some of you guys know absolutely nothing about brakes and how they function

 

Care to enlighten us?

 

I know you're a car guy but

1. I stated that hypothetically, both car going at the same speed. This is so that I can clearly explain my point on how mass has to do with kinetic energy.

2. Some where within my endless ramble, I stated that tires on both cars are of equal so that someone wont come along and tell me brakes don't stop the car, the tires does. If the tires are equal and its grip powers are infinite, the next thing that will help the car to stop quicker will be the ability of the brake to dissipate heat and the pads to maintain the highest possible coefficient of friction.

 

You're ignoring reality. The IS 250 (either of them) will not generate as much kinetic energy as the IS350 because they can't accelerate as hard. Brakes are sized based on anticipated heat load, then market demands for fashion. Big brakes are fashionable which is one of the reasons we see them on cars these days. The other reason is ultimate heat shedding capacity.

The accountants know they can save money on brakes for the IS 250s because the engineers say it's safe to run a less fade resistant brake on the the IS 250, so they put cheaper brakes on it and sell the car for less money. The engineers know it's safe because the IS250 does not accelerate as hard, so it doesn't need the same amount of heat resistance as the 350. The 350 has bigger brakes because it can generate more heat AND (more importantly) because fashion demands bigger brakes on an upscale car. Case in point is the IS-F. The brakes on the IS 350 would work fine on the IS-F, but because fashion dictates bigger brakes, Lexus put on drilled Brembo brakes.

Finally, to put all this to complete rest, at the Lexus Driving Academy, the instructors in the lead/follow event drove IS 250s while the "students" drove IS-Fs. My instructor in an IS 250 made us work to keep up, and he drove the car hard right off the parking lane. He never had any issues with fade, even with the stock 250 brakes.

So, really, the stock brakes on all three of the 2IS are MORE than adequate for the tasks they've been assigned.

 

Yes I am ignoring reality, I was trying to make a point on kinetic energy and momentum, I cant explain that easily if I include all the tire grip, heat dissipation, wind resistances, rotational inertia, lose of friction do to heat generation and all that nonsense.

And yes, 250 cannot accelerate with as much intensity as a 350.

And you prove my point, because I was trying to state originally on page 2, that if I was going at extreme speeds on the highway in a heavier 250, I would definately find a need for bigger brakes. Or not lol (I disproved my point below *smack*)

When you are on the highway people dont tend to drive until their speedo maxes out so they tend to ease off at a certain speed, so I was just trying to say if someone was going at a dangerous speed (which they shouldnt to begin with), it doesnt matter if that person was a 350 or a 250 as they are going at the same speed.

If they are going at the same speed, then the heavier weight of the 250 awd will have a higher kinetic enegry thus it will generate more heat than the 350 (if they use the same rotor and caliper) under braking and in turn will increase its braking distance (though probably very minuet). This is due to the increase in heat reducing the effective coeffiction of friction of the pads, again probably very minuet.

And thank you for stating that its most importantly a fashion statement

 

and I always thought a bigger part of the reason why companies put bigger brake kits on cars (slotted and drilled also applies)(oh and so do 20+ inch rims on sedans) because that's what the people want. More people care about how a car looks and sounds over what it can do.

I think most people don't even use their cars for the purpose they were built for ie sports cars for sporty driving(on or off track) hummers for rock climbing, range rovers for fording river, etc etc.

 

14's way too serious for me

 

I may be talking out of my ***, but just from a physics standpoint, if an object is beginning its deceleration from X to 0, how it got to X is irrelevant. Sure the IS350 would accelerate faster and thus generate more heat, but that energy is primarily being dissipated at the friction between tire and road - not at the point between brake and wheel.

At the instantaneous point of change at the beginning of deceleration, the stopping force (i.e. brakes) does not care about anything that came before because at that moment, the RPM of the wheel and tire are the same in both situations, given that wheel and tire diameters as well as coefficient of friction between tire and road are equal.

 

agreed...12 pistons are just for looks...imagine locking up everytime you brake

 

.....and that's why you'd want to have a proportioning valve to balance the front and rear brakes, not to mention that it would be a wise decision to upgrade the rear calipers and rotors as well.

 

This explanation needs to start with IGNORING REALITY AND PURELY FROM THE THEORETICAL VIEWPOINT. Then it would be exactly correct. HOWEVER, when you account for reality and understand the IS250 is a mid 15 second car, and the IS350 is a mid 13 second car, you'll figure out pretty quickly which car needs the more fade resistant brakes.

To directly address your example - assume both cars start from a stop. Both cars accelerate to the maximum speed they can attain for 1/4 mile. The IS350 is going a little over 100 mph, where the IS250 is going close to 90 mph. Lets assume both cars brake at full braking. Which one generates more heat, and how much more? First it should be intuitively obvious the IS350 will generate more heat, even if the IS250 is AWD (and the AWD car won't even make 90 because it's heavier). Since Ke = (V^2*m)/2, which of the differences will have the greater effect, the 10+ mph faster, or the 100 lbs heavier from the AWD? I'm betting on the squared factor with the 10+ mph...

So if I have to design a braking system to withstand the heat generated by the IS350 and the IS250, which one will need more ability to shed heat? Again, lets consider that the IS350 can do this same superior acceleration repeatedly and finally, let's decide the IS250 does not need as fade resistant braking as the IS350 does. Make sense?

All the stuff about starting from a given speed makes a huge bad assumption - that the IS250 can accelerate hard enough to attain the same speed in the same amount of time. We all know this is not true. So let's call it a day on that front.

AFA 12 piston calipers - if the volume/area ratio of the 12 pistons is the same as the OEM calipers, then no adjustments to anything are needed. They should bolt up and work just like OEM regardless of the number of cylinders because they have the same leverage ratio and volume characteristics as the OEM calipers. If the volume/area ratio is different, then the ABS will be unhappy.

FWIW, there is no proportioning valve in our cars. Brake proportioning is handled by a computer, not a mechanical valve. This is why getting the right sized calipers is critical to the brakes working correctly.

 

The first staement is pretty much the truth. Tires stop cars, not brakes. The latter just convert inertia into heat as best as teh tire road surface coefficient of friction will permit.

To the second, it's mostly increased rotor thermal mass then brake pads and brake fluid designed for higher operating temperatures. Such pads will usually be noisey on the street and result in longer initial stops when they are cold than regular OEM street pads.

Street performace is a different set of parameters from track performance, which is why cars are set up quite differently for each venue.

 

Of course if at one instant car A is going 100mph and car B is going 90mph, and then next instant both start braking, car A is going to generate more heat.

You can replace A and B with any car model out there and it would still be true given their weights are similar and all else remains equal. The difference between the IS250 and IS350 from the standpoint of this argument is the IS350 gets to said speed faster, which again is irrelevant from the brakes' point of view.

 

Right. But it's not irrelevant from the brake engineer's point of view.

 

That 350z is a friend of mines... They are sick in person. way to represent alabama woot woot haha i think i got more pics of the brakes cause i was there when they were doing that shoot.