Non-Boosted Braking; 6 X 13 wheels

[DonPeaslee]

Our hillclimb Rabbit has a braking system without a booster, as it was removed some years ago to accomodate the linkage for a custom pedestal gearshift arrangement. At that time a 17mm master cylinder from a very early Rabbit (unboosted as stock, I believe-?) was installed. This provided adequate braking, but with a very mushy feel. (Soft pedals in Rabbits appeared to be more usual than not when I was racing a GTI in ITB some years ago.) We just finished the installation of a new 21mm master cylinder to try to get a firmer pedal. That goal has been resoundingly met, but I can no longer lock the brakes, which is not acceptable. My question to the forum is: are there master cylinders of, say, 18-19mm available for or adaptable to Rabbits? We have rear disc brakes from a later model; they are fed through individual lines with a master cylinder mounted VW proportioning valve for each line. The cylinder has an outlet hole for each wheel.

My calculations indicate that the 21mm has about 60% of the mechanical advanatage of the 17mm. My guess is that halving that reduction to c. 80% could result in a good stopping, firm pedalled car.


6 X 13 (or 6.5 X 13) wheels, with stock offset. I'm told that VW did not make them. Did anyone? We're in the market, particularly used.

Thanks-

Don Peaslee

[Bill Miller]

Our hillclimb Rabbit has a braking system without a booster, as it was removed some years ago to accomodate the linkage for a custom pedestal gearshift arrangement. At that time a 17mm master cylinder from a very early Rabbit (unboosted as stock, I believe-?) was installed. This provided adequate braking, but with a very mushy feel. (Soft pedals in Rabbits appeared to be more usual than not when I was racing a GTI in ITB some years ago.) We just finished the installation of a new 21mm master cylinder to try to get a firmer pedal. That goal has been resoundingly met, but I can no longer lock the brakes, which is not acceptable. My question to the forum is: are there master cylinders of, say, 18-19mm available for or adaptable to Rabbits? We have rear disc brakes from a later model; they are fed through individual lines with a master cylinder mounted VW proportioning valve for each line. The cylinder has an outlet hole for each wheel.

My calculations indicate that the 21mm has about 60% of the mechanical advanatage of the 17mm. My guess is that halving that reduction to c. 80% could result in a good stopping, firm pedalled car.
6 X 13 (or 6.5 X 13) wheels, with stock offset. I'm told that VW did not make them. Did anyone? We're in the market, particularly used.

Thanks-

Don Peaslee
[/b]


Don,

I'm a bit confused. You put rear discs (I assume off a 16v car) on your Rabbit. You also used the 16v (21mm) master cylinder, and the associated prop. valve (I assume you used the larger prop valve). While you may have increased pedal effort, you have the large MC, so you should still be able to lock the brakes. I assume you've put on stainless brake lines.

Something you may want to look at are your calipers. You may have issues w/ the seals leaking, and not applying the full clamping force. That's probably where I'd look first.

As far as 13x6 (actually, anything wider than 13x5.5) go, nothing OEM from VWoA (might be some from Europe, but I've never heard of any in over 20 years in the VW watercooled scene). There were several choices back in the 80's (BBS, Rial, Enkei, MSW, Ronal, ATS, etc.), but nothing that's probably less than 10 years old. You can get race wheels like Panasport, Joengblood, etc. made, but they're pretty pricey. An alternative are steel wheels from Diamond. You can get wider widths too. The price you pay, is that they're not light. Not only that, you need what's known as a torque plate for VW hubs.

[spiro13]

Don

You might want to check the brake pedal rod. The length is different on a brake booster car than on a car without a booster. If you don't change the rod when you take the booster off you have issues resulting in what you are describing.

Tony

[Joe Camilleri]

There is a 20mm master and I think that i have seen an 18mm, but you may have to lengthen your pedal to get the leverage that you will need to lock them up without the booster.

Aren't those residual pressure valves mounted on the master, designed to keep the shoes out a bit to reduce the pedal travel?

These may help to figure it out. http://www.tiltonracing.com/content.php?page=faq&view=9

http://www.tiltonracing.com/images/pedalrad.gif

[DonPeaslee]

Bill Miller: •previous brakes were adequate with 17mm cylinder, but mushy.
•original car builder concerned that new cylinder is too big. He's right, I think. I can't lock the wheels at any speed.
•reminder: NO BOOSTER. And no place to mount one.

•I found some 13 X 6's! BBS, I believe, from an '81 Scirocco. Their offset is only 1/4" less than an an OEM 13 X 5 1/2.

Tony: investigating your rod length comment.

Joe: residual pressure valves, rather than proportioning valves- ? Looking into it.


Everybody: THANKS for you suggestions. Will let you know how it all plays out.

Don Peaslee

[msogren]

Stock, alloy, BMW 320I wheels, are pretty nice but stick the wheel out more( less back space) . We have used them on many Dubs. They are usually 13x5.5. Some 320IS wheels are ATS 6x13. MM
The straight accumulators , on the MC are to keep the shoes tighter. They suck up pressure and drag the rear pads. MM

[67ITB]

Hi Don,
I here you have Bill R's old car. that thing was wild!!!
Its a rabbit. it will never have good pedal feel

I think you had been talking to Eli about some ronals. Those are some very nice wheels he has.

Matt Bal

[Bill Miller]

Don,

Those BBS wheels were a popular swap for the early VWs, and I think some dealers even sold them as a dealer-installed option. I believe the model is the RS. The FWD offset is ET33, while the RWD (common on BMW 320i, etc.) is ET13. If you go w/ the ET13 wheels, it will increase your track a bit, but you may have fender clearence issues. I had a set if the ET13 wheels that I sold to someone, and I have 2 sets of the ET33 wheels that I might be interested in parting with.

[orlando_wrx]

and if all else fails downsize brake plumbing to 2an...mucho mas pressureo :D

[racer14itc]

and if all else fails downsize brake plumbing to 2an...mucho mas pressureo :D
[/b]

Um, no. But I'm sure you're just kidding, right?

MC :024:

[Eric Parham]

Many (but not all) of VW's rear disk cars used individual pressure limiting valves in each rear brake line. They may look the same as the residual pressure valves used on drum brake cars, but they are internally and functionally different. Check part numbers to be sure. They were used instead of a proportioning valve. If you add a proportioning valve, they should probably be removed.

[Joe Camilleri]

Does anyone have the correct part numbers? I have a master from an '85 8V Scirocco that had drums and two valves on the rear brake lines at the master. I just want to be sure that they are residual pressure valves.

[msogren]

The factory book say that they are proprtioning vavles. inline, inthe rear axle area.
The books do not say anything about any residual pressure valves, under the car, only some mounted on the M/C. drum cars only.
Most likly you have 2# pressure keepers
I take these off and run only a knob style reducer. But the knob is always full rear on..

Some drum cars had two, individual valves, some wagon style cars and German M2 , had the 4tube , spring/ lever style proprtioning valves, just like the 16v disc cars.
Just blow thru the questioned piece, maybe with a syringe.
MM

[Bildon]

>> While you may have increased pedal effort, you have the large MC, so you should still be able to lock the brakes.

Nope, The larger the MC dia.. the less pedal effort required but the lower the pressure will be. So it makes sense you are no longer able to lock the brakes.

[racer14itc]

>> While you may have increased pedal effort, you have the large MC, so you should still be able to lock the brakes.

Nope, The larger the MC dia.. the less pedal effort required but the lower the pressure will be. So it makes sense you are no longer able to lock the brakes.
[/b]

Are SURE about that, Bill? :unsure: P(ressure) =F(orce)/A(rea). If you want a desired Pressure at the Caliper (to provide a force at the brake pad which provides the force at the disc which provides the torque that decelerates the wheel/tire), if you increase the area (larger MC) then you have to apply a larger Force at the pedal. Right?

I had to go to smaller MC's when I started getting Charley Horses trying to stop my GP car. This decreased the force required to lock up the wheels, but increased the pedal travel a little bit.

MC

[Bildon]

Oh crap... typo, should be: a bigger master cylinder means higher pedal effort but the lower the pressure will be.
I think I'm somewhat dislexic .. I read that 3x and my brain was saying one thing while the text said another. :(

OK Engineering 101 time....My classes at PSU were SO long ago...but I still have notebooks of stuff since I related everything then to race cars. I started to question my memory so I dug out my old "race bible" notebook.

p=f/a that's tue.
But are we talking about the Master Cylinder area or the Caliper Piston area?

Lets start with the Master Cylinder.

For example lets say you're thinking of going from a 17mm rabbit MC to a 21mm Golf MC.
However to make the math easier I'm going to cheat and use 7/8" and 1-1/16 MC diameters. These #s do NOT correspond to 17mm and 21mm, but they will suffuce.

The 7/8" MC piston had a surface area of 0.60 square inches (pi*r^2). Pushing on that piston with 100 lbs would generate 100/0.60, or 166.67 lbs per square inch of pressure. p-f/a

The 1 1/16 master cylinder (0.089 square inches) will generate 100/0.89, or 112.36 psi.

So again, a bigger master cylinder means less pressure, and therefore higher pedal effort.

Keep in mind that a bigger MC piston pushes more fluid down the lines, so the pedal won't have to move as far to move the calipers the same amount. A bigger master cylinder means less pedal movement.

Brake Calipers
Now here is where the confusion may be. At the other end of the brake lines we have brake calipers, with a piston in them. I'm not even going to go into the math for the caliper as sliding calipers (VW) act differently than a fixed piston. (basically 2x piston working area)

OK so to make the example real easy: Assume a total caliper piston surface area of 7 square inches. 112.36 psi (from above) x 7 square inches = 786.5 lbs of clamping force on the rotor.
Bigger caliper pistons here would mean even more force, and at the same time, more fluid would be needed to move the pistons.

In summary:
Bigger Master Cylinder dia. means less pedal travel, higher effort, lower line pressure
Smaller Master Cylinder dia. means more pedal travel (spongy), less effort, higer line pressure

Bigger brake caliper pistons means more force and, therefore, less pedal effort, & more pedal movement.

So to increase brake pressure:
Increase rear wheel cylinder dia.
Decrease MC dia.
Increase front caliper bores.

REMEMBER! The amount of fluid displaced is not changed by changing your Master Cylinder. It will displace the same volume in the caliper pistons and rear wheel cylinders.

I found this in my old notes too:
MC diameters:
17mm 2 kinds Rabbit w/o servo Early
20mm 5 kinds, servo Rabbit '80 ATE
21mm w/Bendix servo '79-80
20.6mm 84-> Rabbit, Golf (GTI)
22mm ?
22.2 ?
23mm ?

[DonPeaslee]

Thank you all for your continued interest in my stopping issue.

Bill and Mark, the master cylinder we're using is for a "16 valve" , supposedly introduced when the rear discs were first used. We THINK (for some reason) that it's 21MM, but we don't know it. Bill, your info on MC sizes was much appreciated. Has anyone ever heard of an 18 or 19 MM unit? If ours is in fact 21MM I'm concerned that a stepdown to 20MM won't be a big enough increase in pressure to clear up the problem. We are trying to find someone with the original detail data (such as a dealer might have had) to try and assure ourselves of what we've got. Or pull it, open it up, and measure! (Sigh!).

Mark, your comment about coming down on size was noted. Do you recall what sizes were involved? Were you boosted? Stock or other caliper bores-?

One other possibility is to try some very high initial friction pads and hope that they'll last for a climb before fading to hell.

Any further ideas on possible pads, master cylinder sizes, years, part numbers-- or large bore calipers for stock vented '84 GTI discs?

Bill, we met at Lime Rock 2000/01, I think? Again, thanks to all.

Don Peaslee (ex-Bill Rutan Rabbit hillclimb/autocross car)

[Bildon]

Are you running a bias knob to the rears to cut down on the rear pressure? Depending on how you have your lines run from the master cyl. this can help a LOT to bring the pedal up higher and give you the firm feeling you want. Depending on how much weight you have in the rear you may consider shutting the rear brakes off entirely. A rally type hydraulic cut off lever is good for this.

The change from 21 to 20 will be noticable. 20mm MCs are dirt cheap rebuilt. I'd try that for sure.

Also you mentioned high initial bite pads. I'm going to assume in hill climbs that it takes 1/2 the run to get the brakes up to temp at all. Many pad compounds say "high inital bite" yet still require you to get them very hot first. Initial bite does not equate to cold bite. It's referring to inital bite at working temperature.

some things to chew on....