Front to rear brake bias, 4 whl discs

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Bruce2
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Front to rear brake bias, 4 whl discs

Post by Bruce2 » Thu Jan 31, 2013 2:38 am

My current car has stock Karmann Ghia brakes on the front (40mm pistons), and stock 914 rear brakes (33mm pistons) This was "the way to go" 20-25 years ago. When I first installed them, I was impressed at the braking power. Although it's not hard to impress when you're leaving drums behind. I also tested them at high speed and thought the bias was pretty good.

I was recently talking to a friend who said he used the same rotors and calipers on the front of his Type 3 Fastback, then for the rear, he used another pair of 40mm front calipers. My friend's conclusion of his bias was the same. His thought was that a car with lots of weight at the rear could use big rear brakes.

Next, I have another friend who builds a lot of fake Speedsters. When he uses stock Type 4 front brakes (slightly larger rotors with 42mm pistons), he also uses A3 Golf/Jetta rear calipers (38mm pistons). like my Type 3 friend, he claims this is the perfect brake bias.

What do some of you guys use, and what're your opinions of the bias?

Note, if you are using some gigantic Porsche Turbo S bla bla bla 4-6-8 pot brakes, that's not what this question is about.

Slow 1200
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Re: Front to rear brake bias, 4 whl discs

Post by Slow 1200 » Thu Jan 31, 2013 4:00 am

what about tire sizes?

Ian Godfrey
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Re: Front to rear brake bias, 4 whl discs

Post by Ian Godfrey » Thu Jan 31, 2013 5:58 am

Ghia, 205's rear 195's front, sticky dunlop soft R spec.
42mm dual T3 front, 32mm single rear (fiat) similar to 914. I couldn't get the bias right with my adjuster, too much front brakes (locked way sooner). Went back to 40mm ghia calipers and got the balance right. (fronts locking slightly before rears).
I think you only need big back brakes if you have the cal look skinny's on the front (135's)
Assuming decent front tyres, when you stand on it, the weight transfer at roughly 1 g puts 60% of the weight on the front, so you need about 60% of your brakes on the front and 40% on the back. so if you had similar calipers front and back you would be WAY out :roll:
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Chris V
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Re: Front to rear brake bias, 4 whl discs

Post by Chris V » Thu Jan 31, 2013 10:11 am

We've got two bugs with 42mm Type III's (2-pots) and 38mm (single pot) 944 rears- both have had 225-255's on back and either 195/205mm up front - similar to running Type III drum on back, but better pedal feel (always fitted stock dual M/C.
Last edited by Chris V on Thu Jan 31, 2013 7:03 pm, edited 1 time in total.

JWP
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Re: Front to rear brake bias, 4 whl discs

Post by JWP » Thu Jan 31, 2013 10:13 am

Ian Godfrey wrote:Ghia, 205's rear 195's front, sticky dunlop soft R spec.
42mm dual T3 front, 32mm single rear (fiat) similar to 914. I couldn't get the bias right with my adjuster, too much front brakes (locked way sooner). Went back to 40mm ghia calipers and got the balance right. (fronts locking slightly before rears).
I think you only need big back brakes if you have the cal look skinny's on the front (135's)
Assuming decent front tyres, when you stand on it, the weight transfer at roughly 1 g puts 60% of the weight on the front, so you need about 60% of your brakes on the front and 40% on the back. so if you had similar calipers front and back you would be WAY out :roll:
I agree with Ian, and this is also referenced in new Porsche's. They still run bigger brakes up front and they still have good weight transfer, see below
Image

Not to hijack the thread, but Ian can you share more about your car, setup, suspension etc?

Ian Godfrey
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Re: Front to rear brake bias, 4 whl discs

Post by Ian Godfrey » Thu Jan 31, 2013 7:34 pm

JPW, a well trodden path.....
front: stock width beam, avis adjust, whiteline heavy adjust sway bar, sway away 15% stiffer torsions, beam stiffners (run from the ends of the beam to the 2 bolts on body. 6" wheel, 195/55. stock ghia brakes.
Rear: IRS, 28 torsions, CSP kafer bar, whiteline heavy adjust sway bar, urethane bushes, rear disks, fiat caliper, 7" wheel 205/60.
2387cc, 86B cam, IDA's etc. close 3 - 4 box with a 3.88 R & P

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Piledriver
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Re: Front to rear brake bias, 4 whl discs

Post by Piledriver » Thu Jan 31, 2013 8:41 pm

When you start with a 30/70 weight bias things don't always work as you expect.

73 Squareback, ~stock height, 205/60s on Mahle 5.5s.

I'm running BMW 2002T1 front calipers and 38mm Audi Cabriolet rears (single piston/floating so effectively 2X38) on 914 rotors.
(rears probably same as Passat, same bracket)

The balance seems excellent and the self adjusting nature of the later model rear calipers make a huge difference in braking consistency.

I've been playing with balance somewhat unintentionally as I'm still in progress on fully dual circuiting it.

Right now, the front 1/2 of the MC feeds the front upper pistons, and the rear feds the front lowers and rears.

Could use a touch more front bias, so I have picked up another long line to go to he back and will split it 50/50 like the Swedish dual piston setup.
(front upper/Right rear, front lower+Left rear)

Need to put a new MC in as the present one is marked "16" (mm) Looked several times. It's a 16.

When i had the 2002 4 pots on the front of my 914 I put the original fronts on the back and replaced the adjuster with a "T".
Worked awesome wet or dry. Parking brake... not so much.
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Marc
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Re: Front to rear brake bias, 4 whl discs

Post by Marc » Thu Jan 31, 2013 9:33 pm

Piledriver wrote:When you start with a 30/70 weight bias things don't always work as you expect...
Indeed. In most European nations front discs (single-pin, like early 'Ghia) were mandatory on `67 Beetles if equipped with the 1500 engine, in order to meet the (dry pavement) stopping-distance requirements. With the skinny rear shoes, this gives way too much front bias in the wet - the same car would stop shorter with US-spec front drums in the wet. But the `67 'Ghia, with the exact same brakes as the Euro 1500 Beetle, was manageable in the wet - the `Ghia's extra weight helped keep the fronts from locking up.

Here in Seattle the streets are wet more often than not, so I prefer a little more rear bias than would be considered proper on dry pavement...just enough more to ward off premature front lockup in the rain, not so much that the rear tries to overtake in a dry panic stop.
As Chris described above, with `66-`71 III fronts and 924T/944 rears on a Beetle, the balance is just right (for me, anyway) and the stock 113 611 015BD M/C works just fine, with no extra residual pressure valves...but note that my rear tires are considerably wider than the fronts which helps keep them from locking up in the dry...with matched F/R tires this combo would probably be a little tail-happy for most folks.

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Re: Front to rear brake bias, 4 whl discs

Post by Jadewombat » Sun Feb 03, 2013 12:11 pm

I'm really surprised this topic isn't stickied for as many times as it comes up a year. :roll: :?:

I ran 924T/944 brakes all around on my bug with a CNC drop-down pedal cluster with a balance bar. I thought the usual thing when I started that most of the weight would go to the front like other cars. It really doesn't shift dramatically on a bug and after quite a few adjustments it was right near 50-50 bias for me that wouldn't keep the fronts from locking up. And I drove the piss out of that car.

The weight transfer of a rear-engined old bug shifts to the middle of the car in a stop, not all the way to the front end like a front-engine RWD or FWD car. Mind you my car was stripped inside and had a very small fuel cell up front.

Image

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Piledriver
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Re: Front to rear brake bias, 4 whl discs

Post by Piledriver » Sun Feb 03, 2013 9:04 pm

The height of the center of mass makes a huge difference in weight transfer as well...

Even a Bug has most of the weight pretty low, a Ghia or 914 even lower==less weight transfer.

I have to find a set of Cookie Cutters...
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Bruce2
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Re: Front to rear brake bias, 4 whl discs

Post by Bruce2 » Sun Feb 17, 2013 1:30 am

Sorry about not following up on this topic for so long. Right now I'm about 4000 miles into a much longer road trip through Mex and maybe further south.
Over those latest miles, I've had the opportunity to re-evaluate the braking of my car. You see, in Mexico, they rely more on speed bumps to control traffic speeds than by using cops. In many instances you get no warning when on the highway, so jumping on the brakes full out from 60mph is common. I'm finding that the front wheels lock up a lot easier than I remember from back in the 90s. I'm sure this is due to the much larger dia rear tires I now use. Then, I used 195 & 225/50/15. Now I am using a 205/55 and 215/60/15 rear.
I'm liking the idea of using the 38mm Golf calipers now.

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Piledriver
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Re: Front to rear brake bias, 4 whl discs

Post by Piledriver » Wed May 08, 2013 4:01 am

I finally finished fully dual circuiting the square, BMW 2002 4 pots front. 38mm Audi Cabriolet (38mm/Passat proabbly same) rear.

Set it up so upper front is coupled to RR and lower front plumbed to LR caliper ala VWs dual circuit setup.

Put a new stock master in it and all new long lines on principle, pretty sure i have a good bleed on it (with the help of speed bleeders out back) but it has more pedal travel than I'd like.(although it stops great)

Does the 23mm Meredes M/C referred to all the time have the same size circuits F/R? Anyone know the application so i can order one?

Also: Anyone know what pookie Dorman uses on their speed bleeders?
Seems it dissolves in brake fluid. :evil:

Has to be a better choice.
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Ian Godfrey
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Re: Front to rear brake bias, 4 whl discs

Post by Ian Godfrey » Wed May 08, 2013 8:12 am

I have used a non servo 23mm dual bus master cylinder with good results. It bolts straight into a beetle. not sure of the year, maybe 69ish. Some cars are happy with the 19mm, some need more to keep pedal height, I have never figured out why this is so! It seem independant of the fluid volume of the calipers. May be it has to do with caliper flex :?

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Re: Front to rear brake bias, 4 whl discs

Post by Piledriver » Wed May 08, 2013 9:59 pm

I figure some of it has to do with all the rubber hose I added, 4 rubber lines up front, and 2X relatively long Mk4 Golf/Jetta front rubber hoses to the rear calipers. Still seems to require more pedal (evenly//properly split) than it did last week with the same rubber lines.

I'll probably run another quart of fluid through it before shopping for a bigger MC, as the installed one is new.
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Re: Front to rear brake bias, 4 whl discs

Post by Piledriver » Mon May 20, 2013 2:01 am

After driving it for a week or so, I got used to the long(er) pedal, reels it in with extreme gusto, well tested due to idiots several times in traffic already.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

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