Brakes work by turning kinetic energy into heat hence discs have to be fairly tough and have to withstand ten of thousands of heat cycles in a life time, that is getting very hot under hard braking, cooling, getting very hot again. Steel used to be used for brake discs, i.e. iron with a high carbon content which helped avoid it being abraided by the pads as well as inhibited corrosion, nowdays largely due to the banning of absestos in brake pads the discs abrade at almost the same rate as the pads (by surface area) and cast iron or low carbon steel is generally used, alluminium and alloys thereof would surely be too soft, abraid too quickly. Carbon fibre composite brakes are used in high end motorsport and some supercars, they are in theory more powerful though only work well once up to temperature so not ideal for consistent braking performance on a car that might be driven a mile to the shops on a frosty morning as well as braking from 80mph on a busy m/way.
As it happens I changed the front pads and discs on my Mondeo yesterday, they were the second set of both, the first were changed by the dealer at about 60k, the replacements had lasted another 50k at which point the discs were nearing their 22mm min thickness though the pads had a few thousand miles left in them, I could perhaps have legitimately changed the discs and not the pads.
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I was under the impression that cast iron was the traditional material, but that the recent switch to steel was the cause of both the increased wear rate and modern discs' propensity to rust. Old fashioned drums were certainly made of cast iron, and were very fragile if hammered or dropped.
The following quote from Leeds University engineering dept, first Google hit for "brake disc composition" :
?The rotor (ie the brake disc or drum) is attached to the hub of the wheel and is traditionally manufactured from grey cast iron ?.
Grey cast iron satisfies these requirements but its relatively high density means that the rotor mass is significant (typically over 5 kg for the front disc of a normal passenger car). Lightweight alternatives to cast iron have therefore received serious attention in recent years, notably in the form of aluminium metal matrix..?
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My understanding is that the discs that are designed to be used with contemporary zero asbestos etc pads are either lower carbon steel or cast iron hence they corrode more than steel discs circa 1970/80's.
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My understanding is that the discs that are designed to be used with contemporary zero asbestos etc pads are either lower carbon steel or cast iron hence they corrode more than steel discs circa 1970/80's.
That sounds plausible, but not what my quote from Leeds said. Nor does it fit with my own observations. The discs on my Triumph are the originals, vintage 1964. They don't rust, and they have only done 208,000 miles.
. Neither do those on my Volvo, 1993. The latter are still within wear tolerances, at 351,000 miles. What's going on here? They both use non-asbestos pads, which I find last about 40,000 miles.
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That sounds plausible, but not what my quote from Leeds said. Nor does it fit with my own observations.
My comments DO fit with your observations, I am saying that older discs were more resistant to corrosion.
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Erm - because they would melt. Aluminium melts at about 630 degC; that's why exhaust manifolds are cast iron.
659.
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(Should have been reply to "Why not aluminium disks").
659.
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"Cast iron is very brittle (it cracks easily) but it has a greater resistance
to corrosion (see rusting) than either pure iron or steel."
Extract from gcse.com - in answer to the question "Does cast iron rust more quickly than steel?"
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Apart from some specialist applications, most car disc brakes have always been made from the cheapest and nastiest grey cast iron - usually sand cast. Its all about cost, so the metal quality is pretty poor and then the component is over-spec'd - this leads to the overly-heavy discs that most cars are fitted with.
The other advantage of cast iron is that it is a better conductor of heat than steel - and that is important in modern vented applications where the outside surface can be very hot and the inner webs still cold. Really, the typical cheap car disc brake is a bit of a mess thermally, especially when assymetric pads are used.
I think modern brakes apparently rusting more than in the past is down to pads leaving less deposits on the disc. Older type pads (which had a shorter life) where more 'adherent' and left a very thin film of pad material on the disc which helped protect it. Also the amount of carbon and silicon in the iron may have been changed to increase disc durability with todays long service intervals.
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Also modern alloys,in place of steels with hub caps,mean you can see the discs.
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Apart from some specialist applications, most car disc brakes have always been made from the cheapest and nastiest grey cast iron
Apparently there's an (enforced?) SAE standard:
"Disc brake rotors are commonly manufactured out of a material called grey iron. The SAE maintains a specification for the manufacture of grey iron for various applications. For normal car and light truck applications, the SAE specification is J431 G3000 (superseded to G10). This specification dictates the correct range of hardness, chemical composition, tensile strength, and other properties that are necessary for the intended use"¹.
¹ Extract from wikipedia.org - See en.wikipedia.org/wiki/Disc_brakes
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Mike Farrow
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I think modern brakes apparently rusting more than in the past is down to pads leaving less deposits on the disc. Older type pads (which had a shorter life) where more 'adherent' and left a very thin film of pad material on the disc which helped protect it. >>
It is not only the swept area of recent discs that corrode more, the edges and inners do soo too.
Also the amount of carbon and silicon in the iron may have been changed to increasedisc durability with todays long service intervals.
I would suggest that it is perhaps the contrary, that disc material is now designed to abraid along with the pads due to the eco changes in pad material thus making the disc itself a disposable item along with the pads where as in the past it was only pads that were changed unless discs had warped or corroded from lack of use.
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Still confused. I have old discs (42 and 13 years respectively) but use modern pads. Discs last forever, pads last about 40,000 miles. The obvious conclusion seems to be that the heavy wear and rust now being reported are due to poor quality materials. Volvo and Triumph obviously fitted good quality discs, whatever that means - grey iron, white iron, steel, high silicon, ? That doesn't seem to square with modern "long service intervals".
BTW a Google search shows suppliers of Aluminium discs. So they don't melt, presumably because they have good heat dissipation.
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One thing that has not been mentioned is that modern discs are not particularly expensive and can be be changed as quick as a set of pads.
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So just like old ones then? As long as they are separate castings, and not integral with the hubs.
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>>Still confused. I have old discs (42 and 13 years respectively) but use modern pads. Discs last forever, pads last about 40,000 miles. >>
Newer discs would offer better braking performance with newer type pads though would not last well as your old discs do.
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I'm not so sure that things have changed that much. I've been servicing cars for a long time and 20+ years ago we used to reckon two sets of pads per disc. Typically on an early '80's Merc or Audi the pads would last about 30-40k miles and the discs would last 80k or so.
I think brakes get used harder nowadays (cars have relatively better performance) and people seem to brake more, and harder, than they used to.
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I heard somewhere, and I cant remember where, that brake discs are now made of softer material because ABS works better with softer discs. Would this make sense? Soft discs = less carbon = more rust? Cant remember whether this was from a reputable source, or whether it was an excuse peddled by a service manager at a dealership (this rings a bell, my Dad usually complains that his discs should not need changing).
Also regarding disc wear, cars have got a lot heavier in the last 10 years haven't they.
Rich.
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I'm not so sure that things have changed that much.
>>
Up and until my current vehicle I have never needed any disks changed.
Cortina MK2 well over 100K
Cortina MK 4 130K
Sierra 140K.
I am a gentle braker so that may have affeced the wear rate.
I have no history on my current 98 Mondeo 2l auto which has just had disks changed at 105K.
I bought it at 80K and judging by the rust in the ventilation slots when I first took the wheels off i suspect they were original disks.
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My theory is that for many low mileage drivers, rust is a greater destroyer of discs than usage. Regular car washing, especially with alloy wheels, short runs and driving through puddles will mean on low mileage cars that the discs will rust when parked. It may not be much but a thin layer of iron is then worn away as rust when the car is next driven.
Add to that infrequent usage and light brake usage and rust is often not worn away and localised pitting forms.. which destroys pads and hence discs in a feedback loop.
High mileage cars are often braked hard, less frequently washed and with longer journeys rust has less opportunity to form.
Based on personal experience.. never changed discs on any of my cars, every 5 years (20k -30k miles) on wife's cars due to corrosion induced wear.
madf
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Newer drivers (say in last 20 years) are taught by Driving Schools to apply the brakes.
When I was a learner - (43 years ago) there were no disc brakes and no servo assistance - we were taught to change down and use engine braking as the drums were not the most efficient form of brakes, especially in the wet.
Disc brakes did exist but were only trickling down to say the Morris 1100 - (Cortina in 1963 had drum brakes originally - I think)etc - Morris 1000 (which I learned in) was drums back & front.
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