Brake discs are an important part of the braking system, they provide a surface for the brake pads to work against. They rotate with the wheels and allow the braking force to operate on the tyres causing the car to slow down. Evidently they do this using friction caused by the brake pads being clamped on to them, a biproduct of friction is heat, a lot of it. Energy is converted from one type to another, in this case kinetic(movement) energy is converted into braking and heat. Obviously the more you brake the more heat is created.
You may think that brake discs can handle very high temperatures and they can, but there is no use having brake pads that can handle very high temperatures if your brake discs overheat and crack well below those temperatures, and by the same theory there's no point having huge expensive discs on a car that won't warm them up.
A disc stores and radiates the heat caused during braking, the brake pad creates the heat, it doesn't really hold anywhere near as much heat energy as the disc itself. Therefore the more metal there is(thickness, diameter, vented/solid) then the more capable a cars braking ability is.
Thicker discs: If you make the disc thicker you have more metal to hold the heat therefore a higher specific heat capacity.
Larger discs: If you make the diameter larger(say 266mm instead of 247mm) you are not only increasing the specific heat capacity but also the potential braking force. This is because you are increasing the leverage of the pad on the disc as you are moving the point of leverage further from the centre. One must however consider a disc spinning at say 100rpm, if the disc is larger the outer edge of the disc will be travelling at a higher speed(still rotating at the same revolutions per minute but then the disc is larger so the outside edge is travelling a longer distance) than that of a small disc so it will increase the friction but it will also create more heat. So with increased braking force comes more heat.
Vented or solid discs: A solid disc is just as it says a solid disc which the pads act on. A vented disc is more like a sandwich, where they took the solid disc, chopped it down the centre and put some spacers in between each half so that air can flow between the two halfs of the disc to cool it. Vented discs will have a higher amount of metal in them so a higher specific heat capacity and also they will cool down quicker when they do get hot as their surface area has more or less been doubled. Some vented discs use curved vanes inside them to scoop air from inside the wheel through the disc as it spins.
Popular disc applications:
Saxo base model 3 stud - Solid discs, 238mm diameter, rear drums
Saxo with power steering & 4 stud: Solid discs 247mm diameter, rear drums
Saxo sport models(VTR/VTS & furio): Vented discs 247mm diameter, rear solid discs 247mm diameter.
Peugeot 206 & C2/C3 sport models: Vented front discs 266mm and 283mm diameter, rear solid discs 247mm diameter.
Peugeot 306/Xsara sport models: Vented front discs 266mm and 283mm diameter, rear solid discs 247mm diameter.
As you can see the larger and heavier cars use larger discs on the front but the rear discs are still only 247mm because they are more than adequate for a car weighing 500kg more than a saxo(that's over 50% more!!!)
Furthermore, the newer brake setups(year 2000 and newer) have started using thicker discs, the old 266mm discs are 20.5mm thick, the new are 22mm thick, the old 283mm discs were 22mm thick, the new ones a whopping 26mm thick)
One thing to remember though is that bigger is not necessarily better. Larger discs weigh a lot more, this increases rotational mass as well as upsprung weight, and both of these weights need to be kept as low as possible. Also bigger calipers for bigger discs weigh considerably more, but they don't affect rotational mass. There is no point saving 3-5kg per wheel with oz ultralegerras that cost a lot of money, then adding 10+kg per wheel of extra disc and caliper.
247mm discs:
Fitted to all vented 247mm brake setups(VTR/VTS/Furio/Rallye/xsi/GTi/306s etc)
Disc size: 247mm
Disc thickness: 20.5mm
Disc overall height: 27.9mm
Offset: 7.4mm
266mm discs:
Early type(fitted to cars made before year 2000 and some later cars too, all lucas calipers and some bosch)
Disc size: 266mm
Disc thickness: 20.5mm
Disc overall height: 27.9mm
Offset: 7.4mm
Late type(fitted to most cars after 2000 with bosch calipers)
Disc size: 266mm
Disc thickness: 22mm
Disc overall height: 34.5mm
Offset: 12.5mm
283mm Discs:
Early type(Gti6 & Xsara VTS lucas calipers):
Disc size: 283mm
Disc thickness: 22mm
Disc height: 27mm
Offset: 5mm
Late type(206 GTi180, most 307s bosch calipers)
Disc size: 283mm
Disc thickness: 26mm
Disc height: 34mm
Offset: 8mm
Those are the popular PSA applications and this should allow you to order the correct discs for your big brake conversions.
Disc choices:
Plain discs:
Drilled discs:
Grooved discs(Hispec, EBC plain discs machined)
Combi disc:
Standard plain discs: These are what you'll get if you order from citroen or a motor factors. They are perfectly adequate for all road use with occasional track use, they are cheap to replace too which is why most people use them.
Grooved budget discs: These are cheap plain discs with grooves machined into them. They will offer slightly higher performance but potentially reduced longevity and reliability.
Budget drilled discs: Again cheap plane discs cross drilled. These offer potential performance gains but i would be very wary of the quality of these, they are prone to cracking and distortion.
Combi(drilled and grooved discs): A combination of grooves and drilling to offer ultimate performance gains but with the downfalls of both grooves and drilling.
Quality discs are a different ballgame all together, they are made from scratch and are produced from higher quality metals with higher tollerances as well as better quality machining, real testing done and as a result they will usually outlast the cheaper discs when used hard on track.
What does drilling do to a disc?
Drilling allows the gasses and dust from the pad to escape through the disc and give edges for the pads to "grab" on to, this will increase the available braking power, they also increase the surface area so increase cooling. They do however reduce the heat capacity of the discs and drilled holes make it very easy for the discs to crack. Even porsches which come with drilled discs as standard have problems with their drilled discs, and we're talking £300 a piece discs, not the £100 a pair discs people fit to saxos. I personally never have and never will use drilled discs. These will increase pad wear over plain discs also.
Cracked rotor from a 2 piece drilled disc.
What do grooves do to a disc?
Grooves do more or less the same as drilling except grooves are less likely to cause stress cracks due to their shape and the fact that they do not go right the way through the disc. They allow gasses to escape from under the pad surface, offer improved cooling, the tangential grooves clear the surface of the pads(deglazing) by cutting into them, this increases braking power but also wears the pads much quicker than plain discs. I would recommend decent grooved discs from a reputable company for track use but wouldn't advise their use on road cars as they won't make much difference in road use apart from eating pads and making lots of noise.
Single and 2 piece discs:
Most discs are single piece, they are cast, machined, balanced etc. 2 piece discs feature a cast and machined outer rotor with is then bolted to a lightweight alloy centre bell. This is done to reduce the heat transferred to the hub, improve the cooling of the rotor and also reduce the weight of the rotor.
Interesting that they use plain discs but 2 piece and for a corvette
That is all for now folks.
You may think that brake discs can handle very high temperatures and they can, but there is no use having brake pads that can handle very high temperatures if your brake discs overheat and crack well below those temperatures, and by the same theory there's no point having huge expensive discs on a car that won't warm them up.
A disc stores and radiates the heat caused during braking, the brake pad creates the heat, it doesn't really hold anywhere near as much heat energy as the disc itself. Therefore the more metal there is(thickness, diameter, vented/solid) then the more capable a cars braking ability is.
Thicker discs: If you make the disc thicker you have more metal to hold the heat therefore a higher specific heat capacity.
Larger discs: If you make the diameter larger(say 266mm instead of 247mm) you are not only increasing the specific heat capacity but also the potential braking force. This is because you are increasing the leverage of the pad on the disc as you are moving the point of leverage further from the centre. One must however consider a disc spinning at say 100rpm, if the disc is larger the outer edge of the disc will be travelling at a higher speed(still rotating at the same revolutions per minute but then the disc is larger so the outside edge is travelling a longer distance) than that of a small disc so it will increase the friction but it will also create more heat. So with increased braking force comes more heat.
Vented or solid discs: A solid disc is just as it says a solid disc which the pads act on. A vented disc is more like a sandwich, where they took the solid disc, chopped it down the centre and put some spacers in between each half so that air can flow between the two halfs of the disc to cool it. Vented discs will have a higher amount of metal in them so a higher specific heat capacity and also they will cool down quicker when they do get hot as their surface area has more or less been doubled. Some vented discs use curved vanes inside them to scoop air from inside the wheel through the disc as it spins.
Popular disc applications:
Saxo base model 3 stud - Solid discs, 238mm diameter, rear drums
Saxo with power steering & 4 stud: Solid discs 247mm diameter, rear drums
Saxo sport models(VTR/VTS & furio): Vented discs 247mm diameter, rear solid discs 247mm diameter.
Peugeot 206 & C2/C3 sport models: Vented front discs 266mm and 283mm diameter, rear solid discs 247mm diameter.
Peugeot 306/Xsara sport models: Vented front discs 266mm and 283mm diameter, rear solid discs 247mm diameter.
As you can see the larger and heavier cars use larger discs on the front but the rear discs are still only 247mm because they are more than adequate for a car weighing 500kg more than a saxo(that's over 50% more!!!)
Furthermore, the newer brake setups(year 2000 and newer) have started using thicker discs, the old 266mm discs are 20.5mm thick, the new are 22mm thick, the old 283mm discs were 22mm thick, the new ones a whopping 26mm thick)
One thing to remember though is that bigger is not necessarily better. Larger discs weigh a lot more, this increases rotational mass as well as upsprung weight, and both of these weights need to be kept as low as possible. Also bigger calipers for bigger discs weigh considerably more, but they don't affect rotational mass. There is no point saving 3-5kg per wheel with oz ultralegerras that cost a lot of money, then adding 10+kg per wheel of extra disc and caliper.
247mm discs:
Fitted to all vented 247mm brake setups(VTR/VTS/Furio/Rallye/xsi/GTi/306s etc)
Disc size: 247mm
Disc thickness: 20.5mm
Disc overall height: 27.9mm
Offset: 7.4mm
266mm discs:
Early type(fitted to cars made before year 2000 and some later cars too, all lucas calipers and some bosch)
Disc size: 266mm
Disc thickness: 20.5mm
Disc overall height: 27.9mm
Offset: 7.4mm
Late type(fitted to most cars after 2000 with bosch calipers)
Disc size: 266mm
Disc thickness: 22mm
Disc overall height: 34.5mm
Offset: 12.5mm
283mm Discs:
Early type(Gti6 & Xsara VTS lucas calipers):
Disc size: 283mm
Disc thickness: 22mm
Disc height: 27mm
Offset: 5mm
Late type(206 GTi180, most 307s bosch calipers)
Disc size: 283mm
Disc thickness: 26mm
Disc height: 34mm
Offset: 8mm
Those are the popular PSA applications and this should allow you to order the correct discs for your big brake conversions.
Disc choices:
Plain discs:
Drilled discs:
Grooved discs(Hispec, EBC plain discs machined)
Combi disc:
Standard plain discs: These are what you'll get if you order from citroen or a motor factors. They are perfectly adequate for all road use with occasional track use, they are cheap to replace too which is why most people use them.
Grooved budget discs: These are cheap plain discs with grooves machined into them. They will offer slightly higher performance but potentially reduced longevity and reliability.
Budget drilled discs: Again cheap plane discs cross drilled. These offer potential performance gains but i would be very wary of the quality of these, they are prone to cracking and distortion.
Combi(drilled and grooved discs): A combination of grooves and drilling to offer ultimate performance gains but with the downfalls of both grooves and drilling.
Quality discs are a different ballgame all together, they are made from scratch and are produced from higher quality metals with higher tollerances as well as better quality machining, real testing done and as a result they will usually outlast the cheaper discs when used hard on track.
What does drilling do to a disc?
Drilling allows the gasses and dust from the pad to escape through the disc and give edges for the pads to "grab" on to, this will increase the available braking power, they also increase the surface area so increase cooling. They do however reduce the heat capacity of the discs and drilled holes make it very easy for the discs to crack. Even porsches which come with drilled discs as standard have problems with their drilled discs, and we're talking £300 a piece discs, not the £100 a pair discs people fit to saxos. I personally never have and never will use drilled discs. These will increase pad wear over plain discs also.
Cracked rotor from a 2 piece drilled disc.
What do grooves do to a disc?
Grooves do more or less the same as drilling except grooves are less likely to cause stress cracks due to their shape and the fact that they do not go right the way through the disc. They allow gasses to escape from under the pad surface, offer improved cooling, the tangential grooves clear the surface of the pads(deglazing) by cutting into them, this increases braking power but also wears the pads much quicker than plain discs. I would recommend decent grooved discs from a reputable company for track use but wouldn't advise their use on road cars as they won't make much difference in road use apart from eating pads and making lots of noise.
Single and 2 piece discs:
Most discs are single piece, they are cast, machined, balanced etc. 2 piece discs feature a cast and machined outer rotor with is then bolted to a lightweight alloy centre bell. This is done to reduce the heat transferred to the hub, improve the cooling of the rotor and also reduce the weight of the rotor.
Interesting that they use plain discs but 2 piece and for a corvette
That is all for now folks.