mmmbeer

AndyMC; are you sure the cracks in your picture are actually cracks in the rotor itself and not just in the material left on the rotors from the pads?

When I look closely at the picture it seems like the latter. See the parts of the rotor that are not covered by brake pad material; no cracks there.

But then again, looking at a picture is harder than inspecting it in real life.

AndyMc

Yes that is correct.

I've seen it too many times for it to just be coincidence.

If I use drilled rotors they develop large cracks at the holes and lots of little cracks elsewhere (I believe this is because the holes cause that much stress in the material that cracks appear all over

When I use slotted 2 piece rotors I find I wear the rotor out completely ie it becomes too thin to use, sometime fine cracks will be evident but the cracks have never been the reason for replacement. Further more these rotor will have done three to four times the mileage too.

PS I learnt 15 years ago that drilled rotors don't last as well, the only reason I've had them since is because they were already on a car when I bought it

Andy

AndyMc

My post above was in response to Docks post

mmmbeer

I've just had a close look at them and I can see and feel the smaller cracks where the is no pad material, incidentally a lot of that pad material has disappeared now and more of the rotor looks shiny but I have been driving slowly this last week

Dock

If it's the holes that are causing widespread stress in the material it seems that rotors that do not have holes would not exhibit the same cracking phenomenon (meaning the rotors without holes are not subjected to the same "material stress" that rotors with holes are postulated to be subjected to). But as the picture above of the Viper rotor shows, this is not true.

AndyMc

OK I'm done with this thread now,you can believe what you like, my experience gives me my opinion, you don't seem to have any experience and yet know it all

What I'm going to do

Buy 2 piece slotted rotors and pagid pads and enjoy the full performance of my car and not worry about cracked rotors

Not get into pointless debates on the internet

Andy

Dock

I'm making observations and asking questions. If that doesn't sit well with you then so be it.

You have not asked me anything about my experience/background, and I find it interesting that my questions and observations have led you to opine that I "know it all". Quite a leap.

Dock

This site is called "Rennlist.com discussion forums". I consider asking questions, seeking clarification, and offering ideas "discussion".

mmmbeer

I like to get my information based on facts. Therefor I think it is healthy for a discussion forum to have people that does not buy undocumented claims or hearsay easily.

I have to give some support to the one person who is swimming against the current here. Not that he is right (i can not be the judge of that), but because he ask questions and demands facts over opinions.

The world is full of unsubstantial rumours in which people believe without reason or facts. Do not let technical threads be such a field.

The post above asking something like; "Why is everything with you a debate?" is absolutely priceless. Especially since this is a discussion forum.

Macster

If the air flow were sufficient to remove the brake dust there'd be no build up of it in the holes.

Sincerely,

Macster.

Dock

I would think that centrifugal force, along with the surface friction of the inside of the hole, contribute to trapping some of the brake dust inside the hole. How much brake dust is expelled out of the holes and back onto the rotor surface? I have never seen any hard data addressing this. But based on the rotor design philosophy, and the physics that are being used to support this objective, it seems reasonable that a large portion of the brake dust is ejected out the circumferential openings on the rotor.

Bob Rouleau

Dock, I have to clean the crud out of my rotors after a couple of days on the track. It takes a lot longer on the street mind you. Unless I am mistaken the picture you posted of your rotor showed an accumulation of dust in the holes. In any event, I assure you that pad material does accumulate there and one is well advised to clean it out. The holes were put there for a purpose after all.

Regards,

Dock

I completely agree about the holes accumulating brake dust. I always spray my wheel cleaner (P21S wheel cleaner) directly on all the rotor surface I can get to, then use a sharp, strong stream of water (shot from a hose nozzle held a short distance away from the rotor) to force the brake dust out of the holes and vented area. The water expelled from the openings along the outside edge of the rotor is normally filled with brake dust.

dantzig

The uneven heat generated in the rotor (hotter on the outer surface than on the inside) produces an azimuthal tensile stress. There is also a radial stress, but it is much smaller in magnitude because the rotor is free to expand radially. The presence of the holes concentrates the stress at the top and bottom of the holes, because the force cannot be transmitted through the hole. The "stress concentrastion factor" as it is called depends on the ratio of the size of hole to the thickness of the part. In the limit of hole size going to zero, the amplification factor is 3. It is probably close to 3 for the configuration on our brake rotors. That is why the cracking pattern develops as it does.

More info: look here (no affiliation, just found it in a convenient place)
http://www.mae.ncsu.edu/eischen/cour...Appendix_C.pdf

Clogging of the holes with brake material makes the problem worse because it reduces the ability of the rotor to transfer heat, so they run hotter.

This is less of an issue with slotted rotors because the edges of the slots are near the hub and outer surface of the rotor, which are not as hot as the main braking area. The inner section is cooler because of the mass of the hats, and the outer is cooler because of the proximity to the external surfaces.

I hope this helps.

Jon

Dock

Good information Jon.

Reference my comments contained in the quote in your post above. It is the widespread presence of cracks, not associated with any of the holes, that is the issue. I fully understand how and why cracks of varying length develop at the top and bottom (radially) of holes. Porsche knows these cracks can occur and specify a maximum length of a crack prior to replacing the rotor. I believe they also specify that if a crack connects two holes, or if a crack reaches the edge of the rotor, it should be replaced. The presence of a crack that does not reach Porsche's replacement criteria does not in my opinion mean the rotor needs to be replaced.

My point is that stress cracks can obviously occur on rotors without holes. AndyMc stated that his rotors would have been fine had they not had holes. It is my opinion that the widespread presence of cracks on his rotor(s) that are not associated with or near any hole, were not caused by the holes in his rotor.

dantzig

Dock -

Rotors without cross-drilling will also develop radial cracks. But length and connection of cracks is not nearly the same issue with undrilled rotors. A crack concentrates the stress even more than a round hole, and failure occurs much faster as the length of the crack increases. That's why they work so hard to inspect those other things you drive, and take them out of service if the cracks get to be too long.

Jon