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Old 26th December 2010, 23:27   #301
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Default re: The ABS discussion thread

simple: No ABS, your brakes still work; no servo, even, and your brakes still work, but needing a lot more force from your foot. Of course, there is brake failure itself, when they don't. Scary.

When your power steering fails --- you can still steer
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Old 27th December 2010, 03:28   #302
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Default re: The ABS discussion thread

Continuing the very interesting discussion that took place in pages 17-19, here's some stuff behind the cause of wheel locking.

In some of the posts, the writers had described the cause to be force transferred due to angular momentum of the tires. I could not gather much from that so I set out on a journey to unmask the truth. (A bit melodramatic, I know. But it's going to get boring as you scroll downwards.)

Here're the wheels. I find it impossible to proceed without a diagram.

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When a body is rolling, its angular and linear velocities are bound by the relation

V=wr (V is linear velocity, w is angular velocity, r is the radius of the wheel.)

Hence, at the bottom point, the point in contact with the road at that instant, the forward linear velocity and the backward angular velocity cancel each other, thus at an instant, the velocity of the contact point is zero. (This also explains why @arun4242 found that the friction at the bottom is static friction, and not kinetic friction during rolling). This friction plays the leveling role to support the condition of rolling V=wr. I'll not go into the details of how the balancing occurs.

So when we press the brakes, this action is not directly responsible for the decelerating motion. It merely slows the rotation of the tire.

Ex : If you were launched into space, sitting in the driver's seat of your car, if the wheels are rotating, pressing the brakes will only stop the wheels. It will not retard the motion of the car.

So what happens when you press the brakes is that, w of the wheel decreases. At the contact point, the net velocity (V-wr) is now no longer zero. There is now relative motion between the contact point and the road. Or to say in other words, the wheel is now skidding.

This relative motion is what causes friction (Backwards direction) to spring in. This friction will attempt to play two roles :-

1. Increase the angular velocity (The torque due to friction supports the angular velocity of the wheel)

2. Decrease the linear velocity (Force opposes velocity)

such that the new V-wr=0

Since we keep pressing the brakes until the angular velocity is 0, friction will (Using the equation V-wr=0) bring the linear velocity to zero.

What happens during lock-down

The friction force due to pads becomes so high, that it reduces w to 0 before friction due to the ground has time to reduce the linear velocity to 0.

Hence, the rotations stop, but the linear velocity does not, causing skidding.

Once the rotations have stopped, the friction on pads changes from kinetic to static, thereby increasing the friction force due to pads, which keeps the tires in lockdown until brakes are released.

One flaw of this rudimentary model of mine is that it does tell why lockdowns won't occur at high speeds. This is because I have assumed the friction to be constant of different velocities. The only explanation I could figure out was that at higher speeds, when you brake, there is more relative velocity between the ground and the wheels. This somehow causes a higher friction, which means higher torque on the wheels which prevents the angular velocity from reducing to 0.



Last edited by anku94 : 27th December 2010 at 03:38.
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Old 27th December 2010, 03:35   #303
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Default re: The ABS discussion thread

Some use(ful/less) calcs I did:-

Note to mods : I created a separate post for this to prevent the other one from going too long and hindering readability. Also the content here is more calcs than concepts, so it looks less messier. Please feel free to merge if any issues.

Basically, these calcs are to find the condition of locking:-

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Old 27th December 2010, 21:22   #304
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Default re: The ABS discussion thread

Quote:
Originally Posted by anku94 View Post
Continuing the very interesting discussion that took place in pages 17-19, here's some stuff behind the cause of wheel locking.
.....
Hi,
If the brakes are powerful enough, which they normally are, wheels can be locked at any speed. It's just a matter of forces.

May I suggest a slightly different theoretical approach?

A) Work out the forces if it is a pinion rolling along a rack, and what happens if it is suddenly locked. Theoretically no chance of slippage/ skidding!

Once that is done,

B) tackle the case of a steel wheel rolling along a steel rail. Work out the case (forces, time- velocity graphs) of locking a wheel. This will also answer the question of when the wheel will lock. Assume static and kinetic friction to be the same.

Then

C) Factor in the fact that static friction is more.

Now come to the case of tyres, with its not so self evident properties,

D) esp variation of Mu with %age slippage, and velocity.

Simplying assumptions:
Assume in all cases that the brakes are powerful enough to lock the wheels. Assume that the braking force on the rotor is generated by a 'black box', as these details are not necessary for the current discussion. Assume also the wheel has 0 moment of inertia.

In some cases (at least for me) it might be beneficial to change the frame of reference to that of the axle. i.e. the wheel is rotating about a stationary axle, and the 'road' (load) is a (infinitely) long beam of mass equal to that of the vehicle, being moved by the wheel. (Assume the beam is resting on the wheel)

Regards
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Old 20th January 2011, 11:58   #305
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Default re: The ABS discussion thread

What about the scenario where:
1) Wheels lock up randomly on a number of occasions at no specific speed.
2) Premature warping of discs - 10k to 20k km
3) Excessive brake pad wear on only one side
4) ABS light does NOT come on
5) Overall braking ability doesnt seem to have come down except in scenario if locking happens
6) No specific pulsating feeling from pedal

Is the ABS functioning?
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Old 27th January 2011, 14:55   #306
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Default re: The ABS discussion thread

Found this weird ABS issue in one of Best Motoring's battle, so thought i'd post it here. Watch the vid from 3:40 onwards.

Looks like an ABS sensor failure can permanently lock the wheels. Even when the driver came off the brakes, the wheel didn't unlock.



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Old 29th January 2011, 08:35   #307
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Default re: The ABS discussion thread

I own Maruthi swift vxi with ABS 2006 and its just covered only 32,000 Km. I recently started showing braking warning indicator on the dashboard and I took the car to ABT service center in Adyar. After examining the car they told me that oil in the ABS is leaking and the ABS need to be replaced with a new one and it cost around Rs. 34,000. In addition to that they told me I need to replace clutch and power stearing since its hard and give the total quote for 80,000. I told him not to do anything, just do the oil change and got the delivery. I feel its abnormal to get these problems with the mileage in my car. Kindly share your views on this and give your recommendations.

Thanks,
Rajesh
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Old 29th January 2011, 11:52   #308
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Default re: The ABS discussion thread

@nrajeshkumar, someone at your service center has a serious financial target to meet LOL! Surprising he didn't ask you to replace engine and transmission also .

If any service advisor says "ABS is leaking" if Brake Warning Indicator comes on (along with recommending clutch and PS replacement), it is time to go to a different service center. Brake Warning Indicator can come on due to various reasons, and looks like the SA didn't look carefully before inferring that there is an oil leak. He should have run diagnostics on the ABS ECU to figure out what is wrong.

Next time a service advisor tells you something is leaking, ask them to
a. show you (physically) where it is leaking from, and
b. if the fault is related to a system that has an ECU (injection, transmission, ABS, etc.) ask them to run diagnostics, and show you & explain the diagnostic results (error codes, etc.)

@Shan2nu. read the caption carefully at 4:15, which says "blow-by from the differential oil cooler ruined the ABS speed sensors"! LOL Really amusing, since that oil cooler is a good 3' away from the nearest ABS speed sensor. Whoever came up with the caption is really imaginative. Had that person complained to Nissan about the failure (and reason thereof), Nissan would have told him:
1. that any warranty on the car is void if a car, meant for normal road use, is taken on a race track for whatever reason
2. that he needs to educate himself on where the sensors are located, the reasons why any system or component would fail, and how

Last edited by DerAlte : 29th January 2011 at 11:54.
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Old 29th January 2011, 21:33   #309
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Default re: The ABS discussion thread

Quote:
Originally Posted by DerAlte View Post
@Shan2nu. read the caption carefully at 4:15, which says "blow-by from the differential oil cooler ruined the ABS speed sensors"! LOL Really amusing, since that oil cooler is a good 3' away from the nearest ABS speed sensor. Whoever came up with the caption is really imaginative. Had that person complained to Nissan about the failure (and reason thereof), Nissan would have told him:
1. that any warranty on the car is void if a car, meant for normal road use, is taken on a race track for whatever reason
2. that he needs to educate himself on where the sensors are located, the reasons why any system or component would fail, and how
Irrespective of what the caption writer says, what do you think is happening?

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Old 29th January 2011, 22:04   #310
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Default re: The ABS discussion thread

I have a 2004 Santro Xs ABS and I recently got the brake pads changed , ever since I get a feeling that the abs cuts in too easily .
I showed this to the A.S.S and they say that its normal as the pads are new but I did not have this problem when the pads were changed earlier .
Is it normal or the A.S.S people are trying to misguide me to cover their faulty job ??
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Old 29th January 2011, 22:32   #311
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Default re: The ABS discussion thread

I have attached a graph below which show the variation of coefficient of friction under static and slip conditions for given vertical tire load. You can see that the mu decreases with increasing vertical load.
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The other graphs shows the variation of the longitudinal force with respect to slip kappa at various tire loads. You can see as the slip increases the braking force also decreases.
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And the last graph shows the variation of lateral force which provide the force necessary to make a turn as a function of steering angle alpha and longitudinal slip kappa. Notice at the full lock the cornering force is close to zero. That's why the car keeps going in the direction it was going just before the wheels locked.

Thanks to Dr. Ir. I.J.M. Besselink for these graphs.

Last edited by Pankaj401 : 29th January 2011 at 22:37.
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Old 29th January 2011, 22:57   #312
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Default re: The ABS discussion thread

Quote:
Originally Posted by Sutripta View Post
... what do you think is happening? ...
The car skidded to a stop with tyres smoking? Conjecture doesn't help in such cases, so no point in thinking beyond that. All possible expressions are plausible, but not necessarily correct. The truth can be known only by examining that car, not by intellectual ...

Quote:
Originally Posted by aditya_911 View Post
... recently got the brake pads changed ... get a feeling that the abs cuts in too easily ... A.S.S and they say that its normal as the pads are new ...
They are right. How often do you change brake pads? From the way you put, it sounds like 'quite often'!

Much depends on how you brake. If you are landing up making ABS act often under normal conditions, perhaps you should brake earlier and lighter.
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Old 29th January 2011, 23:00   #313
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Default re: The ABS discussion thread

Well this is the third set in 42k KMS!
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Old 30th January 2011, 10:16   #314
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Default re: The ABS discussion thread

@adita, not sure but that sounds quite high. I have a 10 year old Santro (no ABS), and I have changed the rear pads only once. And that also because brake fluid had leaked onto and soaked the pads.

IMHO you should consciously check the way you brake. Anticipate and brake early, instead of leaving it till late and braking hard.
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Old 31st January 2011, 20:19   #315
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Default re: The ABS discussion thread

Quote:
Originally Posted by Pankaj401 View Post
The other graphs shows the variation of the longitudinal force with respect to slip kappa at various tire loads. You can see as the slip increases the braking force also decreases.
Thanks for posting the graphs. Could you enumerate the parameter values (tyre loads) for the slip vs longitudinal force. And what about a longitudinal force vs velocity graph?

Quote:
Originally Posted by DerAlte View Post
The car skidded to a stop with tyres smoking? Conjecture doesn't help in such cases, so no point in thinking beyond that. All possible expressions are plausible, but not necessarily correct. The truth can be known only by examining that car, not by intellectual ...
Considering that 'truth' has never prevented us from commenting/ speculating/ critisizing, should we stop now?
Would make for an interesting 'intelluctual' discussion, dont you think?

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