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My Brakes

nism0man

By nism0man,
in Western Australia

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nism0man Enthusiast

nism0man

Posted
Posted

wen i break an slow down to a certain point...my car makes a funny noise coming from the front and the car rattles a tiny bit...it sounds like scraping [kinda]

is it my pads that need changing....or a rotor skim??..

if so where can i get it checked out and fixed, and how much

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lunjiaow Community Regular

lunjiaow

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Posted (edited)

so when braking does the steering wheel shake or does the pedal just pulsate?

well if its steering wheel... front brakes/rotor. and pedal is rears...

oh yeah reminds me. Sumies rolla does this really bad.. so if you see her inform her about it.

Edited by lunjiaow
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NA_Goodness Grand Master

NA_Goodness

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I'd say brake rotors causing pulsation? Ian Diffen etc do cheap skimming.

No they dont, they take it off and give it to us :P

My front rotors were badly warped a while back, shaking when braking from 80km/h!! Skimmed them 8 times before they flattened out, took about 1mm+ off them. When you consider most disks need 0.02mm taken off, you can see how bad they really were!!

Take it into a mechanic Andrew, they'de be able to tell you. Soooo hard to diagnose without actually hearing/driving it.

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**RB2530** Proficient

**RB2530**

Posted
Posted

Internet mechanics diagnosing brake problems.....scary thought....

btw

'breaks' is what happens when you drop a glass...'brakes' are what you use to stop...

'disk' is interchangeable with 'disc' however disc is more commonly accepted

Can't sleep so I have chosen to become the spelling police...sorry

Morale of my post is do somebody elses rear bumper a favour and get a mechanic to look at you brakes...

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bubba Grand Master

bubba

Posted
Posted
Internet mechanics diagnosing brake problems.....scary thought....

btw

'breaks' is what happens when you drop a glass...'brakes' are what you use to stop...

'disk' is interchangeable with 'disc' however disc is more commonly accepted

Can't sleep so I have chosen to become the spelling police...sorry

Moral of my post is do somebody elses rear bumper a favour and get a mechanic to look at you brakes...

Fixed.

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**RB2530** Proficient

**RB2530**

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Posted
No they dont, they take it off and give it to us :D

My front rotors were badly warped a while back, shaking when braking from 80km/h!! Skimmed them 8 times before they flattened out, took about 1mm+ off them. When you consider most disks need 0.02mm taken off, you can see how bad they really were!!

Take it into a mechanic Andrew, they'de be able to tell you. Soooo hard to diagnose without actually hearing/driving it.

Something else that can happen with discs is fat spots. Basically brake discs are cast steel that is subsequently heat treated to achieve a harder surface for better wear life...What can happen is that if the heat treatment is not uniform around the disc, the depth of the harder skin varies and so once the disc starts to wear, softer spots get exposed. They then in turn wear faster. This produces fat and thin spots around the disc and causes the bad brake shake...

OEM Ford Falcon discs are notorious for this...Sometimes they do it within 20,000km from new. Machining fixes it for a while but the fact is non-uniform hardness around the disc..They do it again in another 20,000km..Solution- DONT BUY A FORD!!

Disc warp is mainly caused by non-uniform cooling of the disc. ie if the discs is very hot and you dont cool down your brakes, when you stop, the disc area exposed to air cools much faster than the disc underneath the brake pad. This induces high temperature differential and therefore thermo-stress in the disc and you get plastic deformation (cooler metal gets stretched and hotter gets compressed) around the disc which can warp them....

Warped discs can cause brake shudder but also bad for calipers. In opposing piston calipers the pistons will shuttle in and out to suit the disc warp at the wheel rotation frequency..This will accelerate caliper seal wear no end. In floating single and dual piston calipers the floating pins cop it...

anyway back to work...

cheers

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NA_Goodness Grand Master

NA_Goodness

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Posted
Something else that can happen with discs is fat spots. Basically brake discs are cast steel that is subsequently heat treated to achieve a harder surface for better wear life...What can happen is that if the heat treatment is not uniform around the disc, the depth of the harder skin varies and so once the disc starts to wear, softer spots get exposed. They then in turn wear faster. This produces fat and thin spots around the disc and causes the bad brake shake...

OEM Ford Falcon discs are notorious for this...Sometimes they do it within 20,000km from new. Machining fixes it for a while but the fact is non-uniform hardness around the disc..They do it again in another 20,000km..Solution- DONT BUY A FORD!!

Disc warp is mainly caused by non-uniform cooling of the disc. ie if the discs is very hot and you dont cool down your brakes, when you stop, the disc area exposed to air cools much faster than the disc underneath the brake pad. This induces high temperature differential and therefore thermo-stress in the disc and you get plastic deformation (cooler metal gets stretched and hotter gets compressed) around the disc which can warp them....

Warped discs can cause brake shudder but also bad for calipers. In opposing piston calipers the pistons will shuttle in and out to suit the disc warp at the wheel rotation frequency..This will accelerate caliper seal wear no end. In floating single and dual piston calipers the floating pins cop it...

anyway back to work...

cheers

So basically, heavy braking makes them hot, they warp and cool, and therefore shake with heavy braking?

Brke warp for dummies... Like me :)

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**RB2530** Proficient

**RB2530**

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So basically, heavy braking makes them hot, they warp and cool, and therefore shake with heavy braking?

Brke warp for dummies... Like me :(

Basically the harder you brake the hotter the discs get...A moving car has kinetic energy...Energy is always conserved and the brakes absorb the kinetic energy of the car and converts it mainly into heat. Harder braking leads to higher rate of energy transfer into the brakes. The brakes dispate the energy in the form of heat...And if you put energy into the brakes at a much faster rate than what can be disipated they will get much hotter. Bigger brakes have more surface area to dissipate heat plus larger thermal mass...So they can absorb more heat...

Discs buckle due to the following reason...When steel gets hot it expands...If the temperature throughout the disc is even, it expands evenly...so there is no problem with high temperature until the steel starts to soften..Discs will glow before this happens...

The problems occurs when the disc cools unevenly. The disc cools down as soon the brakes release...(because they are no longer absorbing energy). If the disc cools evenly there is no problem as the disc contracts consistently ie Everything shrinks at the same rate (or close enough to the same)

If the disc cools very unevenly some of the steel is trying to shrink more than the neighbouring steel it is connected to...The differnce in shrinkage is directly proportional to the temperature differential...In discs the difference can be 1/100ths to 1/10ths of millimetres at a guess for normal, slightly uneven cooling. Steel is elastic (will recover after being stretch or compressed to a certain point) but if you try and stretch it too much, it becomes plastic. ie stretches or compresses in permanent way..

So if you start with a very hot disc at say 500-600degC and stop the car suddenly, the section of disc under brake pad will not cool nearly as fast as the rest of the disc which is exposed to air...'Newtons law of cooling' comes in here...maybe you could Google that cos I wont explain it here

Therefore the section of disc under the pad in the caliper will remain larger or more expanded than the rest of the disc whcih is cooler. If the difference in shrinkage between the hotter and cooler steel is too great, the stress in the steel will exceed the elastic limit of the steel, and permanent deformation will result. Even when the discs cools down to ambient temperature it will remain deformed and buckled...

The morale of the story is if your brakes are very hot don't stop the car straight away...The idea is too cool the brakes (while they are still rotating) down enough so that when the car is stopped, the difference in rate of cooling of the entire disc is uniform enough such that difference in expansion across entire disc doesn't permanently deform it.

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