AP Racing Brakes? - Page 3 - The Unofficial BMW M5 Messageboard (m5board.com)

REKIII · 29th February 2008, 16:19

Several factors here.

1st is the rediculous difference in front tire width to rear tire width.

Second I would bet you my M5 that there is a proportioning Valve in there reducing the line pressure to the rear brakes.

Again, Brake engineers know what they are doing.

Bob

Ted · 29th February 2008, 17:07

Quote:

Originally Posted by REKIII

Several factors here.

1st is the rediculous difference in front tire width to rear tire width.

Second I would bet you my M5 that there is a proportioning Valve in there reducing the line pressure to the rear brakes.

Again, Brake engineers know what they are doing.

Bob

Honestly sir, I have no idea how these factors you mention have anything to do with brake rotor dimensions. Unless you are really good at explaining these things, I probably need to be having this discussion with David Zeckhausen.

REKIII · 1st March 2008, 00:59

I'm not a brake engineer, so, no I'm not going to be good at explaining them.

I apologize for not being able to better.

If ntohing else, just realize that the brakes to not slow the car, the tires do, and the brakes slow the tires. Tires, and weight of the car have huge factors on braking and are part of the very complicated world of brake engineering.

Bob

Ted · 1st March 2008, 15:39

In that case it's strange that the M5 in stock form has bigger/heavier wheels and tires on the back than on the front yet the the back brakes are smaller than the fronts! There are several cars on the road today whose rear brakes are the same size or bigger than the front ones. I would love to be able to just call up my brake shop and say "I would like 4 15" rotors please" instead 2 of these and 2 of these (that's the balance I'm talking about). You know when we I just got the M5 with the staggered setup wheels the first time I called up a tire store to get tire pricing for 4 new tires they were like "2 what, and you said 2 what??? and what was that other size",etc... I knew it was gonna suck having different sizes front and back. Half the places I talked to acted like they never saw a staggered set-up before! Made me scared to do business with them. But then I got 18x9.5 wheels for the front, and my next tire purchase was beautiful! Walked in, ordered 4 BFG KDW2's 275/35-18, walked out, simple as that. No worries. And that's what I want with my brakes...balance. Same exact rotors and pads front and back. Is that too much to ask? I don't think so.

Super-saloon · 1st March 2008, 16:23

Here is a picture of the front APs fitted.

mottati · 1st March 2008, 20:29

my stoptech kit has the same rotors front and back, 355 mm. It's a one off kit, the prototype stoptech made, but the larger rear rotors won't clear stock wheels, so they re-did it with a 345 mm rotor. Calipers are different, as are brake pads. You don't need the same braking force at the rear, because when you brake, weight transfers to the front. If you had the same amount of braking force at the rear, the car would be unstable, and you'd never be able to brake going into a corner, for example.

I've replaced my front rotors once, in 4 years of having the bbk. It's not like going into burger king, you're not going to walk into your local shop and ask for brake rotors, and if that is a concern, you're best off to leave it stock, which actually works quite well unless you're on the track with any regularity.

I've never had any problems with parts, or tires for that matter, staggering is very common in performance cars, you must be going to the wrong shops.

Quote:

Originally Posted by Ted

In that case it's strange that the M5 in stock form has bigger/heavier wheels and tires on the back than on the front yet the the back brakes are smaller than the fronts! There are several cars on the road today whose rear brakes are the same size or bigger than the front ones. I would love to be able to just call up my brake shop and say "I would like 4 15" rotors please" instead 2 of these and 2 of these (that's the balance I'm talking about). You know when we I just got the M5 with the staggered setup wheels the first time I called up a tire store to get tire pricing for 4 new tires they were like "2 what, and you said 2 what??? and what was that other size",etc... I knew it was gonna suck having different sizes front and back. Half the places I talked to acted like they never saw a staggered set-up before! Made me scared to do business with them. But then I got 18x9.5 wheels for the front, and my next tire purchase was beautiful! Walked in, ordered 4 BFG KDW2's 275/35-18, walked out, simple as that. No worries. And that's what I want with my brakes...balance. Same exact rotors and pads front and back. Is that too much to ask? I don't think so.

DZeckhausen · 1st March 2008, 23:50

Quote:

Originally Posted by Ted

In that case it's strange that the M5 in stock form has bigger/heavier wheels and tires on the back than on the front yet the the back brakes are smaller than the fronts!

Not at all. With the amount of forward weight transfer under heavy braking, the front brakes require significantly more thermal capacity than the rears. That's why E39 M5 front rotors are so much heavier than the rear rotors - 27 pounds vs 17 pounds. More rotor weight equals more thermal mass, and thus lower temperatures, for a given amount of energy stored.

Quote:

There are several cars on the road today whose rear brakes are the same size or bigger than the front ones.

The E46 M3 has rear rotors 3mm larger in diameter than the front. However, this does not mean the rear rotors are "bigger". The front rotors are 28mm thick and weigh 19 pounds each, while the rear are only 20mm thick and weigh 17 pounds each. And several pounds of the rear rotors are dedicated to the integral parking brake drums. The M3 has a large diameter parking brake, requiring a large enough rotor to clear it and still have enough swept area left over for the brake pads.

Some rear engine sports cars, like the Porsche 911, use similarly sized (dimension and weight) rotors front and back. With the engine sitting so far back, the rear brakes get to participate more. There's nothing quite so impressive as a Porsche on racing tires at the limit of adhesion under braking! On a front engine car, the front brakes typically do about 75% of the work under heavy braking.

Quote:

I would love to be able to just call up my brake shop and say "I would like 4 15" rotors please" instead 2 of these and 2 of these (that's the balance I'm talking about).

If you had a StopTech 4-wheel BBK on your M5 using the 4-piston rear calipers, then you would have 355x32mm "friction rings" at all four corners (see: http://www.zeckhausen.com/StopTech/R...eplacement.htm for details) and you would use the same pads front and back. The center aluminum portions ("rotor hats") would be different, of course, because they determine the offset of the rotor and, in the case of the rear rotors, include an integrated parking brake drum. Fortunately, the friction ring is the only part that needs to be replaced, so you could call us up and ask for four 14" rotors. In reality, your rear rotors would probably outlast the fronts by two or three to one.

Quote:

And that's what I want with my brakes...balance. Same exact rotors and pads front and back. Is that too much to ask? I don't think so.

It's quite a constraint on the designers to require the same rotors and pads front and back. It would mean, for example, that the parking brake could not be built into the rear rotors. And the suspension geometry would be constrained by the dimensions of the rotors at the other end of the car.

It seems that your definition of "balance" in this discussion means having the same parts at both ends of the car. It's more of a discussion about convenience than it is about the physics of brake balance. As others have pointed out, there are many reasons why the parts are different, ranging from the performance requirements at each end of the car to the physical space constraints to product optimization.

There are some components that can be standardized on an automobile that make sense, such as fasteners and bulbs. Other components, such as brake rotors, calipers, and pads need to be optimized to meet differing requirements at each end of the car. As for tires, your point is valid and adjustments to swaybar sizing, spring rates, and shock valving could make it possible to derive decent handling balance without resorting to staggered tire and wheel sizes. I'm not a BMW chassis engineer, but it wouldn't surprise me if BMW marketing had some input into the use of wider rear tires and wheels to obtain a more aggressive look.

Ted · 2nd March 2008, 08:46

Looks very good, do they come in cross-drilled/slotted?

Ted · 12th December 2008, 13:09

Thanks to everyone for all the replies, it makes perfect sense now!

rhouse181 · 12th December 2008, 17:36

Dave's the man...