Re: Master Cylinder exchange question (MDVDuber)
Ok, but why then does VW include a larger MC with the rear disc option? Is it only to retain pedal feel? Or is there a safety issue?
First, let's make the incorrect assumption that all wheel cylinders are the same diameter. In this situation, a larger diameter master cylinder would result in better control over the brakes. Yes, this comes at the cost of more pedal force, but the window of when you lock up the wheels gets bigger making it easier for you to closer to that point with more control.
Now, lets correct our original assumption. Rear drums have smaller wheel cylinders than rear disks. Well, let’s ignore the mechanical difference between drums and disks for a moment, and just run the math to see what happens from the master cylinder to the slave cylinders.
Let’s say:
20 lbs foot force + 22 mm master cylinder = 33.9 psi in the brake lines.
Drum slave cylinder = 1” (I don’t know, I’m just using a number) = .785 in^2
Disk slave cylinder = 1.5” (again, I don’t know, I just know it’s bigger) = 1.767 in^2
(33.9 lb/in^2) / (.785 in^2) = 42.18 lbs force (drum)
(33.9 lb/in^2) / (1.767 in^2) = 19.185 lbs force (disk)
As you can see, the disk would require a good bit more foot pressure to have the same force on the brakes. Sounds like you would want the drums, with the small master cylinder doesn’t it? And if you were looking to simply stop the wheels from rotating, you would be right. But braking is A LOT more complex than simply stopping the wheels from rotating. We want to stop the wheels from rotating as fast as possible, WITHOUT locking them up, while the level of force required to lock them up continually changes. And drums, with a small cylinder would give you so much power, it would be hard to approach the limit of lock up, while that limit was continually changing, without actually locking up.
This website: http://scirocco.dyndns.org/faq....html
Indicates that the larger MC is for additional Fluid Volume.......
And clearly, they have a point. But let’s put a value on it. (I haven’t done this yet, so this will be fun) Let’s assume:
4 rotors (all the same) (yeah, I know that’s not the deal, but let’s pretend it is)
Slave cylinder diameter = 1.5”
Master cylinder diameter = .886”
Now what we need to know is how far we need to close a caliper to stop a wheel. As we all know, brakes are never truly “off” the rotor. When we take our foot off the brake, the system relaxes, and the pads back off the rotor as far as they are “forced” back by the working of the system. So, let’s put a value on the distance of movement. Well, the caliper has some play, and flex in it, let’s call this value .030” (at best, that’s an educated guess, no more) now we have compression of the brake pads, and the rotor itself. Let’s add .005” for that. (Again, I don’t know these values, but I feel like they are realistic) Now, let’s run the numbers.
(1.767 in^2) * (.0035 in) = .062 in^3 (this is the volume of fluid necessary to close one caliber enough to stop the wheel)
(.062 in^3) * ( 4 wheels) = .247 in^3
Now, let’s see how much travel in the master cylinder that would require:
(.247 in^3) / (.886 in) = .279 inches, or just over a quarter of an inch. That seems reasonable.
Now, let’s do it for an 18 mm master cylinder
(.247 in^3) / (.709) = .349 inches. Or about 40% more travel.
Interesting, I wouldn’t have considered that a real player, but it’s clear that it’s something that must be considered.
Now, there’s another factor we haven’t considered yet, the brake booster. And since I don’t know any of the numbers on this guy, I’m not really qualified to discuss it in any kind of interesting detail. But I do feel it’s a component that must be considered in this conversation.
So, where did we get with all of this?
- We know that a smaller master cylinder will increase brake line pressure.
- We know that a smaller master cylinder could have a problem moving enough fluid to operate the brakes effectively.
- We know that a larger master cylinder allows for finer brake control, but at the cost of more foot pressure.
- We know that I don’t know much about brake boosters
Suffice it to say, if your car stops hard, why would you want to fix it?
I did an A3 GTi front brake swap onto my A2. As part of it, I put in a 22 mm master cylinder out of a 93 Passat and steel braided brake lines. When I was all done, I couldn’t believe how soft the brakes were. I pulled out the 22 mm master cylinder and put my old 20 mm master cylinder back in. Although I was happy with the fronts, I noticed that I my rears were locking up before the fronts. Before I went to rear disks I thought I would run an experiment. My thinking was that because my car was lowered, and running far stiffer springs, that my rear brake proportioning valve might not be functioning as the rear simply wasn’t lifting enough to actuate it. For $10 I bought a rear brake proportioning valve out of a Passat. My thinking was that with the longer wheelbase, that car wouldn’t have the kind of rear end lift that my Jetta came with stock. Now my car stops quite impressively with a pedal feel that I find confidence inspiring. The rears will lock up, but only after the front has already done so. In fact, I have almost all the “stuff” to do a rear disk conversion, but I’m not going to put it in, as I don’t appear to be having a braking problem, not even on the track.
