davidpowell

I have a slightly tired 85 s2

Brake discs badly worn- I have a nice pair of shiny Sebring replacements. They are a pair but unmarked.

My car has two of the same discs on it ( Ie wrong!)

The Wsm has an incidental photo of the disc showing th e grooves flowing from hub to the rear of the car, but a line drawing showing they should flow from Hub forwards in the direction of travel.

The retailer of my new discs says they should flow back wards, but videos of people doing stuff to their 928s show mostly the grooves flowing forwards but not always!
P

davidpowell

... Can anyone advise me?

MFranke

When wheel spins forward the inner end of the spiral should lead the outer end. Scoops air from the inside and throws it outward.

From your description the wsm photo sounds correct if you are viewing the top half of the disc

mark kibort

think of the rotor as a pin wheel . the air gets scooped up at the inner diameter and ejected rear ward .

Bertrand Daoust

GlenL

The grooves on the outside are immaterial and do not indicate how the inner vanes are oriented.

jon928se

Strictly speaking the air primarily gets ejected outwards towards the tyre.

At the bottom of the rotor it might be ejected forwards a bit and at the top of the rotor rearwards a bit.

GlenL

When going forwards, it's the opposite.

Imo000

Unless you take it to the track, you will never notice the difference. So install it whichever way you like.

BIMMERMIKE

Fantastic photo Bertrand, appreciated.
Thanks

davidpowell

Thanks guys for all the responses.

The answer does seem to be that the grooves are not relevant - the very clear diagram of airflows is what counts.

The diagram also fits with the strange sketchy drawing in the WSM - on their drawing the grooves go one way, in their photo showing brake pad changing they go the other so even Porsche have caused a bit of confusion. It would have been useful if they had said ignore the grooves!

Many thanks.

jon928se

I didn't want to get into the fact that the air being ejected (simplistically to the rear) a the top of the rotor is actually travelling forwards faster than the vehicle at the moment of ejection as well as travelling outwards. I can't even envisage the aerodynamics of what is happening in the wheel well around where the air gets ejected - pretty confused I imagine, hence why Porsche sort of ducted air towards the centre of the rotor from the high presuure zone at the front of the car where the "centrifugal pump" action of the rotor spins it outwards.

Bertrand Daoust

Welcome guys.

GlenL

I was leaving that out as well.

Even though the vanes point one way, the air is moving the other way due to the wheel going around. (assuming the rotor is mounted correctly.)

If you just had the rotor and spun it, the air that comes out would be turning about the axis of rotation just like the rotor. The curved vanes increase the effectiveness of the pumping, and improve the stiffness of the rotor, but they don't make the air go "backwards" faster than the rotor is turning forwards.

And... If the orientation of the rotor isn't important then let's all gang up on the next person who asks for the "best" oil, brake fluid, gear lube, steering ATF, washer fluid, tire air, etc. for their street car.