why do bigger brakes make you stop faster?
Discussion
eldar said:
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.
What cars have you tried?
What nonsense! Most road cars will not have the power to lock the wheels (read engage abs) above say 70mph or so.What cars have you tried?
Higher than that and its just instant brake fade unless you have something beefy.
eldar said:
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?
Bennet said:
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?
Edited by balls-out on Tuesday 10th November 16:20
Bennet said:
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.currybum said:
R8VXF said:
And also provides a larger surface area to transfer more friction to stop the wheel rotating.
Braking force in itself is independent of the pad area for a given material. It is all about heat dissipation and component protection. A pad of 40cm^2 will give exactly the same braking force as a pad of 80cm^2 of the same material and applied force. The smaller pad will get hotter quicker and be unable to dissipate as much heat so will fade/wear/fail much faster than the bigger pad. But the bigger pad will require a bigger disk and calliper so will add weight. As with all engineering it is an attribute/weight/cost/performance trade off discussion.
SkinnyPete said:
What nonsense! Most road cars will not have the power to lock the wheels (read engage abs) above say 70mph or so.
Higher than that and its just instant brake fade unless you have something beefy.
Sure, I get instant brake fade every time I brake from 80mph.Higher than that and its just instant brake fade unless you have something beefy.
Go and try it.
SkinnyPete said:
What nonsense! Most road cars will not have the power to lock the wheels (read engage abs) above say 70mph or so.
I can lock the wheels of either of our cars in the dry at motorway speeds (80-90) and one of them isn't even servo assisted. It takes a fair old shove in the Elise but activating the ABS in the Skoda at 80 takes remarkably little pedal force. s it was a drive video, found it on FB, but it is only a short teaser to the now private YT vid https://www.facebook.com/DriveTV/videos/8708960863...
R8VXF said:
s it was a drive video, found it on FB, but it is only a short teaser to the now private YT vid https://www.facebook.com/DriveTV/videos/8708960863...
Here's the video, I watched it earlier... 
https://www.youtube.com/watch?v=qkIowmqOFRg
SkinnyPete said:
What nonsense! Most road cars will not have the power to lock the wheels (read engage abs) above say 70mph or so.
Higher than that and its just instant brake fade unless you have something beefy.
you can't exchange "engage ABS" for locked wheels - its completely different in terms of where the energy goes.Higher than that and its just instant brake fade unless you have something beefy.
Brake fade is caused by build-up of heat. You will will NOT get "instant" build up of heat and so brake fade.
EricE said:
R8VXF said:
s it was a drive video, found it on FB, but it is only a short teaser to the now private YT vid https://www.facebook.com/DriveTV/videos/8708960863...
Here's the video, I watched it earlier... https://www.youtube.com/watch?v=qkIowmqOFRg
Wondered where it had gone 
eldar said:
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?
However. Do you know that feeling at all when you stand on the brake and experience a moment of fear when you realise you aren't slowing as fast as you need to, yet nor are you sliding or activating the ABS? That's what I'm thinking of when I say that full brake force won't lock the wheels when stopping from a high speed. To be honest, I'd have thought that the ability to lock up at 80mph would cause more crashes than it prevents.
It's also possible I wasn't braking hard enough.
Edited by Bennet on Tuesday 10th November 16:39
The answer to the op is
a) The larger the diameter of the disc the larger the pad can be and the greater friction surface you have also the larger the caliper can be so you can have 2,3,4 or greater pistons inside the caliper. This will be offset slightly by the larger disc traveling faster at it's perimeter than a smaller disc and hence requiring more force to stop it, so the gain will not be linear.
b) Heat dissipation, although not a factor in a single stomp on brakes emergency type situation, a sports car will need to get the heat out of the discs between corners to prevent damage to the pads.
c) Bigger is better, car would look daft with 19inch alloys and a weeny 10inch disc.
a) The larger the diameter of the disc the larger the pad can be and the greater friction surface you have also the larger the caliper can be so you can have 2,3,4 or greater pistons inside the caliper. This will be offset slightly by the larger disc traveling faster at it's perimeter than a smaller disc and hence requiring more force to stop it, so the gain will not be linear.
b) Heat dissipation, although not a factor in a single stomp on brakes emergency type situation, a sports car will need to get the heat out of the discs between corners to prevent damage to the pads.
c) Bigger is better, car would look daft with 19inch alloys and a weeny 10inch disc.
kambites said:
SkinnyPete said:
What nonsense! Most road cars will not have the power to lock the wheels (read engage abs) above say 70mph or so.
I can lock the wheels of either of our cars in the dry at motorway speeds (80-90) and one of them isn't even servo assisted. It takes a fair old shove in the Elise but activating the ABS in the Skoda at 80 takes remarkably little pedal force. Edited by SuperchargedVR6 on Tuesday 10th November 16:48
balls-out said:
Bennet said:
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?
Edited by balls-out on Tuesday 10th November 16:20
But I'm not a dynamics engineer and sadly don't have to think much about engineering these days but here's my stab at it:
To lock a wheel you must reduce it's rotational rate so it's slower than the passage of the road beneath it, which means reducing the inertia of the wheel and other roundy roundy bits, including brake discs, drive shafts, diff, gearbox and at least half a clutch, at higher speed the inertia is a lot higher so the brake must apply more force at higher speed vs lower speed to achieve the same rate of reduction of inertia, therefore it takes more force to lock the wheels.
Also, my daily is ABS free and if I for example brake firmly at 40mph and hold a constant pedal force the car will slow with no locking until about 5-10mph.
Though I must admit to not having put pedal to floor at high speeds, I fear I'd not fare well.
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