limited prep HP rear disc brakes

[evanwebb]

Hi, this is a bit OT for the IT website, but maybe someone can help me out. I have a limited prep H Production (1976 A1) Scirocco and I'd like to convert over to rear disc brakes. What is the easiest/best rear disc brake conversion to do? I guess I need a complete rear spindle setup from some slightly later A1 car that has the rear discs and it will bolt right up? Which ones will clear the 13" x 6" wheels? Will the stock master cylinder for the drum brake car work OK with the disc brakes or do I need to change that too? I'm currently using a Wilwood proportioning valve in the rear circuit so I can control the pressure. What pads would be appropriate? A street pad like Hawk HPS or Hawk Black or a Porterfield R-4S? I'm using the Porterfield R-4 compound in the front. Thanks in advance!

[Bill Miller]

Evan,

The easy swap is to get the rear axle, master cylinder, and proportioning valve from a 16v Scirocco. It's a straight bolt-up.

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MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608

[dave parker]

Evan
Use the rear disc brakes from an A2 golf gti.
Get the whole rear axle assembly and you will have everything you will need.
The wheels should bolt right up(i ran 13x6" wheels on my ITB golf gti) with no trouble.
The big question now will be does the early A1 master cylinder have enough capacity to deal with rear disc brakes. That is the 10k question.
To the best of my knowledge no A1 VW came with rear discs and the small master cylinder used on early A1 cars.....
Sounds like a good project to keep us posted on.
As far as rear pads go get the cheapest rear pads that Autozone sells(9.95 i think) and you will still lock the rear brakes without meaning to.
Cheers
dave parker
wdcr ITC #97

[Greg Amy]

Piece 'o cake, Evan. Simply install the rear brakes from an A2 GTi as Dave and Bill mentioned. All you *need* is the spindles, hubs and brake parts; the spindle will bolt right up to your A1 rear beam, same bolt pattern. If desired, you'll also want the e-brake cables (they're different for disc cars).

Master cylinder and bias are really no problem either; the one from the GTi works fine. The ETKA shows a 20.64mm (13/16") bore master cylinder on the '84 cars, same as any '76 A1 also equipped with a brake servo.

How do I know all this? I've run A2 rear disc brakes on my Rabbit GTi (http://www.gatm.com/cars/gti.html) for over a decade. I didn't really notice a dramatic performance advantage but maintainence was virtually eliminated (except for cold rainy days when the e-brake cables would get wet and freeze.)

Plus, they look cool.

[racer_tim]

Evan, DON'T do it. The rear disk setup is HEAVIER than the drum setup.

We all know how much rear pad wear we have, and we also know how much the rear end of a FWD car does.

How many years have we all raced with the same rear pads on our cars?

I know that they just threw us a bone, but don't bite at it.



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Tim Linerud
San Francisco Region SCCA
#95 GP Wabbit
http://linerud.myvnc.com/racing/index.html

[evanwebb]

Thanks all! Hey Tim, how much heavier is it? I really want to do the swap because I hate dealing with either the stupid self-adjusting adjusters that don't self-adjust, or the stupid star wheel that I have to adjust before every session. After thinking about the pads I'm wondering if a pad that is ineffective until it gets really hot might be the right setup? That way it doeasn't really have much bite initially and probably just starts to work when I'm letting oiff the brakes. Or would that just glaze the pad and rotor because they would never get bedde properly. Or I could bed them on the front and then switch them to the back? Are they the same calipers on the back of the A2 as the front of the A1? Thanks to all you experts who know this stuff...

[Bill Miller]

Dave,

The easy swap is the Scirocco 16v rear axle, since it's an A1 car. Otherwise, you have to do as Greg said, and take the stubs off the A2 axle and put them on an A1 axle. Probably easier to find the A2 stuff than the Scirocco stuff, but the Scirocco axle is by the easy way to go.

Evan,

Like Tim said, the stock VW discs are a bit heavier. If you want lighter, I believe you have to go w/ Wilwoods (or something similar). The rear brakes don't do a whole lot on these cars, but pedal feel and maint. are good reasons to do the conversion.

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MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608

[JLawton]

Originally posted by evanwebb:
I hate dealing with either the stupid self-adjusting adjusters that don't self-adjust, or the stupid star wheel that I have to adjust before every session.

Jumping the thread a little here, sorry..... How do you deal with adjusters that don't adjust or even lock up (other than the usual "grease"). They are a PIA.


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Jeff L
#74 ITB GTi

[racer14itc]

I have the rear discs on my GP Scirocco and the reasons I did it was for the same reasons Bill stated: pedal feel and maintenance. I used the 16V rear stubs, rotor/hubs, and calipers and bolted them on the A1 rear axle. They are a little bit heavier than the drum setup but I like the simplicity of the swap.

I didn't like pedal feel with the drum brakes, it was too hard to modulate them properly under threshold braking. Drum brakes are self actuating and are non-linear in terms of pedal force and braking force. Finding that fine line between threshold braking and lockup was hard, and I tried several different shoe compounds. Once I switched to the rear discs it was so much easier to balance the car under braking, once I found the right pad.

In reply to one of Evan's original question, the stock master cylinder should work out OK with the rear discs. I'm using a 3/4" Wilwood master cylinder for the rear circuit, which is about the same size as the stock master cylinder (if you account for the connector between the front and rear pistons in the stock master cylinder). There should be plenty of fluid displacement capacity available.

Hope this helps,

MC

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Mark Coffin
#14 GP BSI Racing/Airborn Coatings/The Shop VW
Scirocco
Zephyr Race Coaching and Consulting
http://pages.prodigy.net/Scirocco14gp

[shwah]

The small rear drums are lighter, but the disks have less rotational inertia. Not sure which of these is most important. I went to rear drums on my ITB Golf, for light weight, but will likely go back to disks for the ease of use reasons cited by others. My car is underweight anyhow, and maybe I will get some feel for the effect of changing the rotational inertia....

Chris