BMWs (Finally) Get Brembo Brakes OEM...

[Emre]

hhmmm 135i with Brembos.... coilovers, lsd, done.

You forgot "minus 200-300 pounds." The 1-series is massively overweight.

BMW brakes stock are fine, usually all that needs to be done is upgrade the pads. Although while driving an e60 M5 I was able to get a pretty decent amount of brake fade just on the street exiting the highway over and over...

On the street, they're fine. On the track? No way. They just fall apart after a couple of laps, especially on porky but high-powered cars like the E46 M3.

Emre

[matt]

On track, the weight of a car is irrelevant to how hard it is on brakes. High powered cars are hard on brakes.

I agree that BMW brakes generally just need ducts and good pads to work well. It takes a very fast driver to cook most stock BMW brakes with good pads and ducts.

[bmw_e30]

You forgot "minus 200-300 pounds." The 1-series is massively overweight.

On the street, they're fine. On the track? No way. They just fall apart after a couple of laps, especially on porky but high-powered cars like the E46 M3.

Emre

I run stock sized brakes on my E36 M3 track car.

No issues really.

I typically out brake all cars sans the superlight track day cars like Atoms/Lotus 7/Radical.

[Emre]

On track, the weight of a car is irrelevant to how hard it is on brakes.

Not true, I'm afraid.

Force = [mass] x [acceleration]

Entering a corner at higher speed means you need more deceleration. A heavier car means you have more mass to decelerate. The force you need your brakes to generate is a factor of both mass and deceleration.

Let's say you've got a 2000lbs. 2002tii and a 4000lbs. Mustang. They've got the exact same tires and brakes. They enter the braking zone at the same speed. Which car would stop sooner?
Emre

[matt]

I am not talking about braking performance. Obviously a lighter car will perform better in all ways at a given power/brake level.

I'm talking about brake temperatures. All brakes do is turn one type of energy into another. Where does a heavier car get more energy to heat its brakes more?

[Emre]

All brakes do is turn one type of energy into another. Where does a heavier car get more energy to heat its brakes more?

Again, that energy comes directly from it's mass. A heavy object moving at 100 mph has more energy (momentum) than a lighter object moving at 100 mph. Slowing that object down takes more force, which means the braking system has to convert more energy into heat.

That's why bigger cars need bigger brakes...at least on the track. One or two hard stops isn't a problem. But repeated stops at track speeds in a heavy car means lots of heat to shed. That means you want: a bigger a heat sink (larger, thicker rotors); to spread the heat and pressure more evenly (multi-piston calipers); and, to cool the system efficiently (more effective rotor vane designs, brake ducting). It goes without saying that you also want pads and fluid that work optimally at whatever temperature you're going to see on the track.

A car like an E46 M3 is heavy, powerful, and relatively under-braked. It absolutely needs bigger front brakes to hold up to serious track use, especially with R-comp tires and full race suspensions (which directly translate into greater speed that must be shed in the braking zones). Although brake ducting helps tremendously, there's only so much you can do with just ducting and pad selection.

Emre

[whiltebeitel]

I'm going to have to agree with Emre. I know I don't have the tech of either of you, but there's a good reason why all the track cars I help prep and maintain had Stoptech or similar kits. It's not for the bling factor nor the ease of changing rotors.

[Emre]

I run stock sized brakes on my E36 M3 track car.

An E36 M3 is noticeably lighter and significantly less powerful than an E46 M3. Unlike the E46 guys, your brakes are not disproportionately small.

I typically out brake all cars sans the superlight track day cars like Atoms/Lotus 7/Radical.

I've never seen an E36 M3 with OEM brakes that could consistently out-brake my Evo over the course of a 30 minute session...much less a 1 hr. enduro. That's despite the fact that the Evo weighs more (3200 lbs.) and makes more power...often far more power. Mine was making ~400 hp, for example.

So, I guess those 4-piston front/2-piston rear Brembo calipers must be doing something...especially considering the piss-poor ventilation at the front. They're certainly far better than BMW's single-piston sliding calipers and undersized rotors that's for sure.

Emre

[Emre]

I agree that BMW brakes generally just need ducts and good pads to work well. It takes a very fast driver to cook most stock BMW brakes with good pads and ducts.

BTW, more stopping power isn't the only reason people go for multi-piston, fixed calipers. BMW's sliding calipers are notoriously flexy. That makes for lousy pedal feel, which makes modulating the brakes tricky. It's bad enough that modern BMW's have overboosted, grabby brakes. Throw in the caliper flex, and it gets mighty tricky to modulate your brakes when it counts. No fun on a damp track, for example!

On top of that, sliding calipers are known to bind. No matter how you look at it, they're heavily compromised. Not what you'd expect for an M-Car IMNSHO. If I get Brembo F40 calipers on a $35K Evo or STi, I sure as hell would expect them on an M-Car that costs 2-3x as much.

Emre

[matt]

I have the tech. :) From BFC not long ago:

Brakes are just converting the car's kinetic energy into heat energy. If you have two cars with the same average HP but one weighs twice as much - where is the "heavy car" getting the "extra energy" to be harder on brakes?

Short answer is it isn't - you can assume the car as a closed system on the track, and the energy going in is provided by the engine, and the energy going out is friction(brakes, tires, aero drag etc.).

A lighter car might have a bit more aero drag due to higher speeds, and a bit more cooling like techno said since they have more total pressure to work with, but it's basically braking requirements being dictated by engine HP.

Heavier cars have bigger brakes so they get acceptable life out of their pads and for performance reasons. Heavier cars are also more powerful.

I'm going to have to agree with Emre. I know I don't have the tech of either of you, but there's a good reason why all the track cars I help prep and maintain had Stoptech or similar kits. It's not for the bling factor nor the ease of changing rotors.

I assume you mean you work at a shop... Something to do with profits maybe?

Again, that energy comes directly from it's mass.

Huh? The kinetic energy of the car comes from the motor. Nowhere else.

A heavy object moving at 100 mph has more energy (momentum) than a lighter object moving at 100 mph. Slowing that object down at the same rate takes more force, which means the braking system has to convert more energy into heat.

Two cars with the same motor, aerodynamics, and gearing enter a straightaway with the same amount of kinetic energy. One weighs 2000lbs and the other weighs 4000lbs. How much kinetic energy does each have at the same point at the other end of the straight? Where is the extra energy for the heavier car coming from?

Let's say you've got a 2000lbs. 2002tii and a 4000lbs. Mustang. They've got the exact same tires and brakes. They enter the braking zone at the same speed. Which car would stop sooner?
Emre

Lets look at this from the other direction... in order to enter the braking zone at the same speed as you've assumed, the heavier car will have to be more powerful.

[FredK]

Two cars with the same motor, aerodynamics, and gearing enter a straightaway with the same amount of kinetic energy. One weighs 2000lbs and the other weighs 4000lbs.

I'll bite. If they have identical powerplants converting chemical energy into kinetic energy at the same rate, and all other factors are constant,

KE = 1/2mv^2.

For this problem, let's assume m1 = 1000 kg, m2 = 2000 kg, and v1= 30 m/s.

If KE1=KE2, then

1/2m1v1^2 = 1/2m2v2^2.

v2 = 21.2 m/s.

[FredK]

for visual clarity

[matt]

Now do the same amount of work to both of them... which has more kinetic energy then?

[FredK]

neither

[matt]

Bingo. Even though they weigh different amounts, the kinetic energy of the car is always equal to the amount of energy put in minus the amount of energy taken out.