Front disc replacement...drilled, slots or std - Page 2 - Alfa Romeo Forum
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(Post Link) post #26 of 35 Old 4 Weeks Ago
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My twopence is simple:
I have slotted Brembo discs on my GT and they are noisey but work very well. They also produce more brake dust due the the "shaving " effect of the slots.
I am now fitting standard, plain (anti rust coated) Brembo discs on all my cars, as they are excellent quality, not expensive and work fine with good pads. Drilled / dimpled / grooved discs for normal road use are not required. They are a cosmetic upgrade only.
I get my discs from autodoc: https://www.autodoc.co.uk/car-parts/...oaAhM6EALw_wcB

End of my opinion.

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(Post Link) post #27 of 35 Old 2 Weeks Ago Thread Starter
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Thanks UncleJam for the link, Brembo discs it is then..they’re a really good price, thanks again.
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(Post Link) post #28 of 35 Old 2 Weeks Ago
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Quote:
Originally Posted by johnlear View Post
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).
This is actually correct. I found this out when using early EBC 'kevlar' organic pads on a motorcycle. Apart from a nauseating stink of burned fish, they were simply useless, double the braking distance of the sintered metal pads I had been using.

After a couple of weeks use, I could stand them no longer, and threw them out, and went back to Dunlopad sintered metal. To my horror they were now useless too. It took a half day to wear away the kevlar coating that the EBC's had left on the disk, and then I had brakes again.

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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...???
AIUI it's a constant process of deposit, abrasion and replacement of the deposit. Disk material matters here. AIUI cast iron is a better brake material; at a molecular level it's slightly rough, and gives rise to more friction. It's also great at dissipating heat. Unfortunately it rusts like crazy, gaining an oxide deposit in a matter of hours - which is swept away as soon as the brake is used. Cast iron disks also need to be quite thick, for strength.

But people don't like the sight of rusty disks on their expensive pristine vehicles so disks are nowadays almost always alloys. This is more slippery and less efficient, but looks less upsetting. Fully stainless disks don't rust at all, but are low friction, which is why you'll see nice shiny, skinny but very large machined-from-sheet disks on many m/c nowadays.
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It took a half day to wear away the kevlar coating that the EBC's had left on the disk, and then I had brakes again.
It's probably a function of the disc as well as the pad. People I knew who raced Suzukis always said that when changing pads they had to clean the discs with brake cleaner & wire wool to get the new pads to work effectively.



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AIUI cast iron is a better brake material; at a molecular level it's slightly rough, and gives rise to more friction.
For a given organic material the coefficient of friction with cast iron is around 25% higher than with stainless steel.
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(Post Link) post #30 of 35 Old 2 Weeks Ago
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Originally Posted by halftone View Post
Cast iron disks also need to be quite thick, for strength.

But people don't like the sight of rusty disks on their expensive pristine vehicles so disks are nowadays almost always alloys. This is more slippery and less efficient, but looks less upsetting. Fully stainless disks don't rust at all, but are low friction, which is why you'll see nice shiny, skinny but very large machined-from-sheet disks on many m/c nowadays.
Stainless steel discs on cars? Lots on bikes. I agree that cast iron is a better material for discs, even if the disc has to be thicker / heavier for strength. But, this also makes the disc a better heat sink, i.e. the disc takes longer to get hot due to the greater mass. A lightweight probably unvented disc could get too hot during a single heavy brake application from high speed (especially with a heavy car).

Steel, and especially stainless, is not as good as a casting material (compared to cast iron). The casting process is more expensive, the casting cannot be as intricate / complex as for cast iron, the resulting casting is relatively difficult / slow / costly to machine into the precise finished shape required.

For discs with internal vents casting is the method to use (vented discs are pretty standard these days, for good reason, at least for front discs which do most of the braking and get much hotter). Internal vents greatly increase the disc surface area, greatly assisting heat dissipation. With SS the disc is more commonly flat and unvented, with the flat disc bolted to the hub (because the SS disc is typically made from plate, not cast due to cost / difficulty of manufacture). A cast iron disc can easily have the typical 'top hat' shape cast into it, avoiding the complication of having to be a 'floating' disc.

Cast iron does rust quickly, but it tends to only be skin deep. Beyond that it takes a relatively long time for significant corrosion to occur (certainly a lot longer than steel alloys that are not 'stainless'). The rust is 'slowed down' by the typical high carbon and silicon content of the cast iron, i.e. the exposed iron molecules on the surface oxidise quickly, but the encroaching rust then encounters carbon and silicon molecules which inhibit deeper penetration (my non expert understanding).

Regards,
John.
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No, not stainless on cars, but not quite plain old pig iron either, which flash rusts like crazy, and is quite brittle and can shatter. Modern disks are mostly alloys that contain variable amounts of copper, vanadium, titanium, carbon (so mild steel). There is definitely some difference in braking efficiency, and how quickly they rust, resistance to forming hard spots when overheated, between different alloy mixtures. There's no right answer, but I know to my cost that the thin stainless steel disks on bikes are prone to warping easily.
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No, not stainless on cars, but not quite plain old pig iron either, which flash rusts like crazy, and is quite brittle and can shatter. Modern disks are mostly alloys that contain variable amounts of copper, vanadium, titanium, carbon (so mild steel). There is definitely some difference in braking efficiency, and how quickly they rust, resistance to forming hard spots when overheated, between different alloy mixtures. There's no right answer, but I know to my cost that the thin stainless steel disks on bikes are prone to warping easily.
I agree, the cast iron used for brake discs isn't a crude material, but 'malleable' cast iron with carefully 'designed' metallurgy for purpose. Not nearly as 'tough' as steel, but not particularly brittle like cheap general purpose cast iron can be (thankfully). Over the years I have seen a number of fractured discs, but it is impressive that even with a big crack they rarely seem to fly apart (only seen this happen on a race track, which isn't to say it couldn't ever happen to a road car disc...).

Still, disc cast iron can 'flash rust' badly enough, as can often be seen on a cold humid morning, but nevertheless is fairly resistant to deep rust penetration (is my understanding).

I don't know about steel discs, but cast discs don't 'warp'. What is most commonly put down to 'warping' is a thickness variation caused by uneven build up of pad material on the disc faces. This happens in one or more locations on the disc, at the same point on either side of the disc. What then happens is that as the now 'thicker' part of the disc passes between the pads, the pads are both simultaneously pushed very slightly back into their bores, which causes a braking force variation and a pulsation at the pedal. This was (apparently) discovered by Carrol Shelby and his team during development of the ford GT40, and is generally considered to be 'accepted wisdom' these days among automotive engineers (not that I am one...).

Regards,
John.
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Last edited by johnlear; 1 Week Ago at 12:58.
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All good stuff. The bit about new discs causing brake vibration seems to come from less than meticulous cleaning of the hub before fitting new discs.

I've twice had car discs which were less than completely linear. The first time I had the problem sorted by an on-car brake lathe. The second time I changed my brake usage. Before, I tended to use the brakes little apart from reasonably firm pressure at fairly high speed (probably with cold discs). Even with high carbon discs, I got a little brake vibration but this seems to have sorted itself after more urban use and no speed limit downhill firm braking to a stop.

I wouldn't have said the previous use was overly taxing but I wonder if it was similar use/conditions which necessitated the 330mm disc conversion on the faster, heavier GTA. What I will say is that I always found properly warm brake discs seemed smoother than disc which had been heated quickly/unevenly due to a single firm stop from speed which seemed to cause issues between hot rotor but cold hub mounting.
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So imagine a disc that is machined perfectly flat both sides with an even thickness everywhere, but is mounted ever so slightly askew. Put a dial indicator on it and it will tell you that there is disc 'runout' and the diagnosis that is commonly leapt to would typically be; 'warped' disc. But it isn't warped, and the askew disc won't cause braking force variation or pedal pulsation. If these symptoms exist then there will be an additional problem of thickness variation on that disc, or one or more of the other discs.

The slightly askew disc won't cause a braking force variation or pedal pulsation because the caliper will accomodate a slightly 'off square' disc. A 'fixed' multi piston caliper will do this by allowing the pistons on one side of the disc to move inward while the pistons on the other side simultaneously and equally move outward (as the wobbly disc rotates). As the pistons move the pressures inside the system don't change to any significant degree because there is no net change in the fluid volume inside the caliper (just fluid transferring from the pistons on one side to the pistons on the other side). The only resistances to the free movement of the pistons will be piston seal flexure and the inertia and viscousness of the brake fluid (as well as inertia of the pistons and pads), which are extremely minor. A sliding caliper will do a somewhat similar thing, but instead of the piston moving the pins will slide, which is a very minor resistance with a miniscule affect on braking force variation and no affect on pedal pulsation. For a pedal to pulsate there must be a significant rise and fall in overall sytem pressure (which can be felt at the pedal), and this doesn't occur if all the pistons etc move fairly freely in equally positive and negative ways.

So, a slightly askew disc will cause the pads, pistons and the fluid to move ever so slightly from side to side in the caliper, but this won't result in a significant change in the force pushing the pads against the disc or the friction strength at the pad / disc interface. There will be some minor friction where the pads bear against the pad carrier that will try and inhibit the free lateral movement of the pads, but again it won't have much affect.

The symptom that leads to the common diagnosis of 'warped discs' is in reality a symptom of thickness variation of the disc. That is, an uneven build up of pad material on the disc faces. This starts off as a spot of build up on one side of the disc (i.e. one section of the disc at which the disc is effectively ever so slightly thicker than the rest of it), which causes the pads to 'pinch' the disc at that spot with each revolution of the disc. This in turn causes a localised hot spot on the disc (both sides), at which hotter location more pad material is further deposited (than on the cooler parts of the disc). Now we start to have an increasing thickness variation on both sides of the disc at the same location on the disc.

Each time the slightly thicker part of the disc passes between the pads the thick spot forces the pads on both sides to be simultaneously pushed into the caliper body, which forcefully expels some fluid from the caliper (up the brake hose), momentarily increasing the pressure in the system (felt as pedal pulsation), and momentarily increases the friction between the pads and the disc (manifested as braking force variation). The problem just gets worse because it is self generating, and the only cure is to machine the discs or to replace them. The person who made the diagnosis of 'warped discs' then says, "see, told you that would fix it"...

I know from personal experience that an 'askew' disc doesn't cause pedal pulsation or force variation. Racing karts usually have only one brake (on the rear axle), and I've more than once whacked the disc hard on a ripple strip (or something, other drivers helmet perhaps...), and continued racing neither having nor suspecting any brake problems whatsoever, only to discover later in the pits a huge disc wobble because the aluminium disc carrier / hub had been bent. I'm talking a serious out of true here, easily visible with the naked eye, yet no problems at all in use, no symptoms whatsoever.

Regards,
John.
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Last edited by johnlear; 1 Week Ago at 01:00.
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Originally Posted by Fruity View Post
All good stuff. The bit about new discs causing brake vibration seems to come from less than meticulous cleaning of the hub before fitting new discs.

I've twice had car discs which were less than completely linear. The first time I had the problem sorted by an on-car brake lathe. The second time I changed my brake usage. Before, I tended to use the brakes little apart from reasonably firm pressure at fairly high speed (probably with cold discs). Even with high carbon discs, I got a little brake vibration but this seems to have sorted itself after more urban use and no speed limit downhill firm braking to a stop.

I wouldn't have said the previous use was overly taxing but I wonder if it was similar use/conditions which necessitated the 330mm disc conversion on the faster, heavier GTA. What I will say is that I always found properly warm brake discs seemed smoother than disc which had been heated quickly/unevenly due to a single firm stop from speed which seemed to cause issues between hot rotor but cold hub mounting.
New discs quickly exhibiting brake vibration / force variation is probably due to a problem bedding in the pads, I would expect (and hand in hand with bedding in the pads, bedding in the discs, i.e. getting an even coating of pad material deposited). Some pads seem worse in this respect than others, and maybe some particular specifications of cast iron might be more prone than others? Some Australian Ford Falcons seem to suffer from uneven disc pad deposits much more than most other cars seem to (or maybe there are just a lot of easily and cheaply available crap aftermarket pads for these cars?).

Possibly the worst thing that might be done is to get new / newish pads hot and then sit at the traffic lights with the brakes applied before they have adequately cooled? Instead of the hot / softened pad material being evenly smeared onto the disc surfaces as they rotate, it would be held stationary against the disc with the pads applying a stationary pressure, all while the brakes are cooling down. I doubt this would be condusive to an even deposition of pad material...

This is likely a bad thing to do even if the pads have already been properly bedded...

Regards,
John.
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