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488 Posts
Discussion Starter #1 (Edited)
My build threads need to migrate from SportsMaserati, I paid money to a SopranoStu (a sponsor) who didn't fully deliver, voiced my concerns and was banned as a result. I'm still shortchanged, and I'd be happy to share any messages and details so you can make up your own mind if you so please. I'll copy over the more interesting parts from the original thread as well. To any of the people who found this interesting, please do go over to my threads on the other forum and link them so that people who aren't aware that I got banned can keep up with what I'm doing. I am learning this as I go, and enjoy sharing the process so a lot has changed since the first post.

So without further ado!


One of the (many) issues with my car is that it needs new brake rotors. Initially this didn't bother me, the OEM rotors are engineered sub-par and selecting a new rotor would be one of many improvements over the initial engineering, fit and finish. Little did I know that Fiat had intentionally gone and selected a rotor with such unusual dimensions that it was nearly impossible to find a direct replacement.

Never mind, I thought, there will be some aftermarket options!

So as I began looking at what was available I realised that I would be either selecting something with the same issues, or even potentially worse than the OEM rotors. The more I looked into it, the more I realised that the only way I could get a safe braking solution would be to engineer my own brakes.

The Problem with the OEM Rotors:

The first problem was fitment. I made a searchable version of the DBA catalogue in excel to see if there were any rotors where I could modify the PCD to fit. Options were very limited and the handbrake shoes on the rear made it nearly impossible.


In order to understand what is wrong with the brakes you have to understand how brakes work. Most people think that brakes are designed to stop the car, which is the effect that they have, but they are actually designed more like a battery as their job is to store and subsequently dissipate energy. If a car is moving at any velocity it is carrying energy expressed by the equation:


This means that the energy increase with velocity squared. If we needed to go from 24m/s > 0m/s we need to dissipate 483 840 Joules. Kinetic Energy Calculator

So how much energy is that?

Skipping some of the harder maths, about 70% of that energy goes into the front brake rotors, so 35% per rotor being roughly 169kJ. The specific heat capacity of cast steel is about 510J/kg, and our rotors are about 10.8kg. This is enough energy to raise the temperature of the rotor by 32 degrees every time you change your velocity by that much. So going from 200km/hr > 100km/hr regularly on a track for example.

Ah but our rotors are vented and cross-drilled! - but that is part of the problem...

Things get out of hand very quickly so I'll save the explanation for another day, but vented rotors behave like any heat exchanger (radiator). The amount of energy they can dissipate is directly proportional to surface area and air flow. So you can increase the surface area of the rotor for better cooling (more vents) or make them more efficient and flowing air (directional vents, a type of reverse ducted fan). Our rotors use a pillar cooling design to try and increase the surface area. This is the best technical feature of our brakes. Below is an accurate CAD model of what our brakes look like internally.


The cross-drilling however is not quite so smart. The first problem is that the cross-drilling isn't chamfered, which significantly increases the rates of cracking and blocking which entirely defeats the intended goal of cross drilling which is ventilation. Cross drilling is a source for corrosion shortening the life of the rotors, and our rotors have all the cross-drilled holes in the same arc which shortens pad life which roughens the pad surface which in turn shortens the rotor life! And believe it or not, cross-drilled rotors exhibit poorer cooling than flat discs because there is less interface surface between the pad and the rotor anyway. So why do cross-drilled rotors even exist? I'll let DBA answer that...


They look cool, that is why.

If you have a look on their website, and that of any large brand you'll find they all have disclaimers saying cross-drilled rotors are not recommended for high temperature applications. You'll never see it in racing as they're a failure risk. And before anyone says it - yes ceramic rotors are cross-drilled, but those material properties are very different to steel and that is a lesson for another day.

So this is the long way of saying that our rotors are fine for daily use but a liability if you like to do track days. Below is a very loose model of rotor temperatures over time for our car on a track day. The two other curves are a low powered lightweight car, and a high powered lightweight car:


Our rotors can easily get hot enough to fry wheel bearings, crack and glaze pads in 15 minutes of hard track level driving.

So after all that I decided to look into aftermarket options...

2 Piece Rotors Are Intrinsically Unsafe:

The first problem I encountered with the aftermarket options is that the single piece rotors were a direct copy of the OEM rotors, which means that even for street use they weren't satisfactory as they carried over the same design flaws. The second problem was that the 2-piece rotor sets were also cross-drilled which magnify the problems with 2 piece rotors. And the third was an absolute lack of basic specifications on these sets that would allow me to confirm their safety.

2 piece rotors are often called 'big brake kits', and the reason for this is that they're usually bigger than OEM kits. A lot of people like to talk about how the rotors are lighter for unsprung mass, but that is an intrinsic misunderstanding of what brakes do. Big brake kits are bigger because they need to increase their thermal mass as it has been significantly reduced by removing the hub. My explanation earlier shows that the mass of the rotor is the most significant component of keeping a rotor safe. The purpose of a 2 piece isn't to reduce mass, or increase ventilation, it is to protect wheel bearings and control the rotor temperature to match a specific pad. The pre-disposition to a lighter thermal mass, and the extra fasteners introduced into the system create an intrinsic risk of failure making poorly selected 2 piece rotors quite dangerous. Fortunately brakes tend to be made of literal slabs of steel so the odds of messing up so badly your have a catastrophic failure is nearly zero, still the knowledge that people run these systems without understanding why makes me uncomfortable.

In a situation like racing where its impossible to keep rotors at reasonable temperatures, by separating the hub and the rotor you can protect the wheel bearing. The lower mass means a hotter rotor, but that is okay because you can select a pad that matches your rotors operating profile. The below chart is the mu (friction) characteristic curve of pads provided by Hawk Performance. They're currently updating this chart (I asked) so it is out of date. But you can see that if you know your rotor is going to run at 500c (900f) you can pick a pad that matches that operating temperature and still protect your hub by physical separation.


This is why it is very important that when you purchase aftermarket brake parts that they provide you with at a minimum, mass data on the rotor. None of the aftermarket options of our cars provide that information, and therefore the risk is unacceptable to me.

So What Next?

I've been tossing up a few idea, but ultimately I've decided to go for a 2 piece design and delete the handbrake.

I've been speaking with Hawk and they're going to provide me with updated pad curves Premium Brake Pads, Rotors, & Brake Kits | Hawk Performance
I've also been speaking with SpartaEvo and they're going to manufacture my hubs Sparta | Performance Brake Systems
I've also contacted Wilwood and I'll be replacing my problematic handbrake shoes with an electronic handbrake Parking Brake

DBA, Wilwood, Alcon and AP all have publicly available information on their rotors and fitment, and a lot of the fitments are common. My current plan is to design a hub that has a common fitment to prevent being tied into one brand. SpartaEvo don't publicly disclose their rotor fitments because of the bespoke nature of their work but if some of their rotors support common fitment they will be used as well. Even better, some of the common fitments support rotor sizes for 330mm, 355mm and 380mm all at the same time, allowing for 2-piece rotors that match or exceed the OEM thermal mass maintaining a safe street use application. By using common fitments it also allows for using a host of materials, including CC! SpartaEvo know that I'm planning to fit another manufacturers rotors to the hubs I've asked them to make, and they've still agreed to do an internal engineering review and manufacture them, which to be honest is pretty amazing.

By going to an electronic handbrake I completely eliminate fitment issues on the rear making it easy to go to a matched set of front and rear rotors.

Our front calipers have a massive 57mm interface, so they're more than suitable for even the most hardcore application - and in my opinion, evidence that Maserati knew they were pushing the rotors they selected to the edge. They were trying to compensate for high temperatures by increasing the interface area to prevent fade.



This should be fun!

488 Posts
Discussion Starter #2

I have found the data on the offsets, at last. Tarox has them.

Ferrari 456 Front (Offset 62 mm)
Tarox web catalogue-auto
Ferrari 456 Rear (Offset 81.5 mm)
Tarox web catalogue-auto
Ferrari 550 (Front Offset 61 mm, Rear Offset 80 mm)
Tarox web catalogue-auto
Maserati 3200 (Front Offset 61 mm, Rear Offset 80 mm)
Tarox web catalogue-auto
Maserati 4200 (Front Offset 61 mm, Rear Offset 80.5 mm)
Tarox web catalogue-auto
The diameters and the PCDs are all the same. Maserati says that the rear rotors for 3200 and 4200 are the same, so Tarox's measurement may not be reliable, however.


488 Posts
Discussion Starter #4
I was a bit curious as to how strong brake hats really are, so I applied a basic model made of 6061-T6 (This is what Wilwood and DBA make their hats out of) and applied some forces and had a look at what happened.

Below the image has the PCD as fixtures and the rotor PCD as a force in a tangent direction to simulate braking. I went nuts and each of the 10 mounting holes is 1kN, so 10kN all up - about 10T per wheel is applied, with a minimum Factor of Safety of 9. So roughly 90T of braking force would be required to fail the hats... ****... Its not quite so simple as there are rotations involved, but a 1.8T car, so 900kg a side (so lets round up to 1T), so you'd need to stopping at something like 90g to break the hats... I'll try and work it out properly in the morning...


I also did a low effort topology study out of curiosity using 7075-T6, and got the mass down to 400g.

Speaking of mass - this is what the part would weigh if it were Chromoly Steel:


And what it would weigh if it were 6061-T6


Anyway, I'll keep at it - I'm determined to get something made for at least the front wheels this month.

488 Posts
Discussion Starter #5
One of the good things about doing things like this as a hobby is that simple economics of production aren't quite the same level of concern. So instead of just making a mounting hat that can be turned out on a lathe of completed on a mill, I decided to unnecessarily optimise my the mass using the topology study I did earlier and some realising values.



I calculated the loads properly this time:

An F1 can slow down at 5g, so I used that as my starting point.

So for my car I rounded up to 2T, and then split that between the two front wheels assuming 100% of the braking force per wheel., so 1T=10kN per wheel.

That means that to slow down at 5g, each wheel experiences 50kN.

Using the rolling diameter of 0.6m the torque applied (and assuming zero slip) at each wheel is 30kNm.

At the 108mm PCD the force applied is now 3/0.108 = 280kN. I applied this across the 5 bolts, so 56kN at each bolt. I then fixed the rotor PCD so that the whole thing would be getting twisted as if the rotor is stationary and the force is being applied without any slip.

Using the scenario of a two wheel, 2T formula one car slowing down at 5g these hubs have a minimum factor safety of 1. So with a more realistic expectation of ~1g each hub has a FOS >5.

It also weighs in at 645g.

With the rotor I've selected, if the hub were steel the total mass would be ~10.8kg. With the aluminium hats, the total mass is lower than OEM at ~9kg. Due to the large rotor I've selected, the thermal mass is higher than OEM but the total mass is lower.

Is any of this necessary? No.
Are you likely to feel the difference using an unobtanium alloy saving you an extra 150g from something normal like 6061-T6 (which is used on race cars). Definitely not.
Am I getting them quote in 7075-T651 because I can and I'm doing this for fun and idgaf? Yes, yes I am.

I'm scared to see how much this monstrosity costs...


The rotors I've picked are directional, maintain the full 57mm swept area (they have a 61mm annulus) and were used by Holden Special Vehicles for silly cars like the Maloo, Coupe GTO and other things with 2T and 6.0L V8s. So there will be no silliness about using narrower pads, or needing to offset the caliper, no dogbones etc. There was also a suitable rotor by Wilwood, but I went with DBA just because of local supply and a significant reduction in shipping costs.

The rotors typically cost ~$300 AUD each, and the simple hats (I'm waiting to hear back on my monstrosity hats) cost ~$300 AUD each. So about $600AUD a corner (Currently about 300GBP). At least in AU, that will be comparable to the cost of the OEM rotors by the time you factor in shipping. I've been somewhat mindful of costs, looking at what aftermarket stuff exists and so far its still worth the exercise. Time to get onto the rears!

488 Posts
Discussion Starter #6
Reviewing my notes just now, there were actually a bunch of issues with the rear rotor, even deleting the handbrake. The front rotor was easy, Wilwood and DBA both have rotors that are a straight fit, even for the unusually large annulus.

If anyone can help confirm/answer the following it would be super helpful:

1 - What is the diameter of the rear rotor? I measured 312mm but that is an incredibly uncommon size. If it is 312mm, then I'll almost certainly need to pick a rotor of a different size, probably 308mm or 316mm, so it would help to have an idea of the rotor/caliper clearance as well (this is the gap between the top of the rotor and anything it will hit inside the caliper if it were bigger). When I inspected the car initially I took some photos and it looks like there may be a bit of clearance to go slightly larger. It is pretty unusual for the pads to run right up on the edge of the rotor which indicates that the rotor may be undersized for the caliper.


2 - The swept area of the pad. Sometimes you can get it from pad drawings, but the pads in my car don't have a detailed enough drawing when I look it up. Knowing this will help me size the annulus of the rotor if required. The easiest way to work .this out is to measure the wear surface on a rotor, since that is where the pad is sweeping! I used a drawing from Hawk to infer the swept area as ~59mm, which is pretty hefty for rear calipers!

3 - Anyone who has changed the rear rotor/pads with any regularity, if you could comment on how snug the fit was? 30mm thick rotors are far more common than 28mm and I'd be far more likely to fit the OEM hand brake if I can squeeze a fatter rotor in. The line in red dictates the maximum fitment the caliper will allow, yellow the maximum pad width. I looked up the part number of the pad that was in there, DB1356SP, which is a pad used on HSV vehicles. Coincidentally it was the pad that would've been specced for the rotor that I've selected for the front! Anyway, the pad is 16.5mm thick when new, which matches with the thickness of the the Porsche 911 (964) pads which also fit.


Everything is odd in the braking setup of this car, I wish I knew what was going on at the time. They're used front calipers as rear calipers that are effectively the same size (functionally) as the front, which leads me to think the only reason they ran a different caliper was cosmetic.... any help would be appreciated.

488 Posts
Discussion Starter #8
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!

488 Posts
Discussion Starter #9
Turns out I can get my rotor hats anodised any colour I want. What colour would you all pick?


So I have some solid costs now (in AUD):

$288 per front rotor
$334 per rear rotor
$361 per hat (including shipping and finishing)

All up it would be $1578, or about 850GBP. Compared to getting new units from eurospares, it is a little bit more in direct costs as the parts would be $1140, but there would also be around $300 in shipping. So I'll be charitable can say that my bits are $1600 v $1400. If you tried to get the parts through a dealer though.... oof.... Compared to FD, all four would be ~$1600 before shipping for the OEM replacements. I've seen front only kits that are still cross-drilled, or require skinnier pads at between $1200-$1600 for just fronts, before shipping.

Anyway, its looking like its worth my effort, and these rotors will probably last longer than the rest of the car does...

Design wise, everything up front is finished and I just need to step up and press buy. The rears I need to measure a few more things. It would appear that the rear calipers will hold a 30mm wide rotor and if that is the case then I will be able to directly fit a couple of rotors. I also searched through a bunch of pads that would fit, and found a couple a bit thinner which would accommodate the extra 2mm if needed. If I can squeeze a 32mm rotor in then things get a lot better, but I doubt it. If the 30mm thick rotor will fit, DBA52650.1 would be a prime choice to accommodate the OEM handbrake.

I am debating on ordering the front rotors and hats now and resolving the rears later as I can't measure the critical dimensions on the rear till I get home in a few weeks. The other part of me wants to do it all in one go...

488 Posts
Discussion Starter #10
Things are going well!

A wonderful chap has been following this for a while and got hold of me, he is based over in Queensland, and he pulled apart his brakes and confirmed a bunch of measurements for me as well as getting a few things I had missed but didn't realise until I needed to know! He sent me a bunch of photos over the last couple of days, and even took his brakes off the car! Legend!

Right now I'm finalising some choices for bolts lengths, and double checking things, but all the big decisions have been made, so I'll run you through it.

Front Rotor

I found two rotors that would fit all the limitations:

Willwood 150-15256 - This rotor meets all the requirements, is a directional vane rotor and hat an 8 hole mounting pattern.

DBA5048.1L/R - Available globally, but most importantly locally for me. It has a 12 hole mounting pattern, which I like as it would distribute the load more. Availability was the main factor here.

Here is an transparent screen shot of the rotor model:

The OEM annulus is 61mm, and since I measured the area swept by the pad as 57mm there is plenty of area for the pad:


Total height of the rotor and hat is the OEM spec is 60.7mm to ensure it fits with the factory calipers:
Thickness of the hats is 7.4mm, which I measured for both front and rear:

Rear Rotor

I needed to get creative with the rear rotor. The below solution was the best I was able to come up with, and it was only possible with the help from Chris. He verified that the maximum clearance in the rear rotor was 30.5mm, which gave me a half decent choice for the rotor. The difficulty in fitting a rear rotor was ridiculous. I spent maybe four days in total just going through catalogues!

This is the DBA52650.1, it is a 316x30mm rotor. As it is right now, the rotor won't fit. The maximum clearance for the rear caliper is 313mm, and the mounting lugs would
protrude into the handbrake. However, because I can fit the 30mm width, I don't need to play around with the caliper, and it is only slightly oversized.


The solution is to turn down the outer edge by 6mm, and to turn out the original mounting lugs. I'll put 12 mounting points instead of 8 to better distribute the load, and I won't bother remaking the lugs as and just leave it solid.


Shaving it down will just hit the pillars on the outside. On the inside there is enough of the original mounting lugs to use, but I need take off a bit more on the inner edge to allow clearances for tools to tighten the fasteners. Oddly enough the disk will be slightly stiffer this way as the mounting points are closer to the pillars.


Chris also confirmed the area swept on the pad as 50mm. This is a critical dimension, if it was 55mm I would've needed to get creative with the mounting holes.

The blue line is a 50mm swept area:

The blue line is a 55mm swept area:

488 Posts
Discussion Starter #11
This is the whole assembly below:

In order to mount the hat, the fasteners will need to come in from behind and tap directly into the hat:


This transparent render shows how it will mount into the hat:

Total height of the assembly is the OEM 80.5mm

Here is a transparent render so you can see the pillar design, DBA refer to it as the 'Kangaroo Paw'.


It is important to go over things and then check again! I found a discrepancy between some of the dimensions I was provided by DBA, and their drawings. I called up to confirm, and I had been given the wrong dimensions! Very lucky, that would've been an expensive mistake otherwise!


I also did some thermal expansion calcs on the rear rotor to ensure running oversized won't clash with the caliper. Being conservative, the 30mm rotor could expand to 30.2mm if it were to reach 600 degrees, which it never should.

Right now all the design work is done, just making a few little checks and changes. The last thing I need to do before getting my parts is select some appropriate fasteners as they will dictate the thickness and thread length of the mounting points.

I've also spoken with Hawk and they have a whole range of pads that will fit both the front and the rear of the car, and since they provide nice little temp charts, and just recently sent me their most up to date one, I'll select a set from them! Only AP Racing provides as much data, but none of their hardware was going to fit.



488 Posts
Discussion Starter #12
So a big post!

Today I finally resolved the issues with the rear rotor and have finalised the parts and design. I thought I'd take you all through my adventures! I needed quite a bit of help to finalise the last few things, and I harassed DBA until they put me in touch with one of their engineers to answer the remaining questions I had. I had to do this because the packaging at the rear is just too tight.

So the design problem comes from the 338/M138 being old. Before the late 00s, large rims weren't common and so limited the diameter of brake rotors. Our cars hard large rims for the time, but due to the development cycle our cars are a 90s car in terms of technology. When you have a heavy, powerful car and not much space for brakes, how do you make them bigger? In the 90s the solution was to run very tall pads. This means that our rotors have a small (large for the time) outer diameter and a large annulus. This means that the rotor only has limited internal space for mounting hardware. When a rotor is a single piece this is much less of an issue, but for me, and especially on the rear rotor, this turned out to be very difficult! As I've gone through this I've gotten a couple of PMs about people wanting to do this themselves, so this is a loose how-to get all the parts yourself.

Front Rotor


For the front rotor, the larger annulus meant I could only find two off the shelf rotors that would fit:

Willwood 150-15256

As I've said previously, I went with DBA because of the ease of availability. DBA is a global company, with distributors around the world, so you would be able to get these rotors wherever you are.


Contacting them is quite productive and they will put you in contact with distributors. Locally the rotor costs $288AU and it is a directional vaned rotor with a slotted annulus. A thicker 34mm version of this rotor is used on the V8 Supercars.

In order to get correct fitment in CAD, I measured up the original rotor - and with the help of google and other forum members - I created a profile for the height of the original components and drew the hat in those boundaries.


In order not to clash with the hub, I also drew a reference sketch for the hub. I then drew several guide circles in order to ensure clearance. Fitment is quite snug with a clearance of 4mm (2mm per side) as I needed to make room for the bolt holes.



The inside diameter is 163.16mm, the rotor PCD (the diameter that the bolts run through for attaching the rotor to the hat) is 182mm. This means I have 9.42mm to fit the walls of the hat, and a 1/4" (6.4mm) bolt hole.

The solution was to slant chamfer the outer edge to provide as much rigidity as possible. I also needed to create a little cutout to accommodate the head of the bolt. There is <1mm clearance between the bolt head and the edge of the hat.


The bolts DBA use on their setups are NAS bolts. NAS is an aerospace product with yield strengths close to double 306 stainless bolt. The issue I had was that I can't use the hex head bolt due to fitment reasons, and with such a tight tolerance I had have precise parts. As I'm designing my own hats I also needed to make sure that I had the depth set correctly as the below are shoulder bolts. If I had the hole depth set incorrectly, then the nut would hit the grip (shoulder) before hitting the rotor and the assembly would be loose. I set about looking for drawings of the bolts so that I could set the correct thickness of the parts, which turned out to be very hard.


You can see in the below cross-section that if I didn't have the thickness correct, the nut would hit the grip. If I had the bolt too shallow, there could be load from the rotor on the threads which is also bad.


As I needed to select my own bolts, I ended up asking about the choice of thread (UNF instead of UNC), and it wasn't an arbitrary choice. The engineer said:

"They are UNF as the fine pitch is preferred due to its proof load and small lead angle. The crimp nut thread engagement on UNF enables a low profile body.

NAS bolts have a ground shank too.

So bolts. You want to drive the hat/rotor union on a bolt shank not the thread.

So I needed to look for a specific bolt. I eventually came across these guys AAE eStore | Aerospace Hardware and Supplies - AAE eStore who supply aerospace fasteners. They told me about Military Standards MIL-STD, Military specifications MIL SPEC where I could find drawings and technical requirements for every fastener you can imagine. After going through their catalogue I eventually came across a socket head cap screw that is MIL (military) spec that would be suitable. The series of bolts I settled on was MS20004. These bolts have a fixed 0.5" thread, and the rest of the bolt is shank. I then pulled up the current standard, which is from March 1989, and used it to accurately model the bolts so I could ensure the fitment. The document is a bunch of drawings and call outs. It takes a fair bit of back and forth to check the dimensions.



I fed some relevant information into a design table in my CAD program so I could 'test' fit several lengths.

For the front rotor I will need 24 x MS20004-6 at $5ea. I'll use the crimp nuts that are supplied with the rotor as these are the correct thread.

488 Posts
Discussion Starter #13
With the results correctly dimensioned I was able to get a quote on the part (and get it anodised red). $523USD for a pair out of 6061-T6.

I'm really quite pleased with the front rotor - there are no compromises. This rotor isn't thinner, I don't need to go fiddling with brake pads or dog bones, and I get away from the issues and costs of other kits by quite a substantial margin. The cheapest front only kit I could find was ~$1600AUD before shipping. To equip the front setup as detailed above I'm looking at $1448AUD including shipping.

Rear Rotor

The rear rotor was just the biggest pain, and it was made harder as it wasn't until I found a supplier for the MIL spec bolts that I had a way forward. Before that I was using McMaster Carr as all their bolts come with drawings and CAD files, but they didn't have anything quite the right dimensions. More on that in a bit.


I found several rotors that were good candidates, but everything was a compromise. None of the rotors were the correct dimension in every direction.

The closest I could find was by Alcon DIV2235X562C48. This would probably be a decent matched rotor for Zep's kit as it is a racing fish hook design with direction vanes. However, I decided not to go with it as I didn't like the position of the mounting lugs. Ultimately I went for a rotor that was oversized in every direction so that I knew I would have enough material to remove and get a correct fit. I settled with the DBA52650.1. This is a bi-directional pillar rotor used on a number of Subarus. It is a 316x30mm rotor, has a massive annulus and lots of material in the centre for mounting. Due to the changes I would need to make, I spoke with the engineer who designed this specific rotor to find out the minimum thicknesses, and consequences of any changes. Because this rotor is so thick and heavy it turns out that the minimum thickness for the rotor would be ~25mm, so taking this rotor down from 30mm to 28mm for my application would be acceptable. These rotors are $667AUD a pair, and machining will be another $100 per rotor.


The real trouble came with clearing the handbrake. For whatever reason, even the handbrake is oversized. I tried about a dozen variations of the rear hat until I found the MIL spec bolts.

I thought about tapping into the hat directly, but the engineer advised against it:

I thought about mounting from the other side and using little windows to fit the nuts. The problem with this was that it blocked off the ventilation. I couldn't do this from the top as the little holes would conflict with the handbrake surface. The idea itself isn't that cray, Wilwood use it on some products.

The handbrake interface inside the hat:

I tried to make a conventional flange fit and used some clever low profile fasteners, but there was no way it wouldn't clash with the calliper.


488 Posts
Discussion Starter #14
The gap between the calliper and hat is only 8mm!


I tried the inverted mount again, but this time with ventilation cut into it. This would've worked but the machining costs would've piled up....

When I finally found the MIL spec bolts, I was able to come up with a much more appropriate design. I ran the long bolts all the way to the top of the hat, taking off only enough material to allow the nuts to fit. This meant that my costs were reduced slightly as I could use the supplied nuts with the rotor. It also meant that I could keep the shank as the only interface with the hat and not the threads, there is no risk of the threads pulling out of the hat, and because I had accurate drawings of the bolts I could ensure there are no clashes or interference.


Everything is unbelievably snug and it wouldn't have been practical do this conventionally, too many expensive mistakes to make! Being able to test fit parts in CAD has made this viable for me to do. I don't have a current quote for the rear hat, but the last one I had was $600USD. I'm expecting this one to be similar, if not slightly less as it is simpler. So I would have 16 x $5 for the bolts, $863 for the hats and $887 for the rotors. So $1810 for the rears.

All in, front and rear setup would be $1448 + $1810 = $3258AUD. Right now that is about $2300USD and 1800GBP for a full two-piece rotor setup. About double what the OEM rotors would cost, but I get rid of the cross-drilling which is what kicked off this whole saga in the first place. Depending on where you are in the world you might even be able to pull it off cheaper if your local DBA supplier is cheaper.

Yep, I spent 4 weeks worth of full time hours reading catalogues, drawing things in CAD, running FEA and calling strangers for advice and help just because I dislike cross-drilled rotors...

488 Posts
Discussion Starter #15

Some of the more astute viewers may have noticed that the rear rotors are plain. They've sent the wrong ones over, so they'll be getting sent back tomorrow and hopefully the correct ones sent out the second time!

The milspec bolts have arrived in Australia, and I sent off some drawings to get the rear rotors made!




488 Posts
Discussion Starter #16
So the first bit of new content on this forum!

I went around to the car to test fit my parts, and it went well! I've made a few notes for design revisions in the future, but everything fitted as expected.

This photo shows how overworn my OEM rotors are

Here is the OEM compared to my 2-piece

Now here is the big question, does my solution for the handbrake work? Yes! It does! @MAS430 was a big part in this, he emailed me from the other side of the country to help validate all the measurements sot that this solution wouldn't contact the caliper!

There is somewhere around 2-3mm of clearance, so more than enough!

The larger access holes for the handbrake made things a lot easier to adjust.


I'm taking the rear rotors off to a machinist tomorrow in order to get cut to size!

11 Posts
This is a good post, I like. It's cool someone is developing something for these cars, looks awesome. Let alone in your spare time as a pet project. Definitely keep us updated and driving impressions needed lol. Do you think these will hold up well for track days?
Do you happen to have a weight comparison between the Lambe design and oem?

488 Posts
Discussion Starter #19
This is a good post, I like. It's cool someone is developing something for these cars, looks awesome. Let alone in your spare time as a pet project. Definitely keep us updated and driving impressions needed lol. Do you think these will hold up well for track days?
Do you happen to have a weight comparison between the Lambe design and oem?
These will definitely hold up well for track days. I've been getting a lot of help from Hawk about selecting brake pads, and they're sending me over sets of their PCs, HT10s and DTC60s to test with my setup. When it is all ready it will go up on my youtube channel, so if you want to see the testing videos you'll find it all there. ShiftMotoring The rotors aren't fully floating because I intended them to replace the OEM rotors, so if you wanted racing rotors I'd do something different but for regular track use they will definitely exceed requirements. The front rotors are a variation of the front rotors used on the NZ V8 Supercar series, they're a capable component.

I haven't had a chance to weigh them yet, but from the CAD the mass should approximately be:

Front - 11.9kg vs 10.5kg
Rear - 9.9kg vs 7.5kg

All the mass savings should be in the hats as they're 6061-T6. I have picked rotors with thick contact surfaces and a thinner airgap than OEM to increase their overall thermal capacity. Once of the things I learnt during this process (and I learnt this directly from an engineer who holds patents in brake designs) is that all the marketing around 'better cooling' is fictional. Unless you're heat soaking the rotor entirely (in which case you have other problems) most of the heat is in the contact patch and is dissipated through radiation (why they glow red) rather than convection (air flow). That said, there is a modest weight saving overall in the system



I saw your question on the other forum, so tagging you here. The wishbones, they're well on their way to existing. I have all the bushings and such, I just need to finish the CAD work and order things. Feel free to link this post back at SM for the disbelievers.

As for the rotors, they're finished. They're currently at a machine shop having the rear rotors matched to the hubs so that they can be mounted. I'm also having a few small details adjusted on the fronts which became apparent once I had them in person. In fact, I have taken what I've learnt and made a second revision. So if I ever make them again the overall design is improved.

Front Revision:
  • Increased the chamfer on the top outer edge to prevent chipping of the anodising.
  • Cosmetic change with the thickness of the outer edge with how it matches with the rotor.

Rear Rotor:
  • Increased OD by 0.5mm to allow the bolts to be mounted from the top side (this is purely cosmetic).
  • Revised the bolt length to allow bolts to be mounted from the top side.
  • Chamfered the top outer edge to reduce chipping against a hard edge.
  • Revised hub wall thickness by 0.2mm (for centring within the brake caliper). 0.2mm is the thickness of an A4 piece of paper, but it is the principal of the matter!

107 Posts
Great to hear about the wishbones, care to tell which car did you source the bushes from? Hope its not a Ford Falcon :unsure:
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