Stock brakes too weak?

[wparsons]

Originally Posted by BRZ68

I worry about braking ahead of time with this car more than the other cars i'm driving almost every other day..that includes a camry, a Lexus es350, and a Mercedes e350. HP primacy tires looses grip way ahead of time more than Yokohamas and Bridgestones that come stock on those sedans. The difference is night and day. Im not saying that the tires are the only out of so many factors in stopping, but theyre a major one.

I would love to see a side by side comparison... the primacy tires aren't nearly as greasy as people make them out to be when compared to an average all season tire. They suck as a pure summer compared to GOOD extreme summer tires, but compared to an all season they're still a decent tire.

What's probably happening is that your other cars dive a lot more under braking (the camry for sure, my wife has one) so it feels like they're braking harder but they aren't. The FRS/BRZ brakes pretty flat for a stock car.

[troek]

stock yokohamas that come on the 16" wheels over here were surprisingly grippy. i think this car does very well at making full use of a tires potential. i had a bast on those tires keeping up around turns with some cars with much more sticky tires. only reason i swapped em is some high speed understeer i got once or twice kinda scared me. as far as brakes with aftermarket pads the only increase i feel your gona get with bigger brakes is less chance of fading. stronger brakes arnt going to stop the ABS from kicking in.

[Perihelion]

brakes saved me twice today. i think they are okay

[bkblitzed]

I think our stock brakes are fine. I did cook them on my first canyon run, but the car went faster through it than my old STi did so that's a big +1. The tires for being so skinny i think are great too, cant wait till i put in some 245's and have this thing handle like it's on rails.

[IMOA]

These days I usually steer clear of these discussions but I've got some time to kill so what the hell.

Short version - CSG_Mike is right, it's the tyres that determine the stopping distance

Medium version (I've got a very long version with all the maths/physics somewhere from back from the days I was racing and helping people build cars but I've lost it so I'll do the abbreviated version)

There's a number of factors which affect how much braking force can be generated by the braking system, the summary is as follows:

Brake pedal - push harder, more braking force
Brake master cylinder - smaller diameter, more braking force
Caliper pistons - more caliper piston (face) area, more braking force
Pad material - higher coefficient of friction, more braking force
Disk diameter - pad further away from centre, more braking force

Car weight - heavier car, more force required
Car speed - higher speed, more force required

However no matter how much force you generate in the braking system that force only does something when it acts on the ground and that is done entirely by the tyres. In effect the tyres can be in two states, rolling or locked and when they're locked they lose a significant amount of their ability to apply that braking force to the road (yes, yes, we're talking paved roads here, not gravel, snow etc)

So basically you can do lots of things to the braking system to increase the amount of force it is able to generate but if that force is in excess of what the tyres can apply to the road it will go to waste. As a result the tyres create the ceiling of what force can be applied so once you are at that ceiling you cannot apply further braking force to the road therefore changes to the braking system will not reduce the stopping distance.

To be very clear, once your braking system has the ability to lock the wheels at a certain speed then the only way to reduce the stopping distance is to get stickier tyres to raise the ceiling.

The most common mistake that people make is to put bigger brakes/better pads on a car and go for a drive, when they press the pedal the car stops quicker. Instinctively you think 'well if the car is stopping quicker when pressing the pedal this hard, when I really jump on the pedal it will stop even quicker so my braking distance has been reduced'. This kind of makes sense but it ignores the ceiling of the tyres, what it really means is that you are now able to reach that ceiling with less force on the brake pedal. So while it feels like the braking distance has reduced, it actually hasn't.

What this means is on a modern street car travelling at 'normal' speeds (ie, the braking system can generate enough force to lock the tyres) the way to reduce stopping distance is to change to better tyres, upgrading other components may help other factors (pedal feel, pedal length, unsprung weight etc) but you won't stop quicker.

On a track car where you are stopping from much higher speeds on stickier rubber it's likely that the standard brake system will not have the ability to lock the tyres so increasing the braking force will reduce stopping distances. Also on a track car since you're braking heavily from high speeds regularly having a braking system which can both handle higher temperatures and dissipate that heat more quickly becomes far more important than for a street car, and that includes street cars used for spirited country roads.

In short, if you want to stop in a shorter distance change your tyres, changing your brakes will only help once you have enough grip that the wheels don't lock at the point you're pressing the pedal the hardest.

[Grip Ronin]

i just felt that the stock pads didnt bite enough, i just put stoptechs on the front and braking has become great and consistent

[Racecomp Engineering]

Originally Posted by IMOA

These days I usually steer clear of these discussions but I've got some time to kill so what the hell.

Short version - CSG_Mike is right, it's the tyres that determine the stopping distance

Medium version (I've got a very long version with all the maths/physics somewhere from back from the days I was racing and helping people build cars but I've lost it so I'll do the abbreviated version)

There's a number of factors which affect how much braking force can be generated by the braking system, the summary is as follows:

Brake pedal - push harder, more braking force
Brake master cylinder - smaller diameter, more braking force
Caliper pistons - more caliper piston (face) area, more braking force
Pad material - higher coefficient of friction, more braking force
Disk diameter - pad further away from centre, more braking force

Car weight - heavier car, more force required
Car speed - higher speed, more force required

However no matter how much force you generate in the braking system that force only does something when it acts on the ground and that is done entirely by the tyres. In effect the tyres can be in two states, rolling or locked and when they're locked they lose a significant amount of their ability to apply that braking force to the road (yes, yes, we're talking paved roads here, not gravel, snow etc)

So basically you can do lots of things to the braking system to increase the amount of force it is able to generate but if that force is in excess of what the tyres can apply to the road it will go to waste. As a result the tyres create the ceiling of what force can be applied so once you are at that ceiling you cannot apply further braking force to the road therefore changes to the braking system will not reduce the stopping distance.

To be very clear, once your braking system has the ability to lock the wheels at a certain speed then the only way to reduce the stopping distance is to get stickier tyres to raise the ceiling.

The most common mistake that people make is to put bigger brakes/better pads on a car and go for a drive, when they press the pedal the car stops quicker. Instinctively you think 'well if the car is stopping quicker when pressing the pedal this hard, when I really jump on the pedal it will stop even quicker so my braking distance has been reduced'. This kind of makes sense but it ignores the ceiling of the tyres, what it really means is that you are now able to reach that ceiling with less force on the brake pedal. So while it feels like the braking distance has reduced, it actually hasn't.

What this means is on a modern street car travelling at 'normal' speeds (ie, the braking system can generate enough force to lock the tyres) the way to reduce stopping distance is to change to better tyres, upgrading other components may help other factors (pedal feel, pedal length, unsprung weight etc) but you won't stop quicker.

On a track car where you are stopping from much higher speeds on stickier rubber it's likely that the standard brake system will not have the ability to lock the tyres so increasing the braking force will reduce stopping distances. Also on a track car since you're braking heavily from high speeds regularly having a braking system which can both handle higher temperatures and dissipate that heat more quickly becomes far more important than for a street car, and that includes street cars used for spirited country roads.

In short, if you want to stop in a shorter distance change your tyres, changing your brakes will only help once you have enough grip that the wheels don't lock at the point you're pressing the pedal the hardest.

Clear, concise, and correct. Well done.


- andy

[qoncept]

It's still absolutely wrong. Play with the calculator below. You'll see that when you increase the maximum available force you also increase the maximum static (rolling) friction. In other words, if your brakes are stronger, you can apply more force before you lock them up. It's a chain that starts at the pad and ends at the road.

It's right here, clear as day. Unsubbed.

http://www.ajdesigner.com/phpfrictio...imum_force.php

Example:
We'll make up a coefficient for brake pads on rotors since I reall have no idea.. lets say 0.8.

- Stock brakes apply 20 newtons of force, which gives us a max of 16 newtons of friction on the rotor. Tires on road is a coefficient of .9, and the 16 newtons we're applying gives us stopping force of 14.4 newtons.

- Megabrake applies 25 newtons of force. Same coefficient of .8 means the rotor sees 20 newtons of friction, and then the tire on the road sees 18 newtons of maximum static friction.

Of course, the friction required to stop a car increases exponentially. Same deal with power and acceleration.

[celica73]

Originally Posted by qoncept

It's still absolutely wrong. Play with the calculator below. You'll see that when you increase the maximum available force you also increase the maximum static (rolling) friction. In other words, if your brakes are stronger, you can apply more force before you lock them up. It's a chain that starts at the pad and ends at the road.

It's right here, clear as day. Unsubbed.

http://www.ajdesigner.com/phpfrictio...imum_force.php

Example:
We'll make up a coefficient for brake pads on rotors since I reall have no idea.. lets say 0.8.

- Stock brakes apply 20 newtons of force, which gives us a max of 16 newtons of friction on the rotor. Tires on road is a coefficient of .9, and the 16 newtons we're applying gives us stopping force of 14.4 newtons.

- Megabrake applies 25 newtons of force. Same coefficient of .8 means the rotor sees 20 newtons of friction, and then the tire on the road sees 18 newtons of maximum static friction.

Of course, the friction required to stop a car increases exponentially. Same deal with power and acceleration.

I don't think you are using the equation properly.

[Racecomp Engineering]

Originally Posted by qoncept

- Stock brakes apply 20 newtons of force, which gives us a max of 16 newtons of friction on the rotor. Tires on road is a coefficient of .9, and the 16 newtons we're applying gives us stopping force of 14.4 newtons.

- Megabrake applies 25 newtons of force. Same coefficient of .8 means the rotor sees 20 newtons of friction, and then the tire on the road sees 18 newtons of maximum static friction.

Of course, the friction required to stop a car increases exponentially. Same deal with power and acceleration.

Yes obviously more brake force means more brake force, but if the wheel locks up at just 10 newtons of stopping force, it doesn't matter if you have you a 4.4 or 8 or a bazillion newtowns in reserve. Push the pedal harder, you aren't going to stop faster. You also can't lock up the wheels, and then lock them up even more.

The tire doesn't care if your rotor diameter is bigger and/or your piston piston area has increased or even how hard you push the pedal. The ideal amount of force is the force right before the tire begins to slip because as you have pointed out, static mu is more than slipping/kinematic mu. This ideal amount may be easy to achieve (in most cases) on a stock vehicle on stock tires. You just push the pedal really hard and you have more than enough force.

For what it's worth, I've seen many brake tests where bigger brakes with equal tires resulted in longer stopping distances. In some cases due to brake bias...and the fronts getting locked up too easily.

In other words, if your brakes are stronger, you can apply more force before you lock them up

No, you can just lock them up more easily. It's the same amount of force. The force that the tire starts sliding at is not going to increase because you have more force available.

I really don't understand what you're getting at. I understand kinematics quite well.

- andy

[qoncept]

Originally Posted by Racecomp Engineering

Yes obviously more brake force means more brake force, but if the wheel locks up at just 10 newtons of stopping force, it doesn't matter if you have you a 4.4 or 8 or a bazillion newtowns in reserve. Push the pedal harder, you aren't going to stop faster. You also can't lock up the wheels, and then lock them up even more.

The examples I just posted (see the bold) say that the wheels lock up at 14.4 newtons in the stock example, 18 newtons in the upgraded brake example.

The number you're calculating is maximum static friction. Increase force, increase maximum static friction. Stronger brakes increase the threshold of friction between the road and the tire before locking up.

So no, you don't just lock up the brakes easier.

[Dave-ROR]

Originally Posted by qoncept

It's still absolutely wrong. Play with the calculator below. You'll see that when you increase the maximum available force you also increase the maximum static (rolling) friction. In other words, if your brakes are stronger, you can apply more force before you lock them up. It's a chain that starts at the pad and ends at the road.

It's right here, clear as day. Unsubbed.

http://www.ajdesigner.com/phpfrictio...imum_force.php

Example:
We'll make up a coefficient for brake pads on rotors since I reall have no idea.. lets say 0.8.

- Stock brakes apply 20 newtons of force, which gives us a max of 16 newtons of friction on the rotor. Tires on road is a coefficient of .9, and the 16 newtons we're applying gives us stopping force of 14.4 newtons.

- Megabrake applies 25 newtons of force. Same coefficient of .8 means the rotor sees 20 newtons of friction, and then the tire on the road sees 18 newtons of maximum static friction.

Of course, the friction required to stop a car increases exponentially. Same deal with power and acceleration.

Since when does the amount of braking force determine the limits of adhesion for a tire? If you can EXCEED the limits of adhesion than a stronger brake setup nets you ABOLUTELY NOTHING except being able to exceed the limits of adhesion more easily and being able to exceed the limits of adhesion if you increase that limit (ie better tires). However, that will have no effect if the original brakes could also exceed the limits of adhesion that you just increased.

The tires stop the car. Period. If you can lock them up then threshold braking will net you the shortest stopping distance with any brake force >= the force required to threshold brake. You could generate 1,000 newtons, your stopping distance will NOT change (at that increase it might be worse since it would be much harder to threshold brake in that setup most likely ).

[Dave-ROR]

Originally Posted by qoncept

The examples I just posted (see the bold) say that the wheels lock up at 14.4 newtons in the stock example, 18 newtons in the upgraded brake example.

The number you're calculating is maximum static friction. Increase force, increase maximum static friction. Stronger brakes increase the threshold of friction between the road and the tire before locking up.

So no, you don't just lock up the brakes easier.

No, they don't.

[qoncept]

Originally Posted by Dave-ROR

Since when does the amount of braking force determine the limits of adhesion for a tire?

Since the invention of the wheel I suppose. Static friction isn't just a good idea, it's the law.

If you can EXCEED the limits of adhesion than a stronger brake setup nets you ABOLUTELY NOTHING except being able to exceed the limits of adhesion more easily and being able to exceed the limits of adhesion if you increase that limit (ie better tires).

No, you can't exceed the limits of friction and stop any faster. But you can change the limit by changing the force applied to it.