Extreme braking

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Old 30 Sep 2004, 07:21 am   #1 (permalink)
Daniel M. Dreifus
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Default Extreme braking

'94 Camry - brakes replaced couple years ago at the dealer - recent
evaluation showed 90% front pads remaining, 80% rear shoes remaining,
braking is smooth and progressive and quiet - no problems, except . .
..
the few occassions, when pushing the car to higher speeds and then
braking very firmly, usually from 80 mph to a smooth but very rapid
stop, there is a vibration and rumbling from the front - wonder if the
calipers are just desgined for sedate use, and their limits have been
reached, or if the caliper hold down bolts are incorrectly torqued, or
something else is going on.
Again, all normal driving, no problem, racer style deceleration,
there's a humming resonance that seems to say "don't do that" so I
back off and brake with a bit less force. BTW there is no ABS.
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Old 30 Sep 2004, 10:56 am   #2 (permalink)
Philip
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Default Re: Extreme braking

Daniel M. Dreifus wrote:
> '94 Camry - brakes replaced couple years ago at the dealer - recent
> evaluation showed 90% front pads remaining, 80% rear shoes remaining,
> braking is smooth and progressive and quiet - no problems, except . .
> .
> the few occassions, when pushing the car to higher speeds and then
> braking very firmly, usually from 80 mph to a smooth but very rapid
> stop, there is a vibration and rumbling from the front - wonder if the
> calipers are just desgined for sedate use, and their limits have been
> reached, or if the caliper hold down bolts are incorrectly torqued, or
> something else is going on.
> Again, all normal driving, no problem, racer style deceleration,
> there's a humming resonance that seems to say "don't do that" so I
> back off and brake with a bit less force. BTW there is no ABS.



I've run into this problem on numerous cars over the years. From 80+ mph to
a stop, you have quite a bit of time to build up a lot of heat. The milder
semi-metalic pads (the ones that seem to wear forever) ALSO do a great job
of polishing the rotor surface. So you combine hot and highly polish
braking surface with hot low metalic pad and you'll get a low resonance
rumbling from disc brakes as you get below 40 mph (typically) once the rotor
surface becomes highly polished. The vibration you have is the pad now
seizing/releasing (repeat) because the heat generated is nearing the limits
of the pad material. Get a little hotter and the braking will now start to
fade ... quickly. Toyota DOES offer different grades of pads. You will
have to have the rotors resurfaced and better pads installed.

--

-Philip


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Old 01 Oct 2004, 10:26 am   #3 (permalink)
Daniel M. Dreifus
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Default Re: Extreme braking

"Philip" <1chip-state1@earthlink.net> wrote in message news:<O0W6d.13867$gG4.10017@newsread1.news.pas.ear thlink.net>..

The milder
> semi-metalic pads (the ones that seem to wear forever) ALSO do a great job
> of polishing the rotor surface. So you combine hot and highly polish
> braking surface with hot low metalic pad and you'll get a low resonance
> rumbling from disc brakes as you get below 40 mph (typically) once the rotor
> surface becomes highly polished. The vibration you have is the pad now
> seizing/releasing (repeat) because the heat generated is nearing the limits
> of the pad material.


Thanks, Philip.
Had seen this mentioned before, but forgot - perhaps since high speed
forceful braking arises infrequently.
I remember Mercedes recommending occasional rapid stops from high
speed to prevent glazing of their brake pads, and have seen others in
the Toyota forums mention more forceful braking application to avoid
this highly polished rotor/ glazed pad condition.
I'd been glad the rotors look so nice (free of scoring) and the pad
thickness remains good over time, while practising very progressive,
gentle braking application.
So your description of the problem was exactly right.
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