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when I was building the estate......I bought a 33 bar bias....thinking it came with an 18 like my S4 had. Then I checked it..............55!!!! Very odd, but also the single best braking 928 on track I ever drove, given crap tires and stock S brakes. Everyone drove that car at the limit...me, MK, Anderson, Jeff, Sean, Petty.....everyone liked the brakes......all I am saying
It's probably not a bad time to remind folks that what we refere to as the "bias" valve is much more accurately described as a "rear brake pressure limiting valve" for all but the most extreme conditions. It limits the pressure rate of increase under hard braking when loop pressure exceeds the valve's rated pressure. Until you exceed that rated pressure, bias is strictly a matter of MC piston areas, caliper piston areas, pad friction and contact area, and the diameters of the rotors.
MK's question about 'trail braking' is one where the bias valve will have no effect. As Jeff points out, it's a matter of tuning the other variables (like rear pads) to best match up with what's going on in front.
Meanwhile, good ABS offers a tremendous amount of forgiveness if the rear pressure is allowed to go high enough to start locking a rear wheel. It takes a little getting used to the pump 'helping' modulate system pressures under your foot, but that action is probably the easiest empirical-tuning tool available to the average driver who might want a little more rear participation in hard street stopping. If a wet street is the worst condition you drive in, find some empty parking-lot real estate next time it rains, and practice some full-pedal stopping with some steering input added, and see what happens first with the ABS diasbled. Then add ABS back in to see how much it's contributing.
..and/or...be surprised how much MORE BRAKING is available by braking from say 60mph hard without engaging front ABS, and pulling some pressure (steady not instant 100%) with the Ebrake.
That's what you dont have, that you COULD have with changing friction levels and/or bias valve swaps.
I have a recommendation for the rear for the street/autox/occasional track types. A HIGH friction low temp pad that spec class racers love because they cant change bias (via OEM valve or manual adjustment).
Porterfield R4-1. Its a "fix that" pad.
R4-1 – The Vintage compound this pad is designed for vehicles that have trouble keeping heat in the pads. Best suited when temperatures are under 600F and peak temps are not over 1000F. We have also found this to be a very useful autocross pad.
It wont eat the rotor, it will dust more than a normal pad, but the dust will NOT corrode/stick to anything. It's carbon/kevlar dust.
They have to be hand cut for the calipers we use, so when you fit them you may need to hand file some of the backing plate tabs, and they may end up a tiny fraction of a MM too thick, because they are all hand made for our use.
They are only manufactured in some specific larger sizes, and then custom ground per order for everything else.
It's a very unique feeling once you get REAR brakes actually participating in braking.
The way the Rear Brake Bias Valve works on Porsches is...
The rears get the same pressure as the fronts until the changeover pressure of the valve is met, then the pressure increase to the rears is less than the fronts.
With the 5/55 on my GTS and the ABS disabled the rears locked up about 50 ft into braking from 90mph trying to hold braking at threshold. So with my tire sizes brake pad combinations and suspension settings the 5/55 is too high and the rears locking up well before the fronts induces ABS modulation before maximum braking both front and rear are achieved.
On my 90GT with less rear rubber and smaller front brakes than the GTS the 5/55 bar worked great which is why I went to the 5/55. On the 90GT I had tried the 5/33 and there was more rear braking available so went to the 5/55 and worked great without rear lockup like on the GTS.
The difference seems the opposite of what I would have expected. Maybe it is the bigger brakes on the front of the GTS needing less rear bias because it is transferring more weight off the rears sooner. Maybe my GTS is setup with less weight on the rears to begin with. Don't really know without some kind of telemetry. The 90GT only the front rotors turned blue, the GTS both the front and rear rotors are blue after a few autocross laps.
Last edited by RKD in OKC; 08-08-2014 at 05:29 PM.
I'm surprised that folk who go racing are bothering with messing about with a
limited range of bias valves. Also the Porsche ABS system is pretty old technology
and has no place on a race car.
One of cheapest ways of cutting lap times is to optimise your braking.
The best option is to fit dual master cylinders with a balance bar and no ABS.
Failing that pull the ABS fuse and fit a variable bias valve in the rear circuit.
This is a very cheap modification and works amazingly well. My track Triumph Dolomite
Sprint has front discs with stock rear drums. Rear wheel lock up was originally
prevented by a crude antilock system activated by the height of the rear suspension.
Brake hard and nose goes down, tail goes up and pressure to the rear brakes is cut off.
I removed that and fitted a TILTON variable bias valve and now can fine tune the braking
so that the fronts lock up just barely before the rear in any situation, wet or dry.
I'm surprised that folk who go racing are bothering with messing about with a
limited range of bias valves. Also the Porsche ABS system is pretty old technology
and has no place on a race car.
One of cheapest ways of cutting lap times is to optimise your braking.
The best option is to fit dual master cylinders with a balance bar and no ABS.
Failing that pull the ABS fuse and fit a variable bias valve in the rear circuit.
This is a very cheap modification and works amazingly well. My track Triumph Dolomite
Sprint has front discs with stock rear drums. Rear wheel lock up was originally
prevented by a crude antilock system activated by the height of the rear suspension.
Brake hard and nose goes down, tail goes up and pressure to the rear brakes is cut off.
I removed that and fitted a TILTON variable bias valve and now can fine tune the braking
so that the fronts lock up just barely before the rear in any situation, wet or dry.
actallly, i use the ABS about half the time im racing. i dont need it or ever get into it. but i like it at tracks lke sears, were its an early warning system. you hear it and back off.its never hitting the rear brakes , always the fronts. without it, the car drives exactly the same as it should. the only difference is , if i make a mistake, i can get a tire to lock. much of the rear braking is downshifting anyway using the engine to slow and fight lock up.
Originally Posted by RKD in OKC
The way the Rear Brake Bias Valve works on Porsches is...
The rears get the same pressure as the fronts until the changeover pressure of the valve is met, then the pressure increase to the rears is less than the fronts.
With the 5/55 on my GTS and the ABS disabled the rears locked up about 50 ft into braking from 90mph trying to hold braking at threshold. So with my tire sizes brake pad combinations and suspension settings the 5/55 is too high and the rears locking up well before the fronts induces ABS modulation before maximum braking both front and rear are achieved.
On my 90GT with less rear rubber and smaller front brakes than the GTS the 5/55 bar worked great which is why I went to the 5/55. On the 90GT I had tried the 5/33 and there was more rear braking available so went to the 5/55 and worked great without rear lockup like on the GTS.
The difference seems the opposite of what I would have expected. Maybe it is the bigger brakes on the front of the GTS needing less rear bias because it is transferring more weight off the rears sooner. Maybe my GTS is setup with less weight on the rears to begin with. Don't really know without some kind of telemetry. The 90GT only the front rotors turned blue, the GTS both the front and rear rotors are blue after a few autocross laps.
interesting. ive been using my S4 at a little more extreme level, and using GTS rotors up front, and very aggressive pads, i have also 315 slicks up front. this adds a tremendous amount of meat up front to stop with . and , im able to get a decel g loading that stands the car on its nose. literally, the car is decelling at a rate of between 1.5 and 2g. this means very little weight is left on the rear tires. i already start out with 56/44% weight distribution.... during a threshold brake, with hug rear slicks too, the weight on them, is pretty light. maybe 300lbs a tire .dont know how much more the rears can help the decel process, and it would be interesting to see a data acquisition plot of a threshold brake with and without rear brakes...... of course, dont forget the power of the engine on the decel in the high rpm downshift ranges... does anyone remember????? I was able to run A 1:46 LAP at laguna after completely losing my brakes to a blown brake line. that's amazing that you can slow and turn with no brakes with just engine and turning braking and run that fast of a lap.
I do like the idea of using a very low temp racing pad on the rear to increase the rear effectiveness withoiut changing the bias valve.
one other thing to note..... when i have my fade issue... that is at the end of straight to turn 2 at laguna. i end up, after 3 seconds of braking, the last 1second is fade... bad fade, so im pushing as hard as i can. what is that doing to the rear brakes???? kind of over bias , right? during this effect, the rears are having no effect on slow down. But , based on the graphs that were posted, maybe its not supposed to as it looks like the bias valve just pushes all the pressure to the fronts and bleeds it off the rears.
And from your explanation sounds like your bias valve is the 5/18 no doubt. The rebound of the rear dampers also affects the rear braking availability by how quickly the weight lifts off the rear. There is a lot more than just the bias valve in play here. That's why the call it setup and why the driving style matters so much for each setup.
And from your explanation sounds like your bias valve is the 5/18 no doubt. The rebound of the rear dampers also affects the rear braking availability by how quickly the weight lifts off the rear. There is a lot more than just the bias valve in play here. That's why the call it setup and why the driving style matters so much for each setup.
I try to be smooth under braking , to take that variable out of the equation, but even under a quick stab, or more aggressive stap, where the weight trransfer is faster, which in itself doesnt change anything... its when the rear of the chassis stops moving up, (at the end of travel) were it unloads the rear more than just a constant threshold braking action. however, if it doesnt reach end of travel, which it most lkely will not. (because that would be 100% of the braking force on the front, and 0 on the back), how fast it travels upward , does lessen the weight on the rear tires before it gets to steady state where then the weight on the rear tires is a fuction of decel, car weight, and wheel base.
"because that would be 100% of the braking force on the front, and 0 on the back"
---
To be clear, that would be braking potential via line pressure. Line pressure assumes no actual braking deliverable..just that you have pressure delivered.
Force has to include variables such as braking material friction coefficients and tyre characteristics as well.
Okay, first. Overall front to rear balance to me isn't necessarily the actual weight on the wheels. It is the front and rear tires starting to break loose at the same speed slightly under throttle (for maintaining speed to make up for cornering scrub). The allows me to throttle steer where adding and reducing throttle changes the balance to either rotate into a tighter turn or widen out the turn as I 4 wheel drift thru the apex. It has to front to rear weight, front to rear spring balance, tire grip area, and anti sway bar settings.
Braking is all about setting the car up for the corner. First I want the car to slow down enough that I can turn the corner. I want that to happen as quickly as possible so I can brake as late as possible to stay going fast as long as possible. For this to happen the front and rear brakes need to do as much braking as they possibly can.
Technique of applying the brakes or driving style has some to do with it. Jab the brakes to hard to quickly and the front tires will lock up because they do not have enough weight on them to brake as much as you are asking them to. Apply the brakes too slowly and you are losing time and increasing the area need to slow the car. So the best technique is to brake as hard as the tires can take and increase braking force as the weight transfers to the front tires and they are able to brake more.
While managing maximum front braking for weight transfer you want the bias valve to apply less and less rear brakes as weight transfers to the front. If the braking stays the same amount as the weight transfers the rear tires will lock up because they don't have enough weight on them for the braking they are being asked to do. The bias valve helps to control the amount of braking going the the rear tires in relation to the pressure going to the front tires.
However pressure bias is not the only thing that controls the front to rear transfer of weight and the overall rear braking effectiveness. The rebound rate of the rear dampers also controls how quickly the rear lifts. Not how much, but how quickly. Too quickly and yes you can jab the pedal quicker for front brakes to be more effective sooner, but you would also need less rear bias sooner to keep the rear tires from locking up and your front brakes will be doing all the work making them overheat and/or fade sooner. Adding firmness to the rear dampers slows the transfer of weight off the rear tires so you can use a higher rear bias and they can do more of the braking. I initially started adjusting the rear rebound and using higher bias valves to keep from overcooking the front brakes on my 944 Turbo.
The other job of the brakes is to trail brake on turn in. What this means is you slowly let off the brakes as you turn the steering wheel to turn the car. The idea being that if you start the car turning before all the weight that is on the front the lightened rear end will start to slide out or rotate the car towards the apex. Lifting slowly also keeps more of the weight on the front for maximum turning of the front wheels. To stop the rear rotation, when the car is pointed at the apex you add just enough throttle to stop the deceleration and transfer the cars weight to it's neutral balance. If you didn't brake too aggressively this should put the car in a slight 4 wheel drift heading for the apex of the turn under enough throttle to maintain the drift. The higher the rear brake bias the car has the less you need to lift off the brakes for the rear to start rotating. Making the corner turn in rotation easier to manage.
For a reference, all of the Porsche factory 964's and 993's racecars (RSR's, German Cup Cars, and the ill fated American Cup Cars) had the bias valves completely removed.
The ABS system is designed to eliminate one wheel from locking up....and to allow the driver to bring the other wheels closer to lock-up. Only the 993's had a really scary ABS system.....which hurt a few drivers badly, until an "override" system was added. This problem was "fixed" in the next generation of the ABS system, used on the early water cooled RSR's. Current generation racing ABS systems are light years ahead of what we have in our 928's....but the theory is the same....bring all four tires into a state of almost being locked up.
The bias valve is added to make sure a street vehicle will absolutely not lock the rear tires....in rain or snow conditions.
The bias valve can also be used to compensate for when someone does something dumb, in the building of a racecar. "Well balanced" brake systems do not need the bias valve, with ABS.
It was always very difficult to teach drivers to properly use the ABS system on the track....very few knew what to do, when the first signs of ABS begin to appear (pedal begins to jump) and like Mark does, they would release some foot pressure to keep this from occurring. Rookies. The opposite reaction is actually the desired way....as soon as the ABS begins to pulse the pedal....push harder, which brings not just one tire into ABS mode, but all tires into ABS mode. The stopping distances decrease in a huge amount when all tires are put into ABS mode, requiring an entire change in how each corner is handled. This requires a complete "retraining" of how to drive any particular race track.
The drivers that really knew how to use the ABS were always the fastest guys on the track. You could watch cars come in, from an open test session, and know who was really fast, by simply looking at the cars. The guys with the smoking brakes and the smoking rubber on the inside of all four wheels were the fast ones....those were always the cars that we had our own drivers "go to school on" when those drivers were in those cars. The rest of the drivers were just there to fill the field.....and to provide moving chicanes.
Racecars without ABS require a much higher tire "budget", when driven near the limits, by most drivers, due to flat spotting tires under braking. It borders on absurd to have current day racecars not equipped with ABS. The techniques required to very quickly slow the Cup Cars without ABS (without locking the rear tires) would stun most everyone here....and very few drivers are able to operate these cars using these techniques...which is why there is such a huge variance in lap times between people that understand and use these techniques and the rest of the field.....and why so many "club racers" ruin so many tires.
A fun story about ABS.....and trail braking....and when I got the very first clue that I'd better stick to fixing cars and not driving them.
I arranged a private driving lesson at Bob Bondurant with Walt Maas as my instructor. Walt Maas retired from racing and was a part time instructor at Bondurant.....and certainly one of the very, very best Porsche 911 drivers that there ever was. I took over my one year old converted RS America to drive....hoping that they would let me put it on the track, with Walt teaching me what he knew. This car was essentially a very well developed and very fast Cup Car clone (no bias valve, BTW.)
Vic Edelbrock was there, with his daughter, and they had a couple of American Trans Am cars, on their trailers.
At the beginning of the third day, Walt told me to get the RS off of the trailer and we'd take it out to see what it could do. I have no idea what the Bondurant track looks like today, but in those days, it was basically perpendicular to Firebird, with some interconnecting sections that allowed access to that track. At the far end of Bondurant's track, there was a 180 degree turn, with a fence maybe 20-30 feet away from the corner, with a public street out past the fence.
I drove the car around the track (faster than any human had ever driven anything, of course) 10-12 laps, with Walt sitting in the full race passenger seat, secured with another 5 point harness. Walt would make suggestions and point at things, trying to give me clues. Great stuff.
Then Walt took over, to show me some tricks. Walt drove the car around Bondurant's track a couple of times, said he thought he had it figured out, and wanted to try out the ABS, with trail braking.
We went into that far 180 degree corner 40-50 miles per hour faster than I'd ever been. It was absolutely crazy. I thought, without a doubt, that we were simply going through that fence. Walt put the car into full ABS to initially slow it (engaging all 4 wheels into ABS), then did some sort of magical trail braking thing right past the apex, rotated the car perfectly, and drove out the other side....20-30 mph faster than I'd ever been.
All I could do is laugh (while wishing I could get out of the car) knowing that I'd never, ever, be able to do that.
By this time, Vic and his daughter were also on the track with their Trans-Am cars. At lunch, the instructors and the mechanics went out and opened up the connecting roads to Firebird and after lunch, we had a huge connected Firebird and Bondurant course to run. Every mechanic, every instructor, every student was in one of Bondurant's cars. Vic and his daughter drove the Trans-am cars, and I was out there in the Porsche.
Greg, those are the miracles of ABS... ahhhhhh , RACING ABS, not ABS on our poor old 928s.
If i was to go into turn 11 at sears and engage 4 wheel ABS, i would not only run through the fence, i would end up at the ticket booth! I race with guys with the new cup cars, and i see what they can do trail braking, and during the initial braking zones. its amazing to see them just stab on the brakes, no bias, so all wheels are getting max braking, and no lock up due to the frequency of the ABS controller.... racing art tuning to automation. its the only edge ive had for years, and racing one of these cars, only allows steering through the turn as my only advantage.
08-08-2014, 04:25 PM
