Bigger brakes vs drilled discs?

[Fanatic]

I am close to a rotor change and I am thinking what would be better between the two. Due to cost one of the two would be the option as doing both would increase the cost significantly. I cannot say that I have any complaint about the braking power, but mostly the decrease in effectiveness when they get really hot.

So options are:

A) The 2,5 has 286mm rotors in front and solid 280mm back. I could swap the front and go to 300 just with new carriers or 325mm with a set of second hand calipers+Carriers with standard discs. In the back the option is again to install vented larger discs from any E46.
B) Get front and back grooved brembo discs and keep the size the same. Due to the grooves they should dissipate heat better than the standard ones.

Any advice would be much appreciated.

 

[ga41]

Definitely go bigger, if you can only do one of the two.

The only real reason you would do grooved or drilled rotors on a road car would be looks. Performance benefit will be negligible if any. But larger diameter discs mean more braking torque plus more metal mass to act as a heatsink hence better endurance. Won't look as nice as grooved but the benefits outweigh the looks...

 

[David-H]

Put 325mm rotor on the front and get the caliper + carrier from a breakers off a 330 but leave the back as it's not worth doing. That's what I did (new Brembo grooved rotor) on my 3.0i and the results are very good.

 

[Fanatic]

Put 325mm rotor on the front and get the caliper + carrier from a breakers off a 330 but leave the back as it's not worth doing. That's what I did (new Brembo grooved rotor) on my 3.0i and the results are very good.

The much larger brakes at the front compared to the back will not mess up the balance of the car I guess will it??

 

[David-H]

Put 325mm rotor on the front and get the caliper + carrier from a breakers off a 330 but leave the back as it's not worth doing. That's what I did (new Brembo grooved rotor) on my 3.0i and the results are very good.

The much larger brakes at the front compared to the back will not mess up the balance of the car I guess will it??

They work very well on mine and I think that the back one's are tricky because of the handbrake mechanism.

 

[Fanatic]

Put 325mm rotor on the front and get the caliper + carrier from a breakers off a 330 but leave the back as it's not worth doing. That's what I did (new Brembo grooved rotor) on my 3.0i and the results are very good.

The much larger brakes at the front compared to the back will not mess up the balance of the car I guess will it??

They work very well on mine and I think that the back one's are tricky because of the handbrake mechanism.

True but still the 3,0 has 294mm vented rears compared to my 280mm solid rears. So the difference is larger. Theoritically the Z brakes more forward but still the balance could be affected. Hmmmm

 

[Ewazix]

You should upgrade all round to maintain distribution, ABS can cover some of issues but ultimately you could reduce fade at the cost of ultimate stopping ability.
Done incorrectly it's yet another mod with good bragging and cosmetic value, but that might actually reduce performance. Done right it's a very good idea.

a reasonably simple explanation of the science stuff courtesy of MGF guys http://www.mgf.ultimatemg.com/group2/brakes/big_brakes/brake_myth_1.htm

and the full-fat boffins version http://stoptech.com/docs/media-center-documents/the-physics-of-braking-systems

 

[GuidoK]

I dont see any problems in just upgrading the fronts...
The z4's brake bias is -like any other roadcar- pretty much front biassed. Adding more front stopping power makes it even a little bit more front biassed. And that is what you want on a roadcar. You dont want to spin immediately at lockup do you?

Personally if you're already going to overhaul the brakes I'd try and get a 330i front setup (brake caliper, caliper carrier and discs (or buy new discs/pads) and get a 323/325/328 rear caliper+carrier and new rear discs

I dont know how much used caliplers/carriers are but I think pretty cheap? (especially rear ones)
New discs are also fairly cheap if you go for a cheap brand like mtec or so.

If it's because of performance, get the front 330i calipers/discs. I think the front brakes dissipate 80-90% of all the heat/energy I estimate.

 

[Ewazix]

I dont see any problems in just upgrading the fronts...
The z4's brake bias is -like any other roadcar- pretty much front biassed. Adding more front stopping power makes it even a little bit more front biassed. And that is what you want on a roadcar. You dont want to spin immediately at lockup do you?

Personally if you're already going to overhaul the brakes I'd try and get a 330i front setup (brake caliper, caliper carrier and discs (or buy new discs/pads) and get a 323/325/328 rear caliper+carrier and new rear discs

I dont know how much used caliplers/carriers are but I think pretty cheap? (especially rear ones)
New discs are also fairly cheap if you go for a cheap brand like mtec or so.

If it's because of performance, get the front 330i calipers/discs. I think the front brakes dissipate 80-90% of all the heat/energy I estimate.

Farting around with anything that changes the ratio of front/rear braking effort is likely to reduce total braking efficiency and upset driving dynamics. Luckily abs/dsc will save the day preventing lock-up and spin out, but because it has to step in earlier can't restore total braking lost through changing the ratio.

This is a safety issue so go with the maths and keep the front/rear ratio by upgrading both proportionately, it's why BMW don't just up-rate front discs/systems.

Just thinking you might need to inform your insurance of the upgrade as well? (or maybe keep it to yourself if you only do the front :wink: ).

 

[Artful-Bodger]

Brake balance is heavily front biased on all cars, the reason is very simple, weight transfer, if you weigh your car the weights on the wheels will be the static weights, as soon as you move those weights change dependant on acceleration/cornering/braking forces, the car rotates around it's centre of gravity and it's suspension roll centre's putting a far greater load on the front tyres which increases with the speed you started braking, just think of a cyclist braking too hard, they don't fall off the back.

As the weight transfer increases the rear brakes will be doing less and less work as the tyre unloads, but it is still managing the engine torque, all the race cars I built used adjustable brake bias and the balance was always a matter of achieving the highest G without locking the rears, our road cars will never see the braking efforts of a race car so the weight transfer will always be less, so we can use a bigger rear disc without too much worry, but it's the fronts that do the most work and on road cars with our squidgy road tyres the limit is imposed by the tyre wall/pressures which changes considerably from make and model.

The Z4 Si model has the 325mm/290mm setup so it's perfectly sensible to go to this setup without any concern.

 

[GuidoK]

The Z4 Si model has the 325mm/290mm setup so it's perfectly sensible to go to this setup without any concern.

Exactly, just my point.
On a lot of bmw's disc size ratio vary between front&rear on the same car.
For example there are some e46 models with the same engine with 300x294, 325x294 and 325x320 brake configs, although most have 300x294.

Upgrading the front will never result in a spin out during braking. Spinning only happens when the rear locks up and the front keeps rolling, and that only happens when you put huge brakes on the rear and leave the front stock, but only a moron would do that.
(lockup means zero grip and wheels with no grip have the least amount of resistance so they have the tendency to move in front of the car).
IF you go with bigger front brakes, it means you can also put bigger brakes in the rear. To get that last bit of extra stamina on braking on tracks if the front can dissipate much more heat than the rears. Although I didnt find there was much in it when I did that. The rears hardly do any braking compared to the fronts. Thats why loads of cars had drum brakes at the rear for a long time.

When I build my own braking system (both front and rear), there were masses of people on loads of fora shouting "oh no, you'll mess up the bias ratio, it will impact brake performance" etc etc. Most people shouting obviously where people who never build brakes themselves and have no hands on experience. You never get a reaction of someone who did the same and gives a picture with the result being a car crashed in the road barrier :lol:

 

[Fanatic]

I found a rear set of 3,0 calipers with carriers. Are they a direct fit? What is the problem with the hand brake I have read about? Also what brand do you suggest for rotors and brake pads? Are ceramic pads worth the extra money?

 

[GuidoK]

rear calipers/carriers from the z4 3.0i will bolt on directly on the 2.5i with the vented 294mm 3.0i disc.
There is no problem with the handbrake. All z4 hand brake shoes are 160mm (160x20)

From the e46 you can (roughly) use rear caliper and carrier from 318i to 328i, but it has to have the 294 vented disc (for the e46 there are many different combinations). I think anything from an e46 328i is ok (I think the z4 3.0i has the e48 328i brake system specs)