ABS Fundamentals

ABS Fundamentals

"These entries are part of a Special Series on the development of the M3 V8's Bespoke AP Racing Clubsport Brake Kit.

In this entry, we lay down the fundamentals to brake bias, historical evolution of BMW M's ABS and take a more technical dive on it."
- Matt 

Brake Bias 101  

What is it?

In simple terms, brake bias is how much of the stopping power is biased towards which axle. In may be referred as brake balance in some automotive circles as well. 

The Genuine rear brake calipers on the E9X M3.  

Why should you care?

Remember your parents' tips and tricks on how to ride a bicycle?

"Son, use both brakes, always."
- A responsible dad. 

We all still tried it, because that's what kids do. Similar dynamics will happen if you increase front brake bias too much in a car minus the flip, scratches and tears. 

Our BMW M3s will understeer and stop slower. When modifying your brake system, it's important to keep brake bias close to what was engineered by BMW. 

Here's a cartoon equivalent of too much front bias: the rears become useless!

An historical perspective. 

The pre-ABS days and why it doesn't matter. 

Appearances of safety and limited brake technologies forced automotive engineers to develop pre-ABS cars with brake bias drastically shifted to the front. 

Locking up the wheels that you steer with is generally safer, as you can release the brakes and gain back control of the car.

If the opposite happens, the car will begin to oversteer, which is a lot more dangerous and counterintuitive for most Sunday drivers. 

New braking innovations: rotors and ABS.  

Let's take a moment to appreciate our disc brakes: these were introduced by Citron in the 1950s and became the norm in the 70s. 

As tire technology dragged on and braking systems became more powerful, locking up the front tires was an issue. 

Anti-locking Braking Systems (ABS) started making its way into air planes in the 1920s. The innovation was later picked up by automotive engineers and progressively made its way into cars. 

As with most innovations in the automotive world, it starts with luxury cars.

BMW as no exception. They introduced ABS as standard on the E23 7 series in 1977. All BMW M3s produced for the street were fitted with ABS as standard, including the genesis E30 chassis. BMW engineers still kept some of their habits from pre ABS days. The E30 had brake balance close to a 70:30 depending on the master cylinder valve opening. 

A local, tuned E30 M3 currently underdoing restoration back to OE spec. 

A slightly more technical dive on ABS.

To stop the fastest you want the exact percentage of brake torque applied at an axle to match the percentage of grip that axle has, at any given time.

If 60% of the weight of the car is on top of the front axle when stopping and 40% over the rear axle, given same grip conditions, than having 60% of brake torque on the front axle and 40% over the rear axle will allow your car to stop the quickest.

What is grip?

Grip is often associated with the only connection between our M cars and the ground: tires.

Pitch, yaw, roll. 

It's optimal grip is mainly determined by the weight applied to our tires.

M cars are fundamentally dynamic objects: they are bound by the laws of physics (!). 

  1. Accelerating hard shifts the weight rearward.
  2. Braking hard shifts the weight forward. 

This describes pitch

Roll, and yaw also need to be considered, in addition to a myriad of other variables:

  1. Tire sizes
  2. Tire wear
  3. Tire condition
  4. Pad material
  5. Brake bite

How smart are modern ABS?

Welcome to engineering: complex problems are hard to solve, and humans excel at creative innovation. BMW engineers know these things. Their modern braking systems are much more intelligent than the ones from 35 years ago.

They realized ABS could be much more than be a safety feature, they could drastically improve brake performance by influencing the vehicle's dynamics under braking to optimize available grip per axle. 

The E36 M3 marked a considerable upgrade in ABS technology and understanding of its potential. 

Modern BMW M ABS systems juggle brake torque between two wheels of the same axle. To achieve this, they had to develop new components such venting valves, bypass valves and other mechanisms integrated into an intelligent, adaptable system. 

ABS systems are now able to account for various grip conditions as well as adapt over-time to your driving style and often become more pro-active as more of these adaptations are saved onto the on board control modules. 

The E36 M3 looks prehistoric next to the E60, yet barely a decade separate the two. 


Yet, the reality of developing a street car manifested itself.

///Marketing kicks in, the legal department's anxiety flood the engineering desk and the bean counters sharpen their pencils. As you start to unleash the E9X M3's chassis potential, this becomes ever more obvious.

BMW M Engineers have multiple things to think about when they are designing the braking system beyond brake bias.

Engineering for safety (and liability)

BMW M engineers need to account for the worst case scenario: a convertible E93 M3 with an overweight passenger, a skinny driver, on fresh front tires and rear tires on their last breath.

That example of a clapped out fatso needs to stop adequately in adverse conditions. 

I humorously call this the E93 problem.

Engineering for scalability.  

Bean counters at BMW are fans of cross-chassis compatibility. The caliper housing usually needs to be used across multiple models to achieve economics of scale with suppliers. This isn't inherently bad: it is arguably how BMW M engineers played the bean counters and convinced them to make the S85 and S65. 

There are however compromises that are made in performance. The brakes need to cover a wide spectrum of chassis applications. 

As an example, the modern F90 M5 and F93 M8 are built on BMW's Cluster Architecture (CLAR) yet are dynamically different cars: the center of gravity, weight, wheelbase. They still use the same braking system to a T: the caliper, brake booster and master cylinder are identical. 

The solution: smart ABS

Their engineers ended up creating extremely flexible and adaptable ABS systems. BMW M's solution is having to rely on adapting the ABS and valve control within the system over the fundamental hardware itself. 

Engineered for higher thresholds.  

Amongst the numerous variables BMW needed to consider, additional improvement in overall chassis performance were planned and incorporated with the M3 GTS.

The chassis & braking hardware was notoriously improved: bigger brakes, stiffer suspension, solid mounted subframe, stickier tires resulting. 

The GTS' blueprint is common amongst M enthusiasts: we've all come to realize brakes end up being a performance bottleneck. The BMW M smart ABS adapted further more to higher performance thresholds of the GTS.

Running a wider front wheel & tire, improved pad compound along with a tuned suspension was a quick way for me to run into more frequent ABS intervention and overheating concerns, even on backroads. 

I had previously swapped my OE pads to Akebono ceramics for daily use on OE calipers. 

The limits: Competition use. 

BMW M engineered the E9x M3 ABS systems for an extremely wide spectrum of intended use. By narrowing the application spectrum and building upon BMW M's smart ABS, we saw an opportunity to improve braking performance for the enthusiast driver by shifting brake bias to the rear

It is only with the M3 E92 GT4 that BMW M went to a revised, race oriented ABS system optimized for slick use and fast lap times. This is a step beyond the intended goal of the Clubsport kit. 


Up next: How BMW M's smart ABS fundamentals, our research and on-track experience led to the unique technical specifications of the Clubsport kit. 

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