More updates!
With the front rotor I got my quote back for my ultralight hat - unfortunately infinity dollars is too much, so I dropped back to something more sensible. The below design is pretty much standard fare. The scallops around the mounting points isn't there for weight reduction, but to reduce heat conduction through to the aluminium.
I will be making them out of 6061-T6, this is the same grade that is used by DBA and a bunch of others. It knocks the weight down to 1/3rd compared to steel of the same dimensions and is nearly as strong. The V8 Supercars use this as the material for the hats on their rotors, and I imagine its the same for all touring cars.
I included a screen shot of some off the shelf rotors with hats by DBA for comparison. The chamfered edge towards the mounting holes is significantly stronger than going into a flat edge, and it had a negligible effect on costs, so I'll keep it. One little thing I did that was my own idea was chamfering the centre bore. Whenever a rotor sticks to a hub, its always due to the centre bore. The thing is though, the centre bore is never under any load, so it doesn't need to be a significant structural component. By tapering the edge down it will be much easier to get the rotors on and off in the future.
On the inside edge of the hat there is a 3mm fillet (rounded edge) which will prevent stress from focusing in the flat top.
There is a 3mm clearance between the hub and the inside wall. This tiny clearance was one of the most troublesome items. Finding a rotor with a large annulus and a small rotor PCD was very difficult.
Below is the rotor the hat is designed for. These directional vaned rotors are used locally on a range of HSVs, and I believe they are used on Corvettes in the states.
Due to the unusual requirements of annulus and rotor PCD, I was only able to find two rotors that fitted over our hubs without any compromises. One was this, and the other is a rotor at Wilwood. The high vane count of this rotor allows the rotor to be a bit heavier, so a better thermal mass, than the Wilwood and being available locally made it a much cheaper choice for me as shipping is pretty hefty on slabs of steel.
I contacted Wilwood, Brembo, Alcon, AP, DBA and a couple of smaller players and went through hundreds of pages of catalogues. There are other rotors which could fit without modifying the position of the caliper, but they would introduce other compromises such as needing to run a thinner rotor. If I got the pad placement absolutely perfect I could run a couple of other rotors as well, as the minimum annulus is 57mm, but to go down to exactly 57mm on the rotor I'd need to get the centre point perfect otherwise I risk running the pad over the edge, and since I found a couple of fits that worked it wasn't necessary to be that pedantic.
The rear rotors remain somewhat more problematic. The culprit is the highlighted part of the hub, it dictates that the hat must have an internal diameter of 180mm to clear the part, if you include wall thickness then you've got 190mm used up. Working in radii to make the problem a bit clearer:
Outer radius is 160mm
Take away 55mm for the annulus which leaves us with 105mm spare.
Take away 95mm for the hat and you're left with 10mm to mount the hat.
The rotor PCD must occur between 190mm-200mm. Most rotors (regardless of size) have a mounting PCD ~205mm (this is what made it hard to find the front rotors).
I've only found one rotor that meets the bizarre requirements of a huge annulus and small rotor PCD, but there is a compromise in that it isn't 28mm thick but 25mm. The only other candidate I could find was 32mm thick and just won't fit outright, I would need to machine the rotor face. I mean that is fine to do as that is what people do anyway as rotors wear out but it seems a bit of a waste on a brand new rotor. Instead it is easier to fit a thinner rotor and to shim the pads out to prevent the pistons running out so that is what I'm looking at doing currently. I still need to confirm though if the annulus is 55mm or 50mm as that makes a substantial difference to the rotors I can choose from.
I'd really like to know what rotor Zep used on his front kit. It looks like an AP Racing, but I couldn't find anything in their catalogue that would've worked up front on the standard sized pads. If you don't mind letting me know who you went with, they may have a better candidate for a rear rotor since the seem to carry unusual sizes?
A friend of mine suggested just making my own rotor for the rear out of billet. I mean, it would work technically but lol, no.
I also have a sneaking suspicion that the front calipers will fit onto the rear. A very helpful forum user is going to try and measure that up for me, but if that were the case it would open up an interesting possibility. The front and rear calipers appear to be almost the same with effectively identical piston area and pad area. I can really only put down the difference in caliper size to cosmetics because I can't see a practical reason why they're different. It also doesn't make sense that they have quick release pads on the rear but pins on the front. Anyway, getting a set of front calipers on the rear would also address the issue as I've already got a rotor that fits.
For a bit of fun, this was the final version of the ultra light hat:
See what happens!