why do bigger brakes make you stop faster?
Discussion
This may be a terribly stupid question, so bear with me...
In a non-ABS car, when you stamp on the brakes (to carry out an emergency stop for example) the car comes to a halt. It's often advertised that bigger discs/pads allows you to stop quicker, but why? With your foot fully on the brakes doesn't it, theoretically, stop the wheels moving instantly - therefore not affected by the size of the contact?
Surely this is controlled a lot more by the tyres than the 'size' of the brakes?
What am I missing here?
In a non-ABS car, when you stamp on the brakes (to carry out an emergency stop for example) the car comes to a halt. It's often advertised that bigger discs/pads allows you to stop quicker, but why? With your foot fully on the brakes doesn't it, theoretically, stop the wheels moving instantly - therefore not affected by the size of the contact?
Surely this is controlled a lot more by the tyres than the 'size' of the brakes?
What am I missing here?
littleguy said:
With your foot fully on the brakes doesn't it, theoretically, stop the wheels moving instantly - therefore not affected by the size of the contact?
I'm not an engineer but the key point is that brakes in todays normal cars are nowhere near effective enough to instantly lock the wheels at higher speeds (>60 mph). I suspect your train of thought might be correct up to about 20 mph but the additional friction required to instantly lock the wheels at higher speeds is probably not linear.Larger brake pads and rotors offer more friction, more stopping power and thus shorter brake distances. They also dissipate heat better which is important for the track.
A number of factors
1) The larger the disk the more leverage there is.
2) The larger the disk the more cooler the disk gets/stays because there is more time for it to cool down again before it hits the pads again
3) The larger the disk the larger the pads can be without going multi-pad or curved pads.
4) The larger the disk the less (disk) wear there is.
On a related note as well as larger you get wider pad swept area which allows more pad area.
You also get thicker which allows better cooling / less heating.
Mostly though it's about multiple stops rather than any one single stop, if you can lock the wheel at will your brakes are already as good as they need to be but a bigger brake setup will keep doing this while a smaller setup would overheat.
1) The larger the disk the more leverage there is.
2) The larger the disk the more cooler the disk gets/stays because there is more time for it to cool down again before it hits the pads again
3) The larger the disk the larger the pads can be without going multi-pad or curved pads.
4) The larger the disk the less (disk) wear there is.
On a related note as well as larger you get wider pad swept area which allows more pad area.
You also get thicker which allows better cooling / less heating.
Mostly though it's about multiple stops rather than any one single stop, if you can lock the wheel at will your brakes are already as good as they need to be but a bigger brake setup will keep doing this while a smaller setup would overheat.
EricE said:
littleguy said:
With your foot fully on the brakes doesn't it, theoretically, stop the wheels moving instantly - therefore not affected by the size of the contact?
I'm not an engineer but the key point is that brakes in todays normal cars are nowhere near effective enough to instantly lock the wheels at higher speeds (>60 mph). I suspect your train of thought might be correct up to about 20 mph but the additional friction required to instantly lock the wheels at higher speeds is probably not linear.Larger brake pads and rotors offer more friction, more stopping power and thus shorter brake distances. They also dissipate heat better which is important for the track.
littleguy said:
This may be a terribly stupid question, so bear with me...
In a non-ABS car, when you stamp on the brakes (to carry out an emergency stop for example) the car comes to a halt. It's often advertised that bigger discs/pads allows you to stop quicker, but why? With your foot fully on the brakes doesn't it, theoretically, stop the wheels moving instantly - therefore not affected by the size of the contact?
Surely this is controlled a lot more by the tyres than the 'size' of the brakes?
What am I missing here?
It's just your initial assumption, that's all. In most non-ABS cars (all the ones I've driven anyway!), stamping on the brakes at 60mph will not result in the wheels locking, they will only lock at a lower speed. There therefore remains the potential to improve the performance of brakes and therefore stop quicker. Additionally, for track or demanding road use, better brakes will normally allow better cooling, which is needed for repeated stops without fading. Most normal road cars can't lap circuits continually without their brakes overheating, or even tackle a lot of demanding roads at speed, such as mountain passes. Cars are built to a cost.In a non-ABS car, when you stamp on the brakes (to carry out an emergency stop for example) the car comes to a halt. It's often advertised that bigger discs/pads allows you to stop quicker, but why? With your foot fully on the brakes doesn't it, theoretically, stop the wheels moving instantly - therefore not affected by the size of the contact?
Surely this is controlled a lot more by the tyres than the 'size' of the brakes?
What am I missing here?
littleguy said:
With your foot fully on the brakes doesn't it, theoretically, stop the wheels moving instantly - therefore not affected by the size of the contact?
I don't think instant stopping would be particularly pleasant. Shoving an iron bar through a bike wheel springs to mind. littleguy said:
Surely this is controlled a lot more by the tyres than the 'size' of the brakes?
What am I missing here?
I think we've had this debate before. Brake size is often determined by vehicle weight, vehicle performance and repeatability. That's why some Bentleys have 400+mm front discs. Imagine what a 2 tonne car stopping from 200mph would do to Citroen AX front brakes What am I missing here?
Tyres is a good point too. It's no use having monster brakes if the chassis is not able to deploy that stopping power effectively. I think that's why 911s have always been brilliant at stopping. Decent size tyres, decent brakes and a good chassis. In other words, Porsche brakes that a lot of VW scene boys throw onto their Golfs are nowhere near as effective as they are on a 911, simply because a Golf chassis is nowhere near as good as a 911s, if that makes sense?
I think it's all related to geometry, the size of the control arms, where they're anchored to the body, the type of bushings used etc etc. If you ever see a FWD car slam the brakes on in slow motion, the front wheel visibly kicks back towards the rear of the car. That's stopping energy not transferred to the tarmac. Do the same with a 911 and it's solid.
Edited by SuperchargedVR6 on Tuesday 10th November 15:33
Mr E said:
EricE said:
I'm not an engineer but the key point is that brakes in todays normal cars are nowhere near effective enough to instantly lock the wheels at higher speeds (>60 mph).
You're right. You're not an engineer.When I hadn't been driving long I remember imagining that I could lock up the wheels by braking hard enough at any speed. In practice, you can't. So what do you know that we don't Mr E?
littleguy said:
With your foot fully on the brakes doesn't it, theoretically, stop the wheels moving instantly - therefore not affected by the size of the contact?
Surely this is controlled a lot more by the tyres than the 'size' of the brakes?
You don't want to instantly stop the wheels moving - this is a skid, and doesn't help you stop. (ETA: well, at least not as quickly as brakes).Surely this is controlled a lot more by the tyres than the 'size' of the brakes?
- Energy cannot be created or destroyed, only changed from one form to another.
- Any moving mass has kinetic energy, which is (mass x velocity^2)/2
- When a car brakes it is reducing its velocity; obviously its mass stays the same, so it's energy has to reduce. We know from the top sentence that the energy has to go somewhere.
What your brakes are doing is changing the kinetic energy of the car into heat energy (which is also a type of kinetic energy) by friction. If you stamp on the brakes and lock the wheels, the job of generating heat is done by the tyres and tarmac, which aren't as good at it.
The brakes have to remove the heat from themselves either by conduction (air around the discs cools them) or radiation (when disks get so hot they glow orange). As mentioned above, heat dissipation of larger brakes is an important factor in being able to brake hard again and again and again (e.g. when racing). Greater surface area helps conduct heat to the surrounding air, which is also why discs are sometimes drilled, grooved, vented or various combinations of all three.
Edited by xRIEx on Tuesday 10th November 15:41
I could watch this all day https://www.youtube.com/watch?v=UWra93NkrHU
R8VXF said:
I could watch this all day https://www.youtube.com/watch?v=UWra93NkrHU
You'll like this one thenNothing like anything tested to destruction
xRIEx said:
You don't want to instantly stop the wheels moving - this is a skid, and doesn't help you stop. (ETA: well, at least not as quickly as brakes).
- Energy cannot be created or destroyed, only changed from one form to another.
- Any moving mass has kinetic energy, which is (mass x velocity^2)/2
- When a car brakes it is reducing its velocity; obviously its mass stays the same, so it's energy has to reduce. We know from the top sentence that the energy has to go somewhere.
What your brakes are doing is changing the kinetic energy of the car into heat energy (which is also a type of kinetic energy) by friction. If you stamp on the brakes and lock the wheels, the job of generating heat is done by the tyres and tarmac, which aren't as good at it.
The brakes have to remove the heat from themselves either by conduction (air around the discs cools them) or radiation (when disks get so hot they glow orange). As mentioned above, heat dissipation of larger brakes is an important factor in being able to brake hard again and again and again (e.g. when racing). Greater surface area helps conduct heat to the surrounding air, which is also why discs are sometimes drilled, grooved, vented or various combinations of all three.
Don't forget though, that in a wheel, where the mass is also counts, which is handled by the formula above. Mass near the centre of the wheel is moving at a slower velocity than mass at the rim of the wheel. So a mass at the rim has more energy than the same mass at the hub. Remove mass from the wheels themselves also helps in improving braking performance.- Energy cannot be created or destroyed, only changed from one form to another.
- Any moving mass has kinetic energy, which is (mass x velocity^2)/2
- When a car brakes it is reducing its velocity; obviously its mass stays the same, so it's energy has to reduce. We know from the top sentence that the energy has to go somewhere.
What your brakes are doing is changing the kinetic energy of the car into heat energy (which is also a type of kinetic energy) by friction. If you stamp on the brakes and lock the wheels, the job of generating heat is done by the tyres and tarmac, which aren't as good at it.
The brakes have to remove the heat from themselves either by conduction (air around the discs cools them) or radiation (when disks get so hot they glow orange). As mentioned above, heat dissipation of larger brakes is an important factor in being able to brake hard again and again and again (e.g. when racing). Greater surface area helps conduct heat to the surrounding air, which is also why discs are sometimes drilled, grooved, vented or various combinations of all three.
Edited by xRIEx on Tuesday 10th November 15:41
R8VXF said:
xRIEx said:
You don't want to instantly stop the wheels moving - this is a skid, and doesn't help you stop. (ETA: well, at least not as quickly as brakes).
- Energy cannot be created or destroyed, only changed from one form to another.
- Any moving mass has kinetic energy, which is (mass x velocity^2)/2
- When a car brakes it is reducing its velocity; obviously its mass stays the same, so it's energy has to reduce. We know from the top sentence that the energy has to go somewhere.
What your brakes are doing is changing the kinetic energy of the car into heat energy (which is also a type of kinetic energy) by friction. If you stamp on the brakes and lock the wheels, the job of generating heat is done by the tyres and tarmac, which aren't as good at it.
The brakes have to remove the heat from themselves either by conduction (air around the discs cools them) or radiation (when disks get so hot they glow orange). As mentioned above, heat dissipation of larger brakes is an important factor in being able to brake hard again and again and again (e.g. when racing). Greater surface area helps conduct heat to the surrounding air, which is also why discs are sometimes drilled, grooved, vented or various combinations of all three.
Don't forget though, that in a wheel, where the mass is also counts, which is handled by the formula above. Mass near the centre of the wheel is moving at a slower velocity than mass at the rim of the wheel. So a mass at the rim has more energy than the same mass at the hub. Remove mass from the wheels themselves also helps in improving braking performance.- Energy cannot be created or destroyed, only changed from one form to another.
- Any moving mass has kinetic energy, which is (mass x velocity^2)/2
- When a car brakes it is reducing its velocity; obviously its mass stays the same, so it's energy has to reduce. We know from the top sentence that the energy has to go somewhere.
What your brakes are doing is changing the kinetic energy of the car into heat energy (which is also a type of kinetic energy) by friction. If you stamp on the brakes and lock the wheels, the job of generating heat is done by the tyres and tarmac, which aren't as good at it.
The brakes have to remove the heat from themselves either by conduction (air around the discs cools them) or radiation (when disks get so hot they glow orange). As mentioned above, heat dissipation of larger brakes is an important factor in being able to brake hard again and again and again (e.g. when racing). Greater surface area helps conduct heat to the surrounding air, which is also why discs are sometimes drilled, grooved, vented or various combinations of all three.
Edited by xRIEx on Tuesday 10th November 15:41
t. We nearly forgot that.Mr E said:
EricE said:
I'm not an engineer but the key point is that brakes in todays normal cars are nowhere near effective enough to instantly lock the wheels at higher speeds (>60 mph).
You're right. You're not an engineer.Bennet said:
I'm an accountant and I also believe EricE is correct.
When I hadn't been driving long I remember imagining that I could lock up the wheels by braking hard enough at any speed. In practice, you can't. So what do you know that we don't Mr E?
Just about any 21st C. car will lock the front wheels in the dry at 80% of its max laden speed. You didn't press the pedal hard enough - or have EBS.When I hadn't been driving long I remember imagining that I could lock up the wheels by braking hard enough at any speed. In practice, you can't. So what do you know that we don't Mr E?
What cars have you tried?
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