Rear Disk Brakes Pro & Con

[Piledriver]

Pretty sure those MK4 rear calipers are 38mm.
(Identical set of the back of my square, they can even work with the std parking brake cables)

Later MK4s had them in aluminum.

[petew]

I've avoided rear discs as I've had really good experiences with type 3 rear drums. They do take a bit to get used to, but once you get used to bleeding and adjusting them, they work really, really well. This is as against the handbrake issues that come with rear discs. Earlier this year I did a skidpan autocross in a mates Lbug. Great car and I gave it a hiding, but the handbrake system screwed me up all day long... you guessed it, aftermarket discs. I was forever pulling the lever up too high and it was just nasty and weak.

Front drums... hmm, no thanks. I couldn't believe the difference when I went to discs up front, but the rear drums are pretty great.

[VW&MGman]

Hi Everyone,

Just an update on my rear disc brake install. I purchased the CB Performance rear disc kit, pn: 4631.

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

My fears came true! With some rough measuring, it seems that disc brake kit increases the rear track by 11mm per side. While this might be good to promote under-steer, but in my case the tires are going to rubber the fenders when the car is cornering under load. (The rear of my car is lowered by about 1", and I run 195/55/15 tires.)

Couple of questions:

1) Anyone else have this problem of rear tire rub? If so, how did you overcome this?

2) Are there other rear disc kits on the market that does not push the rear wheels out?

3) Are there 1'' wider fenders available in fiberglass? (I don't really want to go this route)

Any other thoughts?

Thanks in advance

[Steve Arndt]

I have CB discs front and rear. Roto hub rear. 2 psi residual pressure valve on the rear circuit. CBs 19mm master cylinder and direct mount reservoir. All stainless lines. Line loc on the front. 205/50R17 front and 215/55R17 rear tires. 2" narrow front beam link pin beam, dropped spindles.

The brakes work wonderful. Perfect balance. Hand brake takes 3 clicks like stock and it applies massive force. I was very skeptical of the parking brakes before installing. There were 10 or so different small areas that needed massaging, shimming, clearancing, etc. to make it all work but boy do they work.

[VW&MGman]

Hi Steve,

Thanks for your input.

Is the rear of your Beetle lowered? If so, by how much?

Do you know the off-set of your wheels?

[Steve Arndt]

The rear is stock height. I want to drop it about 3/4 inch just to close up the fender gap a touch.

Wheels are Porsche 986 fronts all four. 17x7 ET50

Fronts fit up perfect for clearance.
Rears I used a 22mm spacer, narrow early swing axle, to bring the wheel inside away from the shock tower castings. Still has a full hand thickness of clearance to the fender lip.

[H2OSB]

Pile--

What rear disc brake conversion bracket are you using? I am installing, essentially, Gerrelt's exact set up on the rear of my son's '74 super except with slightly turned down T3 hubs instead of turned down SB drums. I was about to pull the trigger on the brackets made by Sharp Built because I was unable to find caliper brackets I knew for sure would work, but they're about $175 before shipping, and I'd really like to spend less. Clearly, you have found the correct brackets. Yours LOOK like the common cast brackets available with many of the rear conversion kits, and I suspected those would work, with, perhaps a little "egging" of the holes, but I was unsure, and these brakes are going on my son's car so I don't want to take any chances.

Aircooledtechguy--

Your brakes are the exact set up I came up with for my '74 Super. Thanks to the similarity betwixt the the front brakes on the na 944 and the Super, I was able to mount the 944 spindle directly with the use of Kerscher 19mm ball joints (sure with I could find an equivalent locally). I was thrilled to discover the 911 M calipers bolts directly to the 944 spindle and is centered over the 944 disc. The 54mm piston caliper of the 944 is just a bit too much front bias for our light little rear-engined cars. The 48mm pistons on the M caliper make a much healthier 1.7:1 bias ratio. I'm very happy to hear the brakes feel well balanced.

johnL (aka H2OSB)
www.superbeetlesonly.com

[Piledriver]

I'm using a home-brewed welded on bracket and rear Passat calipers/brackets (same setup, actually from Audi Cabriolet) with the factory parking brake cables. calipers mildly modified using a 14mm 1/2" drive socket welded on as a retainer cup for the brake cable housing.

T3s have a lot more room in the fenders F&R in general.
I'm running 15x5.5 914 Mahles ET35, 205-60 Toyos. (came off my 914)
Car is lowered about an inch, pulled down by the 4ways preload, and the tires are about an inch shorter than stock.

I trimmed the rear bump stop metal bit for tire sidewall clearance.

If you have the typical T1 empi offset wheels, its often hard to fit any decent width tire as the offset is wrong for ~anything.
I don't know why they make them that way, they just are.

[VW&MGman]

Hi Everyone,

Should I be concerned with rear wheel lock up with this disc brake set up? The car is used in road racing only.
Here is what I have:

Master cylinder: Stock VW 19 mm
Front Calipers: Varga 40 mm pistons and discs, with Porterfield R4-S pads
Rears: CB performance calipers with handbrake with 33mm pistons, with their stock pads. (CB pn: 4631)



Thanks in advance

[Piledriver]

If it does have rear premature lockup issues, the EMPI proportioning valve works great-- installed backwards-- per FJCamper.
(The in/out ports are mislabeled)

[Bruce2]

We have to know your tire diameters first.

[VW&MGman]

The tires are, Toyo R888 195/55 R15. Diameter is 23.4 inches.

[Steve Arndt]

It should be pretty balanced. Mount up a go pro and make a few dozen stops (dry and in the rain).

[Ol'fogasaurus]

It should be pretty balanced. Mount up a go pro and make a few dozen stops (dry and in the rain).

What a great idea. Video/GoPro is one of those tools we have that we forget about as being a possible tool.

Years ago I had my oldest stepson take some video of my buggy going through a rough "Whoops" section to see just what my suspension and shocks were doing. It told me a lot and I made some changes. I then took a GoPro and mounted it to my cage and went through the same area. The GoPro has that image smoothing program which told me that I was riding fairly smooth when I was riding on top of the Whoops (not following the whoops contour like the first video did) but couldn't show me what the suspension itself was doing. I need to relocate the camera to take another video of the suspension itself in action or have a free camera, taken like the first video that would give me a wider view of what the suspension at both ends is doing; working or not working together.

We have talked about the cotton tuffs and videoing the flow of air over the car so see how the air flow was especially when putting a radiator in the rear. If you were to put scoops in the fenders or body (for example) then do the yarn trick then video how the flow to the brakes was working.

What about videoing how your inside controls sit as you drive; e.g., shifter, steering, pedals, gauges, switches, etc.. Do you have to reach, take your eyes off the road to look for something or what... things you might do without realizing that you are doing them; things that could be improved on.

Imagine how good of a tool, with a little imagination, you actually have there.

Lee

[FJCamper]

The 70% Solution
Just because 70% of braking effort is generated at the front wheels does not mean 70% of the hydraulic effort is diverted to the front wheels.

Hi Jason,

Our Blitzwagen started it's competition career with a brake setup very similar to yours now, ATE/VARGA type dual-40mm front calipers and aftermarket rear calipers with pads twice the size of the ATE's, all of this fed by a common 19mm dual circuit master cylinder.

The Blitzwagen had no brake vices, even with the 205 front and 225 rear tires. With absolute novices flailing it around the track.

When we changed to dual-40mm ATE front and rear, we had no lockup problems. I thought maybe the stock m/c had some kind of built-in brake proportioning, but during our first ill-fated experimentation with Wilwood clones (quad 40mm pistons per caliper), in which neither 19mm or 20.6mm m/c's would fully operate the clones, I got to measure brake pressure at the rear ATE calipers under a variety of configurations, and it was always 1500 psi or better. If we had ATE's front and rear, all got about 1500.

That means the m/c was sending all the psi it could to the rear, no proportioning involved. And that told me it was the increased brake load at the front that "balanced" the system. In other words, if while braking somehow magically we had an equal load on all four wheels (+ equal friction and adhesion) that the fronts would lock first, exactly what a stock system tends to do.

Of course, anybody can lock up brakes, race car or street car. Your educated foot has to modulate pedal pressure.

Our worry, like yours, was quad 37mm pistons up front (on pads double ATE size) was going to increase mechanical advantage to the point the Blitzwagen was going to pole vault over its own front bumper the first time someone forgot and really put his foot in it.

Almost happened, too.

We had to install a brake proportioning valve on the front wheel circuit only, and within three brake-it-to-limit test slides, got it adjusted just right.

My educated guess is you won't need a rear proportioning valve, and you can run stock rear pads.

Of course, you could prove us wrong by crashing through a wall and be killed, in which case we must rescind your still-open ride reservation.

FJC