My new friend, Hilton, was kind enough to pass on a service bulletin, given to him by a service manager in a German dealership.
It pertains to BMW position on replacement of rubber brake lines with steel-flex. This will likely fan the flames of STEEL-versus-RUBBER discussion. Go on, lads!
The bottom line of the bulletin is:
- On Integral ABS, steel-flex lines may reduce effectiveness of Residual Braking.
- On ABS-2, steel-flex may change control behavior of ABS.
This is my take on this: This issue seems to be a fallout from the Integral ABS residual-braking fiasco, when BMW was accused of evildoing, because braking power was reduced while engine was off. This caused a lot of bad publicity and a lot of legal hassle for the company, plus a lot of trouble proving over and over again that the system is compliant with applicable safety regulations.
Therefore, even if the change of behavior is minute, BMW is loath to condone any modifications to what is approved.
My opinion: I installed steel lines a year ago on my ABS-2 equipped bike. I did notice harder brakes, as expected, but did not pay attention to ABS behavior (did not have too many occasions). I'll check on it when the riding season begins. Like anything else, this is a trade-off: a possibly different characteristic of ABS operation against the likelihood of burst rubber lines, reported here quite frequently. For the time being, I am staying with steel-flex.
Of course, I am not a lawyer and don't even play one on TV, so don't quote me.
Before you get deep into the text of the bulletin, here is a refresher on Residual Braking - adapted from German Wikipedia:
"The residual braking function is an emergency operation in case of failure of the brake booster, as specified by law. If the hydraulic pump and electromagnets are not active, a valve closes and a Residual Braking piston is moved. The resultant manual brake pressure requires longer and more powerful hand lever movement than during Active Braking. "
Translation of the bulletin, in boldface.
Service Information 3/2003
Technical Customer Service
Possibility of Retrofit with Steel-Flex Brake Lines in Vehicles with ABS
All Models with Integral ABS and ABS2
1. Vehicles with Integral ABS
In vehicles with Integral ABS the whole brake system is optimized for the operation of ABS, of active braking and residual braking. In particular, the internal volume changes of the system (brake lines, calipers, etc.) affect the function of the system. Installation of steel-flex lines modifies the change of volume; as a rule, the volume will change less (hard brakes).
Installation of steel-flex lines in Integral ABS wheel circuit (between pressure modulator and brake caliper) can cause a situation where a very fast increase of pressure reaches the pressure limit in controls, limiting the pressure during residual braking - thus preventing reaching the locking limit.
Furthermore, the retrofit does not bring a performance improvement, since the active brake system pumps compensate for change of volume in brake lines.
Installation of steel-flex brake lines in wheel circuit is not allowed.
Installation of steel-flex brake line in control circuit can cause a change of brake feel; here BMW sees no safety-related limitations with regard to the residual brake function.
2. Vehicles with ABS 2
In vehicles with ABS2, installation of steel-flex lines in the wheel circuit (between pressure modulator and brake caliper) affects ABS regulation, which modulates pressure based on change of volume in wheel circuit. That is, due to a different volume-to-pressure characteristic of the lines, the same change in volume will correspond to a different brake pressure. This changes the control levels of the ABS control.
BMW does not approve of installation of steel-flex lines in wheel circuit.
Hint: Installation of steel-flex brake line between hand controls and ABS Modulator can cause a change in the brake feel; however, BMW sees no safety-related limitations with regard to the residual brake function.
Should you discuss this matter with your customer, advise him about the issues described in this service bulletin. The installation of steel-flex brake lines between ABS Modulator and the caliper is not permitted by BMW.
This also pertains to steel-flex brake lines from third-party vendors.
End of bulletin. Notice the wording:
Integral ABS: Installation of steel-flex brake lines in wheel circuit is not allowed
ABS-2: BMW does not approve
of installation of steel-flex lines in wheel circuit.
I understand this as "characteristics will change and we are not responsible - but, with Residual braking we went through regulatory hell, so don't touch it".
Forgive the poor writing style of my translation. As I switch my mind to Technical German, my English literary prowess ties up in a (Bavarian) pretzel.
Click on thumbnail below for larger-size image.