Is it safe though Steve if done right?
Alloy hubs I have from a mazda but they weight the same
as the stock b110 coupe ones :) and take oversized brakes.

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Well all of the above is strictly OPINION.

1. While strength 'might' be reduced, the remaining strength may well be more than sufficient. Proper testing would reveal the truth.
2. Since the rotor plays no part whatsovever in acceleration [unless inertia is considered] this comment shows a lack of understanding of the dynamic & actual forces involved.
3. Minimal impact? Perhaps but any time you can reduce unsprung mass you are making an improvement & if someone wants to do this, then why should we stop him.
4. Possibly, but the greatest measure of cooling is from the airflow of the moving vehicle over the rotor surface.
5. Again, strictly opinion not founded in demonstrable fact. The term "f*** all" is used to mean zero, nill, nada, zip, zilch, or nothing.
In fact a small, perhaps even tiny or almost unmeasurable improvement must surely result. But again, who are we to whinge, bitch & criticise if someone else is prepared to spend the cash and/or put in the effort.

Personally I suspect that cross drilling would provide a better result for the input of cash & effort.

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I have the greatest respect for your opinions, particularly in respect to automotive brake issues but I don't understand about the first two lines of your reply.

Unsprung mass is always a worthwhile subject to study & act upon where possible & practical.
Pistons reciprocate, but not brake rotors, they rotate, so the issue would be the inertia of the rotating mass. [perhaps I missed something here, Doh']

While I agree that this sort of weight saving may not be worthwhile in terms of cost or effort, that sensible fact never prevented any of us from doing stuff before. [especially me]

Can sit here and argue forever if we want - but at the end of the day it will make little/no improvement, may be unsafe, and may be illegal.

from what they tought me in my uni days... i would think this would not have a massive impact on the strength of the rotor

though i wouldn't do this mod myself, i would purchase rotors that were manufactured intentionally this way (obviously some sort of proof of compliance to ADR would be a huge confidence builder)

but like said, the proper way to do it is 2 piece alloy

its funny how they have gone to the effort of reducing weight that way without attempting any slotting or cross drilling also to improve benefit

MY 2c is that it would be less sturdy like that. We have gotten to the point in car manufacturing where raw material costs are factored into car manufacturers decision making processes - so you don't tend to see thick bore walls (compared to some engines from 40 years ago at least!) on a lot of modern engines.

I think that if the factory could have saved some weight without affecting safety, they probably would have. not for performance sake, but for economic reasons. If they don't (admittedly extra machining can cost more than leaving some material on there) it's not always worth re-inventing the wheel.

If it was a streeter, you'd probably not worry about heat soak. But if it was to be raced, within reason, more disc 'material' (all else being equal) should = a better result, unless it was specifically in an area where it added mostly inertia with little extra heat energy transfer/issues.

It could also be the case that the manufacturing of the discs, whlist ok, isn't necessarily perfect, and having more meat around the stud/hub area is a little extra rigidity in case there is a slight flaw in the material itself. I admit this will be unlikely, but not implausible.

Best advice off the top of my head would be to try and find a cheap set second hand, modify them, and then do some back to back testing and see if there is any measurable improvement in peformance, braking, roadholding, whatever. I'd bank on it not doing zip, but the idea isn't 'insane' or 'out there'. It's just a matter of 'degree' - i.e. how much gain *if any* might be possible (and we could write an essay on just what parameters might be considered for gains and losses too!)

The slotting and drillng of rotors afaik isn't to reduce weight, it's to allow pad material or resin (for want of a better term) that is turning to a gas state with the heat, somewhere to expand and vent without pushing the pads off the disc (as weird as that sounds) and to keep the disc cleaner of debris by giving it an escape route of sorts

I think it was ferdinand porsche who quipped that a racing engine that didn't self destruct while the car was crossing the finish line was over-engineered. The point being that for a given application, lesser longevity can be acceptable and even desirable if it goes hand in hand with a performance gain (even be it short lived).

The reason I posted these brakes is because as many know by now
Im anal for anything that reduces weight.

However these brakes are factory stock replacements for the s12
so I was wondering since my solid Sigma disc could be made a
little lighter without going to expensive alloy hat, it could be
a worthwhile in my eyes for my ultralight b10 project.

I intent to also eliminate the front leaf spring eventually as
well fit at least a 30kg lighter suzuki g13b or nissan hr16de engine.
(prefer a bike engine but prices are exorbitant in Oz)

My mistake...I was tired and wrote reciprocating not rotating

But the point is... it seems like alot of effort and cost for no reward
I know its not about performance (of the brakes) ... its about weight and wank factor

Charlie West used to drill out the centre of bolts and weigh the swarf to see how much weight he saved in an effort to make his cars/bikes lighter
Thats probably why his son was national hillclimb champ a few times... cause Dads a perfectionist/loony :)... (Much respect for Charlie and Gary)