Friday, January 26, 2007 - 07:39 am, by: Juan Landauro(Enigma)
Brake Pedal Pulsation
In the past I have driven through some bumps (or sleeping policeman) on the road at a relatively high speed because the bumps were not visible. Now when I am going at relatively slow speed through a bump the brake pedal seems to have life of its own and pulsates (one short time) as if the brakes have failed to operate instantaneously for a very short time. What could cause this phenomenon? Is there in some way a relationship between the front suspension assemblages and the brake assembly? Is the ABS sensor or wire connection at fault? Could someone give some insight on feasible causes and how to put it right?
Friday, January 26, 2007 - 07:57 am, by: David Vaughan(Davidv)
quote:
when I am going at relatively slow speed through a bump the brake pedal seems to have life of its own and pulsates (one short time) as if the brakes have failed to operate instantaneously for a very short time
I assume, though you do not say, that you are braking at the time so that you feel the pulsations.
Friday, January 26, 2007 - 08:13 am, by: David Vaughan(Davidv)
Longer explanation:
When your wheel comes off the bump, it loses traction and locks up under braking force. You say this is at relatively low speed, meaning that the wheel has little rotational inertia and it is able to lock up fairly readily. The ABS senses the lack of rotation compared with vehicle speed and stops braking. Your sense that they "failed to operate instantaneously" is what happens, except it is deliberate. However, the ABS only releases braking in pulses. By this time the wheel will be more firmly on the ground after the bump and will rotate once more. ABS will observe this and will no longer reduce braking force so normal braking resumes. Find yourself a bit of damp road and brake hard from low speed. Get used to the feeling of those pulsations and remember when you feel them to keep braking, do not release at all.
Friday, January 26, 2007 - 09:10 am, by: Matthew Sharpe(Madmatt)
There was a similar thread about this a few months back. The general consensus was it was due to poor dampening caused by shocks, bushes or both being fequed.
Saturday, January 27, 2007 - 10:10 am, by: Damian Ware(Frozenpod)
Sorry mate but I think you might, I have driven many cars of young mates some with stuffed suspension. I haven't seen many that are that bad to cause the above problem like in your case.
I think you really should seriously be looking at a complete suspension rebuild, or at least new front shocks and LCA bushes.
Dan McColl Goo Roo Victoria Active V8 and a Factory Manual XF.
Saturday, January 27, 2007 - 04:55 pm, by: David Vaughan(Davidv)
Perhaps, Damian. The wheel does not have to come right off the road for the wheel to lock, just lose a bit of traction. It really depends on how hard is the braking, the severity of the bump and speed of travel. For example, at low speed you can get ABS intrusion on only a slight bump while turning into a right angle corner and adding a slightly harder push on the brakes, with nothing wrong with the suspension.
A shock test may be all that is needed to check this.
Sunday, January 28, 2007 - 01:23 am, by: Juan Landauro(Enigma)
Front suspension bushes were changed at 90K miles in a shoddy garage (I was not aware of this) because of a variety of noises when I was reversing or making a turning and making a brake manoeuvre. The ‘experts’ told me that the problem was the front suspension and that the bushes needed to be changed. I had the job done and the noises were not eliminated. As far I can remember the car was smooth at 70, 80, 90 miles/hour before the bushes were changed. Presently the ride is smooth below 70 miles/hour above this speed there is a little tremor through the steering wheel in spite of the fact that the 4 wheels had a weight balance recently. The car had a 4 wheel computerized alignment 2 years ago (i.e. camber, toes and caster). The tremor becomes stronger when the speed increases above 80 miles/hour. I have in my shed 2 brand new LCA’s with their respective ball joints. Someone said that changing only the front bushes is not enough because bad LCA ball joints may cause tremor as well.
Tremor can be caused because of rotor run out, hub deformation, bearing cracking or brake pads damaged. This is what the theory says.
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