Bigger brakes
 

I know I'm getting ahead of myself as I have other maintenance that needs to be done on my car but I don't care. I want to upgrade the brakes on my car and wondering if anyone installed 17z front calipers and Boxster (1st gen) calipers for the rear on the FL S8?

6-piston calipers with no provision for handbrake, on the rear? :Confused:

Not 6-piston calipers for the rear. I'm looking to do 6-piston (17z) calipers for the fronts and 4-piston calipers for the rear. This guy uses a custom hydraulic e-brake setup for his rear 4-piston calipers

oh gotcha +++ I read that wrong!

A hydraulic handbrake is not allowed for road use here. We have to have something mechanical.

Aren't the 17z discs smaller than the stock FL S8 fronts though? 330mm I think, compared to the S8's 345mm. People seem to use a 345mm Mercedes ML disc with the 18z too so also no bigger.

I have attempted to figure out fitting an 18z caliper to a 365mm RS4 front disc, but could see no way for it to work as the mounting lugs for the caliper and upright both occupy the same place, so I'm keen to see what you come up with. It would be much easier with radial mount calipers. That said, it might be possible with the ML disc. Just no upgrade in size, although the Porsche monoblock calipers appear superior to the S8 Brembos in pretty much every way.

It springs to mind actually that I don't think I've tried the 18z caliper with a stock S8 disc. Hmm...

It's ok, no worries.

Bummer to hear. The other option is try and custom fit these for mechanical setup.

Yes they list the 17z calipers to use 330mm discs and 18z calipers to use 345mm discs but I'm sure you can use larger discs for either calipers. After some searching this guy is using 350 mm discs with 17z calipers

I did some more searching and found that the C5 RS6 and B7 RS4 use 8-piston calipers and 365mm discs. So I guess the other option is to try and fit 8-piston calipers for the front and use 365mm discs. Will these fit with 18" wheels?

I've done B7 RS4 brakes on a D2. Thats easy as they are radial mount calipers the same as the D2 Brembos so the adapter is a 10mm thick flat plate with 4 holes in it. I would expect they fit in some 18s as thats what the B7 had stock, although the RS6 had 19s.

The 18z will fit over a 365mm disc no problem - it just needs a litle grinding at the outer edges. But, the mounting lugs then occupy the same place in space as the lugs on the upright so you can't even put the caliper on the disc, let alone contrive an adapter. I'll get a pic tomorrow - its a vexing problem since those calipers are cheap and easily available.

I would think it would be easier to simply swap to D2 A8 spindles and then use D3 brakes? D3 A8 have 360mm rotors. D3 S8 are 385mm rotors.

Yes please post some pictures. It sounds like I might go this route

Yeah but how is 2-piston (which are on the same side) calipers better then 4, 6 or 8 piston calipers!?!

Depends on how you qualify one setup as being better than another.

Well, the 2 piston D3 calipers

• Work properly
• Are simple to service
• Don't seize up if the day has a Y in it
• Don't squeak
• Fit on larger discs than stock
• Are reliable
• Work properly (I know I already mentioned this, but I though it was important enough to mention again)

I am pretty sure the FL S8 Brembos were a fashion item rather than a technical solution, since W12 had larger brakes with sensible 2-piston sliding calipers. The RS 8-pots aren't much better in terms of maintenance, although they are very effective. I do like the Porsche 6-pots from an engineering standpoint, but I haven't ever driven with them.

The downside of the D3 385mm setup is the sheer weight of the discs - 18Kg (40lbs) each :eek2: Thats just stupid and not good for handling so I plan to fit the 360s as they are a reasonable compromise in terms of braking performance v weight v cost. Alas they won't work with the very lightweight RS6 365mm discs, but you can't have everything.

If only the Porsche 6-pots could be made to work with the RS6 discs that would be a perfect solution. I keep holding out for a set of Q7 4m Akebonos, but they rarely come up in pairs, and never when I have any money.

I'm no expert with brakes but my guess is that having pistons on both sides of a caliper have better clamping force then having pistons on the same side.

Nope. All things being equal, it makes zero difference in clamping force to have pistons on both sides rather than just one side.

Clamping force is purely a function of total piston area... on one side of the caliper. If you have two 50mm pistons on one side of a piston in a sliding caliper setup, it will provide the same clamping force as it would if there were two 50mm pistons on both sides of a fixed caliper setup. Yes, there's twice as many pistons, but that means that the pistons get half as much pressure, assuming the amount of force you put on the brake pedal doesn't change. Meaning that the resulting clamping force on the rotor doesn't change either because the twice as many pistons get cancelled by the half as much pressure. (There's obviously WAY more to it than that, but that's a pretty simplified way of thinking about it)

If you purely want your car to stop better, you'd actually be WAY better off buying better tires. Your car's limit in how quickly it can stop is purely a function of how much your tires can grip the road.

Using a more simplified example, here's something you can think about. If you can lock up the brakes on your car (assuming the car doesn't have ABS), then having even more clamping force will provide zero benefit at all. You can't make the wheels stop any more if they're already stopped. The same idea would apply even to a car with ABS. The limit of how fast you can slow down is an issue of how much grip your tires can withstand before they lock up on the road surface. Bigger brakes, more clamping force, etc won't change that at all.

Theoretically, moving the calipers outward (bigger rotors) will result in requiring less pedal pressure to provide the same amount of braking force on the road simply because moving the caliper outward provides a longer effective lever arm to act on the tire. But the bigger reason to move to different brakes is simply for other factors like increased fade resistance due to better cooling or more mass to the rotors. Or a more easily maintained setup. The sliding caliper setup is worlds easier to maintain and resist squeaks and resists seizing than a fixed caliper setup like the D2 Brembos. Not to mention, there's only like 3 companies that make brake pads for the D2 Brembos, whereas there's like 3 million companies that make brake pads for the D3 A8 brakes.

There's also handling changes to consider because adding weight to your brakes is un-sprung weight, which will negatively affect handling. However, if handling was that important... would you really be driving an Audi? Having the entire engine in front of the front axle is a terrible design for handling.

Basically a long post to simply say if you want to increase your braking capability... buy better tires.

I can't lock the wheels on my S8. I can't even trigger the ABS on dry tarmac.

This is what I mean about the 18z calipers btw:

http://forum.a8parts.co.uk/attachmen...1&d=1594823804

http://forum.a8parts.co.uk/attachmen...1&d=1594823804

The caliper lugs and the upright lugs both occupy the same point in spacetime. There's nowhere to create a bracket.

However, this is the first time I've tried with a 345mm S8 disc, and its actually pretty close. I am going to try some C5 A6 hub flanges later which sit 6mm further inboard than the D2 ones, and it might just work. I feel that these calipers would be a worthwhile upgrade from the stock Brembos, even on the same size disc as they tend to work properly for longer, are lighter, and should give more even pad contact. I'm not sure they would be better than the 360mm D3 setup though, even with the associated weight penalty. I'd probably have to try both back to back.

Sooo....

A8 upright, A6 C5 hub, OE S8 345mm disc:

http://forum.a8parts.co.uk/attachmen...1&d=1594831739

To make the S8 disc work on the A6 hub (moving it inboard 6mm) means you forfeit the splash guard, and I think the hub needs a little 'lightening', although it is possible to offset the hub up to 2mm with a laser-cut aluminium spacer so that might be a solution. The 18z caliper fits fine with a spacer washer (maybe offset by spacing the hub) and the usual bolt adapter set with a sleeve for the caliper side. The pad may overhang the disc slightly (since its 2.5mm too small), although I've seen mention of using 17z pads in 18z calipers to solve that. Still, with M12 hardware it would be possible to move the caliper in a couple of mm.

That gave me an idea...

A8 upright, A6 C5 hub, D3 A8 360mm disc:

http://forum.a8parts.co.uk/attachmen...1&d=1594831739

http://forum.a8parts.co.uk/attachmen...1&d=1594831739

This also forfeits the splash guard and definitely needs a mm or two shaving off the upright, as it is exactly centred in the caliper as-is. Its very tight on the caliper and may need a little massaging of the caliper inner radius (or machine 2mm off the disc). Again, with 12mm hardware it might actually be possible to move the caliper out a little.

Alas, none of the lightweight discs I have are even close to fitting as the offsets are completely wrong. It could be done with custom alloy bells, but thats missing the point.

I'm not sure what the pad radius of the 18z caliper is so I've ordered the cheapest Q7 pads on ebay (£26!!) to find out. I will report back at the weekend when they are due to arrive +++

Interesting.....

Thanks for info and pictures MikkiJayne +++. Curious if you have any pictures of the B7 RS4 brakes installed on a D2?

I could get fronts lock up fairly easy on my PF D2 with stock 323mm brakes but they would also heat up and fade after couple of hard braking sessions.

All of my HP2 D2s brake better than my Brembo D2s :(

http://forum.a8parts.co.uk/attachmen...1&d=1594888873

http://forum.a8parts.co.uk/attachmen...1&d=1594888873

Pretty simple really. The adapter is just a piece of 10mm flat bar with four holes in it. Wheel clearance is an issue as the caliper is quite chunky and the disc is quite flat compared to the D2 stock disc offset. I can't remember if 18" Avus clear, but the 20" FE wheels need spacers. ET needs to be 30-35 I think.

They stop very well but these discs are very prone to warping and judder and, at £1000 a set, that can get expensive very quickly! This particular car is going to get updated to C5 RS6 discs which are non-floating 2-piece and which are both cheaper and more reliable, at a small weight penalty. They also have more disc offset so we can move the caliper inboard to make more room for wheels.

The brakes on my B7 RS4 are a real bug-bear, they are just seemingly high maintenence and bloody expensive as no real aftermarket parts.

They do stop, but I'd personally look at other options unless you want to create a new headache for an 8 +++

Thanks again MikkiJayne +++ but now I'm confused. How is the flat bar mounted as there is no way it will mount flat to the upright as see in this picture :Confused: Or is the opposite side of the upright flat that's not visible in this picture?

http://forum.a8parts.co.uk/attachmen...1&d=1594823804

Your picture is from different car. Flat bar was on S8 with Brembo brakes. Different brakes and uprights on S8 compared to A8

Correct ^

The RS4 setup is on an S8 Brembo upright whereas I am showing the Porsche 18z with the A8 HP2 upright.

Thanks for the clarification as that makes more sense.

In the land of Audifan, the C5 RS6 had 18's stock. So assuming they didn't change the brakes, they will fit under 18" wheels.


I like your thinking, but one of the biggest factors for increasing disc size is heat dissipation.
You're right in tyres being a critical factor in how efficient your braking is, and the leverage action of larger discs and calipers, but the larger size disc also provides a critical benefit in heat dissipation.
You alluded to it a bit anyway, so I'm sure you're aware of most of this, but brakes are all about energy management.

When the car is at speed, it has a large amount of momentum which is Potential Energy.
In order to slow the car, you need to convert that energy into another form (conservation of energy).
Cars use brakes to convert all this Potential Energy into Kinetic Energy, in the form of heat.
That heat energy then needs to be added to the surrounding environment through equilibrium as efficiently as possible.

A measure of how effective the brakes are, is how effectively they can shed the heat, which is a measure of exposed surface area.
So:
Ventilated discs are better than solid discs.
Bigger discs are better than smaller discs.
Larger pads are better than smaller pads (within disc coverage limits), and larger/multi piston calipers allow for larger pads and better control.

Then there's the engineering compromises of unsprung mass, physical wheel size, metalurgy, cost etc

But as you said, none of this is any good if your tyres lose grip!

Larger pads you say...

http://forum.a8parts.co.uk/attachmen...1&d=1595235429

Porsche / Q7 18z pad vs S8 Brembo! These things are huge :eek2: Tbh I should have figured that out from the size of the caliper but it didn't really register until I had them in my hand.

Btw, cheapest brake pads on the market for a Porsche Cayenne / Audi Q7? £26 :ROFL:

For reference, this is the 18z caliper on an S8 Brembo upright - no way this will work:

http://forum.a8parts.co.uk/attachmen...1&d=1595235429

http://forum.a8parts.co.uk/attachmen...1&d=1595235429

This is the 18z caliper on the S8 345mm disc - also not going to work

http://forum.a8parts.co.uk/attachmen...1&d=1595235429

However, the pads fit the D3 360mm disc perfectly. The only challenge then is a 4mm offset in the mounting holes:

http://forum.a8parts.co.uk/attachmen...1&d=1595235429

I think this might be possible to solve by boring out both the caliper and the upright an additional 1mm and creating an offset bushing to take an M12 12.9 bolt, something like this:

http://forum.a8parts.co.uk/attachmen...1&d=1595236134

I'd have a plain one in the caliper, and the threaded one in the upright, both knurled slightly to grip the aluminium and not rotate, then they'd hold everything in place nicely. I did this many years ago to get Nissan Skyline 4-pots caliper on my Corrado :) However with this setup, its a compromise between using M12 12.9 hardware vs OE M14 10.9 hardware or removing sufficient material from the caliper to get M14 hardware in without losing strength.

Agree with all of this. +++

Lots of great information. Thanks for sharing.

I've gave it some thought and decided to go with C5 RS6 front caliper and discs since it'll be easier for me to mount the calipers. And for the rears I'm going to go with C5 RS6 rear brakes instead of trying to fit 4 piston calipers from a Boxster and custom install a hydraulic e-brake or try to custom install an additional mechanical parking brake setup.

This upgrade is something that I will eventually do and as of right now it looks like I won't be able to get around to piecing it together till next year. So I'm just looking to get as much information about it now.

The only issues you will have with the C5 RS6 rears is the Handbrake cable. You will need the cables from a LWB D2 A8. They are pretty rare. Maybe Audi heritage?

Good to know. Thanks for the info +++

Anyone have the part number for LWB D2 handbrake cables?

Does the LWB VW Phaeton have a mechanical e-brake or is it electrical?

What about the very early model year D3 LWB e-brake?

4D0609721N (left) and 4D0609722M (right).

Thanks. Good thing I'm not in a hurry on this and have some time to find them.

Another question. Why are the C5 RS6 discs so expensive as they are not 2 piece rotors so why the high price?

They are 2-piece rotors, just staked together rather than bolted like the RS4.

Weights from memory:

RS4 - 5.5Kg
RS6 - 6.5Kg
A8 - 14Kg!!

Interesting.

Those weights are for the front rotors? Either way that's a huge weight savings between the A8 and RS6 rotors. The B7 RS4 rotors have a very low rotor hat offset vs. the C5 RS6 rotors correct? So the RS6 rotors will be easier to deal with then the B7 RS4 rotors?

Is there a difference between the front B7 RS4 calipers and the front C5 RS6 calipers?

Yes, the B7 rotors are almost flat, whereas the C5 has about 6mm more offset iirc. That's all down to the C5 RS6 having more offset on the hub as it uses D2 wheel hubs (which sit 6mm further out than B5 / C5 A models, so the disc is different to accommodate that. The B7 I suspect uses the same upright geometry but the A wheel hubs.

I haven't had the two calipers side by side to compare. The part numbers are very close, and from pictures they appear to be the same.

The conversion I did involved removing the 90 degree adapter plate which comes stock on the RS calipers, and then fabricating the flat plate to maintain the radial mounting. I suspect it may be possible to retain the adapter and use A8 uprights with axial mounting, although it may need some spacers and / or A wheel hubs. I was to find that out this year until corona messed everything up.

Referring back to earlier comments does the increased mass of the A8 versions improve heat dissipation so reduce brake fade and other issues?

No, bigger mass can absorb more energy so takes more time/longer braking to heat up by the same amount of degrees but it also takes longer to dissipate it as it holds more energy, it may be better for single braking episode but not for several successive occasions like track use. Dissipation is more related to airflow and surface area.

Thanks for all the great info everyone +++

This is my dumb question of the day. I'm guessing the C5 RS6 calipers are made from aluminum?

Yes. All the Brembo multi-piston calipers are aluminium. The D2 S8 and RS versions are split calipers meaning they are two halves bolted together. The Porsche 17z and 18z are monoblocks so a single casting (hence a lot lighter but also more expensive to make).

Many of the Audi sliding calipers are a combination of aluminium and steel. The HP2 on later D2s for example has an aluminium piston housing but steel slider and pad carrier. I think the D3 360s are mostly aluminium too.