Which way are your discs fitted?

For the M Powered Z3 derivatives
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Which type of disc

Disc A
9
69%
Disc B
4
31%
 
Total votes: 13

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pingu
Joined: Fri 30 Apr, 2004 16:01
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Which way are your discs fitted?

Post by pingu »

I've just got a new set of discs and there is some confusion about which is left and which is right.

So, M owners, which way are your vented discs fitted? Do they draw air towards the hub, or do they vent air away from the edge of the disc?

Is it like this...

DISC A
Image

or like this...

DISC B
Image


N.B. Non-Ms do not have directional vanes on their discs and they are interchangeable left to right.
Pingu
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Gazza
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Re: Which way are your discs fitted?

Post by Gazza »

Edge to centre in direction of rotation

This is the front right

Image
Gazza

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siwilson
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Re: Which way are your discs fitted?

Post by siwilson »

Agreed method 'B'

See here
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pingu
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Re: Which way are your discs fitted?

Post by pingu »

siwilson wrote:Agreed method 'B'

See here
Si,

Who are you agreeing with? The link disagrees with what Gazza has shown.

Can you tell me how the disc is fitted on your car?
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siwilson
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Re: Which way are your discs fitted?

Post by siwilson »

Sorry, I must have looked at Gazza's post before he added the picture. I don't agree with him :(

Mine are fitted as per Zeckhausen with the fins pointing backwards.
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pingu
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Re: Which way are your discs fitted?

Post by pingu »

siwilson wrote:Sorry, I must have looked at Gazza's post before he added the picture. I don't agree with him :(

Mine are fitted as per Zeckhausen with the fins pointing backwards.
Is their guidance general, or M Roadster/M Coupe specific?
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pingu
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Re: Which way are your discs fitted?

Post by pingu »

I am being deliberately vague because I don't want to influence anybody's answers.

Ideally, someone will have bought a set from BMW and they can tell me (odd part number is on the left).
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pingu
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Re: Which way are your discs fitted?

Post by pingu »

Gazza wrote:Edge to centre in direction of rotation

This is the front right

Image
Gazza,

Are yours BMW or aftermarket?

Why did you fit them that way round?

Were the previous ones fitted that way as well?
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Deano1712
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Re: Which way are your discs fitted?

Post by Deano1712 »

B is the right way. Air gets pumped radially outwards.
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gookah
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Re: Which way are your discs fitted?

Post by gookah »

B

centrifugal action takes the air from the centre to the edge. the edge throws the air out, not tries to suck it in. It is in effect an impeller as in a centrifugal pump.. (Google the images)
The air accelerates as it goes from the centre to the edge creating a vacuum at the centre of the disc which draws cold air in.
Last edited by gookah on Wed 12 Aug, 2015 21:34, edited 2 times in total.
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pingu
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Re: Which way are your discs fitted?

Post by pingu »

Deano1712 wrote:B is the right way. Air gets pumped radially outwards.
You won't get many marks from teacher if you don't answer the question :D .

Or maybe you will if you're a naughty boy :shock: .

Can you remember what way were yours fitted when they were standard? Did you notice if they were "wrong" when you replaced them?

I'll be having a good look at all the Ms on Sunday. See you there :wave .
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pingu
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Re: Which way are your discs fitted?

Post by pingu »

gookah wrote:B

centrifugal force takes the air from the centre to the edge. the edge throws the air out, not tries to suck it in.
Again :head: .

I don't want to know how anyone thinks that they should be fitted. I want to know how they are fitted.

I have a very good reason for asking the question which I will reveal later.
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pingu
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Re: Which way are your discs fitted?

Post by pingu »

gookah wrote:B

centrifugal force takes the air from the centre to the edge. the edge throws the air out, not tries to suck it in. It is in effect an impeller as in a centrifugal pump.. (Google the images)
BTW, I agree with you. It's just that the question is not "how should they be fitted?"
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Gazza
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Re: Which way are your discs fitted?

Post by Gazza »

pingu wrote:I am being deliberately vague because I don't want to influence anybody's answers.

Ideally, someone will have bought a set from BMW and they can tell me (odd part number is on the left).


Odd part number is left http://www.realoem.com/bmw/enUS/partxref?q=34112227738
Gazza

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pingu
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Re: Which way are your discs fitted?

Post by pingu »

Gazza wrote:
pingu wrote:I am being deliberately vague because I don't want to influence anybody's answers.

Ideally, someone will have bought a set from BMW and they can tell me (odd part number is on the left).


Odd part number is left http://www.realoem.com/bmw/enUS/partxref?q=34112227738
Yes, but which way do the vanes go?
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siwilson
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Re: Which way are your discs fitted?

Post by siwilson »

Those ones Gazza has are Genuine BMW. I gave the to him some time ago, but they may not have been in the correct boxes then. I did have a look at another spare set I have and can't see a BMW part number. There was a number 7 on it and if positioned as a left hand disc the vanes were facing backwards. The part number for left end in 7 and the one for right ends in 8. Standard BMW practice for even numbers left and odd number right.

Mine a facing backwards and have been that way ever since I bought the car from BMW. I have changed them twice that time, but made sure to keep the orientation correct.

Hopefully we have wrestled this one to death now :)
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c_w
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Re: Which way are your discs fitted?

Post by c_w »

The vanes are spun like a catherine wheel "fanning out", if that make's sense, drawing air away from the hub - it would make no sense to draw air in towards the hub, not only as it would increase hot air to the wheelhub, but air pressure probably would make it difficult. The brake disc backplate has an "intake" to allow air through the centre of the disc and pulled out through it.

Image

Gazza's photo shows a front left, not a front right.

They are marked L and R on the OE discs anyway. The only other 2-piece discs I know of, made by ATE, have the part numbers on the edge of the disc.
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pingu
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Re: Which way are your discs fitted?

Post by pingu »

Now for the reason for the post.

I got a set of EBC discs that were marked 979L and 979R. Both had the vanes facing forwards when fitted as per the part numbers. The ones that I took off the car were facing backwards (as we all seem to agree they should do). I contacted EBC and their technical department said that they should face forward. I queried this, but was assured by them that they were not labelled wrong. They even gave the example of some pairs of Mercedes discs being directionally vaned, but could be fitted to either side of the car, so one side would suck and the other side would blow. They also suggested that the discs rotate so slowly that the venting may even be a gimmick.

I suspect that there may be an element of truth that they are a bit "gimmicky", as there is not proper venting of air to the centre of the hub.

Anyway, I agree with the internet and those that have stated here that the vanes should point backwards, so I have fitted the disc marked L on the right and the one marked R on the left.
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pingu
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Re: Which way are your discs fitted?

Post by pingu »

I've just done an experiment which shows that most of the air is moved by the disc rather than the vanes inside it. The disc was spun at 950rpm (equivalent to around 70mph).

As the air is moved away from the centre by the spinning disc, I think the vanes should face backwards to take advantage of this. I really don't see why EBC (or Mercedes) would want to fight nature and have forward-facing vanes.

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Southernboy
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Re: Which way are your discs fitted?

Post by Southernboy »

The purpose of the vanes has nothing to do with venting by virtue of the spin of the disc. It is designed to allow more rapid cooling of the metal by providing what is essentially 2 discs per side. Each pad operates on a single surface vs a solid disc where both pads are operating on a single disc. The vanes between the discs acts as a heat sink and holds the two discs together. The increased surface area of the discs allows for rapid cooling. More effective cooling promotes a cooler brake fluid temperature, which in turn promotes more solid and safe braking performance.
The mechanics of vane facing forward is incorrect. It will force heat to the center hub by positive air pressure. By rotating with the vanes facing back, a negative pressure is created which draws heat out. If an escape for the air was provided at the base of the vanes, then it would be moot which way to fit the discs.
As per Pingu's experiment, the air moving away from the center of the disc creates a low pressure inside the vaned areas, with a high pressure area outside the disc in the atmosphere. Add the expansion of heated air between the two discs, and the pressure difference increases further. If anything, one would want inlet holes close to the center of the discs to allow cool air to be drawn into the vaned compartments and exhausted at the outer edge.
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pingu
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Re: Which way are your discs fitted?

Post by pingu »

Southernboy wrote:If anything, one would want inlet holes close to the center of the discs to allow cool air to be drawn into the vaned compartments and exhausted at the outer edge.
I agree with everything you said about additional surface area and I think that this is the main advantage of vented discs.

The vents on my car go to the centre of the disc (so there is an inlet). They are on the inside of the disc (internally vented). Some cars (apparently) use air from the outside (externally vented). I think this would be better, as the wheel style could be shaped (like the McLaren SLR) to vent air towards the centre of the hub. I think it would be much more easy to control the air flow from the outside than the inside. The air vents in the bumpers just allow more air into the wheel arch area. The air is not directed other than by a small scoop in the stoneguard that is surrounded by the wheel.

If overheating brakes ever becomes a problem to me I will investigate modifying the scoop to direct air towards the centre of the hub. But as it's not a problem, there's no need to do anything yet.

Personally, I think the directional vanes on the disc produce a minimal additional effect to that achieved by straight venting (unless there is good control of the air flowing to the centre of the disc), but it seems pretty conclusive that if fitted, they should face backwards.
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c_w
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Re: Which way are your discs fitted?

Post by c_w »

I've got ducting to my discs, utilising the existing "inlet" on the disc backplate.

The factory brake ducts are pointless really, so the back of mine are blocked, but the intakes in the bumper are used to force air down the ducts which follows the anti roll bar to the back of the discs.

Southernboy probably makes a great way of thinking how directional vanes can help (or not); that is if the disc is spun ins such a way that it will catch the air and draw it towards the hub then that is obviously wrong, so vanes facing "backwards" would at the very least stop this flow, and if they disc actually sucked air out from the centre then that is a bonus.
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pingu
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Re: Which way are your discs fitted?

Post by pingu »

c_w wrote:I've got ducting to my discs, utilising the existing "inlet" on the disc backplate.
Did you buy the mod, or make it?

If I were to do something, it would probably involve some ducting that flowed air from the bumper vent around the wheelarch and into the centre of the hub.

But, as far as I'm concerned, the problem isn't there (happily) :) .
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Brian H
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Re: Which way are your discs fitted?

Post by Brian H »

pingu wrote:Did you buy the mod, or make it?

If I were to do something, it would probably involve some ducting that flowed air from the bumper vent around the wheelarch and into the centre of the hub.
I made a set of cooling ducts out of an old pair of fog lights, I took the glass off the front, replaced this with mesh, stripped the internals out and I have two hoses that connect to the back of the foglight housing, the hoses route in to the back of the hub where I have a small aluminium bracket holding the hoses in place, I had the hose and fittings so the mod cost me £20.00 for a set of old fog lamps :)

The 3.0 does not have any brake cooling ducts as standards :!: the brakes used to get hot!.
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Re: Which way are your discs fitted?

Post by Southernboy »

Good mod Brian ! I notice the "clicking" of cooling brakes every time I get back and park in the garage. The back dust plate is what does the clicking rather than the discs, but it indicates the temperature transfer to those plates is considerable. The actual discs are obviously hotter, and the brake fluid in the area at least as hot. Definitely not "cool".... 8-)
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Brian H
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Re: Which way are your discs fitted?

Post by Brian H »

Benefit of looking OEM too Barry :)
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Re: Which way are your discs fitted?

Post by mnbrennan »

Model b

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