote" is not "data".
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To be completely honest, Dave, I am struggling to answer the points you raised. Clearly you have greater knowledge, indeed even specialist, so I can only hope you work/have worked for Ferodo perhaps.

I cannot unearth the article where I read that single pressing gives a more consistent pressing. The article stated that a batch pressing could be less consistent, presumably due to perhaps not homogeneous pad material mixture, variation in pressure of a press doing say 20 pucks at a time. Certain motorsport applications will be very low volume so will surely be single pressings. Are they inferior to mass production that way?

I thought all pads were multi cavity.

Curing? The Ferodo footage and EBC, perhaps the Akebono seem to oven cure for a few hours. Other sources seem to heated pressure plate cure the pads. All other sources seem to use heat and pressure. The Ferodo footage is a bit older though so perhaps there are now more cost effective techniques.
Not the most scientific but interesting.

https://www.akebono-brake.com/englis...ial/index.html

https://www.tirereview.com/163702/

https://www.remsa.com/products/brake-pads/
I think Remsa make for various brands.

https://www.apecbraking.co.uk/Produc...t-Results.aspx
I'm not sure if Apec make their own stuff or simply box it.

A non- scientific look at Borg & Beck who contract manufacturers to produce for them.

As said, I cannot find what I was looking for but if anyone can post supporting links one way or another, that would be useful. My intention is not to mislead anyone.

Last edited by Fruity; 14-06-19 at 18:25.
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Good post, John!

Just a couple of comments.

The differentiation between "hard" & "soft" pads is much less marked with modern friction materials. A lot of work has gone into, & continues, in developing pads which have acceptably high friction levels over a very wide temperature range.

Friction level is dependent on rubbing speed; again, modern pads have reduced this effect to almost insignificant levels.

Developing friction materials isn't easy - it's still as much a black art as it is a science!
Yes, but even so I think there is still a significant difference between 'performance' pads and 'commuter' pads (as fitted to most cars). Of course any pad material must generate reasonable friction when dead cold. A racing car doesn't need pads that work all that well when cold, but for any pad intended to be used on a road car the pads do need to work at least acceptably from cold (even if this means the pedal pressure might be a bit high).

Modern pads may well be better than less modern pads, but I'm still not convinced that they are as good as they were when asbestos was a major ingredient of the material. Modern pads are certainly much harder on the discs than when asbestos was used, with discs now being a routinely consumable item, when years ago (in the mists...) disc wear was much less and discs only rarely needing replacement. To be clear, I'm not advocating a return to asbestos based pad compounds...

A little story; Some years ago I was driving a taxi (for my sins) and was carrying a passenger who turned out to be a dangerous psychopath (possibly under the influence of meth...). This person threatened to physically assault me if I did not drive him to a destination at increasingly high speed. This journey was on a rural motorway late at night with minimal other traffic, and the speeds reached became frighteningly high, maxxing at 220kmh (while being continually threatened with violence...). This is the maximum speed that a 2.4 litre Camry can reach, pedal to the floor (and the handling becomes very 'indeterminate' as the air lifts the front end...).

Anyway, approaching an open service centre at about 200kmh, I slammed on the brakes as hard as possible (both feet), entered the service centre and exited the vehicle (while actually being attacked). Taking refuge in the shop, the attendant locked the doors and called the police, while the mad person trashed the car on the forecourt.

The point of this story is what happened when I hit the brakes so hard from such a high speed. The brakes would have had standard pads fitted, which would have been cold at initial application, for about two seconds, being about as long as it took to cook the pads. From there the braking effect almost instantly became very poor. I only barely managed to slow the vehicle enough to enter the service centre, at a substantially higher speed than I had intended. I could smell the strong aroma of well cooked pads. I'm fairly sure that 'high performance' pads would have worked somewhat better in this scenario...

On the other hand, when I was racing karts I'd use discarded 'standard' road car pads (fairly 'soft' compound) cut down to fit the kart caliper (single brake on the rear axle), rather than the expensive 'racing' pads that everyone else used (fairly 'hard' compound). I'd have very good braking from the first application at the first corner, and from there never suffered from the pads 'fading' during the race (because they never did).

Regards,
John.
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Last edited by johnlear; 15-06-19 at 00:38.
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Salt is a lubricant.
I've never heard of salt having lubricating qualities. I'm not saying it doesn't, just never heard it. How much road salt gets onto the braking surfaces, and how long does it stay there? I have no idea. I've never had to deal with salted roads.

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As we all know it corrodes ferrous metals. Rust or blackened metal of a grey iron brake disc has a lower coefficient of friction than good shiny metal. That's why discs in good condition are so (and so massively under-appreciated) much more efficient at braking quickly. The MOT brake roller test is such a minimal standard for brakes and doesn't take into account the effect disc degradation has in a dynamic, high speed hard brake application. Not only is initial brake response delayed but so too is the maximum braking effect and the speed the brake reaches maximum retardation (which then needs ABS intervention) can be very much lower, or may simply not be reached at all.
But, it's my understanding that the pads are not in contact with the disc metal, but rather are in contact with a microscopically thin film of pad material that has been transferred onto the disc surface in normal use. The braking effect (friction) is between deposited pad material on the disc and undeposited pad material on the pad (though this is an over simplification because material is constantly being 'swapped' back and forth between the pad and disc surfaces). The layer of deposited material is so thin that it is more or less invisible (though can possibly be seen if there are abnormally thick deposits on the disc, usually uneven).

So, assuming this to be absolutely correct and not an over simplified understanding, it shouldn't matter what the disc surface actually is, whether it be steel or cast iron, or whether it is oxidised or not (shiny bright or darkened), as long as it forms a sound foundation for the deposited layer of pad material...

Having said this, there must be times when and conditions in which there is 'clean' metal on the disc faces, otherwise the disc would never wear and become thinner over time...???

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Originally Posted by Fruity View Post
If anyone cares to measure braking distance of corroded brake discs from highway speeds against that of bedded in new brakes, the result will evidence my point.
Unfortunately it is that same reason why many people often think the new brakes just fitted are an upgrade. Even rubbish quality new brakes will be far better than expensive old brakes which were in poor condition.
There may be other factors with the 'old' brakes vs new that might be affecting how well they work. Or, may affect pedal travel and feel and how this may influence the perception of the performance of the system. This might include such things as glazing of the pads and / or disc, pad taper (in one or two axes), disc taper (thinner near the outer edge, typically), looseness in caliper mounting pins, pad sticking etc.

Regards,
John.
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Yes, but even so I think there is still a significant difference between 'performance' pads and 'commuter' pads (as fitted to most cars).
Different usage, different pads. As with just about any man-made product, it's a matter of tweaking the compromises to suit the needs of the user.


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I cannot unearth the article where I read that single pressing gives a more consistent pressing. The article stated that a batch pressing could be less consistent, presumably due to perhaps not homogeneous pad material mixture, variation in pressure of a press doing say 20 pucks at a time. Certain motorsport applications will be very low volume so will surely be single pressings. Are they inferior to mass production that way?
You're grossly overthinking this. The production process will be whatever is needed to produce a satisfactory end product. You don't invest the same amount of money in a product which has annual sales measured in tens of sets as you do in a product which is selling in the tens of thousands annually.
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Thanks for all your advice guys, really appreciated. I’ve decided to stick with std discs & go for decent pads & fluid. It’s seams the logical thing to do as I do a lot of miles on my commute & feel std discs will handle the abuse longer.
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