ABS behavior question - 95 R1100RS

[Christo]

Greetings,

 

I have asked this before years ago but in a different way. Problem still persists. I suspect my ABS pump is not working properly. Here are the symptoms:

- ABS flash normal on startup and go out after moving.

- When hard stopping, front ABS works great. No way to lock up front wheel. Just pulsates imperceptibly to maintain static friction between rubber and road.

- When hard stopping on rear, if tire 's friction reaches limit, the ABS will engage (with a woosh) in a single loss of rear braking that seems to last until I reapply the rear brake. In other words, significant loss of rear brakes until it decides to reengage, rather than the continuous stopping force with pulsations that you'd expect from ABS.

 

This has dangerous side effect: When coming to fast stop with both brakes applied hard, if say I hit a bump or ridge in the pavement, the front will be fine, but this bump/skip will be enough to make the rear wheel slip and engage the rear ABS. When this happens, it takes some % of my stopping force away, sending me lurching forward of where I expect to stop.

 

Is the rear ABS pump bad?  Replace ABS unit?

 

BTW, I removed the tank and bled the ABS ports thoroughly and then the calipers. All new fluid. Same problem.

 

Thanks!

[Jerry Duke]

Sounds like air in the rear pump. Did you bleed both of the ABS pumps?

[Christo]

Yes, I didn't just bleed the two pumps, but did two pints worth of flush through it and the rotors.

 

Although...  you just reminded me. To bleed the ABS (and rotors) I connected clear tubing so as to have air-free flow out of nipples. On the rear wheel ABS pump bleeder, when I would pump brakes, the brake fluid would flow, but there were very tiny bubbles streaming from it, as if there was a small leak in the air hose. But I double checked that. I believe the bubbles streamed even when I closed the bleeder, pressurized the fluid with the brake lever, and opened the valve.   

 

Attached is a video.

 

The front wheel ABS bleeder and rotors, no such bubbles.

[dirtrider]

13 hours ago, Christo said:

Greetings,

 

I have asked this before years ago but in a different way. Problem still persists. I suspect my ABS pump is not working properly. Here are the symptoms:

- ABS flash normal on startup and go out after moving.

- When hard stopping, front ABS works great. No way to lock up front wheel. Just pulsates imperceptibly to maintain static friction between rubber and road.

- When hard stopping on rear, if tire 's friction reaches limit, the ABS will engage (with a woosh) in a single loss of rear braking that seems to last until I reapply the rear brake. In other words, significant loss of rear brakes until it decides to reengage, rather than the continuous stopping force with pulsations that you'd expect from ABS.

 

This has dangerous side effect: When coming to fast stop with both brakes applied hard, if say I hit a bump or ridge in the pavement, the front will be fine, but this bump/skip will be enough to make the rear wheel slip and engage the rear ABS. When this happens, it takes some % of my stopping force away, sending me lurching forward of where I expect to stop.

 

Is the rear ABS pump bad?  Replace ABS unit?

 

BTW, I removed the tank and bled the ABS ports thoroughly and then the calipers. All new fluid. Same problem.

 

Thanks!

 

Morning Chris

 

Unfortunately what you are seeing is somewhat normal with the early BMW ABS-2 systems. Your ABS really doesn't have a rear pump as it uses a spinning-motor/shaft/clutch's/displacement-piston  system for ABS  control, not servo pumps like the later I-ABS system.

During an ABS event the motor spins a shaft, then the appropriate clutch engages (front/rear/or both), then the driven side of the clutch (or clutches)  pulls  on a chain  (or chains)  therefore moving a displacement piston (or pistons)----  (obviously a very slow to react system)

 

The basic problem with the early ABS-2 is  that there is a built in rear wheel anti-lift algorithm that prevent excess rear wheel lift (prevents excess stoppie lift).  This would work pretty good if the darn system wasn't so slow responding. 

 

All works as intended UNTIL you stop hard, using the rear brake pedal hard, on a washboard road, or on rough pavement. At that time the ABS system sees a dramatic (quick)  rear wheel spin-down as the rear wheel instantly loses traction as it skips over the rough pavement due to both the rough road surface & vehicle weigh transfer to the front tire. (ABS instantly thinks that the rear wheel has lifted off the road) 

 

Part of the rear wheel anti-lift control is to ALSO allow the partial  release of the front brake to bring the rear wheel back down. It's that front brake release that gives you that gut wrenching heart pounding lurch. 

 

It gets better when you release then re-apply the rear brakes as that re-sets the ABS control so by the time it re-sets you are slow enough to usually not have the problem re-appear. 

 

There is really no easy way to  get rid of the problem as it is inherent with the ABS-2 system, but you can tame it down some by using less rear brake apply on rough surfaces when stopping hard, then also  slightly releasing rear brake pressure as you add more front braking pressure  during a hard stop (you need to practice this).   

 

The later ABS-2 systems were better (had a better programming) & the later yet 1150 I-ABS (servo systems) pretty well eliminate the problem. 

 

I had an early 1100RT that I finally removed the entire ABS  system & got rid of that nasty lurch for good.   

 

 

[Christo]

Thank you DR!  This is a great explanation of the issue. You have explained the mechanics of the rear ABS (which I find fascinating) and also preventing me from replacing the pump which would have had no effect. Mucho thanks!

 

You described the sensation very well as a 'gut wrenching heart pounding lurch' . It usually happens as I approach an intersection.

 

I like the front ABS. Is it possible to disable just the rear ABS? I'm good with plumbing the brake lines directly from the master cylinder to the rear caliper, but am not sure how to fake out the rear wheel sensor. How about attaching or wiring it in parallel to the front?? 

[AndyS]

If it is any comfort it still did this with the iABS fitted to the 1150's. 

[dirtrider]

2 hours ago, Christo said:

I like the front ABS. Is it possible to disable just the rear ABS? I'm good with plumbing the brake lines directly from the master cylinder to the rear caliper, but am not sure how to fake out the rear wheel sensor. How about attaching or wiring it in parallel to the front?? 

 

 

Afternoon Chris

 

You might ( I stress 'might' on this) be able to do that (I always thought about doing that on my first 1100RT) but never got around to it. Once I removed the ABS I was so happy with the  brake modulation & feel that it I never thought about it again. 

 

You would need to leave the  rear wheel speed sensor SIGNAL input operational (so the ABS system doesn’t default). Without a functional rear wheel speed sensor INPUT the ABS system would go into default & not work even on the front.

 

Then, plug off & fully bleed the rear hydraulic channel coming from the rear master cylinder to the ABS module & the one that went from the ABS module to the rear caliper  (basically deadhead both)

Then plumb a hydraulic line directly from the  rear master cylinder to the rear caliper.  

 

Hopefully the ABS  system would then not see any rear brake input so think that you were only using the front brake. (big 'maybe' on this though)

 

But there is (still)  a good  chance that if you  over-brake with the rear  that the ABS system would still see the sudden spin-down rate on the rear wheel & STILL release some front brake to  prevent rear wheel lift. (I can’t remember seeing any internal pressure sensors inside the  ABS-2  module so there is a good chance the above won’t fully stop the problem).  

This all hinges on IF the ABS system pays attention to rear brake input pressure & wheel speed, or just wheel speed (on this I don’t know). 

 

To get more complicated yet, but have a much higher chance of  it working correctly, then probably  install a 2nd wheel speed sensor on the front wheel  then use that 2nd  speed sensor signal in place of the original rear wheel speed sensor (that way the ABS system would not  see the rear wheel suddenly slowing during hard rear braking. (I'm not sure if your present front wheel speed sensor could drive 2 separate module inputs, would be iffy)

 

One other thing that is easy to do (I did this on my 1100RT’s) is to replace the rear metallic brake pads (if your RS bike presently has them) with the 1100R  organic rear brake pads.  This takes a lot of aggressiveness out of the rear braking so the rear wheel doesn’t spin down so quickly on a hard stop. (I’m not sure if your RS came with metallic pads or not, or if someone has since installed aggressive rear brake pads, but this is something easy to do as a first step).

 

The other alternative is: If you change your braking input to use more front brake & less rear brake, or learn to let up on rear braking input as you add more front during hard braking then the  lurch problem will become much less of a problem. 
If you are braking real hard then the rear brake doesn't do much anyhow as the rear wheel should be real light to even bouncing up off the  road when the weight transfers to the front wheel. 

 

[Christo]

DR,

 

Thanks for the technical ideas!

 

So when you removed the abs and liked the results, do you ever have problems with locking front wheel during hard stops? I locked the front wheel on my VFR a couple of times in emergency stops and had to release and reapply.

 

As for configuring the ABS sensing to the front wheel only and bypassing the rear pump, it would be easier to wire both leads into the single sensor already on the front, and possibly use diodes to channel current, but it would depend if the computer didn't like the change in current/voltage, as you hinted to.

 

And I suppose simulating a pulsating signal to the rear sensor would be difficult because the computer looks at the speed wheel via the AC signal the sensor produces, correct?

 

Another possible option: What if you move the rear sensor to the front and do nothing else (leave the rear pump intact)? Would the tiny front wheel stop/starts during max braking (as opposed to the sudden stop of rear wheel lock up) cause the computer to enact the same shutdown of braking?

 

[dirtrider]

Afternoon Chris

 

So when you removed the abs and liked the results, do you ever have problems with locking front wheel during hard stops? I locked the front wheel on my VFR a couple of times in emergency stops and had to release and reapply.-- Not really as I ride pretty hard so am used to braking right to the threshold of lock (usually with the rear wheel dancing) . Every now & then I will get a short front wheel  lock if really stopping hard (especially on dirt or gravel)  but I just release a little brake pressure & it spools back up. But I do practice threshold  braking  often as I ride a lot  of  high speeds dirt/gravel with non ABS motorcycles (or if on an ABS bike I usually turn the ABS off). I grew up on non ABS drum brake motorcycles so I can easily get by without ABS. My preference is to not have ABS on a motorcycle unless it is a very modern (very fast response)  system that doesn't interfere with, or interfere early with, my normal braking control, & better yet have some sort of lean angle sensing. I absolutely hate ABS on an off-road motorcycle as it won't allow any (or much anyhow)  weight transfer on loose surfaces.   

 

As for configuring the ABS sensing to the front wheel only and bypassing the rear pump, it would be easier to wire both leads into the single sensor already on the front, and possibly use diodes to channel current, but it would depend if the computer didn't like the change in current/voltage, as you hinted to. -- You can try but keep in mind that the wheel speed sensors are only outputting millivolts & each diode usually drops circuit voltage .7 volts  per diode so I have my doubts. Probably better  off to just figure on adding a 2nd front wheel speed sensor.

 

And I suppose simulating a pulsating signal to the rear sensor would be difficult because the computer looks at the speed wheel via the AC signal the sensor produces, correct? -- I don't think this would work as the ABS computer does compare front/rear wheel speeds for some functions.  Probably be way ahead to just use 2 front sensors. Plus if the front & rear speed inputs are way off with each it will probably trip a wheel sensor fault & your ABS will probably go into failure mode. 

 

Another possible option: What if you move the rear sensor to the front and do nothing else (leave the rear pump intact)? Would the tiny front wheel stop/starts during max braking (as opposed to the sudden stop of rear wheel lock up) cause the computer to enact the same shutdown of braking? -- I think doing this will bring on other side issues, when the front wheel spin down rate read-ahead  triggers an ABS event then that will probably also dump the rear braking pressure thinking that the rear wheel is also locking up. Safe thing to do  is just plumb rear brakes to not go through the ABS module control.    

 

 

[Christo]

Thanks again DR! All good insights.  I will ponder your suggestions and see what I want to do. In the interim, I could try the organic pads on the back and see how much harder it is to activate the ABS. But the main thing is knowing that it's not malfunctioning and that piston chain Mechanism it's slower to react. That save me a ton of waste of time and money.