E92 - AP Racing Brake Kit Upgrade

By John Thorne

We love BMW; they create some of the greatest drivers cars on the planet. Cleverly styled, impressively engineered, expertly marketed and perfectly packaged as the ultimate drivers machine. So why on Earth can’t they provide an M sport car with a decent set of brakes? I shouldn’t complain of course, without BMW fumbling the ball on brake set ups on M3’s then companies like Thorney Motorsport might not be as successful. Their failure is our gain, but even with this it still stuns me to this day that there’s not been an M3 for over ten years that had a braking system worthy of the M Sport badge. The E30 M3 was great, but it weighed naff all so didn’t need decent brakes. The E36 was a step closer to cut price brakes but it was the E46 M3 that really won the award for the crappiest brakes on a sports car award. How in the World BMW could justify a £40k car with a single sliding pot calliper is beyond me. Is the E92 M3 cut from the same cloth?

 

Well, no. The latest M3 is a lot better than its predecessor for brakes, the callipers are larger, the pads offer greater bite and overall the system is better than the previous model. However, the E92 is a lot larger and a lot heavier so the net result is still the standard BMW offering – passable but not up to hard abuse on road or track, in short improved but still crap for a £50k car.

 

Before going through the options we’ve developed on brakes it’s probably a good idea to go into how they work. Brakes are the perfectly example of wanting your cake and eating it, the fact you’ve bought a magazine called “Performance BMW’ I’m going to assume that you know about how brakes work, disks passing through a calliper on the wheel yada yada but the overall issue with brakes is that they work by generating friction, friction is a good thing as that’s what causes the car to slow down, however friction generates heat and heat effectively acts against the efficiency of friction so you effectively have a vicious circle -

 

Braking = friction = increase heat = less braking.

 

What we do in upgrading brakes on cars is try to maximise friction whilst at the same time minimising heat, as the Meercats would say ‘simples’.

 

To identify how to improve braking we need to break (geddit?) down the system, brakes are made up of:

 

Disc (or Rotor for our American cousins) – this is the big round bit mounted to the hub

Calliper – the big metal bit that sandwiches the disk that holds

Pads – the actual bits of metal that contact the disk

Hoses – these contain the hydraulic fluid that move the pistons in the calliper when you press the pedal

Master Cylinder – Regulates the amount of fluid and pressure in the system

ABS – The computer that controls the anti lock braking system in conjunction with the brakes themselves.

You – the driver, as how you drive and use the brakes will have a direct effect on how good they are.

 

The first port of call for brake upgrades is the pads, the cheapest, simplest most direct way to improve braking on any car.  The idea with pad changes is that the higher temperature tolerance of the different materials means the brakes maintain their operating efficiency at higher temperatures, ie use them more and get them hotter and they still work as well. Standard pads on the M3 aren’t awful, compare them to Porsche and they are still woeful but overall they aren’t too bad in general driving, but press on a bit and pretty soon the pedal will feel softer and you’ll notice that braking distances will increase, sometimes dramatically, especially on track. There are numerous brake pad suppliers around, all will tell you that their offerings are better than the rest with sintered this and metal matrix that but the only way to really tell which is best is to test them. Thus far we’ve tested two types from Pagid – RS421 and RS29. The RS421 pad (also known as Pagid Blue) is an uprated road pad, good for road use and should be seen as a better option to standard, limited brake dust, limited squeal but still not really up to any track work without getting too hot (remember heat is bad). The RS29 (Pagid Yellow) is a full race pad, much harder with higher heat tolerance, we’ve been using these on the Safety Car duties, downside is more squeal when the car is used in town driving (goes away when you get them hot again), both work well with the standard disks. We’re still testing more pads but these two certainly stick out as the best so far.

 

Next up on the upgrade path are hoses. Brakes are a hydraulic system, ie its controlled by the expansion and contraction of a fixed amount of hydraulic fluid in a sealed volume. As with any hydraulic system the more it moves the greater the increase in fluid temperature. Remembering of course, heat is bad (getting bored of that yet). With standard rubber hoses the outer casing are prone to expansion and therefore increasing heat of the fluid so replacing these hoses with stronger items that flex less will help. Goodridge and Earls have both developed braided hoses for the E92, the effect is not dramatic in terms of power and is more about braking feel but it’s a nice upgrade and sensible as part of larger changes. Changing the fluid is also a good idea, brake fluid is hydroscopic (absorbs water from the atmosphere) so does degrade over time, get the brakes hot or even boiling the fluid will cause the brake fluid to degrade faster and lessen its efficiency, we’d recommend to replace brake fluid more regularly than the BMW schedule, especially if driving on track.

 

So, you’ve tried better pads, you’ve tried better hoses but still not enough? Not surprising, the E92 is a big old lump and we’ve found that for spirited driving or any real time on track pads and hoses are not going to cut it so now we get to the real upgrade options – brake kits.

 

Brake kits are as they sound, complete system upgrades to improve every aspect of the braking system from top to bottom. Typically the only things they don’t change (or affect) is the master cylinder and the ABS control, everything else is up rated. There are three or four decent manufacturers on brakes for the M3, Alcon (installed on the CSL Cup race cars, the E46 M3 and CSL), AP Racing, Brembo and Stoptech to name a few. We have a long history with AP Racing, we’ve developed several kits with them and they are one of the few companies with a very professional view on product development so we were delighted when they called us about their new kit for the E92. Some R&D was needed both in fit and application so the Safety Car role for the CSL Cup for our car made perfect sense as a test vehicle.

 

Before going into the details of what the AP kits comprise its worthwhile going through each area of braking performance. The two main ways to improve braking performance are the reduction or increased dissipation of heat together with maximising the pad/disk interface, in other words – larger disks combined with larger pads. The AP kit increases the size of the disk to 378mm at the front and 350mm at the rear. Perhaps more importantly, disc width is increased to 36mm at the front and 26mm at the rear. This increase in width together with cleverly designed ‘vanes’ inside the disk are what allow the disk to cool faster than a solid or narrower disk, keeping cool = better brakes. There are two types of front kit available for those with 18” or 19” Wheels; the discs on the 18” are 10mm smaller in diameter. The discs are grooved to keep the pad surface clear (not as some people think to keep the disk cool). The pad size is increased by 60% over the standard pad so combined with the larger disk the overall pad/disk interface is over 200% larger than standard. The discs (both front and rear) are separately mounted from the hub fixings on an aluminium bell, aluminium to save weight but primarily to avoid the heat from the disk affecting the performance of the hub and vice versa, separate belled disks are a crucial part of any brake upgrade. Finally there are the callipers, which are frankly, huge when compared to standard. The front kit has 3 pistons on each side (so is known as a 6-piston calliper, also known as 6 pot) and the rear has a 4-piston calliper (4 pot). These pistons are variable in size to allow the pad to contact with the disk in  a progressive manner, having the pistons the same size can mean the brakes have limited modulation and become on/off switches – not a good idea.

 

So, enough with the technical detail – what are they like? Bloody amazing is the answer. Milton Keynes is one of the best places to test cars for brakes and handling (we love roundabouts) and the improvement in braking generally is a revelation, smooth progressive use generates no fade whatsoever and braking can be left later and later with complete confidence. Aggressive braking manoeuvres come down to how brave you feel such is the huge increase in confidence they offer.  Most impressive is the overall feel, its almost as you can feel the disk pass through the calliper in use, its very tactile in use and really connects you to the road, all this improves the cars feel and confidence on what you can do with it. Torque balance is perfect (front and rear balance) and whilst there is always some dive at the front – you cant change the laws of physics the car brakes very level allowing extensive use of trail braking. Running the Ferodo DS2500 road pad (there is an option to run RS29 pads) there is minimal brake dust and no squeal even after extensive time in traffic, and in bad boy red they are hard to miss, especially under the 19” Rays wheels.

 

On track the improvement is even more impressive. The ability to stab the brakes to scrub speed is nicely handled and full race pace running didn’t show any issues whatsoever; it really is a very impressive set up.

 

Sadly all this performance does cost, the kits can only be fitted at a full front and rear upgrade and so together cost £4,513.50, but they really are worth it.

 

Next month it should be some power upgrades, will be eye opening I promise you.

 

John