Bigger Brake Question
05-31-2002, 11:19 AM
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Bigger Brake Question
I have upgraded front rotors/calipers on my A4. Porsche 996 calipers and S4 rotors. What will this do to front/rear brake proportining. If I upgrade the rear rotors and calipers to something larger, again, how will that affect proportioning?
Thanks,
-Brent
Thanks,
-Brent
05-31-2002, 12:49 PM
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If you kept the abs...
And the MC, then it is a crapshoot. The Bosch 5.x abs system constantly assesses brake pressure and proportioning, but its 'judgements' are based on the stock parameters.
If you keep the same rear piston, the relative proportion will be the same, with just a touch of less dive.
If you keep the same rear piston, the relative proportion will be the same, with just a touch of less dive.
05-31-2002, 01:38 PM
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IMO, not a good choice.
The Stoptech web page has a few things to read on the subject. But (and this does depend a bit on model year I suppose - traction control and all that) the problem is that bigger rears can lock up quicker. Especially since with bigger fronts the car will transfer weight forward more/quicker under braking - because of that nice first bite.
So when the rears start to lock, the ABS kicks in and pulses them. Thing is, since left front and right rear share circuits (and RF and LR) for safety reasons, you are also losing front braking power when this happens, because you are now pulsing them, but they are not yet in need of it.
The result is a longer stopping distance.
Now ABS able to individually pulse each wheel may exist on some model year. But don't confuse it with systems designed to BRAKE a given wheel. To my (admittedly limited) knowledge, all our cars still share crossed lines.
Really, with Stock ABS and front upgrades you get mostly an improvement ih heat dissapation and the ability to stop hard over and over again without fade. Actual braking distance may not actually improve much. The stock brakes can lock up wheels at 80 (if ABS were off) so how much more braking power is gonna help?
YMMV. This is a holy war subject with some.
So when the rears start to lock, the ABS kicks in and pulses them. Thing is, since left front and right rear share circuits (and RF and LR) for safety reasons, you are also losing front braking power when this happens, because you are now pulsing them, but they are not yet in need of it.
The result is a longer stopping distance.
Now ABS able to individually pulse each wheel may exist on some model year. But don't confuse it with systems designed to BRAKE a given wheel. To my (admittedly limited) knowledge, all our cars still share crossed lines.
Really, with Stock ABS and front upgrades you get mostly an improvement ih heat dissapation and the ability to stop hard over and over again without fade. Actual braking distance may not actually improve much. The stock brakes can lock up wheels at 80 (if ABS were off) so how much more braking power is gonna help?
YMMV. This is a holy war subject with some.
06-01-2002, 04:32 AM
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zain made very good points. Remember that during hard braking 80% is done by the fronts...
more or less. If you assume 60-40 weight distribution(A4), max braking transfers enough weight to the fronts (and removes it from the rears) that 80+ % is on the fronts. With better than stock tires (traction) the car has the ability to generate more braking force at the road, so even more weight is transferred to the fronts.
In most cases, upgrading the size of the rears (on 60-40 car) hurts more than it helps.
Of course a 40-60 car like Porsche gets pretty close to 50-50 or maybe closer to 55-45 during max braking. That's a good reason for approximately equal front/rear brakes.
My $.02
In most cases, upgrading the size of the rears (on 60-40 car) hurts more than it helps.
Of course a 40-60 car like Porsche gets pretty close to 50-50 or maybe closer to 55-45 during max braking. That's a good reason for approximately equal front/rear brakes.
My $.02
06-01-2002, 06:53 AM
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All true, but ...
.
there's more than one way to skin the cat.
The coefficient of friction of different pads can vary quite a bit. For example, the friction value of the Pagid RS4-2 Blue is lower than the RS4-4 Orange.
On the track, if a front only brake upgrade leaves the rears under utilized (definitely not good), running Blue in the front and Orange in the rear might solve the problem. If upgrading the rear brakes results in too much rear bias, you could try Orange in the front and Blue in the rear. You'd also need a special pad setup for the street.
It would be simpler, though, to have the correct brake bias in the first place; the experimentation alone could be rather costly.
The link shows the amount of weight transfer during 0.9 g braking with my S8.<ul><li><a href="https://forums.audiworld.com/racing/msgs/22637.phtml">Weight transfer during braking and cornering</a></li></ul>
there's more than one way to skin the cat.
The coefficient of friction of different pads can vary quite a bit. For example, the friction value of the Pagid RS4-2 Blue is lower than the RS4-4 Orange.
On the track, if a front only brake upgrade leaves the rears under utilized (definitely not good), running Blue in the front and Orange in the rear might solve the problem. If upgrading the rear brakes results in too much rear bias, you could try Orange in the front and Blue in the rear. You'd also need a special pad setup for the street.
It would be simpler, though, to have the correct brake bias in the first place; the experimentation alone could be rather costly.
The link shows the amount of weight transfer during 0.9 g braking with my S8.<ul><li><a href="https://forums.audiworld.com/racing/msgs/22637.phtml">Weight transfer during braking and cornering</a></li></ul>
06-01-2002, 08:12 AM
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Yep, that's the problem ...
.
Brake bias is determined by
1. Master cylinder front/rear hydraulic pressure proportioning (which isn't easily changed)
2. Caliper piston area
3. Rotor diameter (length of the "lever arm" from the center of the pad to the center of the hub)
4. Pad coefficient of friction
The master cylinder is fixed, but you can easily estimate the effect of a brake upgrade. For the stock brakes, calculate the ratio of front (piston_area times lever_arm) divided by rear (piston_area times lever_arm). This number is proportional to the brake bias that Audi designed for the car, and you should stay very close to this value. For my S8, the stock ratio is 1.96:1. [Don't forget that additional bias is added by the master cylinder, which probably brings the total front/rear bias close to 4:1.]
With this, I can now estimate the effect of a rear only upgrade. If I increased my rear rotor from 11" to 12", my calculated bias would fall to 1.76:1, I'd have about 10% too much rear brake, and the rears would lock up early. I'd either need to find a rear caliper with 10% smaller piston area, or use a pad with a 10% lower coefficient of friction.
Hope this helps,
Brake bias is determined by
1. Master cylinder front/rear hydraulic pressure proportioning (which isn't easily changed)
2. Caliper piston area
3. Rotor diameter (length of the "lever arm" from the center of the pad to the center of the hub)
4. Pad coefficient of friction
The master cylinder is fixed, but you can easily estimate the effect of a brake upgrade. For the stock brakes, calculate the ratio of front (piston_area times lever_arm) divided by rear (piston_area times lever_arm). This number is proportional to the brake bias that Audi designed for the car, and you should stay very close to this value. For my S8, the stock ratio is 1.96:1. [Don't forget that additional bias is added by the master cylinder, which probably brings the total front/rear bias close to 4:1.]
With this, I can now estimate the effect of a rear only upgrade. If I increased my rear rotor from 11" to 12", my calculated bias would fall to 1.76:1, I'd have about 10% too much rear brake, and the rears would lock up early. I'd either need to find a rear caliper with 10% smaller piston area, or use a pad with a 10% lower coefficient of friction.
Hope this helps,
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06-01-2002, 10:06 AM
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And in the case of my Stoptechs...
Stopech claims the brake bias is not changed very much by the front upgrade. A bit more up front but not a pile.
The idea being too much bias up front locks them up too quick and when ABS kicks in the rears on the same circuit pulse too - and you lose rear braking help - and increases distance.
Similar to my original comments about rears locking up and the fronts getting their power reduced - a much worse thing for overall stopping distance.
As others have said, you can only address proportionality by adjusting it at the main hydraulics. And you can only deal with the ABS cross circuits my removing it an replumbing the whole thing: not a good thing in a street car anyway.
Careful matching of rotor sizes, pad compounds and cylinder cross sections would also work. If you really knew what you were doing and had time and safe place to do some dangerous braking experiments. I do not. Hence the stoptechs.
Which are really designed more for good performance under lots of heating, and do not claim a stopping distance improvement. The claim maintaining the proportionality for the most part. So I didn't mess with my rear rotors, except to us Tarox G88 slotted ones to help vent gasses. On the G88 the slots are very fine, so I don't much surface area change.
- Z
The idea being too much bias up front locks them up too quick and when ABS kicks in the rears on the same circuit pulse too - and you lose rear braking help - and increases distance.
Similar to my original comments about rears locking up and the fronts getting their power reduced - a much worse thing for overall stopping distance.
As others have said, you can only address proportionality by adjusting it at the main hydraulics. And you can only deal with the ABS cross circuits my removing it an replumbing the whole thing: not a good thing in a street car anyway.
Careful matching of rotor sizes, pad compounds and cylinder cross sections would also work. If you really knew what you were doing and had time and safe place to do some dangerous braking experiments. I do not. Hence the stoptechs.
Which are really designed more for good performance under lots of heating, and do not claim a stopping distance improvement. The claim maintaining the proportionality for the most part. So I didn't mess with my rear rotors, except to us Tarox G88 slotted ones to help vent gasses. On the G88 the slots are very fine, so I don't much surface area change.
- Z
06-01-2002, 04:14 PM
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Re: And in the case of my Stoptechs...
.
What does StopTech recommend for front and rear pads for street and track?
Running with a high friction track pad in the front, the generally low friction stock rear pads, and your back to problem #1 ... the fronts lock early under hard braking.
Also, are you sure that ABS always releases two wheels? I agree that the main hydraulic circuits are diagonally redundant, but I'm reasonably certain that the ABS circuits are completely seperate from this, and if necessary, ABS can release one locking wheel individually.
What does StopTech recommend for front and rear pads for street and track?
Running with a high friction track pad in the front, the generally low friction stock rear pads, and your back to problem #1 ... the fronts lock early under hard braking.
Also, are you sure that ABS always releases two wheels? I agree that the main hydraulic circuits are diagonally redundant, but I'm reasonably certain that the ABS circuits are completely seperate from this, and if necessary, ABS can release one locking wheel individually.
06-01-2002, 08:37 PM
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I'm not sure...
when I was last at their site they had two pads, one less "street" than the other. I have the plain old street-able ones.
As for the ABS pulses, again, in my post below I was unsure how the cross over and ABS pulses interact. Indeed I just went back to the stoptech site and noticed I had merged two papers they wrote together in my mind. And messed myself up.
Indeed, modern ABS can separate front from rear. And pulse any given wheel. But there are still several things to worry about since you cannot really adjust all those proportionalities.
Stoptech's blurb on the rear upgrades is the link.
Their general blurb on ABS and big brake interation is at:
http://www.stoptech.com/whitepapers/abs_bigbrake_122701.htm
- Z
- Z<ul><li><a href="http://www.stoptech.com/whitepapers/rear_brake_upgrade_is_bigger_better120601.htm">Sto ptech on rear upgrades...</a></li></ul>
As for the ABS pulses, again, in my post below I was unsure how the cross over and ABS pulses interact. Indeed I just went back to the stoptech site and noticed I had merged two papers they wrote together in my mind. And messed myself up.
Indeed, modern ABS can separate front from rear. And pulse any given wheel. But there are still several things to worry about since you cannot really adjust all those proportionalities.
Stoptech's blurb on the rear upgrades is the link.
Their general blurb on ABS and big brake interation is at:
http://www.stoptech.com/whitepapers/abs_bigbrake_122701.htm
- Z
- Z<ul><li><a href="http://www.stoptech.com/whitepapers/rear_brake_upgrade_is_bigger_better120601.htm">Sto ptech on rear upgrades...</a></li></ul>
