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  #16  
Old August 25th 2008, 17:03
mabbo mabbo is offline
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dude

cheers for that

so, what sort of ratio should i be aiming for?

would you just stick with the stock bug master cylinder?

its a street car, there are loadsa people using the empi kit with stock master cylinder id imagine - the bias must be acceptable?
if the reason my pedal was 2 soft wasnt because of the 19mm bore (im sure it wasnt), it must have been because of a faulty master cylinder...

mabbo
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  #17  
Old August 26th 2008, 06:05
mabbo mabbo is offline
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just a thought... eventually i was considering upgrading to some wilwood 4 piston calipers front and rear. i could do this sooner rather than later if i could use a bug master cylinder with them successfully, as it would save buying the uprated CSP master cylinder and fitting kit.

the calipers i want have the choice between piston diameters of 1.00" (25.4mm), 1.25" (31.8mm) or 1.38" (35.1mm) for the fronts, and the rear handbrake caliper has 1.00" (25.4mm) pistons.

if i used a stock bug 19/19 master cylinder, to keep the bias right, would you suggest something like the 1.25 for the fronts, and 1.00s for the rear, or the same front and rear then make adjustments using a regulator valve?

mabbo

Last edited by mabbo; August 26th 2008 at 06:10. Reason: added piston sizes in mm's
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  #18  
Old August 26th 2008, 08:40
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evilC evilC is offline
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Quote:
Originally Posted by mabbo View Post
dude

cheers for that

so, what sort of ratio should i be aiming for?

would you just stick with the stock bug master cylinder?

its a street car, there are loadsa people using the empi kit with stock master cylinder id imagine - the bias must be acceptable?
if the reason my pedal was 2 soft wasnt because of the 19mm bore (im sure it wasnt), it must have been because of a faulty master cylinder...

mabbo
I would aim for a caliper/mc ratio of around 8:1 for both front and rear although as previously said the rear can be larger because the pressure can be adjusted with a proportioning valve.

I would stick with a standard bug m/c but make sure its all working 100%.

In a panic situation locking up the front brakes is infinitely preferable to locking up the rear so the Empi kit is intrinsically safe. However, you can't get maximum redardation because the rear never comes up to full braking force 'cos the front locked up long ago. Ideally, maximum stopping power will be acheived with both wheels on the verge of locking up. Using a balance bar arrangement is the best way to dial in the front/rear proportions.

Quote:
Originally Posted by mabbo View Post
just a thought... eventually i was considering upgrading to some wilwood 4 piston calipers front and rear. i could do this sooner rather than later if i could use a bug master cylinder with them successfully, as it would save buying the uprated CSP master cylinder and fitting kit.

the calipers i want have the choice between piston diameters of 1.00" (25.4mm), 1.25" (31.8mm) or 1.38" (35.1mm) for the fronts, and the rear handbrake caliper has 1.00" (25.4mm) pistons.

if i used a stock bug 19/19 master cylinder, to keep the bias right, would you suggest something like the 1.25 for the fronts, and 1.00s for the rear, or the same front and rear then make adjustments using a regulator valve?

mabbo
Choosing the 4 pot wilwood calipers, I would go for equal sized units front and rear assuming that your discs (rotors) are of similar diameter? However, I would get the best out of your present system unless there's a fashion statement being made here. With your set up you could purchase/fabricate a balance bar unit that would allow you to adjust the braking effort front and rear to maximise the effectiveness of your current system. If after that you must have such braking force as provided by the Wilwoods then you will also have the best system to dial the Wilwoods in. A standard brake proportioning valve is a relatively crude unit that just limits the rear line pressure but doesn't apprortion the line pressure front to rear as will a balance bar set up.

I often see brake systems in bugs that are hugely over spec'd and more appropriate to very fast and heavy track racing cars. Most of these installations have poured money into just one end of the braking system without considering the whole (mainly I think that 'cos most of it is not visible).

evilC
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  #19  
Old August 27th 2008, 17:24
mabbo mabbo is offline
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cheers for that mate.

good news that the stock master cylinder should be fine for what i have, and work ok with the wilwoods should i choose to upgrade at a later date.

i have had a quick look at the balance bar setups and to be honest - im not sure if id fancy undertaking that on my bug. its a street car and im struggling with the brake theory as it is, never mind with a balance bar - they look pretty hardcore!

the reason for considering the wilwoods was that they fit the discs i already have (stock bug fronts, empi kit rears... both similar size) and i thought they would be relitevely easy to make brackets for, and i could keep my 15" 4 stud wheels... with the added bonus of improved braking should i ever need it over what i have at the min. once my setup is working as it should with a better suited master cylinder i may be happy with them.

thanks for your help
mabbo
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  #20  
Old August 30th 2008, 17:48
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Angelo Amato Angelo Amato is offline
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brakes

interesting read,

my maths too are bad maybe i should stay away from formulas !

Angelo
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  #21  
Old August 31st 2008, 15:58
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Wally Wally is offline
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Originally Posted by Angelo Amato View Post
interesting read,

my maths too are bad maybe i should stay away from formulas !

Angelo
Which is agood idea, as there are too many variables!

I tried reasoning with it too and it is totally useless trying to calculate it, although I agree with most of EvilC's post ;-)
Just try out one combo and change according to your experience. Works much better for me.

I now have 993 rear brakes (30/34 cups) at the rear and GT2 rear brakes (330x28 with 28/30 cups) at the front with stock MC.
While the cups are much BIGGER at the rear, I still have quite a noticeble front bias as the front still locks up (too) easy and I have really the most front weight beetle of most here...
How you explain that?
So, just try and adjust, try and adjust
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Last edited by Wally; August 31st 2008 at 16:02.
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  #22  
Old August 31st 2008, 21:01
flat flat is offline
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Originally Posted by Wally View Post
I tried reasoning with it too and it is totally useless trying to calculate it,
Good point Wally, I've come to the same conclusion. On paper it shouldn't work, or the bias should be way off but in practice it works great!

Also, when working out a brake system, don't get tied up on the hydraulic bias calculations because overall you are concerned with brake torque. There is a hydraulic component to the brake toque calculation, but there are additional variables like moment arm (disk diameter) and the friction coefficient of the pad.

I've got the Boxster S brakes (318x30, 40/36 F and 298x24, 30/28R...IIRC) with a stock 19mm dual circuit and I do have a slight front bias. Works great, I don't think I could improve this setup with aftermarket MC's...

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Last edited by flat; August 31st 2008 at 21:02.
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