Dusterbd13
Dusterbd13 New Reader
12/15/10 9:54 p.m.

front calipers 1.77 bore size

rear calipers: 1.687

master cyl. bore: 15/16

front rotor diameter: 13 inches

rear rotor diameter: 11.1 inches

front tire height: 25 inches

Rear tire height: 27 inches

so, now that we have all the specs in play, can you help me figure out if this is a good idea? seems like ill have about neutral bias without the bias adjuster in play. is this correct? let me know what you think. im hoping to get a great braking, predicable car out of this swap.

Michael

oldtin
oldtin Dork
12/15/10 10:25 p.m.

Calculating it out is a good start but there's no substitute for testing it out. http://dsr.racer.net/brake_bias.htm

RoosterSauce
RoosterSauce Reader
12/15/10 10:29 p.m.

Suspension, tires, and weight distribution are such huge factors that it's hard to say anything just going off that.

grimmelshanks
grimmelshanks HalfDork
12/16/10 1:29 a.m.
RoosterSauce wrote: Suspension, tires, and weight distribution are such huge factors that it's hard to say anything just going off that.

+1!!! you gotta think front engine vs rear engine, how high it sits, how hard it will dive under breaking etc...

Giant Purple Snorklewacker
Giant Purple Snorklewacker SuperDork
12/16/10 6:22 a.m.

Like has been said - available grip, weight distribution and roll characteristics come into play as well.

If the bias is adjustable find a lonely stretch of road, turn the rear way down and then creep it up until the fronts just lock up a hair before the rear - then - for street use - back it off a little more so when conditions change you don't get a big surprise.

It helps to have a buddy be a spotter if you can't feel them locking until you see the cloud.

AngryCorvair
AngryCorvair SuperDork
12/16/10 10:33 a.m.

yep, without knowing the weight distribution and the wheelbase, we're throwing darts to predict balance. we can guess at the CG height just by knowing the type of vehicle.

imagine a Big-Block Vega and a 911 side by side. total weight might be the same, but front and rear brake sizing will be totally different.

aircooled
aircooled SuperDork
12/16/10 10:48 a.m.

Of note (and Angry will like know more about this) is that neutral bias is not what you want in a car.

You want the fronts to skid first, otherwise you end up with a car that wants to swap ends in almost any skid.

Dusterbd13
Dusterbd13 New Reader
12/16/10 8:22 p.m.

thecar is a 70 duster with as much lightness added up front as i could afford. aluminum heads,intake, water pump, radiator, fiberglass hood, battery in trunk.

tires are 255 size on 17X8 with a 340 treadwear rating.

my concern is that even though the fronts are 2 piston with a lot bigger rotor and pad, the single piston smaller pad rears are about similar in overall bore size. seems like it may be a recepe for end swapping;

i havent stared the install yet, but ive laready bought all the parts. just want to make sure im not doing something stupid by doing this.

Michael

AngryCorvair
AngryCorvair SuperDork
12/17/10 9:19 a.m.
Dusterbd13 wrote: my concern is that even though the fronts are 2 piston with a lot bigger rotor and pad, the single piston smaller pad rears are about similar in overall bore size. seems like it may be a recepe for end swapping;

When you say "2 piston" do you mean

A. 2 pistons next to each other on the inboard side of the rotor, or

B. 1 piston on each side of the rotor, opposite each other?

in a front-engine car, generally speaking A is a recipe for end-swapping and B is not.

if you could post a pic of each caliper, that would help my understanding.

ReverendDexter
ReverendDexter SuperDork
12/17/10 11:40 a.m.

Remind me why, "set it at 80%, test it, and move braking power away from the side that locks up first until the fronts lock up just a picosecond before the rears" isn't the answer here?

AngryCorvair
AngryCorvair SuperDork
12/17/10 1:29 p.m.
ReverendDexter wrote: Remind me why, "set it at 80%, test it, and move braking power away from the side that locks up first until the fronts lock up just a picosecond before the rears" isn't the answer here?

I think it's because the OP wanted to know if he'd have a neutral system, and we don't have enough info to answer that question. and it's possible that "set it at 80%" might put him backwards in a ditch the first time he tries to find out which end locks first.

Dusterbd13
Dusterbd13 New Reader
12/18/10 7:12 a.m.

fronts are 2 piston side by sie from 99 corvette. rears are singles from an 88 300zx turbo.

AngryCorvair
AngryCorvair SuperDork
12/18/10 1:07 p.m.

In reply to Dusterbd13:

that'll be a pretty good start. i'll make a front/rear assumption based on stock V8 duster weights if i can find them, and i'll do some calculations for you.

erohslc
erohslc Reader
12/18/10 3:05 p.m.

You still don't have enough info to calculate, also need: 1) Centroid of the 'footprint' of brake pads. 2) Location of that centroid on the disk, ie radius from disk center to centroid. 3) Coefficient of friction of front pad material, and rear pad material

The shortcut version is this: for each lb of hydraulic pressure, how much ft-lb of torque does each brake create.

Carter

AngryCorvair
AngryCorvair SuperDork
12/18/10 7:23 p.m.

In reply to erohslc:

but after 20 years of brake systems design, development, and validation experience, i can make some educated guesses.

Shaun
Shaun Reader
12/18/10 10:20 p.m.

I set a car up to have "neutral" brake balance as tested on dry level road and swapping 3 different compounds in the rear until I made it darn near happen with the fronts locking just before the rears. The first time I drove the car in the wet with that setup down a moderately steep road I am very familiar with the rear went very light under moderate braking and all sorts of arm spinning just saved me from the proverbial ditch or worse. It was then I discovered something.

Dusterbd13
Dusterbd13 New Reader
12/21/10 7:04 p.m.

so, angrycorvair, had any time to formulate an educated guess?

kb58
kb58 Reader
12/21/10 7:49 p.m.

Use this: Mike's Brake Spreadsheet. It requires inputing everything necessary to give a very good first start. You'll have to guess on a couple things, such as brake and tire coefficent of friction, but it'll get you close.

Disclaimer: I'm not Mike, nor I did not create this utility. He doesn't know that I'm hosting it and may not appreciate it, but attempts to contact him have failed. He also no longer provides this util anywhere on his earlier site. Mike, if you read this and it bugs you, I'll remove the app.

AngryCorvair
AngryCorvair SuperDork
12/21/10 10:32 p.m.

my only real worry is that your master cylinder bore is a little small to be feeding so many pistons. your travel will be slightly long as a result, but you will be able to build decent pressure. do me a favor: measure the distance from the pedal pivot to the MC pushrod attachment, as well as the distance from the pedal pivot to the center of the foot pad on the pedal.

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