MAiNTENANCE CoRNER

Breaking the Brakes

By Tony Gray, Member #3905

Last month I introduced our community to the latest addition to the Gray household - err that would be the bike garage. The 1997 BMW R1100GS has now been officially christened ‘The Red Baron’ (RB). Yes he is Red and is a solid lump of kit so has assumed the male gender. Now RB came onto my radar in very unfortunate circumstances with his previous owner deceased and he sitting forlornly in a large BMW dealership with a ‘no brakes’ sign on his forehead. I therefore have no understanding of how RB lost his brakes or in what circumstances. The workshop for reasons known only to them had replaced the disc rotors and pads both front & back but there was no fluid in the system. It appeared that no one was prepared to investigate and resolve the ‘no brakes’ dilemma with the ABS pump the assumed culprit. So there he sat unloved and unwanted.

ABS Unit with 4 hydraulic lines – In/Out to Front and In/Out to rear brakes.

I had a close look at the original equipment rubber brake hoses and observed what appeared to be a split in a front hose. Could this have been the source of the fluid loss? My next stop on the path to restoring life into the braking system was to get a full understanding on how the brakes worked. The 1100’s were equipped with the first iteration of the BMW ABS system and it was an option on the models through the 90’s and early naughties. The Haynes & Clymer manuals both describe the system in detail but do not venture into the ABS pump as if it’s a taboo subject. The various brand specific internet forums and Youtube videos tend to confuse rather than enlighten. The main confusion appears (from my perspective) to be between the 1100 and later 1150 models which came equipped with the 2nd gen ABS which was servo assisted. This style of pump lasted until 2006 on both 1150 and 1200 models. If this pump ‘failed’ you would be left with very little braking (some suggest as little as 5%). To see how this works, experiment (no, on 2nd thoughts don’t try this at home kiddies) by rolling your ABS power assisted braked car down the driveway without engine power and see if you can stop it! On the 1100 however without power assistance the bike would lose the ABS function but should retain full manual braking even if the pump fails!

The power assisted unit on the 1150/1200 is a lot bulkier and heavier than that fitted to the RB. Once removed I weighed the ABS unit at 4.856 kg or about 10lb in the old money. This compares with suggestions of 20lb on the net for the servo pump unit. I took the top plate off the elect motor hoping to find something as simple as worn or stuck carbon brushes. No such ‘luck’. Apart from blowing out some carbon dust there was nothing else to do as the motor was fine with plenty of ‘meat’ left on the carbon brushes. I took the next

step towards enlightenment and had a word to Les Fitzpatrick who generously offered to have a look at the unit in his fully equipped workshop. Les performed several tests and spun up the motor confirming it was all OK. The unit casing below the motor where the elect motor engages with the 2 pumps is contained within a crimped pressed metal casing. We decided to open this and have a peek. We had entered uncharted territory.

The 2 clutches and (slightly obscured) the hyvo chain.

The vertical stator shaft of the elect motor has a bevel drive gear that engages a gear wheel. This drives a shaft through 2 clutches with two pistons for the separate front and rear brakes. If either ABS wheel sensor detects a locked wheel then the elect motor pulls down a piston via a clutch that releases brake fluid in the ‘locked’ front or rear brake line. The piston is returned to its resting position by a very compact ‘hyvo’ chain. This all happens in the merest fraction of a second. Everything looked to be operating in accordance with its design but it could not be ‘triggered’ unless connected to a pressured brake line which we could not provide on a workbench.

The bevel drive at the base of the electric motor engages with the fibre gear wheel. That grease is 25 years old.

Replacement braided SS brake hoses will be ordered from HEL at Sandgate in Brisbane and then the system can be reinstated and tested to reveal any fault. Watch this space for the next exciting instalment on the path to braking enlightenment.