Temporary loss of ABS during a stop

[Nevets]

I have noticed on a few occasions how it seems that the brakes on my 2003 RT-P "pause" momentarily during a fairly aggressive stop when I encounter a bump on the road. By “pause”, I mean it feels like I let up on the brakes (front lever). It causes no overall problem in my ability to stop, but my heart does skip a beat or so . My assumption was that I was actually losing traction for a moment because of the road bump, so this was somehow affecting the ABS. I have seen at least one other post that mentions the "ice patch" phenomenon. Is this some kind of "they all do that " issue? I have not had the dealer check this issue out yet. Any explanations would be appreciated.

 

Don’t know if it’s related, but I also have a strange clunk occur when I really engage the brakes strongly. I posted a question back the middle of June ("Strange noise when using the rear brake on a 2003 RTP") but was unable to get this resolved/understood.

 

By the way, this web site and forum is a GREAT resource! I can’t imagine how difficult it would be without the information exchanged here.

[Marty Hill]

That's what abs does. If the lumps were wide enough for all wheels to hit, you car would do the same.

[Jerry Johnston]

I have a Pontiac G Am that did the same thing. Just had the cars brakes inspected for wear in the lining a week earlier so I figured the brakes were screwed up by the car dealer. To make this a shorter story I found the battery was five years old and replaced it - it's been a month now with out a problem.

[ShovelStrokeEd]

Simple answer is they all do that.

 

Reason follows, but for clarity, you didn't lose the ABS, all that happened is that the ABS computer momentarily released the brakes because it sensed the impending lockup of one or both wheels.

 

When you encounter bumps, in particular with the rear wheel while braking, it is pretty easy, due to weight shift, for the rear wheel to come off the ground, or at least get light enough so that it is no longer provided with any turning force from the bike's speed. Well, it is easy to see that any braking force is going to cause the wheel to slow radically, possibly even stopping. The ABS computer, which is using a Hall effect sensor for measuring the time between pulses on a toothed wheel on the brakes, sees this as a radical change in the rate of acceleration of the wheel. There is a limit built into the computer or learned by the computer during each stop (I don't know which) beyond which the computer will start to release the brakes in an effort to keep the wheels rolling. This is what ABS does.

 

On a bike with linked brakes, such as your 03 RT, since you are applying signal with the front brake lever and the front brakes automatically apply the rear brake, if the system senses either wheel about to lock it will release both brakes. Your bike should actually have semi linked brakes in that application of the front alone will activate the rear but application of the rear alone will not activate the front brakes. Some have stated that this effect can be minimized by application of both brakes at once. I have no direct experience of that although I have quite a bit on what you described.

 

I do feel this to be a defect in the implementation of the braking system, not in the system, its working as it was designed. I would have thought that BMW would have been able to sense which wheel was locking and only modulated that one. I certainly feel that I can do a better job of braking over stutter bumps on a non-ABS equipped bike. If only because I am willing to put up with momentary wheel locking while the ABS system is not.

[smiller]

On a bike with linked brakes, such as your 03 RT, since you are applying signal with the front brake lever and the front brakes automatically apply the rear brake, if the system senses either wheel about to lock it will release both brakes.

 

The ABS on oilheads is single-channel and always operates on both wheels at once? I didn't realize that, and if so that may provide some illumination as to why some riders consider the BMW ABS system to be too touchy and a PITA while others seem to have no problems. I'm one of the latter group and almost never experience any troublesome ABS activations, but then again I don't have a linked-brake bike. If you have a linked brake system activating the rear brake when it isn't intended or desired and activation of the rear ABS will also activate the front (whether the front wheel is calling for ABS action or not) then I can see how that could easily cause a problem. It's not very difficult to lock a rear brake and if you are 'forced' to use it when you shouldn't (by a linked brake system) and that in turn causes the whole bike to go into ABS then... well, that's really not such a great idea.

[Boffin]

Ed, I'm pretty sure that the ABS does know which wheel has stopped, but BMW in their wisdom decided that, if the rear stops and does not restart imediately after the rear brake pressure is modulated, the bike is doing a stopie. To prevent this the ABS then releases the front brake in order to allow the rear wheel to drop back to the pavement. I think.

 

Cya, Andy

[Ken/OC]

...the bike is doing a stoppie. To prevent this the ABS then releases the front brake in order to allow the rear wheel to drop back to the pavement. I think.

 

Andy, that's my understanding also, and it certainly matches the experience of the "ice patch" effect. However, it does seem to make some sense to program the brakes this way. Why don't other ABS-equipped bikes suffer the same problem? How do Honda and Yamaha handle potential stoppies?

[ShovelStrokeEd]

Andy, you may be right. I know it can tell which wheel is responsible for the locking and will modulate that wheel first. The anti-stoppie thing doesn't really hold much water with me though. None of the BMW's I have ever ridden will, on good pavement, get that rear wheel very far off the ground. That includes, BTW, my 1100S. It will slide the front before the rear leaves the ground. Now, being a Boxer Cup Prep, it is the lightest bike with the shortest wheel base and the highest CG of any in the fleet. If my S won't do a stoppie, none of them will. Actually, on really, really grippy pavement, with a good, fresh tire on the front, I can get the rear wheel up a couple of inches. Nothing approaching dangerous.

 

For my take, I just think they took the easy way out with the software and didn't develop the linkage thing far enough.

[Stan Walker]

I just think they took the easy way out with the software

 

It wouldn't be the first time some software guy said "hey, check out this neat thing I can do!!!!" DAMHIKT

 

Stan

[ghaverkamp]

The anti-stoppie thing doesn't really hold much water with me though.

 

Well, I happen to know for a fact that the rear wheel can be lifted up on a K12RS. I know this mostly because of how noticeable it is when it comes crashing back down to the ground. I wasn't moving too fast, which might have had something to do with it. I was in stop-and-go traffic just above my threshold for splitting, looked down or away or otherwise lost my focus for a second, and then noticed the car ahead of me slam on his brakes. I followed suit. After stopping, my rear wheel then proceeded to land. You do know that if metal is showing, you should get new rubber, right?

 

I haven't spent sufficient time on fully integrated systems to know the differences from my (former) partially integrated system. I was certainly able to make the rear ABS cycle without the front ABS cycling. Outside of the ice patch effect, I can only think of twice on the street where my front ABS cycled. One was my fault for riding angry (moron Porsche driver wouldn't get out of my way on one of my favorite stretches of road, so I tailgated him through it, when he came upon moron Bimmer driver making a u-turn in a blind corner) and the other was on one of those hideous vinyl lane markers than was damp.

 

As I've written before, however, I could consistently induce the ice patch effect in one particular spot. It was a steep downhill a few turns from the parking garage at school. Front brake activated with the rear either activated or not (didn't make a difference), with what I'll refer to as a frost heave (true cause in the middle of San Francisco is not likely to be a freeze.) Rear bounces up, all brakes release. Same thing would happen a lot on my really back backroads I tend to like to ride up in the coastal mountains.

 

Anyhow, on a K12RS, which has a pathetically worthless rear brake, slow with the front and stomp on the rear. The rear should cycle while allowing the front to continue to brake normally. Make that rear wheel pop up off the ground, and you'll make the front release.

 

Greg

[ShovelStrokeEd]

Yeah, you can get it off the ground a few inches to maybe even a foot. That is not dangerous, or even very sporty by squid standards. Point is, there is nothing in particular that a BMW will do under those conditions that would create the need for this whole anti-stoppie reaction thing. Yeah, you can make a GSXR or R1 or the like mousetrap you but it ain't gonna happen on a Beamer. Not even if your trying. I don't consider this to be a planned action but rather a side effect of what is described below.

 

Now as to the ice patch effect, that is triggered by repeated bumps (stutter bumps) on the pavement such as one encounters where there is, or has been, heavy truck traffic and a series of stop lights. That is quite disconcerting and is one of the places where the ABS just cannot out perform a trained rider who is willing to put up with a certain amount of wheel sliding. I think the software implementation here is to blame with not enough thought applied to the stutter bump thing as it is rarely encountered on European roads.

[ghaverkamp]

Now as to the ice patch effect, that is triggered by repeated bumps (stutter bumps) on the pavement such as one encounters where there is, or has been, heavy truck traffic and a series of stop lights. That is quite disconcerting and is one of the places where the ABS just cannot out perform a trained rider who is willing to put up with a certain amount of wheel sliding. I think the software implementation here is to blame with not enough thought applied to the stutter bump thing as it is rarely encountered on European roads.

 

My point was, however, that I don't believe it's just a matter of lacking independent cycling of either wheels. Instead, there does appear to be a component of the system that releases the front of the rear is off the ground. Call it anti-stoppie or call it something else, but the effect seems to be the same.

 

And Moike, an occasional poster here, has posted photos of himself doing stoppies on several BMW models; it definitely can be done, even if it's unlikely to happen by accident.

 

Greg

[ShovelStrokeEd]

Heck, I don't doubt it can be done, but , you really have to try. I can get my S up a few inches, maybe even further if I wanted to try at low speeds, that brings up the question why? MY point is, that I doubt BMW ever intended this as an anti-stoppie device. More likely it is a mistake in the software or an insurmountable problem regarding the hydraulics. I know that at least the control circuits on a linked brake bike are physically connected. My '94 never does any of that kinda stupid schiznit.

 

Greg, I don't think we disagree at all here, merely a matter of semantics and approach to the why of the entire thing.

[Nevets]

Like usual, you all have done an excellent job of explaining it. ...and I'll know what to watch for when riding, and I won't have to get a pacemaker to restart my heart after all. Thanks for all your thoughts.

[Bill Dennes]

It seems that this problem is less likely to happen if the brake pedal has been applied too, instead of just the brake lever. It seems to bias how the ABS "thinks."

 

I gave up on that and unplugged my sensors and relay, and taped over the warning bulb. Now I have no ABS, but I do have brakes that come on when I apply them and stay on until I let 'em off.