Brake upgrade- UFO strut

[Hanuman]

I have a set of UFO struts here, for which I have been planning a brake conversion for some time. It keeps getting put off due to other important issues.
Anyway, I've started.

There are 3 caliper options that I want to focus on, for various reasons.

HP2 setup.

The caliper mounting lugs are M12x1.5 on the UFO strut, and there are 3 lugs, not 2.
Compare this to the S6-HP2 struts which were M14, and only 2 lugs.

I have a set of HP2 calipers spare. As standard, these are drilled for the M14 bolt. I had considered drilling and re-tapping the strut lugs, but have decided to go with a reducer instead.
The next issue is the hub diameter, which needs to be turned down to accommodate a standard disc. Or be replaced with another hub.

Hopefully, that will all get done in the next week or so.

Brembo
Using an adaptor bracket to fit a set of porsche style calipers.
Again, the hub needs to be turned down.

I've yet to choose a disc, but there is a lot of scope to have a nice big disc, and several options exist, including Mercedes.

Q7/porsche cayenne style 6 pots
Anyone have a couple that I could borrow?

[v8quattro42]

Look at here
http://http://www.s2forum.com/forum/showthread.php?t=57629&highlight=urs6+cayenne

[Hanuman]

I like the cayenne option, but the calipers are fetching a premium! I did find a set cheap, but they were in a shocking state, possibly from accident damage, so I opted out.

For now, it's Hp2 as I have them available.

[Hanuman]

(Shamelessly copying my own reply from another thread...)
I've had a bit of spare time recently, what with international travel being so bloody painful. So instead of going home, I've been abusing things in the garage, and doing a bit more work on the Brake Upgrade that I started so long ago.

When I started thinking about brake mods to get rid of the UFO discs, HP2 seemed an obvious choice- used and new spares are plentiful, and the system is already proven in the same or similar vehicles. But I also always had a mind towards a Brembo setup as well.

HP2, because this might have given the quickest result, with the fewest component changes, and minimal fettling. I quite like the idea of just being able to rely on mass-manufactured components.
A Brembo setup would be so much lighter, though. And there is a lot of scope for something even better with larger wheels.

HP2 calipers

There appears to be 2, maybe 3 or even 4 distinct types.

All steel cylinder body, frame and bracket
Aluminum alloy cylinder body, steel frame and bracket.

then there's the variant that uses spring clips to retain the pads, and the other type that use inboard Pads clipped into the piston recess, and a torx-head retaining screw for the outboard pads.
The most obvious difference is 314mm discs, which were used on ur-S6 and A8, (ur-S4 as well?) and the 323mm disc used on the S6plus and S8s.

I bought 2 brand new calipers with a possible part number of BCT120, but I did have the option of going with a slightly different part number. The ONLY discernible difference that I could see from the photos was that the hub mounting holes were drilled slightly more offset, radially speaking. I didn't have the option at the time to measure them, but it did seem obvious in the photos. I think I went with the 314mm version, clipped and screw retained.

The discs have the same pad face and contact area, but there is a difference in the inner discs dimensions which move the friction area outwards. We're only talking a radius change of 4.5mm.

Original UFO discs were 310mm x 25mm, and obviously gripped by the inside out single pot caliper. It's a love hate thing.
I like the idea as it solved a technical difficulty, albeit short lived. Big disc in a small space. 15" wheels, 310mm disc.

urS4/S6 has a 314mm x 30mm disc, HP2 calipers, which are heavy. The other version HP2 using a 323mm x 30mm disc.

I hate the aesthetics. Nice wheels can look like they're just fancy wheel trims on steel wheels; there's something visually powerful in good brakes.



And then the fun begins, because all is not as it might appear.

The S6-HP2 and UFO struts are different, some obvious, and some subtle differences.
The hubs very different.

These differences are not insurmountable, but might easily be missed if your approach to converting with mixed components also involves using an angle grinder and the gearbox as an adhoc lathe...

The mounting hole centres are not the same.
The UFO caliper mounts on 3 points, one at the bottom, two at the top, using M12x1.5 bolts. HP2 mounts with 2 M14x1.5. The hole centres are 145ish mm. So, both struts would use the bottom hole. The UFO strut has a superfluous top hole/lug (which is often cut off, as it can interfere with the HP2 caliper.) But the lower of those two top holes is not the same distance radially from the hub centre, as the bottom one. As far as I can tell, the lower hole is 94mm, while the upper hole is 100mm. All measured in a cold wet garage, so accuracy is not assured.




If the HP2 caliper mounting holes were to be sleeved for a M12 bolt, and then mounted to the strut, as far as I can tell, the upper mounting would have the caliper sitting slightly out of true.
As for the hub itself, it’s slight thicker, giving a small offset on the disk mounting face. The hub is slightly offset also, giving a cumulative difference of about 3.5 mm.

The OD of the hub is 143mm, compared to the standard 131mm. So the UFO hub does not fit inside the hat of a standard S6 314mm disc.
All is not lost though, with regards the discs, as A4-B8/B9 cars have a 142mm OB hub. And there is a 318mmx30mm A4-B9/Q5/A5 disk. I have two on order to trial my prototyping.

Hat height of the A4 discs is 57mm, and the S6 disc is 59-60mm. Quick maths suggest that this will give a disc centre error of about 5-6mm, which is huge in terms of caliper positioning.


One potentially useful fix is that the rear hubs on the V8/urS6/S6plus cars do fit the fronts. In the C4 chassis, the same hub is used front and rear. (441501653B). So if worst comes to worst, I’ll just swap the hubs, as I have some spares. They are also available at a silly usedprice on fleabay.

But this does not resolve the immediate issue of the caliper positioning.
As my brake conversion is likely ultimately going down the Radial mounted Brembo route, all I need to do is fabricate suitable mounting brackets. But I would like to come up with a fix for the HP2 caliper.

That’s all I’ve got for now. And I know it’s a long winded reply in a parts wanted thread, but I’ll fix that in coming days.