Notices
928 Forum
Sponsored by:
Sponsored by: 928 Specialists

928 brake discs - which way do grooves go?

 
Old 07-14-2016, 07:10 PM
  #1  
davidpowell
User
Thread Starter
 
Join Date: Feb 2016
Posts: 14
Likes: 0
Received 0 Likes on 0 Posts
Default 928 brake discs - which way do grooves go?

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 is offline  
Old 07-14-2016, 07:12 PM
  #2  
davidpowell
User
Thread Starter
 
Join Date: Feb 2016
Posts: 14
Likes: 0
Received 0 Likes on 0 Posts
Default

... Can anyone advise me?
davidpowell is offline  
Old 07-14-2016, 07:26 PM
  #3  
MFranke
User
 
MFranke's Avatar
 
Join Date: Jul 2012
Location: Powder Springs, GA
Posts: 477
Likes: 0
Received 0 Likes on 0 Posts
Default

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
MFranke is offline  
Old 07-14-2016, 08:01 PM
  #4  
mark kibort
Addict
Rennlist Member
 
mark kibort's Avatar
 
Join Date: Feb 2003
Location: saratoga, ca
Posts: 29,754
Likes: 0
Received 1 Like on 1 Post
Default

think of the rotor as a pin wheel . the air gets scooped up at the inner diameter and ejected rear ward .
mark kibort is offline  
Old 07-14-2016, 08:07 PM
  #5  
Bertrand Daoust
Addict
Rennlist Member
 
Bertrand Daoust's Avatar
 
Join Date: Oct 2005
Location: Gatineau, Québec, Canada
Posts: 4,196
Received 1 Like on 1 Post
Default

Bertrand Daoust is offline  
Old 07-14-2016, 08:31 PM
  #6  
GlenL
Super User
 
GlenL's Avatar
 
Join Date: Jul 2003
Location: Minneapolis, Minnesota
Posts: 7,625
Likes: 0
Received 0 Likes on 0 Posts
Default

The grooves on the outside are immaterial and do not indicate how the inner vanes are oriented.
GlenL is offline  
Old 07-14-2016, 09:05 PM
  #7  
jon928se
Addict
Rennlist Member

 
jon928se's Avatar
 
Join Date: Nov 2003
Location: Sydney NSW, OZ. ex NZ, ex UK
Posts: 2,582
Likes: 0
Received 0 Likes on 0 Posts
Default

Originally Posted by mark kibort View Post
think of the rotor as a pin wheel . the air gets scooped up at the inner diameter and ejected rear ward .
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.
jon928se is offline  
Old 07-14-2016, 11:37 PM
  #8  
GlenL
Super User
 
GlenL's Avatar
 
Join Date: Jul 2003
Location: Minneapolis, Minnesota
Posts: 7,625
Likes: 0
Received 0 Likes on 0 Posts
Default

Originally Posted by jon928se View Post
At the bottom of the rotor it might be ejected forwards a bit and at the top of the rotor rearwards a bit.
When going forwards, it's the opposite.
GlenL is offline  
Old 07-14-2016, 11:46 PM
  #9  
Imo000
Captain Obvious
Super User
 
Imo000's Avatar
 
Join Date: Aug 2003
Location: Cambridge, Ontario, Canada
Posts: 21,222
Likes: 0
Received 5 Likes on 4 Posts
Default

Unless you take it to the track, you will never notice the difference. So install it whichever way you like.
Imo000 is offline  
Old 07-15-2016, 12:52 AM
  #10  
BIMMERMIKE
User
 
BIMMERMIKE's Avatar
 
Join Date: Aug 2005
Location: Ontario, CANADA
Posts: 1,484
Likes: 0
Received 0 Likes on 0 Posts
Default

Fantastic photo Bertrand, appreciated.
Thanks


Originally Posted by Bertrand Daoust View Post
BIMMERMIKE is offline  
Old 07-15-2016, 05:26 AM
  #11  
davidpowell
User
Thread Starter
 
Join Date: Feb 2016
Posts: 14
Likes: 0
Received 0 Likes on 0 Posts
Default

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.
davidpowell is offline  
Old 07-15-2016, 06:01 AM
  #12  
jon928se
Addict
Rennlist Member

 
jon928se's Avatar
 
Join Date: Nov 2003
Location: Sydney NSW, OZ. ex NZ, ex UK
Posts: 2,582
Likes: 0
Received 0 Likes on 0 Posts
Default

Originally Posted by GlenL View Post
When going forwards, it's the opposite.
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.
jon928se is offline  
Old 07-15-2016, 07:49 AM
  #13  
Bertrand Daoust
Addict
Rennlist Member
 
Bertrand Daoust's Avatar
 
Join Date: Oct 2005
Location: Gatineau, Québec, Canada
Posts: 4,196
Received 1 Like on 1 Post
Default

Welcome guys.
Bertrand Daoust is offline  
Old 07-15-2016, 11:31 AM
  #14  
GlenL
Super User
 
GlenL's Avatar
 
Join Date: Jul 2003
Location: Minneapolis, Minnesota
Posts: 7,625
Likes: 0
Received 0 Likes on 0 Posts
Default

Originally Posted by jon928se View Post
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 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.
GlenL is offline  
 


Thread Tools
Search this Thread
Quick Reply: 928 brake discs - which way do grooves go?


Contact Us - About Us - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: