The Z4 M Coupe comes with E46 M3 Competition Package (CP) brakes as standard, which perform better than most BMW stock brakes under track conditions. Heat build-up and eventual fade can be managed with braking techniques and quality after-market pads, but there's no getting away from the mushy, uncommunicative pedal feel at the limit, caused by its sliding calipers.
It was necessary, and more in keeping with its racy nature, that the M Coupe be fitted with aftermarket fixed-caliper brakes. This project, with its quantitative back-up, came about from driving a friend's E46 M3 CP, both before and after a Brembo fixed-caliper upgrade. At the track, the seat-of-the-pants change in pedal feel, effectiveness and consistency was readily apparent. As a scientist, I felt compelled to measure this difference and back it up with real numbers.
For any big brake upgrade, the main considerations are: what is available? And how much wheel clearance is there? Fortunately, Brembo launched its big brake upgrade kit for the Z4M earlier this year. I decided to stick with 18-inch wheels, due to the anticipated amount of track time, so clearance was an important issue. When the custom forged ZX3-R wheels were still at the planning stage, Forgeline contacted Brembo to ensure that the new brakes would fit behind them properly. After an exchange of CAD files, the correct offset was chosen for front caliper clearance.
Knowing there needed to be some custom trimming of the inner rotor mudguards to accommodate the larger fixed calipers, I decided to have the four-wheel upgrade installed professionally. Tunerworks Performance in Calgary did the work, along with a full brake fluid flush and system bleed. Each Brembo GT big brake kit has a pair of four-piston fixed calipers, Brembo HP1000 performance brake pads, vented and slotted two-piece floating rotors (14x1.3 inches up front and 13.6x1.1 inches in the rear) and includes all necessary adapter brackets and hardware, as well as stainless steel flex lines. The HP1000 brake pads are designed with improved control, friction levels and resistance to fade, and have a longer life when compared with OEM pads.
Each front set-up saved 3.5 pounds of unsprung weight, even though the OEM front rotor size is much smaller (13.5x1.1 inches). Because of the increase in rotor size from 12.8x0.8 inches, there was no net weight change in the rear. Note that the weight is saved or balanced with the Brembos, despite the increase in rotor size, by using aluminum hats and calipers instead of heavy cast iron (as with the OEM parts).
Although rain prevented data collection on our subsequent track date (it's crucial to have similar conditions), this became an ideal opportunity to get a feel for the Nitto Invo tires on a closed circuit under wet conditions. According to the Traqmate GPS system, lap times where understandably slow, about 18 seconds off the dry pace. However, the Invos proved to be excellent, instilling confidence through a high level of relative grip and an ability to channel away standing water. When playing around in some of the slower corners, even with the car's strong power, one had to be deliberate with the throttle and/or steering to loosen the Invos' grip.
When the dry weather finally arrived, we hit the track with the new Brembo brakes and the results were excellent. Again, same driver as for the original testing, similar line, and no in-car time display for motivation. At the previous dry outing, (described in Part 2), with both the front strut bar and the wheel/tire upgrade in place, the average lap time was about 95.1 seconds (the fastest lap being 93.9 seconds and represented by the blue line).
With the Brembo upgrade, that average was reduced to 93.1 with a fastest lap of 92.8, demonstrating faster, more consistent lap results. Also, the average total time under braking per lap was reduced from 15.9 seconds to 12.7 and the average peak braking g-force before turn one increased from 0.86g to about 1.05 (with a spike at 1.3-anomalous, but interesting). Looking at the graph (bottom right), where the red line represents the addition of the Brembo upgrade, one can see the improved brake effectiveness for turn one (from 125 mph down to 75 mph, a real brake test). Braking is now later and deceleration is increased as shown by the shifted and steeper negative slope.
Brembo engineers each kit for a specific model application, with unique piston size variation for optimum brake balance, increased brake torque and thermal capacity. With these larger rotor diameters, braking force is now more apparent due to the improved leverage. Also, this increased rotor thickness promotes efficient heat dissipation. I have yet to experience any fade or fluid issues whatsoever, even after several consecutive hot laps. Pedal feel is now constantly firm and linear, requiring less effort and providing improved confidence and control. In the next installment, the M Coupe will receive a few suspension upgrades from KW Suspensions and TC Kline Racing.
Behind the scenes of Project Z4M: bitten by the family dogThings don't always go as planned. I was invited to instruct at an advanced driving school, with the opportunity of some free track time before the start of the event. I did my usual diligent pre-track inspection (fluid levels, brake pad thickness, tire condition and cold pressure) and gathered my gear (air pump, tire gauge, extra fluids, mechanic's gloves and a few hand tools). I have well over 2000 laps under my belt here at my local track, but that was no help on this particular evening.
Excited about an empty track and the opportunity to collect some additional data for Project Z4 M, I went out for a warm-up. Things went wrong early on the second lap, when I went into an abrupt, seemingly unprovoked and extremely scary spin in turn one, the fastest corner of the track. A review of the GPS data revealed that I was traveling at about 60 to 65 mph when I spun, well below my usual 75 to 80 mph after braking from over 125 mph. I had been 'bitten by the family dog.'
So what happened? Why did the faithful 'dog' bite? After a thorough mechanical inspection, nothing appeared to be wrong before or after the incident. This, of course, rightly pegs the blame on the driver. I can't put my finger on the precise cause of the spin, but I can list several things it could have been: Fatigue: heavy work schedule with international business trips Poor choice of driving apparel: loose-fitting work shoes Complacency: am I not invincible at my home track? Rushing: not warmed up mentally and physically to begin pushing the car
I was lucky to have a 'free' lesson and it could have been a lot worse. Project Z4 M could have been delayed by several months. Track time is great fun and can be rewarding, but it's not without risk. It's up to you, and only you, to be completely prepared for driving events like this. It is also important to give yourself time to adjust to this exhilarating activity by working into it slowly. That night, my students may have not been terribly pleased to have me as their instructor, but I did have a lot of fresh and humble advice for them.