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Discussion Starter #1
i have the 4 pot calipers on my car.( mazda calipers and vented disk ). they work well:).i have a extra set of 4 pot calipers. thinking about putting them on the rear.:eek:..brake balance..hum, that the problem..:confused:. my thought.. lets say the stock front brake pad is 48sq. cm. and the rears are 75% of that...i am thinking about putting the 4 pots on the rear..and trimming the friction part of the pad to 75% of org. sizr. leaving the metal plate the same size..:rolleyes::confused:. is this a workable idea?
 

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Do you still have the proportioning valve in the rear circut?

That'll have bearing too. (as in the rear pads aren't just 75% of the front AFA friction surface, they also have less pressure applied to to begin with)

Not even going to get into the complication of mounting the spare calipers (which I'm presuming are of the same size as the fronts you're using now) on the smaller rear rotor.....
 

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Discussion Starter #3 (Edited)
my car is a 84, really do not know about the valve...but the rear disk will remain stock, the fronts are increased from 270 mm stock to 292 new vented...and yes the calipers are of the same size....one of my prosche freinds looked at the set up, and siad' if you hit your brakes hard enough, you will be pulling your teath out of the stearing wheel':):)
 

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The valve, unless it's been removed, will be on the passenger side kinda tucked up into the area where the trialing arm ties into the chassis.

Easy enough to find actually, just chase the softline on the diff toward the front of the car. (the softline ties into the valve)


I'm of a mind the caliper size itself is going to give you fits, as even if the gap is OK, it's just not correct for a smaller rotor. (if you put bigger rotors yet on the front and it fits those properly, chances are incredibly slim that they'll fit a smaller rotor and work safely. They may even allow the pads to hang over the edge of the rotors if they are off enough)

Then the size of the caliper alone will create some kind of mounting issue, as even if they bolted right up to the front, the rear calipers are smaller in general and have a proportionately smaller bolt pattern, so you'll have to work out some way to bracket it, which in turn will throw off the caliper alignement among other things.
 

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Discussion Starter #5
allready mad the mounts for the calipers, front and rear. and yes the mount front and rear where diff. but not really a problem,( in fact the rears where easyer to make) durring my down time with my alfa, due to acct. i took of the rear calipers to re-build them and measure them, as for the pads being to large for the smaller rear disk. i was planing to run them for a week, take the rear pads off see if the is any 'overhang' and just trim the xtra pad friction mat. off with my trusty dremal tool.:)
 

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Might be better to try and find pads that actually fit rather than trimming.

I'd hate to hear how you got wadded up after a brake failure because something went wrong as brakes are not really something one should just cobble at for amusement.
 

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Whoops, my bad: the proportioning valve doesn't tie directly to the softline at the rear, but its darn close.

Area shown is the main pump and filter on an S3.
 

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i have the 4 pot calipers on my car.( mazda calipers and vented disk ). they work well:).i have a extra set of 4 pot calipers. thinking about putting them on the rear.:eek:..brake balance..hum, that the problem..:confused:. my thought.. lets say the stock front brake pad is 48sq. cm. and the rears are 75% of that...i am thinking about putting the 4 pots on the rear..and trimming the friction part of the pad to 75% of org. sizr. leaving the metal plate the same size..:rolleyes::confused:. is this a workable idea?
I would not do it if I where you.

One of the many design factors that goes into the development of a base braking system in OEM cars is the "bias" or "balance." It's a pretty simple concept to grasp: for vehicle stability under braking, it is required that the rear brakes do NOT lock before the front brakes. Simple, right? Most of you probably knew that already.

So what governs the 'lock up' point of the rear brakes? :

1. tire tractive capability (friction)
2. tire normal force (weight on the tire)

This can be proven from looking that the fundamental relationship for maximum sustainable tire force: F=µN, where:

F = the lock up point, or peak force
µ = tire-road coefficient of friction
N = normal load sitting on the tire

When the OEM is designing a brake system, they size the system components (calipers, master cylinder, rotor OD, etc.) to generate the proper amount of torque at both ends of the vehicle so that the front brake force ('F' above) exceeds its peak traction first. At this point, the front brakes lock and the car slides in a nice, stable straight line.

Fortunately (from a safety standpoint anyway), when most big-brake suppliers adapt a ginormous rotor and caliper package to a vehicle, they end up actually increasing the FRONT bias. How? By increasing the effective caliper piston area and the rotor effective radius, these two factors work together to increase the 'mechanical gain' of the front brakes, building more torque for the same pressure, everything else being equal. So, from a bias perspective we are not pushing the vehicle toward instability, but rather just the opposite - we are underbraking the rear axle! The obvious impact would be an increase in stopping distance - probably the one thing the new owner was actually hoping to reduce. Ironic. So, say you chose to install these big brakes on the front axle but want to maintain the OEM bias. What's the answer? Well, one way would be to invest in big rear brakes too which increase the rear mechanical gain to the point that the system is balanced once again.
Well, let's look at why we upgraded the front brakes in the first place. Contrary to popular belief, the real reason sports- and racing cars use big brakes is to deal with heat. Period. There has been a bunch of stuff published which will disclaim this, but when you look at the braking system from a design standpoint, making them 'bigger' doesn't fundamentally do anything for stopping distance. It's all about the heat. So, you upgraded the front brakes because of thermal concerns but as a hidden surprise got a shift in brake bias. As a band-aid to this condition, you now spend thousands more on a rear brake upgrade because the front system was not sized correctly in the first place. Sure, it looks great, but there is another option.

When upgrading your front brakes, it is possible to size the caliper pistons and rotor effective radius to maintain the original brake system's pressure-torque relationship. Yea, it takes more engineering know-how and you can't sell the same part to everyone anymore, but you are not altering the base brake balance from what the OEM intended. Now, if you sized the front brakes correctly, why would you need to change the rear brakes? Good question. If there are no thermal concerns with the rear brakes (and on a front-engine street car there rarely are) then by installing a rear big-brake kit all you are doing is (a) spending money and (b) adding unsprung weight. This is not usually viewed as favorable, unless you like driving a heavy car that won't perform anymore.

Under an OEM bias condition, the rear brakes only contribute only about 15-20% of all the braking force the vehicle generates, and when you install sticky tires you actually DECREASE the amount of work they need to do. Why? Because at the higher deceleration levels afforded by race tires, there is more weight transfer taking place, reducing the normal force on the rear tires and increasing it on the front (remember F=µN from above?). If anything, we now want to decrease the rear effectiveness. Ironic once again.
Of course, if you decide to upsize your rear brake system components you can also impact the front-rear torque relationship, and consequently you can "bias" the "balance" more toward the rear. Go too far, and the rear brakes could lock before the fronts even with brake proportion valves. Again, not the end result you were expecting, right?
So I suggest that you don't bother with them. Spider's have pretty good braking out of the box. Buy better tires and pads to increase your braking capabilities. I understand your using the same rotor diameter but the facts still hold using a larger caliper.

I think the vented rotor in front and caliper should be plenty braking enough for that car. Especially for normal street driving.
I think your going over board with the rear.
Jason
 

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bianch1,
I can't see the benefit in what you are doing at all... If you depress the brake pedal, and can lock all the wheels, then there is no more "stopping" that the braking system can do... The rotation of the wheels has been stopped fully - that's it... Jason is absolutely correct when he says that brake upgrades have everything to do with the heat consequences encountered on the track. Also, racers change the components to lighter ones to reduce the weight and improve handling, especially for cornering. None of this is relevant on a street car, no matter how fast you drive and how hard you brake... It is cool to upgrade your brakes to a track spec, and while you won't gain anything on the street, you also won't hurt anything, if you use proper components. It's your car, and you are certainly entitled to modify it as you wish, but I would be very careful about implementing what you've described here...

Best regards,
 

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Discussion Starter #13
.. hi , thanks all for the reply.. as for the weight question..i am useing mazda calipers. they are aluminum... so they weigh a lot less the the iron calipers that i am replacing..both front an rear.the calipers weigh.. aprox 4.083 pounds each.. the iron calipers come in at almost 11 pounds each in the front and 7 pounds in the rear..and for the front disk this also have been increased in size..and they now are vented... i am keeping the rear rotor stock..and if i must i have a 'willwood' brake adjuster i can insstale in the rear line..i plan to do some testing in the local cal expo parking lot,or a back road at our local retiered air force baces we have around here ..with a video camera to see how the brakes are working..
 

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Discussion Starter #15 (Edited)
......alfissmo....check streetfame..page 33/32 post no 482/467...on his v6 convrsion...he is right..i am wrong.?.his rotors are 13" front and rear...
 

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......alfissmo....check streetfame..page 33/32 post no 482/467...on his v6 convrsion...he is right..i am wrong.?.his rotors are 13" front and rear...
Whatever man!
Do what you want. I guess you did not read anything I wrote.
Wait one second here, We are talking a highly highly modified Spider over yours a fairly standard one. Thats like comparing a standard 159 to a DTM 159. Makes no sense!
I am not too sure it was really needed on the rear either BTW although he has 19" wheels which will need more stopping power oh and that pesky 24V motor, coil overs, etc and so on but still if used on the street and not the track it is sort of a waste of money. But it's your car your cash and I am sure none of our experiences and knowledge will mean much when you have it set in your mind that the big brakes will be better and more "bling"!

Good luck with it.
 

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Discussion Starter #17
...not to be rude here.. but is not 60 mph the same on streetfames v6 as in my modifide 4 cly?, are the coil overs on streetfames car really much diff. then the stock? as the springs support the car and the 'shocks' only dampen the springs?from what i have seen on streetfames cool car, his car will be for the street..why else would he bother to make the new centre counsle for his car.. track cars would not need that because of weight..as for cost so far.. 2 vented rotors..$75.00 4 mazda calipers..$40.00, rebuild kit for said rotors, $19.63( i work for mazda..)and for the alum. adapter kit i made.. cost of alum..$13.56.. and for the machine work,, did myself at freinds machine shop...agin not to rude..:):)
 

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...not to be rude here.. but is not 60 mph the same on streetfames v6 as in my modifide 4 cly?, are the coil overs on streetfames car really much diff. then the stock? as the springs support the car and the 'shocks' only dampen the springs?from what i have seen on streetfames cool car, his car will be for the street..why else would he bother to make the new centre counsle for his car.. track cars would not need that because of weight..as for cost so far.. 2 vented rotors..$75.00 4 mazda calipers..$40.00, rebuild kit for said rotors, $19.63( i work for mazda..)and for the alum. adapter kit i made.. cost of alum..$13.56.. and for the machine work,, did myself at freinds machine shop...agin not to rude..:):)
sigh....

I don't know how else to explain it to you! I thought my reply was very easy to understand.
The biggest point of the whole discussion was that the fronts contribute to 80-85% of the braking, leaving 15-20% for the rear and EVEN LESS WITH A STELLAR TIRE!
Here are some factors to help in your decision.
As mentioned You will be sorry if your car is more rear biased.

Factors that will increase front bias
Increased front rotor diameter
Increased front brake pad coefficient of friction
Increased front caliper piston diameter(s)
Decreased rear rotor diameter
Decreased rear brake pad coefficient of friction
Decreased rear caliper piston diameter(s)
Lower center of gravity
More weight on rear axle
Less weight on front axle
Less sticky tires (lower deceleration limit)

Factors that will increase rear bias
Increased rear rotor diameter
Increased rear brake pad coefficient of friction
Increased rear caliper piston diameter(s)
Decreased front rotor diameter
Decreased front brake pad coefficient of friction
Decreased front caliper piston diameter(s)
Higher center of gravity
Less weight on rear axle
More weight on front axle
More sticky tires (higher deceleration limit)
(Information borrowed from my [email protected])



I will tell you right now you do not want a rear biased car.

As you go about modifying your car for the street or for the track, be aware that changes in the braking system as well as changes in the car’s ride height, weight distribution, or physical dimensions can swing brake bias all over the place. The only sure-fire way on knowing if your final bias has been optimized is to measure stopping distance both before and after your modification(s).

Finally, your tires certainly still stop the car, but if your bias is out in left field you might not be able to use everything they have to offer. Your braking system is just that – a system – and keeping an eye on brake bias effects during modification will go a long, long way to assuring the correct and safe braking for your car. Of course, selecting the proper kit from a manufacturer who has already done the hard part for you can make it that much easier!

Good luck with the "upgrade"

Jason:)
 

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Discussion Starter #19 (Edited)
hi:)..i have aready tested the stock brakes on my car.painted 2 white stripes opp. each other on the front and rear tyre..used a video camera to see how the brakes work on each wheel..reg. stop from 30mph.. and from 70 mph....i have video now( as of a few months ago acc.) of the wheels lock up or don't..i will test the brakes after i am done in the same manner, and if i need to i will add a willwood brake bias adjuster...as for my tires. i am running 205/50/16 rubber..i am testing this at an old air force base we have here..on sunday..nobody out there at all...and lots of runout room..as for stock stoping distance from 70 it is..158 feet on ave.. at the moment i do not have the dist. from 30 mph..trying to find it..maybe at work..i really do understant how brakes work ... i work at audi/vw as a tech untill i was dig. with parkensons.....i am not one to just put untryed parts on a car and go in to traffic and not know what will happen...like some of those other car guy, that just stuff big motors in there car, or stuff a turbo on the civic and wonder why the motor fails after a few runs.i am looking for a well balanced car..and i have good realtions with a guy how owns a porsche repair shop, and he also races his porsche 914/4....he said i could reduce the pad area to the same ratio as stock and retain the 4 pots on eather end....by the way.. he also put the same mazda calipers on his car, after he saw how light they are..( he did not know untill i brought a pair fo the calipers in to his shop.. he was going to get some willwood or some other make of calipers. but at $40.00 a pair at local auto wreckers.. he thought it was a good buy.. and to rebuild them only $ 19.00 at the dealership where i work as a car sales person...i am not going to endanger myself or anyone else....i was just asking a question.. just toss some thoughts to the wind..:):)
 

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Discussion Starter #20
..oh yes..i am very impressed how the stock brakes work and how alfa romeo, took all the dive out of the front end...she stops nice and flat..:)
 
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