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Old 26th March 2010, 05:28   #1
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Question Disc brakes - A small query.

I was wondering if anyone could tell me if in disc brakes, the pistons are always in contact with the disc at all time? A mechanic told me it was so and I was thinking - Wouldn't it cause excessive wear on the discs?
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Old 26th March 2010, 06:26   #2
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The piston needs to be in contact with the disc all the time, the difference being HOW much contact, i.e. the pressure of the piston on the disc (via the brake pads).

If the piston was'nt in contact all the time, your brake pedal travel will be too high, you may even need to pump the brake pedal a few times to commence stopping!
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Old 26th March 2010, 08:05   #3
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Quote:
Originally Posted by 1self View Post
The piston needs to be in contact with the disc all the time, the difference being HOW much contact, i.e. the pressure of the piston on the disc (via the brake pads).

If the piston was'nt in contact all the time, your brake pedal travel will be too high, you may even need to pump the brake pedal a few times to commence stopping!
This is a news to me that the piston is always in touch with the disc plates 'cause my understanding is that the brake pads, that are attached to the end of piston, do not touch the disc plates; There's also a small gap between both the pads for the disc to be inserted within; the braking happens when the pressue is applied from the brake cylinder to the piston which in turn causes both the pads to rub against the disc plates from the left & right sides. See here
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Old 26th March 2010, 08:24   #4
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Quote:
Originally Posted by 1self View Post
The piston needs to be in contact with the disc all the time, the difference being HOW much contact, i.e. the pressure of the piston on the disc (via the brake pads).

If the piston was'nt in contact all the time, your brake pedal travel will be too high, you may even need to pump the brake pedal a few times to commence stopping!
As I said, wouldn't this result in wearing of the brakes?
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Old 26th March 2010, 08:32   #5
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As I said, wouldn't this result in wearing of the brakes?
That's how the pads and the rotors are designed to work. The wear when the pads are in light contact with the rotor is negligible.
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Old 26th March 2010, 09:01   #6
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To prevent premature pad wear (and higher consumption) disc brakes do not like any residual pressure on the master cylinders. On the other hand drums do not mind. This was the failing in the early NE118s which kept on generating a lot of dust in the front wheels, and also used up the pads real fast. It was only after 15 months that Premier recognised the problem, and did a recall to fix it. Of course the loss due to premature replacement of pads (2 sets in 13000km) was never addressed.
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Old 26th March 2010, 10:01   #7
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Yes, brake pads are always in contact with the rotors. There is no pressure being generated though. So they are "floating" so to speak. The upside is that braking is instant and the effect of a wet rotor is negligible.
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Old 26th March 2010, 10:52   #8
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Another query:-

Now, I know race cars glow their discs under hard braking, but, is it possible in a normal road car? Also, what kind of temperatures are reached in a normal road car under such braking?
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Old 26th March 2010, 10:56   #9
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Is there any alignment for this? Can this alignment lead to stronger/weaker breaks (not considering the pedal depression needed, for example, under quick breaking)?
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Old 26th March 2010, 11:12   #10
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Quote:
Originally Posted by smashnerd View Post
Another query:-

Now, I know race cars glow their discs under hard braking, but, is it possible in a normal road car? Also, what kind of temperatures are reached in a normal road car under such braking?
Racecars use ceramic discs, very tough, very fragile, and capable of very high temperatures (say up to 800C). Normal cars do get hot (above 100C but I have never seen the discs glow. At least in the dark you should be able to see the glow at above 450C
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Old 26th March 2010, 11:15   #11
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I will tell you in a bit of theoretical language!.
Frictional force depends on two things,mu and R,they being frictional coefficient between brake pad material and metal Disk!,and other being Normal reaction( force as applied by pistons pushing brake pads onto disk) .
now although pads are always actually touching the Disk surface,but normal reaction is Zero( well almost,baring force due to weight of liquid coloum and piston retracting inertia,which are negligible low).

F=mu x R.now because R is zero,there is no force ,hence no wear,well theoretically, but practically also it is very close to zero.

Disk brakes do not need to be adjusted, as the brake pads wear out due to the normal use, the pistons advance further, still maintains the touch of disk to pad,and additional liquid (brake fluid) travels from reservoir to caliper to fill up the gap.

stronger brakes or weaker brakes usually depends on type of pads one uses,as low quality pads have lower coefficient of friction,and low quality hoses for brake lines( poor quality hoses expand under pressure,thus not transmitting entire pressure to caliper,leading to weaker braking efficiency).

anything else about brakes,ask me!!hehe
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Old 26th March 2010, 13:30   #12
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Quote:
Originally Posted by smashnerd View Post
Now, I know race cars glow their discs under hard braking,
Whether it is a good thing or a bad thing, it is interesting to see glowing red hot discs in those cars!
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Old 26th March 2010, 13:47   #13
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Originally Posted by clevermax View Post
Whether it is a good thing or a bad thing, it is interesting to see glowing red hot discs in those cars!

Those cars are equiped with ceramic brakepads, so it provides much more brakeing power and also good heat decipation properties of ceramic allows them to cool faster as well.


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Old 26th March 2010, 15:25   #14
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Can anyone tell me how much a disc brake costs? (I mean for a small car)

I can't understand why manufacturers still stick to drums on the rear on models costing upto Rs. 6 lakhs or so.

And, what is the actual reason for brakes squealing on low force braking?
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Old 26th March 2010, 15:55   #15
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Quote:
Originally Posted by smashnerd View Post
Can anyone tell me how much a disc brake costs? (I mean for a small car)

I can't understand why manufacturers still stick to drums on the rear on models costing upto Rs. 6 lakhs or so.

And, what is the actual reason for brakes squealing on low force braking?

Having drum breaks in the rear is more of a cost cutting measure, all cars now days big or small comes with disks in front, the more expensive and powerful ones have them in rear as well.

However retrofitting rear disks is possible as most of the models of indian cars selling abroad have rear disks these days.

As far as the cost is concerned, it depends on the car is question.

Pramod
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