ABS locks up at slow speed?

[username]

all right, something werid happened last night. i went out for a quick ride around the hood and the campus. i was pulling up to a stop sign, going maybe 5mph. i have been working on, and apparently not doing a good job of avoiding using too much rear brake. (long story, but my service tech informs me that i have the rear brake fairy. it turns out i do, i use it too much, and often without realizing im doing it!) as i approached the stop sign, the rear of the bike wiggled a little. nothing scary, but like the tire was on ice or something. like an idiot, i did not go back to see if there was gunk in the road, i just kept going, because i noticed i was pressing the rear brake, and shouldnt have been.

so my question is, can the rear wheel lock up a low speeds like that, especially if it hits some funky pavement on an ABS equipped bike?

and that leads me to...how the heck do i verify that my ABS is working properly without scaring the crap out of myself? :) i do panic stop practice in a parking lot every so often, but it's on nice clean, smooth blacktop.

[Rad]

Originally posted by username
all right, something werid happened last night. .....

so my question is, can the rear wheel lock up a low speeds like that, especially if it hits some funky pavement on an ABS equipped bike?
..

I would bet dollars to donuts that yur ABS engaged and yur rear tire did not lock up. If you have never felt the ABS engage in the rear it can give the impression of sliding cause ya quit slow'n for a second or two, weird feel'n.

Go out and try engaging yur ABS, it is good to know what that feels like.
If ya know what ABS engaging is like and yur sure yur rear locked up I would then bet it locked from compression braking. ABS will not stop that kinda lock up.

It is true that below about 5 miles per hour the ABS may not engage but I have never had that happen to me, have’n three different BMW ABS bikes.

[username]

so i feel dumb asking, but whats the best way to test it? should i just wet down the road in front of my house and brake hard? my neighbors must already think im a wacko, so this is not a big deal. :)

[Rad]

Originally posted by username
so i feel dumb asking, but whats the best way to test it? should i just wet down the road in front of my house and brake hard? my neighbors must already think im a wacko, so this is not a big deal. :)

Very good question

I teach MSF and I always talk to the students about practicing braking on their own bikes. I recommend finding a deserted section of town, like a light industrial section on a Sunday morning. Practice progressively using more and more brake at slow speed until you find that ABS threshold.

Relax, look well ahead and just keep the bars pointed straight ahead. Do not grab the brakes, just progressively apply them harder each run.

Disclaimer, you will be put'n yurself at some risk but I've always felt it best to learn yur braking threshold in a controlled environment rather that that first emergency situation.

good luck

[HANKB]

[i]
so my question is, can the rear wheel lock up a low speeds like that, especially if it hits some funky pavement on an ABS equipped bike?


My rider's manual states that ABS will not work below 2.5 mph (on a '95 K75RT.)

[JimVonBaden1]

I feel the same thing on my 94 R1100RS occasionally if I use the rear brake agressively. It's the ABS kicking in and out. Very odd sensation.

I tested it to be sure that was it by finding a parking lot, doing about 15mph and hitting the rear brake only fairly hard. It locks up easier than you might think, but immediately engages the ABS. Gives a feeling like you are sliding, but you are not. No rubber left on the pavement.

I haven't had the nerve to try the front like that yet! Though I think it engaged yesterday when I was looking to enter the freeway and they guy in front of me decided to stop instead of merge, even though there was plenty of time. It also felt like a bit of slipping.

I would say test it, but it sounds like it's fine.

Jim:brow

[gandalf777]

just a small experiance on my 2003 clc...pulling down a steep incline....blacktop is a little wet...I put my brakes on (maybe 2 miles an hour) brakes lock then LET GO...the next thing I know is I'm out in the middle of the four lane....the only thing I have found out to help the kamakazi brakes is to not use your clutch (use engine compression for stopping)..look for a dry patch of pavement....cross your fingers and hope that no cars are coming...I love the power part of my brakes...but antilock is best used for cars.......

[Andy VH]

I used to work on ABS systems for heavy duty trucks, and the ABS principles of operation are the same for cars. bikes, trucks, trains and planes. Here's a quick primer on ABS operation.

ABS works on a principle of 20% "slip", meaning the actual wheel speed of an ABS controlled wheel is designed to allow a 20% slower wheel speed versus the actual vehicle road speed before ABS control kicks in. That's why, during any ABS controlled stop, you will hear some tire squeal and see tire marks on the pavement. Maximum braking is achieved at the 20% slip ratio.

The two ABS wheel speed sensors send their speed signal to the ABS modulator, which determines the Vehicle Reference Speed (VRS). When braking, during any event when a tire "slips" to a speed less than the VRS, the ABS modulator kicks in and modulates that brake until that wheel speed sensor signal is within the 20% slip thresehold.

All ABS systems are set up based on a preset "modulation" rate. Usually it is about 5 cycles per second (cps). Lessor systems may be only 3 cycles per second. The early Delco ABS systems on Chevys had pronounced less ABS performance because the 3cps rate could not match the performance of the 5cps common to Bosch and Wabco systems. Delco has since improved to beyond the 5cps rate.

But inherent in any ABS braked system, is natural wheel/brake hysterisis. A fancy word to describe the natural dynamics of a braked wheel system. As wheel mass increases, the hysterisis of that wheel also increases. The type of brake system also affects hysterisis. But since all ABS braked BMWs are disc braked we can treat them all the same. The hysterisis of the braked wheel determines the minimum speed thresehold at which ABS cannot modulate the wheel because even if the ABS releases the brake pressure, the latent "lag" in the brake system maintains braking effort on that wheel.

On many ABS equipped bikes, the low-speed thresehold of ABS operation is below 10mph. Below 10mph, the ABS cannot release the braked wheel fast enough to overcome the hysterisis in the system at the 5cps rate. So at slow speeds the wheels can lock up. This is also why on off-road applications the wheels can "lock up", because the loose dirt/gravel surface can "dam up" in front of the tires and actually cause enough drag to slide a wheel even though the ABS is working.

This is why it is important to know how your ABS equipped bike brakes, especially during an ABS event. I to am a 14 year MSF instructor and I try to educate my students that ABS systems are simply for bike stability and really have nothing to do with shorter braking distances. Practice your high-effort braking and know what your bike can do.

For more info on this, check out the Motorcycle Saftey forum at Motorcycle-Usa.com in the message boards section. Enjoy the ride!

[gsinmaine]

I used to work on ABS systems for heavy duty trucks, and the ABS principles of operation are the same for cars. bikes, trucks, trains and planes. Here's a quick primer on ABS operation.

ABS works on a principle of 20% "slip", meaning the actual wheel speed of an ABS controlled wheel is designed to allow a 20% slower wheel speed versus the actual vehicle road speed before ABS control kicks in. That's why, during any ABS controlled stop, you will hear some tire squeal and see tire marks on the pavement. Maximum braking is achieved at the 20% slip ratio.

The two ABS wheel speed sensors send their speed signal to the ABS modulator, which determines the Vehicle Reference Speed (VRS). When braking, during any event when a tire "slips" to a speed less than the VRS, the ABS modulator kicks in and modulates that brake until that wheel speed sensor signal is within the 20% slip thresehold.

All ABS systems are set up based on a preset "modulation" rate. Usually it is about 5 cycles per second (cps). Lessor systems may be only 3 cycles per second. The early Delco ABS systems on Chevys had pronounced less ABS performance because the 3cps rate could not match the performance of the 5cps common to Bosch and Wabco systems. Delco has since improved to beyond the 5cps rate.

But inherent in any ABS braked system, is natural wheel/brake hysterisis. A fancy word to describe the natural dynamics of a braked wheel system. As wheel mass increases, the hysterisis of that wheel also increases. The type of brake system also affects hysterisis. But since all ABS braked BMWs are disc braked we can treat them all the same. The hysterisis of the braked wheel determines the minimum speed thresehold at which ABS cannot modulate the wheel because even if the ABS releases the brake pressure, the latent "lag" in the brake system maintains braking effort on that wheel.

On many ABS equipped bikes, the low-speed thresehold of ABS operation is below 10mph. Below 10mph, the ABS cannot release the braked wheel fast enough to overcome the hysterisis in the system at the 5cps rate. So at slow speeds the wheels can lock up. This is also why on off-road applications the wheels can "lock up", because the loose dirt/gravel surface can "dam up" in front of the tires and actually cause enough drag to slide a wheel even though the ABS is working.

This is why it is important to know how your ABS equipped bike brakes, especially during an ABS event. I to am a 14 year MSF instructor and I try to educate my students that ABS systems are simply for bike stability and really have nothing to do with shorter braking distances. Practice your high-effort braking and know what your bike can do.

For more info on this, check out the Motorcycle Saftey forum at Motorcycle-Usa.com in the message boards section. Enjoy the ride!

now I ride an 05 gs-a and find my self useing the frount brake more so than the rear unless off road (then it's w.o.t.)

[cwsenn]

I have played (tested) my ABS on some paved roads while thoroughly wet after constant rain. I get going about 15 - 20 mph, make sure no cars are in sight and really get on them. I usually use the front brake as they are integrated with the rear but have done both rear and front at different times, it was a good experience. It is a totally different feeling than my Chevy truck. It felt more like the brake locked and then let go and locked and let go over and over. My truck makes more of a growl and never seem to quite lock. The Chevy and the RT systems are both strange feeling but totally diffferent from each other. I felt the BMW ABS system was more effective than that of the Chevy truck or least I felt I had more control. Here in the western WA it rains plenty and I am really glad to have the ABS.

[Andy VH]

I have demo'd the ABS on my bike for my MSF students. On the range, at about 20 mph I apply the brakes qucikly and progressively until I know the ABS will engage. My students have said they can easily see the ABS brake modulation effect on the tires as they can see the tire momentarily almost stop rotating and then speed up again and slow again as the ABS modulates the brakes.

This is more obvious at slower speeds because as I near the low speed thresehold of ABS modulation the momentary "almost stop" action is greater simply due to the time it takes in milliseconds for the brake to release and the tire to speed up again.

Motorcycle ABS systems do not use a "feedback" circuit to send a high frequency pulse back into the master cylinder (either the hand lever or the foot brake). But on cars and trucks this is a common practice used to alert the driver that an ABS event is occurring. On a bike, the feedback from the tires and bike is so direct that an additional feedback circuit is not required or beneficial.

If your bike has ABS, it is important to practice your high energy braking stops to get familiar with the actual point of ABS engagement. Doing this will get you over the initial surprise feeling of the ABS engaged.

[bikerfish1100]

I feel the same thing on my 94 R1100RS occasionally if I use the rear brake agressively. It's the ABS kicking in and out. Very odd sensation.

I tested it to be sure that was it by finding a parking lot, doing about 15mph and hitting the rear brake only fairly hard. It locks up easier than you might think, but immediately engages the ABS. Gives a feeling like you are sliding, but you are not. No rubber left on the pavement.

I haven't had the nerve to try the front like that yet! Though I think it engaged yesterday when I was looking to enter the freeway and they guy in front of me decided to stop instead of merge, even though there was plenty of time. It also felt like a bit of slipping.

I would say test it, but it sounds like it's fine.

Jim:brow

i would strongly recommend that you test the ABS fully, front and rear. Not primarily to see that it is working (tho that is valid reason enough), but to become familiar with the feeling of how the bike responds with the ABS cycling. It is not what i would call "scary" (not much need therefore for "nerve"), but it is a bit disconcerting, and certainly a different feel than normal. Use good technique (head & eyes up, feet on pegs, knees in, progressive squeeze, etc), but after getting comfy with some good 20-25 mph brake runs, make a pass or two with the only change being overly strong brake application- and let the ABS do it's thing. You want to know what it feels like when it kicks in on the road- as that is not the moment to experience it for the very first time.

[Montana]

Just to clarify which ABS system I'm referring to here, my ABS II system will let my bike lock up and it will slide a bit when I'm pulling up to the bank teller drive through window, and there's always a bit of oil there. Physics always wins.

[deilenberger]

One non-threatening way to "feel" the ABS working is look for an empty hard packed gravel parking lot. One with a very hard surface and a thin layer of tiny gravel on top. It's fairly simple to trigger off the ABS on that surface since the thin layer of gravel works like ball-bearings on a hard surface.

I try to do this several times a year - both to exercise the ABS and to remind myself what it feels like..

[PUDGYPAINTGUY]

Considering how much it costs to fix damage on your bike and person when dropping it, would it not be more cost effective to simply have the dealer run the diagnostic?

[bikerfish1100]

Considering how much it costs to fix damage on your bike and person when dropping it, would it not be more cost effective to simply have the dealer run the diagnostic?

well, as both Don and I (and others) have indicated, one of the primary reasons for triggering the ABS in a "safe" enfivornment is so that you know what it feels like when it activates. you'll never get that on a bench test. the other point here is that ABS is designed to help you NOT drop the bike, as it does not allow the skidding that occurs from locked-up wheels. if your general braking technique is good, you will stay upright. and we ALL need to regularly practice full-on emergency braking- so that when we need it, we will have it in the arsenal.

[Jamming]

I've triggered the ABS on my ST twice. First time on the MSF range taking the ERC. and the second time throwing out the anchor, speed brakes, slats, flaps, and the dirty laundry cause a herd(pack)? of Javelina(wild Pig) ran across the freeway at 0400.
No escape route, save for the desert and the ST is no GS, except to stop. I was doing 85, and she stopped HARD! and straight.

I practice evasive manuvers, and hard braking. As a pilot or motorcycle ride we practice, or should, emergency procedures all the time. It must be so ingrained as to be normal. You live longer that way.

Rog

[Andy VH]

One caution about testing your ABS equipped bike on gravel surfaces:

The "damming" effect of gravel in front of the tire contact patch can/will cause monentary lockup of the tire due to a response lag after the ABS has released the brake and the tire has "plowed through/over" the piled up gravel.

Just be aware that the tire, especially the front, can still "lock up" on gravel if the gravel is deep/loose enough that the damming effect happens.

A safer way to practice this is:
1) Use only the rear brake on gravel to first get the feel of the reaction w/ABS
2) Then do the same stops on clean, dry hard pavement to get that feeling.
3) Then move on to doing front brake stops on clean dry hard pavement to get that feeling.
4) Once you have determined the "feel" of impending lockup, you can move to less stable surfaces for low speed (less than 20mph) practice.
5) But, be aware that on soft loose surfaces the front tire can still lockup to some degree and the loss of stability is immediate.

It definitely helps if your have dirt riding experience. In fact, dirt riding is an excellant way to improve your pavement riding capabilities. After 30+ years of street riding I have gone back to dirt riding to further improve my skills.

[deilenberger]

One caution about testing your ABS equipped bike on gravel surfaces:

The "damming" effect of gravel in front of the tire contact patch can/will cause monentary lockup of the tire due to a response lag after the ABS has released the brake and the tire has "plowed through/over" the piled up gravel.

Just be aware that the tire, especially the front, can still "lock up" on gravel if the gravel is deep/loose enough that the damming effect happens.
Hence my suggestion of a THIN layer of gravel (something like 1/8" for so is fine.) It's a fun exercise - you can see the locking and unlocking pattern of the brake very clearly in the gravel surface.. (and with thin small gravel the locked time isn't long enough to build up a dam in front of the tire.. it's usually less than 18" or so before the ABS unlocks it.)

A safer way to practice this is:
1) Use only the rear brake on gravel to first get the feel of the reaction w/ABS
2) Then do the same stops on clean, dry hard pavement to get that feeling.
3) Then move on to doing front brake stops on clean dry hard pavement to get that feeling.
4) Once you have determined the "feel" of impending lockup, you can move to less stable surfaces for low speed (less than 20mph) practice.
5) But, be aware that on soft loose surfaces the front tire can still lockup to some degree and the loss of stability is immediate.

It definitely helps if your have dirt riding experience. In fact, dirt riding is an excellant way to improve your pavement riding capabilities. After 30+ years of street riding I have gone back to dirt riding to further improve my skills.
I'll agree on dirty bike riding.. I did only dirty bike riding for about 4 years before getting my first street bike. It's a wonderful way to learn bike control in a non-auto-threatened environment.

One thing about trying the ABS on "clean, dry hard pavement" - it would be nice if that was the ONLY situation where ABS is needed, but it's actually fairly likely that adverse traction conditions are where the average rider is going to experience ABS use.

It might make sense for people to continue with their experimenting a bit after becoming comfortable in dry conditions - to trigger at least the rear ABS (on non-rear-linked-to-front brakes) in less than ideal conditions.

Learned reactions are what can save you in a bad situation. You embed learned reactions by doing them repeatedly a situation simulation where you hope they come into play. (Think of the MSF "swerve" exercises.. and their braking exercises for that matter..)

Just a thought.. interesting and useful discussion..

[gnavecky]

I had a similar feeling on my 2000 RT one time going only 5-7MPH. it may be just the ABS cycle time\rate. When I was taking MSF RiderCoach training I had many chances to test my ABS. when we did our braking test I had the fastest trap speed with the shortest stopping distanced.

[Andy VH]

Exactly right Deilenberger. A thin layer of dirt gravel is reasonable. My point is only if someone is thinking thin gravel is ok that deeper gravel is better is the caution.

Naturally, after getting familiar with the ABS on dry pavement it is good practice to try it on lower traction surfaces.

Like you say, we can't certain of clean dry pavement wherever we ride so it does pay to be ready fro wherever you ride.

But always, Enjoy the Ride!

[PacWestGS]

all right, something werid happened last night. i went out for a quick ride around the hood and the campus. i was pulling up to a stop sign, going maybe 5mph. i have been working on, and apparently not doing a good job of avoiding using too much rear brake. (long story, but my service tech informs me that i have the rear brake fairy. it turns out i do, i use it too much, and often without realizing im doing it!) as i approached the stop sign, the rear of the bike wiggled a little. nothing scary, but like the tire was on ice or something. like an idiot, i did not go back to see if there was gunk in the road, i just kept going, because i noticed i was pressing the rear brake, and shouldnt have been.

so my question is, can the rear wheel lock up a low speeds like that, especially if it hits some funky pavement on an ABS equipped bike?

and that leads me to...how the heck do i verify that my ABS is working properly without scaring the crap out of myself? :) i do panic stop practice in a parking lot every so often, but it's on nice clean, smooth blacktop.


Why "Shouldn't you have been" using the rear brake while pulling to a stop?

Use both, more front and get a feel for available traction through your rear tire when making straight line stops.

I'm sure your ABS did what it's designed to do but that doesn't always translate to what you think it should have done. They have various weaknesses and some situations mess them up bad. Your's is one of them.

There is a great discussion going on here and people have described how ABS works and why our bikes have ABS but there is an absence of the "Limitations of ABS".

How, certain conditions can make ABS a killer instead of a life saver?

How, speed variations between each wheel sensor and the speedometer can effect the ABS controller? Making it default to standard or residual braking.

How, the application of brake pressure is applied and controlled by the user, and how that can adversly affect the ABS controllers function?

Let's talk about that.

Giong out to "test" your ABS on a gravel parking lot will not tell you much about how it's going to work in the real world. But it will give you a feeling helplessness if it can't do what it was designed to do.

Try going out to a cleanly paved parking lot or low-traffic raod and starting at 20-mph find a spot of water or sand/gravel about two-feet wide and approach that with hard braking. Do nothing as you pass over the danger and see and feel how your brakes react to that situation. Build up your speed until you are going 60 (at your descretion).

My experience says that below 10-mph the ABS will release the brake while it's trying to figure out what the other sensors are doing and in that process send you about 20 feet down the road without any brakes. (That's the primary purpose for the GS's being able to deactivate the ABS system. It senses wheel lock and releases, bad things happen next.)

JMHO
YMMV

Russ

[Andy VH]

Ok, I'll give it a go: partly quoted from an earlier post, with replies:

There is a great discussion going on here and people have described how ABS works and why our bikes have ABS but there is an absence of the "Limitations of ABS".

How, certain conditions can make ABS a killer instead of a life saver?
ABS only works with the bike straight up or with VERY limited lean. If you think you can apply the brakes in a lean like you can straight up and think the ABS will save you, IT CAN'T. ABS systems have no "lean angle sensors" or "steer anlge sensors" to work with the ABS and limit brake application while leaned over. Remember, even a moderate lean angle uses a lot of traction. If you add braking while leaned over you can still loose traction and ABS can do nothing to save your butt.

Keep in mind, ALL ABS systems are intended to maintain vehicle stability and have nothing to do with reduced braking distance. If you think ABS systems will make you into some kind of Uber-Braking God you could be rudely awakened by a meeting with a Buick grill. The braking is still up to you, ABS simply improves your odds of survival.

Now, THIS IS CRITICAL, and really think about this: in a car with ABS, you can mash the pedal all you want and steer the car clear of an obstacle. On a bike with ABS, if you hammer on the brakes and engage the ABS, and then think you are gonna steer around the grey-hair's Buick bumper, YOU ARE GOING DOWN!!!!!! Remember that bike ABS systems have no lean sensor to modify the ABS function while leaning, and a bike HAS to lean to manuever.

So gramma turns the Buick in your path and stops. You hammer the brakes and the ABS kicks in. Then you try to swerve to avoid impact while still on the brakes?
Sorry, traction gone, face plant on the hood or ground. ABS or no ABS, the old rule of seperating braking and swerving STILL RULES! This is one scenario where I think ABS could be a killer if you think it does the same control functions as in a car.

How, speed variations between each wheel sensor and the speedometer can effect the ABS controller? Making it default to standard or residual braking.
As far as I know, there is no link between the ABS controller and the Speedo. As I described in my earlier message, the ABS system determines bike speed by getting signals from both speed sensors and calculates a MUCH more accurate real vehicle speed from that info. A speedo is way to inaccurate for that.

ABS is ALWAYS comparing wheel speeds any time the bike is moving just over walking speed. When braking, a variation in wheel speed greater than 20% less than the vehicle speed will enact ABS on that wheel.

If you were to fit one oversize (diameter) tire on your ABS equipped bike, it could mess up the ABS if the resultant speed sensor info was not within expectations of the controller algorithms. If both tires were increased the same diameter it should not matter. But an error signal from one sensor, say due to an oversize tire could cause a fault and the ABS would revert to standard braking.

Speed variations between each wheel sensor is exactly how the ABS determines whether or not to engage. Let's say you are very adept at spinning the rear wheel through a low traction turn (sliding the bike feet up, no braking), the ABS would sense a broad speed differential between the sensors, but because it also would not sense applied brake pressure (also a function for ABS engagement) it should record any fault. Same for a fast take-off with rear tire spining.

How, the application of brake pressure is applied and controlled by the user, and how that can adversly affect the ABS controllers function?
Again, the ABS control is based on 20% wheel slip DURING BRAKING for either tire. No matter HOW you brake, gentle or hammer it on, if the ABS senses 20% wheel slip it will engage ABS control. this applies for any surface, dry road, asphault, concrete, snow, leaves, sand, gravel, oil, anti-freeze, you name it. If the bike is vertical and braking, and one or both wheels slip at the -20% compared to the bike speed, ABS will engage.

Because the amount we brake is dependent on our skill level, preference, traction available, and all the other factors, ALL the ABS knows is whether one or both wheels are "slipping/sliding" under the 20% thresehold below the bike speed.

[AntonLargiader]

IMO, screw all this stuff about sand/water/gravel etc. Just find a good parking lot and first do a rear wheel only ABS stop and then work your way up to a front wheel ABS stop. I test the ABS on just about every bike I ride. Yeah, be ready to let go if it feels like the front is locking (you might feel like doing that anyway if you've never felt the ABS kick in before) but you really should know how a full ABS stop feels before it happens to you unexpectedly. Plus you might get a better appreciation for the braking power of your bike.