Rear Disk Brakes Pro & Con

[Piledriver]

My (long) weekend project

front-dualcircuit.jpg

T3-rearaudicalipers.jpg

Those are 14mm 1/2" drive sockets, perfect snug fit for the parking brake cables, and cable length works out.
Dual circuited the existing BMW 2002 4 pots and added Audi cabriolet (97) 38mm rear calipers and 914 rotors.
(pulled rotors for clarity, they just clear the calipers for removal)
I secured the hoses/cables and cleaned up the welds some after this pic.

Kinda spongy feeling with the 16mm master and the split layout (and ~3X the rubber line, std T3 front hoses, Mk4 Jetta front hoses on rear, may have to run another brakeline to the rear and split it like the Swedish 4 pot front T3 setup.
Right now the std front circuit only runs tge 2 top ~30mm pistons, and the rear runs the other 2 and the back.

I may also need to try bleeding again, I have a mityvac but it seems pretty useless in suck mode.

(EDIT-Fixed that by cutting an old brake line, that bleeder is 10x1, yanked it, put the cut brake line in the bleeder hole, snugged down and sucked on THAT with the Mityvac, no leaks.
That high spot in the rear caliper hose traps a LOT of air, pedal/brake balance is ~good now, but may still split rear circuit to even things out)

Is the volume the same on the F&R circuits of the master cylinders?

[VW&MGman]

Hi Everyone,

I'm thinking of converting to rear disks on my '69 road race Beetle, but would like to get your thoughts on a couple of my concerns. My current plan is to purchase a new 20.6 mm master cylinder and the CB performance disc brake with handbrake kit.

http://www.cbperformance.com/ProductDet ... tCode=4631

Here are my questions:

1) Should I be concerned with rear wheel lock up with disc brakes?

2) For those that are racing there VWs, what rear pads are you using?

3) Would this kit change the rear 'track' or the car

4) Is it actually a 'bolt-on' item, or will it require any machining (I don't always believe advertisements )


Thanks in advance

[Bruce2]

My current plan is to purchase a new 20.6 mm master cylinder .............

Why do you think you need a larger MC?
What front brakes do you have?

[VW&MGman]

I currently have the original master cylinder installed. I was going to install the larger MC based on this write up and the experience of others.

In terms of the front brakes, they are the VW air cooled discs with Varga calipers and performance pads.

[Bruce2]

You don't need a larger MC. The only thing it will do for you is increase pedal effort.

[Piledriver]

I'm running comparatively huge calipers on my Squareback.(vs the front "Ghia" style Vargas)
I suspect the CB rears are similarly sized.

On my setup is seems very well balanced wet or dry, but the CB setup is a tad more rear biased due to the tiny front calipers, no experience with CB setup (rears are about same as mine IIRC)

Once I finally picked up speed bleeders and got all the air out of the system I have a high hard pedal and as much stop a the tires can handle, almost at will, with great feel.

Stock 19mm master cylinder.

I'm looking at doing vented Wilwood rotors up front, as I may track the car occasionally in the future, but the rotor upgrade would be for cooling and reduced unsprung weight (and the ability to replace the rotors for $35), not for added braking power, that's not needed IMHO...

OTOH the slightly larger master won't make that much of a difference, probably more a matter of taste.
Try the 19mm first.

[aircooledtechguy]

You don't need a larger MC. The only thing it will do for you is increase pedal effort.

I agree.

When you step up a size in the bore you move more fluid for a given distance. This means a loss of leverage at the pedal or a harder pedal feel. It's NOT always needed.

I'm running Porsche 911 "M" calipers front and standard 944 rears on my type3 with the factory T3 disc/drum MC (I don't remember the size). Works fantastic; I love the feel and control. I get compliments from everyone who drives it on how it stops and how the pedal feels. It's firm, but it's easy to modulate under hard driving. Just a plain hard pedal makes it more difficult to modulate the brakes on the edge of lock-up without locking them up. WAY better than my factory 914 brakes (those are a joke compared to these).

I to use speed bleeders and have no problem getting all the air out the first time, which I think is why many think they need to go bigger on the MC. What a great product!!

[Piledriver]

The only thing not "perfect" about speed bleeders is the dry thread sealant they use doesn't seem to be brake fluid proof. (at least not the Dorman ones I bought)

One could probably wrap some Teflon tape around the threaded area instead as long as there was zero chance it could end up in the plumbing.
Some sort of nylon or teflon solid collar that would self thread and act as a thread locker/sealant would probably be more ideal.

This might be worth it if you track your car and replace your brake fluid ~every weekend, rather than annually.
They make it an easy one man job.

[4agedub]

I have not thoroughly read through the entire thread, but my experience on our race cars are:

Rear disc brakes is a must

Only problem we have found is that you cannot adjust the brake bias to add more braking in the rear than in the front. With the brake adjustment taps you can only remove braking force in the rear. With the stock single cylinder master and the tandem dual master you will have this problem.

Thus with our new car I'm investing in a pedal box with separate master cylinders for the front and rear so that you can go past 50/50 and maybe run 60% rear and 40% front.

With the stock style master cylinders I have never had lockup in the rear, except when you don't blip the throttle going down gears whilst slowing down

[VW&MGman]

Hi 4agedub,

what type of master cylinder (19 mm or 20.5mm), rear calipers and pads are you using?

[buildabiggerboxer]

I have not thoroughly read through the entire thread, but my experience on our race cars are:

Rear disc brakes is a must

Only problem we have found is that you cannot adjust the brake bias to add more braking in the rear than in the front. With the brake adjustment taps you can only remove braking force in the rear. With the stock single cylinder master and the tandem dual master you will have this problem.

Thus with our new car I'm investing in a pedal box with separate master cylinders for the front and rear so that you can go past 50/50 and maybe run 60% rear and 40% front.

With the stock style master cylinders I have never had lockup in the rear, except when you don't blip the throttle going down gears whilst slowing down

4agedub tells it like it is^^^^^^^^, but I'm now firmly in the KISS camp and enjoying racing without keep on playing with dual masters and such, mine are on the shelf now were they can grow old,,, but there is a way,,,, what I did was over brake the rear with big calipers, then you can shift bias to the front with an adjuster valve, I have brakes to die for wet or dry, in the rain, I pull back 3 clicks and go, I use 20 mm golf m/cylinder to feed Tarox 6's on front, and Mk 2 Golf Gti fronts on the rear, they have huge pad area and wear is not a factor.
I'm done with complex cars, I'm done with testing, parts left out don't go wrong, cost nothing and weigh nothing. I'm just home from good wood where the hill record has just been shattered by the simplest car ever, it beat la ferrarris, veyrons, fi mclarens, ALL the new prototypes the big makers were hoping to win with pre production,,,,, not a hope, the NOBLE won because it's simple, no traction control, no anti lag, no ABS, nothing, there were some sick (sad) faces after the spanking, stop bolting crap on guys.

[Ol'fogasaurus]

deleted

[4agedub]

Hi 4agedub,

what type of master cylinder (19 mm or 20.5mm), rear calipers and pads are you using?

If have tried a single piston 19mm and 20.5mm before. The 19mm seemed to work better between those two. At this stage we are using a master cylinder from a VW Golf mk1. The golf mk1 master cylinder has tandem 20.46mm pistons



The rear calipers are from a VW Golf mk3 with Ferodo street pads and solid rotors. I'm not sure about the piston size on the caliper

[4agedub]

4agedub tells it like it is^^^^^^^^, but I'm now firmly in the KISS camp and enjoying racing without keep on playing with dual masters and such, mine are on the shelf now were they can grow old,,, but there is a way,,,, what I did was over brake the rear with big calipers, then you can shift bias to the front with an adjuster valve, I have brakes to die for wet or dry, in the rain, I pull back 3 clicks and go, I use 20 mm golf m/cylinder to feed Tarox 6's on front, and Mk 2 Golf Gti fronts on the rear, they have huge pad area and wear is not a factor.
I'm done with complex cars, I'm done with testing, parts left out don't go wrong, cost nothing and weigh nothing. I'm just home from good wood where the hill record has just been shattered by the simplest car ever, it beat la ferrarris, veyrons, fi mclarens, ALL the new prototypes the big makers were hoping to win with pre production,,,,, not a hope, the NOBLE won because it's simple, no traction control, no anti lag, no ABS, nothing, there were some sick (sad) faces after the spanking, stop bolting crap on guys.

Lol, I understand your point as well.
With our new car I'm going all out. I want the baddest bug around, with all the electronic aids we never had before. Most probably we'll even fit ABS adapted from a BMW e46. These ABS control units are self sustainable and only need power, ground and the wheel speed sensors.

[buildabiggerboxer]

They are some neat ABS units, please keep the build progress posted for us, and it's good we get all the varying inputs anyway, and the main thing with brakes is confidence with them, here's one for everyone, I had a Beetle cup customer come over to my shop for setup etc, I did lots of work on his car and put the watch on him at a class test day, car was right there at the sharp end of the class lap times, I asked how he got on with the brakes, they were 12/1300 DRUMS, the only car still like it, every one else had the stock discs allowed, I was keen to get him to swop to discs, but he said he was happy with the drums, so I went out in the car for giggles, well I was staggered, and to this day it still haunts me how good the brakes were, it was progressive, smooth and had better feel than my own disc car, long story short was we left the drums on, the car ran at the front for years and finished in the top 4 of the series over a 3 year life span, it was totalled in year 4 after being cannoned into the barriers flat in top gear, the driver was out cold for a bit , had me worried, but survived to race again, so stock jurid shoes, Brazilian cylinders front and rear, dot 4 fluid, I can't say why they were so good, just a little more work adjusting them lots.