f1rocks

Why would someone want to cross drill AND slot a rotor....the slot is simply there to get the gasses out which the holes will do.....

Someone with extra time on their cnc machining center looking for the next snake oil remedy.

zerMATT951

Agreed. It's all for looks... just like painted calipers. "Look at my bling"

f1rocks

Now....I painted my calipers too, but I used a heat dissipating coating instead of powder coating them (which actually holds in the heat.)

So I got a little bling but the calipers actually dissipate heat 20% better than uncoated aluminum....that is cause for a double!!

RKD in OKC

Well, believe what you like from sales literature, I am reporting what I saw in person on my stock from the factory 2001 Boxster S.

sweanders

Did you see the rotors being manufactured? There are different quality rotors but those that have holes are drilled, not cast. MovIt claims to have rotors with cast holes but I am not sure if I trust it since I don't know of any manufacturers that makes rotors with cast holes..

sweanders

Btw, the drilled rotors on a 951 do have ~40% more surface are which means that there is more surface area for heat to escape from. I believe that this is the main reason for drilled rotors, so if you never get your brakes really hot without enough cool down periods you have no use of the holes except for possibly a rotor with lower weight.

toddk911

Why Drilled AND slotted?

Drilled allows better cooling/surface area, and slotted allows trapped gases to escape easier.

Jeremy Himsel

My Porsche 993TT rotors have clear indicators that the holes are cast unless there is a drill bit on the market that finishes the fin area (between outer and inner rotor surfaces) that leaves casting marks. They are however post-cast drilled during the final machining process for chamfering purposes.

Zimmerman rotors do appear to be drilled.

Chris White

History lesson –
The first use of ‘rotors with holes’ was the 917 – it was done to make the rotors lighter – not for all the other reasons people like to make up.

The original Porsche 993 turbo rotors used with the big red conversions are cast holes. I use them for street and track use with no issues. They will develop cracks around the holes and eventually all the way to the edge (replacement time!) but this is under extreme duress. I get a couple of seasons out of a set of rotors.

Chris White

RKD in OKC

Honestly, I have absolutely no idea who makes what for who aftermarket or OEM. All I know is I saw a rough cast bevel between the hole and the surface of the rotor on the 2001 Boxster S I bought with 500 miles on it from my local dealershp and that there is no machine in our shop that could reproduce that bevel.

HIGHBOOST

If you get some rotors that are the same size as stock, but they are drilled. Then they will actually heat up faster due to the lesser mass of the rotor.

Oddjob

This is absolutely true, if the two rotors (one cross drilled and one solid) were placed on a hot plate (conductive heat transfer).

But keep in mind, if the rotors are in use on a car that is moving, and therefore having some airflow to and through the rotors, the convection heat transfer (cooling) is greater for the cross drilled rotor than for the solid rotor. This is because the holes increase the rotor's surface area which will allow it to transfer more heat to the air stream.

You would actually have to do a lot of testing under various conditions (distance, speed, ambient temps, etc.) and brake pad compounds to determine where the trade off is between the reduction in mass versus the increase in surface area with respect to the overall heat flux.

azmi951

Grassroots Motorsports has a VERY good article on this topic that takes all the guess work out of it.

Synopsis,
Drilling a rotor is for clearing moisture and contaminates (they cited mud/dirt form off road racing), also for lightening of a large rotor. With a huge rotor you do not need as much thermal capacity but do need the surface area for convective heat transfer. It is an engineering ballance and I dont think we have the ability to properly reengineer the properites of a rotor. They claimed there is very few vehicles that need drilled rotors.

Slotting is to wipe the pad of glazed (melted/burnt/galled...) material and allow gasses to escape. Grassroots claimed this is very useful and practical, it is also used frequently. (look at a bunch of race cars with iron rotors, they will have slots before drilled holes)

Plain faced rotors are just cheap to make and work for 99% of cars out there.


I was having a small problem with pad glazing. I layed out some slots on my stock rotors (symetrical and crossing multiple vanes) and cut them using a cut off saw and a guide. The performance increased and pad glazing was much less.

buxton

The surface contact space with the pad on slotted vs. cross drilled should be similar shouldn't it? And crossed drilled should cool quicker than slotted since the holes go all the way through and allows more surface space when you figure the inside of the hole area. I don't believe the rotors are thrown away after a day of track use on the Carrera GT, 911's, or the Corvettes - at least not by the guys that come to the track with me. And if cracking were a problem with the cross drilled rotors I would suspect that the manufacturers like Porsche and Chevy would stop using them and go to a slotted or solid rotor. It seems like the people here that have slotted think they are best and people that have crossed drilled think they are best. I am going to stick with what Porsche and Chevy do as I think they probably know a little more about brake rotors than most of us here do.

Just my opinion.

zoso_turbo

First and foremost...thanks to all for your feedback...I apprecaite the feedback and hope that othjers are benefitting from these posts.....

So...at this point here is a quick summary of the varioujs posts:
* Slotted and Solids seem to be more ideal for "hard braking" scenarios - track events
** Cross drilled will perform better in wet weather, but may not hold up to track days as the slotted and/or solids will, they may develop hairline cracks.
*** Ceramic is not in the same category to compare, seem to be too costly for this comparison.

One observation I would like to point out is this....It seems that when you compare the type of rotors I have mentioned, using data points from "supercars" seems to be inapplicable. I am having a difficult time thinking about spending more than the typical price of a slotted or solid rotor from ZImmerman or Brembo. So, anyway, like I said, I am open to any and all relative feedback.

Thx again.....

Please feel free to add/update my summary.