why have bigger brakes?
22 July 2013, 01:34 PM
#91
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Without it you have less weight to decelerate and the lack of downforce at the rear might give you more traction on the front wheels which is where the majority of braking occurs?
22 July 2013, 02:15 PM
#92
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Sent this link to my dad to comment on...he is retired now but was a supervising engineer at Ford specialising in brakes. One of his projects was the mkI Focus ESP and another ABS for the transit, so I thought you lot might find this interesting.
"If the cars weigh the same and stop from the same speed then there is no difference in the energy absorbed by the brakes. However, its always best to match all the features of the car. After all you legally must have tyres with a speed capability commensurate with the performance of the car even though there is a 70mph limit. Its the same with the brakes. The more power the engine has the more power the brakes potentially must absorb. A higher power car has potential for both higher speeds and greater acceleration making for greater power going into the brakes. You need to think about power not energy. The scenario of different power cars braking from the same speed in a one off event is simple. Both cars have the same energy so the brakes will generate the same amount of heat the brakes converting the kinetic energy into heat. However the higher power car has the ability to repeat the stop more frequently reducing the cooling time for the brakes, the temperatures will soon ratchet up. The other important factor is the speed. Kinetic energy is proportional to the square of the speed; if you increase the speed by 50% the energy rises by 225%. And finally there is the human factor. Someone who spent time and energy and money to boost the power of the car and then doesn't drive any harder than the original power is either a saint or not completely honest.
P.S. grooves and holes in the brakes increase the surface are so increasing the cooling rate (but not much)."
I might be able to get him to reply to specific questions if there are any
"If the cars weigh the same and stop from the same speed then there is no difference in the energy absorbed by the brakes. However, its always best to match all the features of the car. After all you legally must have tyres with a speed capability commensurate with the performance of the car even though there is a 70mph limit. Its the same with the brakes. The more power the engine has the more power the brakes potentially must absorb. A higher power car has potential for both higher speeds and greater acceleration making for greater power going into the brakes. You need to think about power not energy. The scenario of different power cars braking from the same speed in a one off event is simple. Both cars have the same energy so the brakes will generate the same amount of heat the brakes converting the kinetic energy into heat. However the higher power car has the ability to repeat the stop more frequently reducing the cooling time for the brakes, the temperatures will soon ratchet up. The other important factor is the speed. Kinetic energy is proportional to the square of the speed; if you increase the speed by 50% the energy rises by 225%. And finally there is the human factor. Someone who spent time and energy and money to boost the power of the car and then doesn't drive any harder than the original power is either a saint or not completely honest.
P.S. grooves and holes in the brakes increase the surface are so increasing the cooling rate (but not much)."
I might be able to get him to reply to specific questions if there are any
22 July 2013, 02:18 PM
#93
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OK, first can I say sorry if I have missed mention of the point I'm about to make in this thread, but from the skim I've had I can't find it.
The NASIOC thread guy seems to have missed the key point that most 'you don't need bigger brakes' people miss. He basically says that ultimately tyres are the limiting factor and as long as you have the power to lock the tyres you can't stop quicker. This is NOT correct. First off you have to consider what is locking - it's not so much the tyre as the brake disc being squeezed by the calipers. so why does it lock? It locks because it has a pressure exerted on it that is sufficient to overcome the forces casuing it to rotate - fairly obvious. What is less obvious to some is that you can apply more pressure without making the disc lock. How is this? Well, I'll use a simplified example. If you have a two 'pot' caliper squeezing pads against the disc then you have a point on each side of the disc with maximum pressure being exerted which lines up with the 'pots'. It may not seem obvious, but pressure applied by the pad on the discs is not even accross the whole surface of the pad, instead the pressure is greatest where the piston is directly behind the pad. By increasing the number of 'pots' you help to spread the braking force more evenly across the pads and this is where you see improvement in braking performance as you don't have one single concentration point of the pads force on the disc, but rather a more evenly distribusted force over the pad. How does this stop you quicker? Well with the force more evenly distributed you can apply a greater force on the pad before a locking of the disc occurs. It's not the simplest point of physics to explain in words. Essentially though the best brakes are ones that can distribute braking force as equally as possible over the pad area so as to allow for more focrce to be applied before a pressure point great enough to lock the disc is created. If brakes are upgraded to setups that allow this then yes, you can stop the same car from the same speed on the same tyres in a shorter distance. I hope that makes sense. Ultimately the only point I am making is that the belief that 'if the brakes have enough force to lock the wheel then you can't stop quicker with bigger brakes' is not correct and that it may indeed be entirely possible to stop better. Then obviously you can get onto brake fade etc etc. As to whether or not most drivers will benefit from bigger brakes, that depends on the driver. I'm just saying that physics allows you to stop a car quicker by upgrading its brakes even if the standards had the power to lock its wheels.
The NASIOC thread guy seems to have missed the key point that most 'you don't need bigger brakes' people miss. He basically says that ultimately tyres are the limiting factor and as long as you have the power to lock the tyres you can't stop quicker. This is NOT correct. First off you have to consider what is locking - it's not so much the tyre as the brake disc being squeezed by the calipers. so why does it lock? It locks because it has a pressure exerted on it that is sufficient to overcome the forces casuing it to rotate - fairly obvious. What is less obvious to some is that you can apply more pressure without making the disc lock. How is this? Well, I'll use a simplified example. If you have a two 'pot' caliper squeezing pads against the disc then you have a point on each side of the disc with maximum pressure being exerted which lines up with the 'pots'. It may not seem obvious, but pressure applied by the pad on the discs is not even accross the whole surface of the pad, instead the pressure is greatest where the piston is directly behind the pad. By increasing the number of 'pots' you help to spread the braking force more evenly across the pads and this is where you see improvement in braking performance as you don't have one single concentration point of the pads force on the disc, but rather a more evenly distribusted force over the pad. How does this stop you quicker? Well with the force more evenly distributed you can apply a greater force on the pad before a locking of the disc occurs. It's not the simplest point of physics to explain in words. Essentially though the best brakes are ones that can distribute braking force as equally as possible over the pad area so as to allow for more focrce to be applied before a pressure point great enough to lock the disc is created. If brakes are upgraded to setups that allow this then yes, you can stop the same car from the same speed on the same tyres in a shorter distance. I hope that makes sense. Ultimately the only point I am making is that the belief that 'if the brakes have enough force to lock the wheel then you can't stop quicker with bigger brakes' is not correct and that it may indeed be entirely possible to stop better. Then obviously you can get onto brake fade etc etc. As to whether or not most drivers will benefit from bigger brakes, that depends on the driver. I'm just saying that physics allows you to stop a car quicker by upgrading its brakes even if the standards had the power to lock its wheels.
22 July 2013, 02:42 PM
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OK, first can I say sorry if I have missed mention of the point I'm about to make in this thread, but from the skim I've had I can't find it.
The NASIOC thread guy seems to have missed the key point that most 'you don't need bigger brakes' people miss. He basically says that ultimately tyres are the limiting factor and as long as you have the power to lock the tyres you can't stop quicker. This is NOT correct. First off you have to consider what is locking - it's not so much the tyre as the brake disc being squeezed by the calipers.
The NASIOC thread guy seems to have missed the key point that most 'you don't need bigger brakes' people miss. He basically says that ultimately tyres are the limiting factor and as long as you have the power to lock the tyres you can't stop quicker. This is NOT correct. First off you have to consider what is locking - it's not so much the tyre as the brake disc being squeezed by the calipers.
This is why it is easier to lock up in the wet.
22 July 2013, 02:58 PM
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But nowhere did I suggest it was. What I explained was that you can apply a greater force to a brake disc without causing it to lock using a better brake setup. Try to forget tyres locking and think about is as the brake disc locking casuing the tyre to then lock.
Here is perhaps a better worded explanation from another forum.
"Physics:
If you apply an increasing force to a 1 cylinder caliper with 2 pads thats are 10x15cm, at some point the pad will apply so much force to a so small area that the material will physically lock cause of the massive amount of friction.
If you apply an increasing force to a 4 cylinder caliper with pads that are 10x15cm each, the force will then spread across the full area of all the pads.
So when the increasing force reaches 10N the total amount of friction will be the same in both setups and thus the braking power will be the same at 10N. But since the 10N is spread across a larger area the specific friction of each pad is less on the larger setup compared to the smaller. When the smaller system locks the larger one wont cause the friction is spread and will not surpass the materials friction/area limit."
Hope that makes what I was saying clearer.
22 July 2013, 03:03 PM
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But nowhere did I suggest it was. What I explained was that you can apply a greater force to a brake disc without causing it to lock using a better brake setup. Try to forget tyres locking and think about is as the brake disc locking casuing the tyre to then lock.
Here is perhaps a better worded explanation from another forum.
"Physics:
If you apply an increasing force to a 1 cylinder caliper with 2 pads thats are 10x15cm, at some point the pad will apply so much force to a so small area that the material will physically lock cause of the massive amount of friction.
If you apply an increasing force to a 4 cylinder caliper with pads that are 10x15cm each, the force will then spread across the full area of all the pads.
So when the increasing force reaches 10N the total amount of friction will be the same in both setups and thus the braking power will be the same at 10N. But since the 10N is spread across a larger area the specific friction of each pad is less on the larger setup compared to the smaller. When the smaller system locks the larger one wont cause the friction is spread and will not surpass the materials friction/area limit."
Hope that makes what I was saying clearer.
Here is perhaps a better worded explanation from another forum.
"Physics:
If you apply an increasing force to a 1 cylinder caliper with 2 pads thats are 10x15cm, at some point the pad will apply so much force to a so small area that the material will physically lock cause of the massive amount of friction.
If you apply an increasing force to a 4 cylinder caliper with pads that are 10x15cm each, the force will then spread across the full area of all the pads.
So when the increasing force reaches 10N the total amount of friction will be the same in both setups and thus the braking power will be the same at 10N. But since the 10N is spread across a larger area the specific friction of each pad is less on the larger setup compared to the smaller. When the smaller system locks the larger one wont cause the friction is spread and will not surpass the materials friction/area limit."
Hope that makes what I was saying clearer.
Wheels lock 'cos:
Friction between disk and pad > adhesion between road and tyre.
It really doesn't matter if that friction between the disk and pad is over 10 cm2 or 20 cm2 in terms of stopping power.
Last edited by tony de wonderful; 22 July 2013 at 03:07 PM.
22 July 2013, 03:17 PM
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Ok but that doesn't increase stopping powers it just says you need to press less hard on the big brakes to get the same stopping power. Either way the grip between the tyre and road is the ultimate limiter in non track scenarios.
Wheels lock 'cos:
Friction between disk and pad > adhesion between road and tyre.
It really doesn't matter if that friction between the disk and pad is over 10 cm2 or 20 cm2 in terms of stopping power.
Wheels lock 'cos:
Friction between disk and pad > adhesion between road and tyre.
It really doesn't matter if that friction between the disk and pad is over 10 cm2 or 20 cm2 in terms of stopping power.
22 July 2013, 03:19 PM
#101
Right, I've read the first few posts, then skipped to the end
And I'm probably going to get shouted at for this.
But I've done what the OP has done on the original question.
I've gone from 230 to 350bhp ish. Never expected the power to be 'that' high when I finished, but started the modding with Godspeed discs, and orange stuff track pads.
On many many occasions, my friend in his version 7 sti (380bhp) is on brembos with Mintex 1144 pads. We've been out till our turbos are glowing, but the brakes still won't give up?
Now yes, better brakes stop you quicker, also taking into account, that when power is raised. The momentum force is greatly increased. Thus more force is required to stop 1.4 tonne of accelerating mass.
However, if I had the money, yes I'd upgrade to brembos. But I can't so I'm doing ok with what I've got. Just Remind yerself not to drive like a ***** all the time lol
Anyway, that's my 2p worth.
And I'm probably going to get shouted at for this.
But I've done what the OP has done on the original question.
I've gone from 230 to 350bhp ish. Never expected the power to be 'that' high when I finished, but started the modding with Godspeed discs, and orange stuff track pads.
On many many occasions, my friend in his version 7 sti (380bhp) is on brembos with Mintex 1144 pads. We've been out till our turbos are glowing, but the brakes still won't give up?
Now yes, better brakes stop you quicker, also taking into account, that when power is raised. The momentum force is greatly increased. Thus more force is required to stop 1.4 tonne of accelerating mass.
However, if I had the money, yes I'd upgrade to brembos. But I can't so I'm doing ok with what I've got. Just Remind yerself not to drive like a ***** all the time lol
Anyway, that's my 2p worth.
22 July 2013, 03:19 PM
#102
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And I'm not saying more pots gives a better brake caliper. There are a whole load of factors that govern how good a brake setup is, but in highly simplistic terms what you want is braking force distributed as evenly as possible over the contact patch between pad and disc and maximum ability to regulate temperature. It's all down to design. Regardless, it still stands that just cause a brake setup can lock tyres, doesn't mean you can't stop the car quicker on the same tyres.
22 July 2013, 04:01 PM
#106
"If the cars weigh the same and stop from the same speed then there is no difference in the energy absorbed by the brakes. However, its always best to match all the features of the car. After all you legally must have tyres with a speed capability commensurate with the performance of the car even though there is a 70mph limit. Its the same with the brakes. The more power the engine has the more power the brakes potentially must absorb. A higher power car has potential for both higher speeds and greater acceleration making for greater power going into the brakes. You need to think about power not energy. The scenario of different power cars braking from the same speed in a one off event is simple. Both cars have the same energy so the brakes will generate the same amount of heat the brakes converting the kinetic energy into heat. However the higher power car has the ability to repeat the stop more frequently reducing the cooling time for the brakes, the temperatures will soon ratchet up. The other important factor is the speed. Kinetic energy is proportional to the square of the speed; if you increase the speed by 50% the energy rises by 225%. And finally there is the human factor. Someone who spent time and energy and money to boost the power of the car and then doesn't drive any harder than the original power is either a saint or not completely honest.
To the other posters, yes, if you drive on the road, and don't do repeated high speed stops, your standard brakes will cope. It has NOTHING to do with the horse power of your car, but rather what speed you're braking from, and how often. If you use the car to e.g. accellerate up some dual carriage way, to NSL+a bit, then bigger brakes will not make you stop any faster if you have to brake, assuming that they have not got stuck pistons, and assuming that you're not running such sticky tyres that the brakes cannot overcome the friction between road / tyre (very unlikely, unless you run hot slicks).
If you're doing track driving, then yes, due to the repeated high speed stops, you will benefit massively from bigger brakes.
Also, if you drive the car hard enough to experience brake fade on the std 4 pot setup down e.g. some b-road, you *will* be driving recklessly and endangering yourself and other road users. Get yourself on track (and get bigger brakes - see above).
22 July 2013, 04:44 PM
#109
To the other posters, yes, if you drive on the road, and don't do repeated high speed stops, your standard brakes will cope. It has NOTHING to do with the horse power of your car, but rather what speed you're braking from, and how often. If you use the car to e.g. accellerate up some dual carriage way, to NSL+a bit, then bigger brakes will not make you stop any faster if you have to brake, assuming that they have not got stuck pistons, and assuming that you're not running such sticky tyres that the brakes cannot overcome the friction between road / tyre (very unlikely, unless you run hot slicks).
If you're doing track driving, then yes, due to the repeated high speed stops, you will benefit massively from bigger brakes.
Also, if you drive the car hard enough to experience brake fade on the std 4 pot setup down e.g. some b-road, you *will* be driving recklessly and endangering yourself and other road users. Get yourself on track (and get bigger brakes - see above).
so are you saying if someone brakes from 80 mph to zero in an identical car other than one has standard wrx brakes and the other a big brake conversion the stopping distance will be identical??
22 July 2013, 05:56 PM
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Sorry you wasted money on big brakes.
22 July 2013, 06:03 PM
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The limiting factor will be the tyres (assuming indentical tyres fitted to both cars). Either brake set up is capable of locking the wheels, and only a rotating wheel can effectively stop a car.
Where the big brakes score is on repeated hard brake application.
22 July 2013, 06:11 PM
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Suppose that's why all the manufacturers don't fit AP's. The only time you need to "keep" braking from high speed is on a track and most manufacturers expect that if you put your car on a track you will take that into account. I agree that acceleration and speed are all on the upside and what this argument is about is the downside, i. e. stopping, it doesn't matter how big your breaks are surely, it's the tyres that will let go.
Last edited by AndyBaker; 22 July 2013 at 06:12 PM.
22 July 2013, 06:19 PM
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Sent this link to my dad to comment on...he is retired now but was a supervising engineer at Ford specialising in brakes. One of his projects was the mkI Focus ESP and another ABS for the transit, so I thought you lot might find this interesting.
"If the cars weigh the same and stop from the same speed then there is no difference in the energy absorbed by the brakes. However, its always best to match all the features of the car. After all you legally must have tyres with a speed capability commensurate with the performance of the car even though there is a 70mph limit. Its the same with the brakes. The more power the engine has the more power the brakes potentially must absorb. A higher power car has potential for both higher speeds and greater acceleration making for greater power going into the brakes. You need to think about power not energy. The scenario of different power cars braking from the same speed in a one off event is simple. Both cars have the same energy so the brakes will generate the same amount of heat the brakes converting the kinetic energy into heat. However the higher power car has the ability to repeat the stop more frequently reducing the cooling time for the brakes, the temperatures will soon ratchet up. The other important factor is the speed. Kinetic energy is proportional to the square of the speed; if you increase the speed by 50% the energy rises by 225%. And finally there is the human factor. Someone who spent time and energy and money to boost the power of the car and then doesn't drive any harder than the original power is either a saint or not completely honest.
P.S. grooves and holes in the brakes increase the surface are so increasing the cooling rate (but not much)."
I might be able to get him to reply to specific questions if there are any
"If the cars weigh the same and stop from the same speed then there is no difference in the energy absorbed by the brakes. However, its always best to match all the features of the car. After all you legally must have tyres with a speed capability commensurate with the performance of the car even though there is a 70mph limit. Its the same with the brakes. The more power the engine has the more power the brakes potentially must absorb. A higher power car has potential for both higher speeds and greater acceleration making for greater power going into the brakes. You need to think about power not energy. The scenario of different power cars braking from the same speed in a one off event is simple. Both cars have the same energy so the brakes will generate the same amount of heat the brakes converting the kinetic energy into heat. However the higher power car has the ability to repeat the stop more frequently reducing the cooling time for the brakes, the temperatures will soon ratchet up. The other important factor is the speed. Kinetic energy is proportional to the square of the speed; if you increase the speed by 50% the energy rises by 225%. And finally there is the human factor. Someone who spent time and energy and money to boost the power of the car and then doesn't drive any harder than the original power is either a saint or not completely honest.
P.S. grooves and holes in the brakes increase the surface are so increasing the cooling rate (but not much)."
I might be able to get him to reply to specific questions if there are any
I know white Ford Transit drivers (the colour refers to the vehicle, before anyone says anything) often think they're behind the wheel of an awesome machine but................
Anyway, as your dad is so kind, my daughter's fezza has packed up.
I wonder if.............
(All done in the best possible taste
)
22 July 2013, 06:24 PM
#117
I've got to laugh at some of the comments some people make on this thread,i myself think its dangerous to put out info saying your fine with standard brakes if you don't do track days and drive to the speed limit,Some people do believe all they read.Who buys a Subaru to drive at just 30mph?who modifies a Subaru to 350+bhp to not use it?there is a lot of misinformation on this thread Ian of Godspeed brakes has already made a post on this thread and as he is a Specialist in that field if your not sure ask him what he thinks or Alyn of AS performance.Running the standard Subaru brakes with big power and hoping that it will stop the same as KSPORT,Porsche brakes or AP's is wishful thinking,I'm just waiting for the thread titled:Help i tried to stop my 350 bhp Subaru with standard brakes and crashed.SJ.
22 July 2013, 06:35 PM
#118
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One revolution of an 18in wheel travels TT (Pi) inches farther than 1 rev of a 17in one.
Given that brakes are designed to stop wheel rotation, let us assume that a set of brakes can bring a car to a dead stop in x revolutions of the wheel.
If car A has a set of 18in wheels it will stop in 18x inches. Car B with 17in wheels will stop in 17x inches.
As I've already said, each revolution of the wheels of car A will be TT inches farther than that of car B.
Car B will therefore come to a halt TTx inches shorter than Car A.
It therefore travels less distance, and all things being equal retardation must have been greater (same initial speed, same end speed but less distance covered).
Therefore it isn't the size of the discs that count but the size of the wheels.
Unless someone has another view.............