why do bigger brakes make you stop faster?
Discussion
currybum said:
Many people just assume more area more friction.
As a thought experiment have a think about what the world would be like if contact area was a function of linear frictional force...Skiing wouldn't me much fun ;-)
Most people assume friction is the perfectly behaved thing they were taught in school, where area makes no difference because for a constant force the reduction in pressure cancels it out. Real life isn't like that.As a thought experiment have a think about what the world would be like if contact area was a function of linear frictional force...Skiing wouldn't me much fun ;-)
R8VXF said:
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?
Maybe not the most even of surfaces though right enough...
Patrick Bateman said:
I've been near enough 3 figures and had that happen without completely stomping on the pedal.
Maybe not the most even of surfaces though right enough...
M3 and A329M are my most memorable. Decent surfaces for both of those. Apart from the last part of the A329M one where I had to steer into the central reservation. Maybe not the most even of surfaces though right enough...
Bigger brakes don't stop you faster - thats determined by friction on the road surface.
What they do give you is more 'bite' and the ability to dissipate heat faster.
If you think of the point where the pad contacts the disc is as a lever with the pivot on the axle the bigger the brake the longer the lever so the more effect it has (all other things being equal). This is why they tend to have more bite assuming same caliper, pad material and disc material and design.
The increased metallic mass gives a larger surface area to dissipate heat and if vented the potential for larger cooling vents. The relative area covered by the pad is also smnaller in relation to the disc surface area.
The larger diameter also allows bigger pads (negating the last to an extent) which will also give further 'bite' however a larger pad will also assist cooling by dissipating heat out the the caliper (which is also larger) better. A larger caliper can run larger piston(s) thus increasing the hydraulic leverage of the system (relationship between cross sectional area of the piston in the master cylinder to the cross sectional area of the piston) reducing the level of pedal effort (but not travel) for a given level of retardation. All of these factors will give the illusion of being able to stop faster but ultimately as long as you have enough braking effort to cause the tyre to break traction you won't actually stop any faster. In fact you're probably more likely to cause a lockup and extend braking distance as a result.
What they do give you is more 'bite' and the ability to dissipate heat faster.
If you think of the point where the pad contacts the disc is as a lever with the pivot on the axle the bigger the brake the longer the lever so the more effect it has (all other things being equal). This is why they tend to have more bite assuming same caliper, pad material and disc material and design.
The increased metallic mass gives a larger surface area to dissipate heat and if vented the potential for larger cooling vents. The relative area covered by the pad is also smnaller in relation to the disc surface area.
The larger diameter also allows bigger pads (negating the last to an extent) which will also give further 'bite' however a larger pad will also assist cooling by dissipating heat out the the caliper (which is also larger) better. A larger caliper can run larger piston(s) thus increasing the hydraulic leverage of the system (relationship between cross sectional area of the piston in the master cylinder to the cross sectional area of the piston) reducing the level of pedal effort (but not travel) for a given level of retardation. All of these factors will give the illusion of being able to stop faster but ultimately as long as you have enough braking effort to cause the tyre to break traction you won't actually stop any faster. In fact you're probably more likely to cause a lockup and extend braking distance as a result.
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?
I don't think you are missing anything. Bigger brakes simply don't make you stop any faster. If you can lock the brake with your foot then you've locked the brake no matter how big or small the disc is.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?
The advantage of bigger brakes is the 'feel' you get with them. With the larger lever you can have more control in the grey area between going, and locking. Its better for racing because of the extra feel. Not much else
Any car on sale these days will meet ECE Reg13h, which demands something like 0.6g from 62mph.
Realistically, anything from a respectable manufacturer will activate ABS and get over 1g, which pretty much means it's tyre/road adhesion that's the limiting factor. Fitting bigger brakes won't get shorter stopping distances.
Fitting bigger brakes will help with heat management, which is usually tested with repeated high speed acceleration and braking. Think Autobahn 250kph down to 100 kph a few times.
Realistically, anything from a respectable manufacturer will activate ABS and get over 1g, which pretty much means it's tyre/road adhesion that's the limiting factor. Fitting bigger brakes won't get shorter stopping distances.
Fitting bigger brakes will help with heat management, which is usually tested with repeated high speed acceleration and braking. Think Autobahn 250kph down to 100 kph a few times.
This test appears to demonstrate bigger brakes do reduce stopping distance. Whether the test is scientifically valid or not, I couldn't say
http://youtu.be/qkIowmqOFRg
http://youtu.be/qkIowmqOFRg
julian64 said:
The advantage of bigger brakes is the 'feel' you get with them. With the larger lever you can have more control in the grey area between going, and locking. Its better for racing because of the extra feel. Not much else
Probably different in racing, but in passenger cars, disc size is determined mainly from heat dissapation/cooling (and marketing!) requirements. Feel is mainly tuned with booster ratios and "jump in" (basically how hard you press the brakes before you get assistance).blank said:
Oh, and as the test is done with cold brakes and a limit on pedal effort (500N), you often get better stopping distances from an everyday shopping car than something like a carbon ceramic fitted supercar.
They've proved that a big brake setup can apply the same force with a reduced effort at the pedal which is correct. It does not prove you will stop quicker though when trying to apply the maximum force under proper braking conditions. You may be able to do the same with both setups.Wills2 said:
R8VXF said:
Topic is useless without a picture of some big brakes
For reference, those are 20" wheels.
Pah these are big disks! (20" wheels) For reference, those are 20" wheels.
Mind you, such a lard arse she needs all 400mm of them. 
Damn you and your extra 15mm! Yup big cars need big brakes. The Cayman s has comically small brakes for the wheels.
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). 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.
I've triggered the ABS a few times at motorway speeds with standard brakes in an average car with very good tyres on. The brake upgrades are useful if your into spirited driving and use the brakes heavily and repeatedly. 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.
yonex said:
J4CKO said:
but it just seems like manufacturers dont seem to scrimp on brakes like they used to.
Ahem, apart from BMW Bennet said:
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.
I think that'll be it. And it's exactly why most recent cars have "brake assist", where the car detects that you're doing an emergency stop and applies the full braking force you should have been applying but aren't...It's also possible I wasn't braking hard enough.
Almost any car in the last 50 years can lock its wheels or trigger ABS when stopping once from basically any speed. If it can't, it's broken.
Repeated stops (eg accelerate, brake, accelerate, brake on a country road), or long downhills car cause heat build up and brake fade, and larger brakes help with that, but that's a different thing.
Edited by AJB on Tuesday 10th November 21:01
Frustrated with this topic.
Bigger discs with same calipers = same force at bigger radius = more braking torque.
But bigger calipers = more force = even more braking torque.
Bigger discs = more surface area = better heat dissipation.
So bigger disks and calipers = better braking.
Bigger disks = less fade.
Also Bigger pads = pad lasting longer.
But bigger discs and pads = more weight and more expense.
I am an engineer, and yes I understand heat dissipation, and torque and all the other stuff, but wityhout putting maths to it, this relationship will hold true.
Drilling and grooving discs - can't see much point TBH. Tiny bit more surface area, and / or a tiny bit less weight? Not sure.
Bigger discs with same calipers = same force at bigger radius = more braking torque.
But bigger calipers = more force = even more braking torque.
Bigger discs = more surface area = better heat dissipation.
So bigger disks and calipers = better braking.
Bigger disks = less fade.
Also Bigger pads = pad lasting longer.
But bigger discs and pads = more weight and more expense.
I am an engineer, and yes I understand heat dissipation, and torque and all the other stuff, but wityhout putting maths to it, this relationship will hold true.
Drilling and grooving discs - can't see much point TBH. Tiny bit more surface area, and / or a tiny bit less weight? Not sure.
bearman68 said:
Frustrated with this topic.
Bigger discs with same calipers = same force at bigger radius = more braking torque.
But bigger calipers = more force = even more braking torque.
Bigger discs = more surface area = better heat dissipation.
So bigger disks and calipers = better braking.
Bigger disks = less fade.
Also Bigger pads = pad lasting longer.
But bigger discs and pads = more weight and more expense.
I am an engineer, and yes I understand heat dissipation, and torque and all the other stuff, but wityhout putting maths to it, this relationship will hold true.
Drilling and grooving discs - can't see much point TBH. Tiny bit more surface area, and / or a tiny bit less weight? Not sure.
You can't really fit bigger discs without changing (at least moving) calipers. If the calipers and mounting stay the same the biggest disc in the world won't give any more torque.Bigger discs with same calipers = same force at bigger radius = more braking torque.
But bigger calipers = more force = even more braking torque.
Bigger discs = more surface area = better heat dissipation.
So bigger disks and calipers = better braking.
Bigger disks = less fade.
Also Bigger pads = pad lasting longer.
But bigger discs and pads = more weight and more expense.
I am an engineer, and yes I understand heat dissipation, and torque and all the other stuff, but wityhout putting maths to it, this relationship will hold true.
Drilling and grooving discs - can't see much point TBH. Tiny bit more surface area, and / or a tiny bit less weight? Not sure.
The point is that most of the time the limiting factor is tyre/road adhesion and not braking force.
Drilling/grooving can actually make a big difference to cooling.
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