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Disc brake "shields"


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#1 rory57

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Posted 08 November 2011 - 18:41

Any road-car brake engineers out there?
please enlighten me: What are those sheet steel disc brake shields really for? Some road cars have them, some don't.
If you throw them away, everything works as normal, no overheating, just the same in the wet etc (as far as I have noticed)
Are they one of those parts that we have just because we had them last time or what. Must admit I have become obsessed with reducing unsprung weight and these are first on my hit-list.

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#2 fredeuce

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Posted 08 November 2011 - 21:00

Any road-car brake engineers out there?
please enlighten me: What are those sheet steel disc brake shields really for? Some road cars have them, some don't.
If you throw them away, everything works as normal, no overheating, just the same in the wet etc (as far as I have noticed)
Are they one of those parts that we have just because we had them last time or what. Must admit I have become obsessed with reducing unsprung weight and these are first on my hit-list.


As a former mechanic my view about that is that they are there to protect the disc from water exposure.

Once a disc is wet , which they can experience, then before any braking can occur then the disc needs to be dried. That is to say brakes won't work with water between the pad material and the disc.
Application of the brake pedal will achieve that however there is a brief delay before the brakes will have any effect. On public roads you don't want that.

I also understand that hot discs exposed to contact with water can cause warping. The older style solid disc are , I believe, more prone to this than the vented variety which have a much more rigid construction.

#3 Greg Locock

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Posted 08 November 2011 - 21:42

As a former mechanic my view about that is that they are there to protect the disc from water exposure.

Once a disc is wet , which they can experience, then before any braking can occur then the disc needs to be dried. That is to say brakes won't work with water between the pad material and the disc.
Application of the brake pedal will achieve that however there is a brief delay before the brakes will have any effect. On public roads you don't want that.

The other possibility i can think of is dirt shields. A lot of brake wear occurs in freeway conditions where the dirt entrained on the disc gently grinds the disc and pad away if they are in close proximity.

#4 NeilR

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Posted 08 November 2011 - 22:15

imagine a small rock (locally basaly or quartz is used on roads) of a couple of mm thrown up and trapped between the disc and pad. This would effectively stop the pad hitting the disc until it wears away...could be the reason for the shield

#5 sharo

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Posted 08 November 2011 - 22:20

Also, IMO, the vented disks actively pump air, the suction being done at the inner side of the hub, facing the shield. So there may be some predefined flow between the disk and the shield. Also the shield may act as a heat deflector, protecting hoses and cables which pass on the other side. On some cars I've see additional heat deflector plates, guarding the ball joints of the steering arms.

#6 Magoo

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Posted 08 November 2011 - 22:23

Any road-car brake engineers out there?
please enlighten me: What are those sheet steel disc brake shields really for? Some road cars have them, some don't.


To prevent biggish objects like rocks and gravel from getting lodged between the rotor and caliper.

On many vehicles they also have a cooling function in that they are designed into the the air management system around the rotors -- conceivably, by removing them you could spoil the airflow pattern through the rotors and produce overheating.


#7 Lee Nicolle

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Posted 09 November 2011 - 11:49

They are purely a dirt sheild, nothing else. Some manufacturers do put a small slot in them to direct air into the centre of the rotor. Ok on a low speed passenger hack but if any speed is expected bin them to get more air in. Generally 'performance' models have the dirt shields deleted. They are not driven on muddy dirt roads normally where the sheilds do make a lot of sense. Most 4wds actually have quite extensive shields.

#8 jatwarks

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Posted 09 November 2011 - 12:50

I believe dirt shields and airflow management are the correct answers.

For the more exotically equiped drivers a comment during the Spanish MotoGP round revealed that shields are sometimes used on MotoGP bikes to keep carbon discs up to operating temperature in cold conditions.

#9 bigleagueslider

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Posted 11 November 2011 - 04:53

On production cars, I would agree with fredeuce. Water splashing onto front brake discs is a problem for two reasons. Wet discs hinder brake performance, and this is especially problematic with the ABS systems used on most production cars now. Water splashing on hot brake rotors also warps the rotors. And brake squealing due to warped rotors is still a common problem with production cars.

#10 Lee Nicolle

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Posted 12 November 2011 - 01:10

Water splashing on discs centrifuges it self away quickly, and the pads wipe the water away anyway.
Brake squealing has nothing to do with warped discs, that s glazing pure and simple. A warped disc pulsates, and gets worse the hotter it gets.
Think about basic pysics when it comes to water on a rotor. Those sheilds are there to keep out dirt and mud.
For a city car take them off, they do little apart from hold some residual heat, which in traffic may be an advantage with some pad material. But stop the air flow at the other end of the scale of motoring

#11 Magoo

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Posted 12 November 2011 - 12:16

They're rock shields. They are designed to send away gravel and pebbles. If you look at them for a bit, you will see they would be useless for keeping out water, dirt, or dust. If a stone gets wedged between the rotor and spindle or rotor and caliper, it 1) will make a hell of a noise) and 2) can do serious damage.

Also, rotors don't really "warp" in the conventional sense. It's possible but exceedingly rare. What really happens is buildup aka pad deposition. Friction material is fused to the surface of the rotor in non-uniform patterns, producing a thickness variation. This also produces hot spots aka hard spots. The region under the buildup can't cool properly and auto heat treats, essentially.

#12 cheapracer

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Posted 12 November 2011 - 12:43

Also the shield may act as a heat deflector, protecting hoses and cables which pass on the other side. On some cars I've see additional heat deflector plates, guarding the ball joints of the steering arms.


I've been wondering about them all my life, no where near as hard as I wonder about woman or even why wet teabags stick to smoko room roofs though.

The above is also what I believe, they are heat shields and Sharo, you forgot shockers that also need protection from heat in your list.


#13 primer

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Posted 12 November 2011 - 12:51

To prevent biggish objects like rocks and gravel from getting lodged between the rotor and caliper.


Umm, have you seen the narrow gap between the rotor and the brake pads?

Edited by primer, 12 November 2011 - 12:52.


#14 primer

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Posted 12 November 2011 - 13:01

The above is also what I believe, they are heat shields and Sharo, you forgot shockers that also need protection from heat in your list.

What about the heat generated within the damper when you drive your car over rougher roads? Is there a shield against that kind of heat in the hyrdaulics, or are we only concerned about the heat from the brake assembly?

#15 Magoo

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Posted 12 November 2011 - 13:39

Umm, have you seen the narrow gap between the rotor and the brake pads?


Yes. Now please examine the space or gap between the shield and rotor and shield and wheel. Obviously, the shield is not designed to keep out dirt, dust, or water. It would be useless for these purposes. The gap is much too large -- by many orders of magnitude. Obviously, the shield is intended to keep out objects larger than the clearance between the components -- namely, rocks and stones.






#16 rory57

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Posted 12 November 2011 - 14:35

The first time I thought about -and removed- a disc brake shield was when a stone got trapped between the shield and the disc.
Magoo is right: It does make a hell of a noise and it did damage the disc. (SAAB 99, about 1980)

#17 primer

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Posted 12 November 2011 - 15:25

The first time I thought about -and removed- a disc brake shield was when a stone got trapped between the shield and the disc.
Magoo is right: It does make a hell of a noise and it did damage the disc. (SAAB 99, about 1980)

How did the shield help in this case, then? :confused:

#18 Catalina Park

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Posted 13 November 2011 - 05:41

How did the shield help in this case, then? :confused:

Stones come in various sizes.

#19 24gerrard

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Posted 13 November 2011 - 11:38

From my own experience the disk covers are essential for off road use.
I took them off a range rover I modified (350 chev shortened 6 inches, mazda pick up shell) for some desert racing.
The brake pads lasted about five minutes.
We put stronger ones on.

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#20 Lee Nicolle

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Posted 13 November 2011 - 22:18

From my own experience the disk covers are essential for off road use.
I took them off a range rover I modified (350 chev shortened 6 inches, mazda pick up shell) for some desert racing.
The brake pads lasted about five minutes.
We put stronger ones on.

As usefull as they may be I have never seen a rally car or offroader with them. And they never seem to have brake problems that are not of a machanical nature, or a bent rim tearing the calliper off!

#21 Chickenman

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Posted 13 March 2012 - 17:33

Water splashing on discs centrifuges it self away quickly, and the pads wipe the water away anyway.
Brake squealing has nothing to do with warped discs, that is glazing pure and simple. A warped disc pulsates, and gets worse the hotter it gets.
Think about basic pysics when it comes to water on a rotor. Those sheilds are there to keep out dirt and mud.
For a city car take them off, they do little apart from hold some residual heat, which in traffic may be an advantage with some pad material. But stop the air flow at the other end of the scale of motoring


Sorry...you are 100% wrong about the water issue, at least in respects to a daily driver. I live in the Pacific Northwest where it constantly rains. I dare say rather more often than where you live...which is in Adelaide from your profile. Not a flame...but a fact.

I can tell you from personal experience of 35+ years of racing and 40+ years of street driving in an extremely wet weather Province, that removing the brake shields has an immediate and detrimental effect when driving in wet weather. I have tried this on many cars, and it can cause significant delays and instability on initial brake application. Brake delay can approach 1 second before the pads wipe the rotor clean of water. It is not something that you want to do on a street driven car in a climate that experiences a lot of rain ( Pacific Northwest for example ).

Audi had a recall on late 1990's production A4's. My 1998 Audi A4 originally had very small disc shields that only covered about 1/3 of the rotor. Audi had to redesign the disc shield and put out a TSB for wet weather climates. The new disc shields are full coverage and the difference in initial braking actuation and stability in rain is amazing. All models since 2000 came with the new design, full brake shields for Canadian market cars.

I have been a dealership Partsman ( Nissan, Honda, Chrysler ) for over 17 years. Vehicle specifications for different countries or continents vary greatly. What may be specified for a dry, hot weather climate, such as California, is entirely inappropriate for a Northern climate such as Canada. What you may experience in a climate such as Adelaide may not hold true at all in a climate like Vancouver BC, where it has been raining for the last 3 months steady :(

Wet weather braking performance is such an issue here, that I often base my brake pad choice ( on my Daily driver ) on how the pad performs in wet weather. There can be a huge difference in compounds. Some grip right way..some you have have your sphincter contracting into a black hole before they grip!! :eek:

Edited by Chickenman, 13 March 2012 - 17:39.


#22 Amaroo Park

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Posted 28 April 2012 - 13:16

Working in the Industry as I do I know for a fact that GMH say there's are to shield the discs from water. They even have technical bulletins on the detrimental effects remove will have on brake performance. I have seen over the last 20 plus years a huge amount of cars with stones caught between the caliper/pad and discs and all of them have had shields fitted.

#23 rory57

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Posted 29 April 2012 - 16:16

Working in the Industry as I do I know for a fact that GMH say there's are to shield the discs from water. They even have technical bulletins on the detrimental effects remove will have on brake performance. I have seen over the last 20 plus years a huge amount of cars with stones caught between the caliper/pad and discs and all of them have had shields fitted.

If the purpose of these shields is to keep water off the disc, ok but what about the disc surface facing the wheel? The fashion is for very open wheel styles, to show-off the brakes. Does the airflow around the brake keep the outer disc surface dry or is having one dry disc surface enough to improve braking response time compared to both sides wet?

#24 Ray Bell

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Posted 02 May 2012 - 06:17

I would guess that large amounts of water suddenly hitting the face of a very hot disc might cause cracking...

That would be more likely on the inside than the outside, or am I wrong there?

#25 hogits2

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Posted 02 May 2012 - 07:05

There is a lot of spray under cars with one tyre 'squirting' water at the other. I presume it is related that I find more corrosion/ridging etc. on the inner faces of discs than the outer faces.

Hot discs & water don't seem to be a problem - rally cars with glowing discs crossing creeks comes to mind. Cold brakes can be affected - at a local ford there is a sign after the crossing "Check your brakes" .

#26 Ray Bell

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Posted 04 May 2012 - 00:33

That might be more appropriate for drum brakes...

They were a real drama after fording creeks or similar when we had them. They'd fill with water, it was absolutely necessary to get them dry before you relied on them again.

#27 bigleagueslider

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Posted 04 May 2012 - 04:35

If you understand the principles of hydrodynamic fluid films it's easy to see how a thin film of water between the pad and the rotor surface could inhibit braking. Crank journal bearings depend on this same principle for operation. Given the pad surface area and the relative surface sliding velocity, a thin film of water splashed onto the rotor at the leading edge of the brake pad could easily create a hydrodynamic fluid film between them. This fluid film would significantly reduce friction and braking capacity for a brief period.

So I'd have to agree with the shield being a splash guard. :)

#28 Ben Wilson

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Posted 04 May 2012 - 08:25

Came across these pics on another forum and I thought of this thread.

I've never seen shields on a rally car before, but these are WRC cars and I assume they are for stone protection.

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#29 Magoo

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Posted 04 May 2012 - 09:35

That might be more appropriate for drum brakes...

They were a real drama after fording creeks or similar when we had them. They'd fill with water, it was absolutely necessary to get them dry before you relied on them again.


Yep, water on disc brakes can inhibit brake operation to some degree, but wet drum brakes will not work at all.

Youngsters who have not experienced this will not be able to appreciate it. You have a nice, high pedal that feels perfectly normal, but nothing happens. Nothing. When you push on the pedal the vehicle seems to speed up. Amusing or terrifying, depending on the situation.



#30 Tony Matthews

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Posted 04 May 2012 - 11:48

And yet the Mercedes W196 had oil injection fascilities to stop the huge inboard drums from binding, possibly due to overheating. I confess to not knowing the details. Still, seems rather alarming...

#31 MatsNorway

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Posted 04 May 2012 - 14:45

My old bicyle with front drumbrake had mega good bite after some rain.

#32 gruntguru

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Posted 05 May 2012 - 04:49

Ah yes,nothing like a good rinse removing dust and grime to improve the grip. Old trick when traction loss occurs on a chassis dyno - apply water. Grip disappears for a short time but as soon as the water dries, the grip is noticably improved.

#33 Lee Nicolle

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Posted 05 May 2012 - 11:40

Most race cars duct air into the hub of the rotor to push the air through the vented rotor.Some are very sophisticated and some are just a duct into the hub of the rotor. Most race rotors have dierctional vanes that pull said air through the rotor more eficiently.
As an aside a few years ago V8 Stupid cars were grooving the rotors stupidly,, eg 54 grooves on a 14" rotor. It looked like a bastard file!Really I do not know what they were thinking as you had considerably less rotor face to brake with!And they were grinding the pads away. Would be impervious to water though.These days they use the style in the pics above which is a very good design.

#34 MatsNorway

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Posted 05 May 2012 - 16:59

I don`t like the design on the discs pictures above.. could someone explain it.

I would prefered a slice that gives more aero gains.

#35 bigleagueslider

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Posted 06 May 2012 - 00:21

Most race cars duct air into the hub of the rotor to push the air through the vented rotor.Some are very sophisticated and some are just a duct into the hub of the rotor. Most race rotors have dierctional vanes that pull said air through the rotor more eficiently.
As an aside a few years ago V8 Stupid cars were grooving the rotors stupidly,, eg 54 grooves on a 14" rotor. It looked like a bastard file!Really I do not know what they were thinking as you had considerably less rotor face to brake with!And they were grinding the pads away. Would be impervious to water though.These days they use the style in the pics above which is a very good design.


I recall some IMSA GTO race cars back in the late '80s using water cooling of the front brake rotors. The rules required steel brakes and the cars were relatively heavy but also quite powerful and fast. Thus the front brake rotors tended to get very hot. One approach used was to spray water into the cooling airflow being ducted to the center of the vented rotor. It was very effective.

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#36 Tony Matthews

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Posted 06 May 2012 - 05:54

And it's used a lot in truck racing - very impressive, too!

#37 Lee Nicolle

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Posted 06 May 2012 - 08:50

I recall some IMSA GTO race cars back in the late '80s using water cooling of the front brake rotors. The rules required steel brakes and the cars were relatively heavy but also quite powerful and fast. Thus the front brake rotors tended to get very hot. One approach used was to spray water into the cooling airflow being ducted to the center of the vented rotor. It was very effective.

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Still used quite a lot at tracks where brake temp can be a problem. Quite simple really, install a washer jet into the brake ducts and washer pump and bottle.. When the brake light actuates so does the washer. Usually with a switch so it only turns on after a few laps. Otherwise you can over cool the brakes. Used in F1, Most Touring Car catergorys, anything fast or underbraked and usually only on tracks that are very hard on brakes.
Very effective, I have used it myself, only problems is the wheels tend to get very dirty!
Mats, the little hooks are there to disapate the gases that form between the pad and rotor.Most rotors have 6-8 simple grooves that have to start and end over the vane in the rotor. But it still promotes rotor cracking. That design does not.But it is considerably harder to put on a rotor. Simple grooves are usually done when being made though are not hard to install later with a mill. Or even a die grinder.
Oh and never drill a rotor, they will always crackEven with moderate temperatures.

Edited by Lee Nicolle, 06 May 2012 - 08:55.


#38 Tony Matthews

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Posted 06 May 2012 - 09:37

Used in F1

Eh?

#39 Lee Nicolle

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Posted 06 May 2012 - 12:44

Eh?

Back in the 80s, Also a ballast tank to make weight


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#40 Tony Matthews

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Posted 06 May 2012 - 14:53

I don't think it was ever used to cool the brakes, and it certainly was not its primary function. It was a good old-fashioned scam!

#41 desmo

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Posted 06 May 2012 - 18:29

Could be useful in F1 if it allows smaller brake ducts to be fitted for an aero benefit. Surely illegal though like everything else.

#42 Ben Wilson

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Posted 07 May 2012 - 07:56

I don't think it was ever used to cool the brakes, and it certainly was not its primary function. It was a good old-fashioned scam!


I can remember video of the Group B Quattro's with huge bursts of steam coming out of the wheels every time they hit the brakes, not sure how long the water supply lasted though...

#43 Tony Matthews

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Posted 07 May 2012 - 08:01

And, as I mentioned, in truck racing, very impressive - in fact, the first time I saw it on TV, without knowing about the water coolig, I thought there must have been a major mechanical failure!