Operation of I-ABS-gen2 system if ABS is not working

[dduelin]

 

Thanks to Dirt Rider for the steps to perform an operational test of the integrated (linked) I-ABS-gen2 brakes that he provided in an unrelated thread. ( Bike on center stand, key on but engine off, spin rear wheel,  then apply hand lever while rear wheel rotates, observe rear wheel stopping rotation upon brake lever application) I have had mechanically linked brakes on Hondas and know how to spot check their linked systems but I did not know of a way to check the 2007-2013 RT's electronically linked system for front-to-rear integration.

 

Is it possible for you to describe how or if the I-ABS- gen2 system varies the pressure sent to the rear caliper under different braking situations? Could the front brake remain linked to the rear brake if the ABS was inoperative for instance and if so is there a proportioning device in the module that would vary the amount of pressure to the rear brake if ABS was inoperative? It seems that if the ABS was inoperative and the brakes remain linked the opportunity to lock up the rear wheel with moderate pressure on the lever would create a dangerous situation.

 

Thanks in advance,

[dirtrider]

16 hours ago, dduelin said:

 

Thanks to Dirt Rider for the steps to perform an operational test of the integrated (linked) I-ABS-gen2 brakes that he provided in an unrelated thread. ( Bike on center stand, key on but engine off, spin rear wheel,  then apply hand lever while rear wheel rotates, observe rear wheel stopping rotation upon brake lever application) I have had mechanically linked brakes on Hondas and know how to spot check their linked systems but I did not know of a way to check the 2007-2013 RT's electronically linked system for front-to-rear integration.

 

Is it possible for you to describe how or if the I-ABS- gen2 system varies the pressure sent to the rear caliper under different braking situations? Could the front brake remain linked to the rear brake if the ABS was inoperative for instance and if so is there a proportioning device in the module that would vary the amount of pressure to the rear brake if ABS was inoperative? It seems that if the ABS was inoperative and the brakes remain linked the opportunity to lock up the rear wheel with moderate pressure on the lever would create a dangerous situation.

 

Thanks in advance,

Afternoon Dave

 

This is about the BMW I-ABS gen2  system, not the earlier I-ABS (wizzy system)_ 

 

The BMW I-ABS gen2  braking system is kind of simple in it's simplest form & very complicated in it's actual electronic functions.  

 

In it's base form (think  no power to the ABS module) the front brake lever operates the front brake just like older conventional brake systems & the rear brake pedal operates the rear brake just like older conventional brake systems. 

 

Once you plug in the power & wheel speed sensors then the  real magic happens. 

 

With B+ power & wheel speed going into the ABS module, when you pull the front brake lever you get normal non-powered front braking that is directly in proportion to the pressure that you put into the hand lever.

 

BUT, that hand lever controlled front brake pressure also goes to an internal pressure sensor & some smart electronics inside the ABS module.  At this time the rear brake electronics in conjunction with the pressure sensor turns on the ABS module's servo pump (motor) that goes to the rear brake. The amount of front brake pressure (using the pressure sensor & electronics) modulates the rear braking pressure based on BOTH front brake pressure & internal logic to not only vary the rear braking pressure based of front braking pressure but due to very smart electronics and both the front & rear wheel speed inputs it can independently vary or modulate the rear braking pressure based on past riders braking history & the amount of weight or load on the motorcycle (basically the ABS controller learns braking habits & learns front to rear braking traction then adjusts the front to rear braking bias based on that. (this works best when no (or low) rear brake pedal input is used)

 

More_  With B+ power & wheel speed going into the ABS module using ONLY the rear brake pedal you still only get rear braking (no front linking). The rear braking pressure is directly proportional to the foot pedal pressure applied.  (this is REAR BRAKE pedal apply only). 

 

More yet_ With B+ power & wheel speed going into the ABS module, if a rider uses both the front hand lever & rear brake pedal at the same time it starts getting more complicated as in basic brake system hydraulics you can basically only have one input pressure source at at time going to a brake caliper. So there is a valve in the rear brake circuit. I call it an either/or valve (my terminology)  as using the term either/or makes it slightly easier to understand. 

 

So what happens basically is, he who supplies the most pressure wins the contest to see who controls the rear brake at THAT time. 

 

So you are peacefully riding along & something darts out in front of you so you grab a handful of front brake lever to avoid. When you do this that front lever pressure applies the front brake directly & also turns the rear servo pump on to apply the rear brake. The harder you pull the front lever the more front & rear brake apply you get. (somewhat simple so far)

 

But now as you do the above you also step on the rear brake pedal, that does not add any more pressure to the rear brake though UNLESS the rear pedal pressure that you apply adds more pressure than the servo pump is already supplying. If the front servo pump is supplying 50 psi to the rear brake but your rear brake pedal pressure is only supplying 40 psi then your rear pedal pressure is dead-headed & does nothing. Now if you step on the rear brake pedal harder so that now supplies  60 psi that shuts off the either/or valve so now the servo pump pressure is deadheaded & your rear pedal is controlling the rear brake directly. (basically what is supplying the most pressure to rear brake is leading the dance) this can jog back & forth as you vary the lever pressure and/or vary the hand lever pressure. 

 

Problem is, if a rider starts adding too much rear brake pedal apply too often then that sort of screws up the front/rear braking bias that the ABS module has learned.   Not at lot but it does slightly skew the ABS modules learning of proper wheel spin-down vs braking pressures. 

 

A rider can still use the rear brake pedal for normal braking to keep in practice of using both lever & pedal  (most riders do) but if you keep the rear apply pressure low it will just stay dead-headed (not really do or add anything) so won't mess with the ABS module's computer brake bias learning. 

 

There more to this but the above pretty well covers the normal (non ABS) function & operation. 

 

I will try to add the actual ABS function & anti-rear-wheel-lift part tomorrow.  

  

 

 

[dirtrider]

ABS part_

 

The ABS operation is pretty straight forward & operates about like other motorcycles in not allowing either wheel to lock up. It's in how it does that is slightly different than older BMW systems.

 

(I-ABS gen2 system)

 

The ABS for the rear brake uses the servo pump for makeup fluid as required throughout the ABS event. 

 

The front ABS function is slightly different as it doesn't have a servo pump or displacement piston like the older systems but instead uses quick reacting valves then dumps the released fluid into & out of a storage accumulator. Very fast reacting system and no servo pump to deal with so those are both pluses for the I-ABS gen2 systems. 

 

There is also an anti-rear-wheel-lift (anti-stoppie)  algorithm designed into  ABS module that monitors both front & rear wheel spin-down rates then can modify braking pressures to the front brake to prevent excessive rear wheel lift under heavy braking.   If a rider tries hard enough they can still get a little rear wheel lift at extremely heavy braking but it usually doesn't go very high or for very far.

 

It is a fast system so that helps with quick brake release & smooth ABS modulation before total wheel lockup, plus with the quick response it keeps the wheels spinning a little even on extremally slippery surfaces as a fully stopped wheel probably doesn't have enough traction or enough time to start spinning again once totally stopped on a very slippery surface (an automobile can deal with with a short duration locked wheel but a motorcycle usually can't).