The Great 987 Cayman Master Cylinder Debate Thread.

[Slippydiff]

A much (some might say far too much) debated topic round these parts ...

So for those that care about these things, does the fitment of a 997 GT3 master cylinder to a 987 Cayman fix the problem of the long brake pedal and in the process transform the brake pedal travel/feel and its "end point"?

If you think it does, please elucidate your thoughts further, here.

For those that don't care, please feel free to enjoy browsing all the other threads in the PH Porsche section, unhindered by this particular topic :

As the great Bamber Gascoigne would say "Here's your starter for ten" Please confer as much as you want to.... :

anonymous said:

[redacted]

[boxsey]

Porsche911R said:

boxsey said:

D, you really do need to stop telling us that the MC makes no difference to the feel. And you know I am saying this in a friendly way. With new discs, RS29s and new fluid (I know how to bleed it properly), the pedal was just too long within a couple of hot laps. It felt like I was burying my foot into the carpet on the big stops. I should be doing that with the throttle pedal, not the brake pedal! Horrible and scary at times feeling. The bigger MC has completely changed the feel. Everyone else that tracks their R and has fitted the bigger MC says the same. Kevin will be changing his soon too but because despite new discs, pads and fluid at our last Oulton day he twice had to take to the escape road (at the bottom of hill top) because he lost confidence in the brakes.

It's not that simple, we can have a chat over a cup of coffee at Donnington at some point ;-)

The Cayman brakes are a catch 22 issue, based around 2 bigger issues, "heat" and this poor "end point" feel

For your and Kevins pedal to go long after a few hot laps, that is a heat issue , you do agree ?

This brake issue has been going on for 8 years in this platform, if it were as easy as a MC fix then all would have been done and dusted 7 years a go.
But alas no ones solved it. most gave up in the USA and bought GT4's, GT3's etc as not many tracked R and Spyder back then.

but if you want me to just say yes a MC feels different then yes it does of course.

I phrased that poorly, so need to try again. With the standard MC and brakes in good condition the pedal was immediately too long (for my comfort) in the hard braking zones. i.e. it goes further than I like or would prefer it to. It's not traveling further because of heat at that stage in the session. It's simply going too far for my liking when I'm using the brakes hard. Of course as the session progresses the brakes get hotter and the pedal will travel a little bit further but that's also true of the GT3 MC because after all, the brakes themselves are performing and heating up exactly the same regardless of which MC is fitted.

The simple reason I prefer the GT3 MC over the standard one is that my foot doesn't travel as far when braking hard. In the hardest braking zones, it feels like my foot is almost to the floor with the standard MC but it feels like I'm about a couple of inches away from the floor with the GT3 one. None of this has been measured....it's just a feel factor. For me the brake feedback is better with the GT3 MC when using the car on track. And that's all I want from it.

[bcr5784]

At the risk of teaching grandma to suck eggs, it may be helpful to show a typical brake system so that we are not always talking in the abstract.

This is only a single brake example - and obviously in the real world there would be (at least) 4 slave cylinders - not necessary all of the same size.

On any practical brake system you couldn't exert enough on the discs by hand/foot directly (even on a push bike!) - so you need increase the pressure either by mechanical levers or hydraulic means.

In the diagram you will see that in this case there is a mechanical advantage of 4 at the brake pedal itself - the pressure on the master cylinder is 4 times that on the pedal (=4x/x = 4) So the pedal will travel 4 times as far as the master cylinder rod, but extert 4 times the pressure.

In addition the master cylinder in this case is only a 1/3 the diameter of the slave cylinder so the pressure at the wheel cylinder is 9 times (3 squared) that exerted at the master cylinder. So the pressure (for this single brake example) exerted on the disc will be 4 x 9 =36 times that at the pedal and the travel at the pedal will be 36 times that at the wheel cylinder.

NOW if you change the master cylinder for one of a larger diameter the hydraulic multiplier will change from 9 to something less. If for example the master cylinder became half the diameter of the slave the hydraulic multiplier would fall from 9 to 4. In that case the pressure at the wheel cylinder would only be 4 X 4 = 16 times that at the pedal AND the pedal would only move 16 times as far as the wheel cylinder.

Hence if you make the master cylinder larger you will make the brakes heavier and the pedal travel shorter.

When it comes to "sponge" in the system by making the master cylinder larger (and hence pressure on the pedal greater) you may increase flex in the pedal and pedal box proportionately too. Whether that is detectable depends on the rigidity of the pedal/pedal box and any linkages.

One of the other sources of sponge in the system are the flexible hoses joining the (rigid) brake pipes to the wheels and it's often the case that going for better hoses reduces sponge in the system and hence brake travel.

I hope that helps and is non-controversial.

Edited by bcr5784 on Wednesday 14th June 19:47

[ras62]

Small point but it's not the size of the master and slave cylinders but their respective surface area that determines the ratio.

http://www.964uk.com/Documents/Pano%20Brake%20Arti...

[lowndes]

I'm afraid I can't add anything with regards to the idiosyncrasies of Cayman master cylinders.

However as one who is enjoying reading the debate, I think some of the arguments would be easier to follow if posters differentiated between force and pressure. When people write about the pressure on a brake pedal I suspect they are referring to the muscular effort exerted (force) rather than pressure pre se. As others have pointed out the pressure in the system is a function of the force exerted divided by the area over which it acts.

To that extent therefore the larger the diameter of the master cylinder the larger will be the muscular force needed to deliver a given pressure. The corollary being that as a larger piston displaces more fluid than a smaller one the pedal movement needed to bring the pads to bear is less.

Whether that translates into a harder feel I can’t comment but if for given retardation you have to exert a greater force with less pedal movement that might well feel like a hard brake pedal.

[bcr5784]

ras62 said:

Small point but it's not the size of the master and slave cylinders but their respective surface area that determines the ratio.

http://www.964uk.com/Documents/Pano%20Brake%20Arti...

Yes - that's why the hydraulic (dis)advantage is 9:1 (3 squared) rather than 3:1 in this example.

[petej]

I must be lucky.. 987 .1 and the brake action, feel, modulation and stopping ability is brilliant. When I first drove the car my first thought was the car had a dead pedal, but once I weaned myself of off the familiarity of the high servo assistance of my other and previous cars, I realised just how great they are.

[bcr5784]

lowndes said:

I'm afraid I can't add anything with regards to the idiosyncrasies of Cayman master cylinders.

However as one who is enjoying reading the debate, I think some of the arguments would be easier to follow if posters differentiated between force and pressure. When people write about the pressure on a brake pedal I suspect they are referring to the muscular effort exerted (force) rather than pressure pre se. As others have pointed out the pressure in the system is a function of the force exerted divided by the area over which it acts.

To that extent therefore the larger the diameter of the master cylinder the larger will be the muscular force needed to deliver a given pressure. The corollary being that as a larger piston displaces more fluid than a smaller one the pedal movement needed to bring the pads to bear is less.

Whether that translates into a harder feel I can’t comment but if for given retardation you have to exert a greater force with less pedal movement that might well feel like a hard brake pedal.

I thought I was clear, but in case not - with a larger master cylinder you will have to exert more pressure - and have less travel - for the same retardation, which will be (what I would regard) as a harder pedal. I'm trying to make it clear that force and (total!) pressure (not pressure per unit area!) ARE the same (value) at the each point - the pedal, the master cylinder and slave cylinder, though they obviously differ between points - pedal, master cylinder and slave cylinder. Apologies to mathematicians (including me!) - I'm trying to keep it simple, and inevitably you make things a bit fuzzy in doing so.

I could go into the effects of multiple, differently sized slave cylinders - but I think that would be VERY confusing and unhelpful.

Edited by bcr5784 on Wednesday 14th June 19:59

Edited by bcr5784 on Thursday 15th June 08:37

[Pterri]

So what do we all think about BREXIT? *hides*