Team-BHP - ABS for bikes: How different from cars?

Quote:

Originally Posted by gthang (Post 2487726)

Take a read of this article about ABS by Kevin Ash. Puts some new light on this ABS story.

How ABS works | Ash On Bikes

Cheers

Ride Safe.

Read it, but have doubts regarding its authenticity.

Ash claims that ABS works by measuring wheel decelleration (rather than relative front/rear wheel speed) and kicks in when some pre-set maximum decelleration limit is crossed.

However, this cannot be the full story, as the max safe decelleration will vary wildly with road surface (compare dry tarmac and icy roads, say) and no one limit will meet all conditions. (Also, when the wheel locks it will have zero decelleration, and ABS will not be used when it is most needed!)

I believe the basic mechanism of ABS still is the comparison of relative wheel speeds (as all the others sources say), but the decelleration may be also computed to add refinements to the basic mechanism.

WelL the wheel retardation does not always relate to vehicle retardation,if you set a max retard value for the wheel ,you dont have to worry about varity of surfaces. In a sense the guy may be spot on.
Involved riders still do a much better job of handling the brakes of a bike than present gen ABS. Abs is required when you are not involved with the road, surroundings and your machine.
People are lost in curves of ladies and ipads and money while they are on there perfect companion. (read motorcycles) ,as a result they almost stand on the brake pedal in a desperate situation.
We have seen bikes for over 100years,all the while without abs and other electronic aids.
And suddenly every one wants AI to manage there brakes for them.
Imo if you need a computer to apply and release a brake for you,you might soon need artificial insemenationation to have a kid of you own.
It is ok for commuters tryin to make there way to office,and there grey matter lost in a sales presentation.

Quote:

Originally Posted by gthang (Post 2500816)

New light with respect to this thread.

I realize his explanation leaves a lot of unanswered questions, but how is it erroneous? Is there something in that article that is wrong?

Cheers

Determining whether a wheel is locking up or not is not trivial in a car, let alone on a bike. Tried broaching this before (you'll find it somewhere in the thread), but the consensus of the forum was " just check one wheel against the other. Nothing to it."

Ash's article forcefully puts across the point that just comparing one one wheel against the other is not sufficient. But then he oversimplifies it and says that all one has to do is check deceleration values. By itself, this too is an erroneous statement. (We wouldn't have needed anything beyond the Dunlop system).

Unfortunately, we have not even touched on what I wanted to discuss when I started this thread: the dynamics of a bike, its relationship to the rider, the effect of rider assists and overrides on the rider. IMHO, this is where it is/ should be so very different from the relationship of car with driver.

Regards
Sutripta

Hi,
Im no expert, and not in good enough terms with any R&D fellow to know too much, but here's what i think. Im not writing what i found on the internet (Info is limited there,the manufacturers wont spill all the beans for obvious reasons). These are just my thoughts on ' how i'd work if i were an ABS system'.

First, I'd throw some light on what ABS is supposed to do (Yes...AGAIN!!), and then on how its done. Then will try to analyze the different needs of a 4wheeler and a 2 wheeler considering their dynamics.

What ABS does
(sorry if im writing what most people here already know, but bear with me)

Function of ABS sounds simple. It doesn't allow the wheels to lock up under braking.
Why bother with that? Its because of friction. Friction is the force that keeps you ride on the road in a controlled and stable way. The source of friction between a vehicle and road is the humble Tyre.
Now, a tyre can only generate a particular maximum amount of friction ( or grip) The friction generated is the force that makes a vehicle brake, accelerate or change direction. Also, a tyre generates maximum friction when it is rolling, not when it is being dragged(wheel locked) on the road. Also, a locked tyre cannot be used to steer: it will go on straight because of the vehicles directional momentum.
So, basically,ABS aims at maximizing friction by keeping the tyre rolling and not allowing it to lock up. Sounds simple on paper.

But life is hard in the real world.

Change is the only constant- in life and on the road

ABS sounds simple on paper, but to work effectively, it has to take into account various variables.
Friction = Coefficient of friction x Normal reaction
1.Coefficient of friction changes with different road surfaces.
2.Normal reaction (which is equal to weight on the contact patch) also varies as the weight shifts around quiet a bit while the vehicle is in motion (for eg, while cornering). Also it will vary at each wheel with the number of passengers and the amount of luggage.
3.Amount of frictional force available for braking: As mentioned above, a tyre can only generate a certain amount of grip (by means of friction) Say a particular tyre can generate 1000N of frictional force. it utilizes this force in two ways- longitudanal force (accelerating/ braking) and lateral force (changing direction) For example: If you brake hard, the tyres produce their maximum F force (1000N) to stop the vehicle ASAP. But if you are braking hard and steering at the same time, the force gets divided in components. Longitudinal force for braking (say 700N) and lateral force (say 300N). what i mean to say is amount of force available for braking reduces as it is diverted towards steering the vehicle.

An ABS system needs to keep track of all that.

PS: didnt read all 7 pages of this topic, so pardon me if i'm repeating things.

How ABS Works

Not going to mention the mechanics, but the data inputs it would need to 'know' when the wheel is locked or not.

Firstly, it needs to know the speed of the vehicle. vehicle speed and RPM of tyres is directly proportional. so is the deceleration of the vehicle and the tyres (wheels)
So if the rate of deceleration of vehicle is less than that of wheels, its a sure sign that wheels are locking up.
I dont think it is possible to make an ABS system work to its full potential by measuring wheel speed or their relation (front to rear) alone. one needs to know the deceleration of the vehicle as well to compare to. Although no source mentions it, i suspect an accelerometer is used to measure this, and the ECU then compares deceleration of wheels (fed by wheel speed sensors), dec of the vehicle (fed by the accelerometer), compares them, and makes amends.
Traction control works in a similar manner. A sensor tells the ECU that wheels are spinning up faster than the vehicle is accelerating and cuts power to help tyres regain friction.

Similar story with Electronic stability control. On board sensors sense the yaw, pitch and roll rates. these figures can in no way be determined from a skidding wheel.

Now the actual question.
Difference between cars and bikes.

Cars have four wheels and don't lean (they do roll, but nothing compared to a bike) they also dont have a risk of falling over.

Bikes on the other hand, have to lean in order to change direction. leaning changes the size of contact patch (hence available friction). Also weight transfers are more pronounced, as the rider shifts his weight a lot to control the bike. also front and rear brakes are independent of each other unlike a car
If an ABS system overdoes it a bit ( these systems are fast, but not instantaneous- the computer takes a finite time to calculate and implement things depending on its speed) for a fraction of a second it can go unnoticed in a car. but in a leaned over bike, it will lead to a washout and a dirty crash. The ECU would be need to be programmed to take into account every possible situation and process it real fast (say the bike goes from tarmac to gravel while leaned over and braking hard- the ABS has to detect the sudden change in friction coefficient)

I think the variables in bikes change a lot more than in a car and more risks are involved and hence a bike's ABS controller would be a lot more complicated. maybe the reason why they showed up on bikes pretty late.

Haven't experienced an ABS equipped bike yet. But im amazed that it is possible, and curious to know its limits.I don't know deep details about how it works, but here are the complexities i can see from common sense.

Quote:

If the concept of simply comparing front and rear wheels is indeed true, here is my hazy take on it.

Most of the braking effort comes from the front wheels (because of weight transfer). Is it possible that rear wheels (which are receiving very less braking force compared to front ones and not enough to lock them up) are rolling freely with the road and their speeds relaying actual deceleration to the ECU?

But again i said this theory is hazy as it would mean limiting the 'help' of rear wheels in braking. Also doesn't makes sense with modern cars equipped with all wheel disks and heavily loaded SUVs and trucks where rear wheels still share significant braking forces.

Well ABS is boggling my mind now.