docmirror

Gah, I hate PO's. I was just going to swap my new wheels, and paint the calipers and what's this then? I glance in to look at the front pads and I see the rotors are on the wrong side.

It never ends with these cars. Now I gotta jack up both sides, and pull the calipers, and swap them and hope they bed in to the pads. I am willing to bet a trillion dollars that it was a discount shop that did it.



I blame you.

hacker-pschorr

Don't be so quick to judge....

I've run into 928 owners at track events with their rotors on backwards (also having brake fade issues.....) who argue with me about them being wrong.
Actually it's unfair to only point this finger at 928 owners, I see it rather often all over different makes / models.

Many do not understand the concept of the rotors being an air pump starting from the inside pushing the air out the edges. Most seam to think the rotors are designed to force air in from the outer edge and install the rotors accordingly.



Probably a good time to go over this again since all 928's have direction rotors (I think..?)

Adk46

Gosh, I've never thought to look - this is very useful.

As long as we're at it, what exactly are the holes for? Cooling, or giving water or steam a place to escape?

hacker-pschorr

It's an air pump, sucking in fresh air from the inside / middle and the spinning of the rotor forces it out the edges.

This is why air cooing ducts channel air to the inside:

Bilal928S4

Doc, I think you need to blame the person who did the PPI.
OH, I forgot you did your own PPI. Hee Hee

docmirror

Unfair edit protocol. You cut off the emoticon. You're fired.

The guy who did the PPI is also fired.

Now that we have all this free time, lets go get a frosty malted beverage.

docmirror

Well, the theory was that the holes would improve airflow cooling. After some exhaustive testing on high speed airplanes, under brutal conditions, there was no benefit to drilling rotors.

So, it's for looks, to psych out your competition. Unlike the bright red paint I'm putting on which will surely reduce braking distance by 50-60%. Yah, right...

FredR

A most useful post- I know the two front discs have different part numbers but I never stopped to think why- now I know.

Popped outside to check my front rotors and they are installed correctly [they damm well ought to be] according to Hacker's post.

Regards

Fred

docmirror

Grrrr some more. Thanks Fred, I just got the right wheel off and it's the correct rotor. So, I have a right rotor on my left side, now I gotta spend money. Roger! Hep!

FredR

That's a bit of a pisser- at least you were sharp enough to pick it up!

Just goes to show that much as we know quite a bit about our cars there is an awful lot still to be learnt.

With a bit of luck some other poor SOB who reads this post might also have two left feet to compliment your two right feet and you can then do a swap to get one of each?

Just as a matter of interest have you noticed any braking issues? I can imagine something a bit odd happening on a track day when pushing the envelope but I wonder if it is noticeable for your regular driving profile whatever it may be. I doubt the braking performance will be compromised unless you do a succession of heavy stops back to back- it will be interesting to read what others think [not that I am recommending this].

To be fair it would irk me knowing something like this was on the brakes so I can well understand the drive to correct- I do like to ensure my 928 has all the capabilities Porsche designed into it and a few more.

Rgds

Fred

Randy V

Good info, Hacker - first I've ever seen that explanation.

How can you tell looking at the rotor exterior the correct orientation?

Otto Mechanic

Well, that's the theory of course and as you've pointed out that theory has failed to find empirical support, however, in practice, the holes have unequivocally proven to aid the rotors in exploding violently when overheated, thereby saving the front suspension and magnesium alloy wheels from bursting into flame.

hacker-pschorr

If you have nice open wheels, reach in and stick your finger in a top vein and see which way it's "pointing".
The top most opening you should be able to feel it pointing towards the rear of the car, see the red arrow illustration above.

The openings in the 87+ rotors are large enough you can usually get at least the tip of your finger in there and feel which way they are going.

Friend of mine likes to go to car shows and point out to the reps when their rotors are on wrong, and surprisingly it's quite often. Few years ago an AMG Black had the rotors on wrong at the Chicago Auto Show.

docmirror

Rotor explosion under heat.

Check.

Bonus

Adk46

Wait! If we add blue airflow squiggles to the bottom of the wheel - also left to right - I think we find that it is only the rotation that's important, not the prevailing wind. We just need the same orientation that any centrifugal compressor uses.

Oh, crap, that's not quite right either: the prevailing wind is faster on the top of the wheel than the bottom, so it does matter. Draw shorter blue squiggles on the bottom. Unless the prevailing wind is totally shielded by the wheel and fenders, which is plausible.

At GE, we'd have to go find an "aero guy" elsewhere in the cafeteria to be further educated. We'd normally avoid them; aerodynamics is weird.