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Andy VH
Where is the earth shattering kaboom!?



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   Posted 1/29/2007 1:59 PM (GMT -8)    Quote This PostAlert An Admin About This Post.
I posted this on a BMW cycle web forum and thought it was good stuff for our forum here. This is a lengthy post, but hang in there. 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 as much as 20% "slip", meaning the actual wheel speed of an ABS controlled wheel is designed to allow up to 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. I believe most cycle ABS systems function at no less than the 5cps rate, and probably at a higher cycle rate.

Inherent in any ABS braked system, is natural wheel/brake hysterisis. Hysterisis is a fancy word to describe the rotational natural dynamics of a braked wheel system. As wheel mass increases, the hysterisis of that wheel also increases. The hysterisis of a wheel assembly also affects how well an ABS system can control that wheel. An ABS system on a Victory Hammer would require different algorithms to control the skinny front tire versus the giant rear tire. The type of brake system also affects hysterisis. But since all ABS braked bikes 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 preset cycle persecond 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 can also apply on grass, say going downhill.
 
One thing about ABS is practice to get used to the feeling. I suggest initial practice at about 20mph on a straight dry paved surface. Some people suggest to practice on a low traction surface like a thin coating of gravel on a hard-packed surface. 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.
 

 
 


Training, the best safety and performance "equipment" you can get!
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Post Edited (Andy VH) : 11/29/2008 8:28:47 PM GMT

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Andy VH
Where is the earth shattering kaboom!?



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   Posted 1/29/2007 2:13 PM (GMT -8)    Quote This PostAlert An Admin About This Post.
So now, the meat of the message, what your ABS equipped bike CAN'T do:

Can certain conditions 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" 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. Leaning and ABS braking combined could PUT YOU DOWN!

Keep in mind, ALL ABS systems are intended to maintain vehicle stability and have nothing to do with reduced braking distance. On a bike, ABS ONLY applies to straight line stopping. It is still up to you to keep your head/eyes up, and apply steady progressive braking effort. 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 angle sensor to modify the ABS function while leaning, and a bike HAS to lean to manuever. Leaning in all its glory requires a lot of traction. If you think you can use any of that traction for an ABS stop I pray for your survival. ABS is solely for straight up stops.

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.
 
Here's another one. Let's say you are riding a nice straight stretch of road, when suddenly a truck ahead pulls out from the Sand/Gravel Pit driveway in your path. You apply the brakes on the clear dry road and sense the ABS engage. But as you get closer and still braking, the bike is now on a sloped sandy apron near the driveway. The front tire now slides down the slope with ABS doing all it can, and the bike has achieved a slight lean. This is a situation where the front tire could now lock up if brake pressure is maintained.

Do speed variations between each wheel sensor and the speedometer 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 on any bike. As I described in my earlier message, the ABS system determines bike speed by getting signals from both wheel 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 and record a fault code. It is also possible that a fault code repeated over and over to the ABS controller could become PERMANENTLY burned into the ABS system memory. In essence "frying" it. I uh, found out the hard way.

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 (brake system pressure is also a function for ABS engagement) it should not record any fault. Same for a fast take-off with rear tire spining.

How does brake pressure applied and controlled by the user affect ABS, and how can that 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. You are the real brains in the braking system. So it is STILL up to you. ABS helps, but don't bet your life on it.


Training, the best safety and performance "equipment" you can get!
Get MSF trained, check out: http://www.msf-usa.org

Post Edited (Andy VH) : 11/29/2008 8:29:57 PM GMT

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CurtP
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   Posted 1/29/2007 3:10 PM (GMT -8)    Quote This PostAlert An Admin About This Post.
Excellent Andy. Even I understood it. This should be required reading for anyone buying a bike with ABS.

Curt
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Smitty
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   Posted 1/29/2007 6:44 PM (GMT -8)    Quote This PostAlert An Admin About This Post.

Lot of work into that one Andy & I am going to have to sit back & read it again.  Just so glad I do NOT have ABS brakes on my m/cs because my 60 yrs of riding m/cs has been with drum to disc brakes manually operated by myself as the rider.

I remember I bought a Mazda 2500 4X4 small truck, to have ABS brales on the rear & even in slowing down to a stop sign or reason to stop, but no panic, I could heard the darn rear brakes locking & releasing & yes I carried a much heavier load then the average truck driver with basicallly nil in the box.  Several friends of mine also bought Mazda two wheel trucks of around the same year & all cursed the rear end locking & releasing be they at a slow of fast pace when stopping.  All of us felt the ABS should have been at the front.

That was some 15 yrs ago & feel things have progressed for my Subaru Forester has ABS at the front, but then have to admit it might be at the rear & to 'ell with it I do not feel like going out to look them over, BUT can say with the change in 15 yrs the wheels do not grab & release. 

Still never will I buy a m/c with ABS brakes.


Remember all the others on the road are crazy & out to kill you.

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Andy VH
Where is the earth shattering kaboom!?



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   Posted 1/29/2007 11:04 PM (GMT -8)    Quote This PostAlert An Admin About This Post.
Thanks for the replies and sorry for the long posts, but there is an awful lot about ABS systems that riders either don't know or have misconceptions about.

Smitty, this 94 is my first bike ever with ABS. It came with it when I bought it used (at 3,000 miles) so it wasn't my choice. In 98 I found I had fried the ABS controller, so I just yanked out the power relay and rode for another seven years without ABS. Heck, I hadn't needed ABS for the first 18 years of my riding background, so I could ride without it.

But I had always said if I found a replacement ABS controller cheap enough ($250 on ebay versus $1600 new) I'd replace it. It just bothered me to have something on my bike that didn't work. That's not my way with my bikes. So last year I replaced the controller with a salvage part and I'm back to practicing my ABS stops. And that's the whole point, like other aspects of your bikes, practice your skills so you know what you and your bike can do before you need it.


Training, the best safety and performance "equipment" you can get!
Get MSF trained, check out: http://www.msf-usa.org

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Andy VH
Where is the earth shattering kaboom!?



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   Posted 12/8/2007 2:12 PM (GMT -8)    Quote This PostAlert An Admin About This Post.
As an update, I need to find out what the actual maximum pulse rate is for the current crop of cycle ABS systems. I suspect it much higher than five cycles per second, and may be nearer 10 cycles per second.

The higher the pusle rate the finer the control feel of the ABS system to the rider, while also creating less reaction feel at the handlebar.

Pulse frequency or pulse rate is one factor of control, as is maximum pulse strength, or how "deep" each pulse is relative to wheel speed. A very deep or strong pulse would still cause wheel lockup. A weak or low pulse would cause ineffective braking effort and increase stopping distance as a result.


Training, the best safety and performance "equipment" you can get!
Get MSF trained, check out: http://www.msf-usa.org

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Andy VH
Where is the earth shattering kaboom!?



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   Posted 11/29/2008 12:26 PM (GMT -8)    Quote This PostAlert An Admin About This Post.

I recently found in a BMW technical journal on the current ABS system, that the cycle rate of the ABS is indeed eight cycles per second. That makes sense given the much lower mass of motocycle wheel assemblies compared to the wheel assemblies of cars or trucks. So the Bosch ABS systems can now cycle the brakes on and off, for each wheel, individually, up to as many times as eight times per second.

 


Training, the best safety and performance "equipment" you can get!
Get MSF trained, check out: http://www.msf-usa.org

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Deacon Blues
The Imaginary Director



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   Posted 9/30/2011 11:39 AM (GMT -8)    Quote This PostAlert An Admin About This Post.
The K75RT has ABS equipped... but so far I've not had to use it. Thankfully.


"Lane splitting will never be accepted in those areas where driving is considered a martial art."

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Andy VH
Where is the earth shattering kaboom!?



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   Posted 9/30/2011 1:41 PM (GMT -8)    Quote This PostAlert An Admin About This Post.
Yup, depending on the year/model of your K75, it will have the 1st gen ABS or the 2nd gen ABS like that fitted on my 94 R1100RS.

If you have a bike with ABS, it is best to practice high effort braking stops to get used to when the ABS engages and what it feels like when the ABS does kick in. Better to practice it and find out before you need it.


Training, the best safety and performance "equipment" you can get!
Get MSF trained, check out: http://www.msf-usa.org
 

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Deacon Blues
The Imaginary Director



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   Posted 9/30/2011 2:15 PM (GMT -8)    Quote This PostAlert An Admin About This Post.
Roger that.

Thankfully, Texas has lots of big parking lots with very few cars in them on weekends.


"Lane splitting will never be accepted in those areas where driving is considered a martial art."

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