brake bias fitting change on 87
08-13-2014, 03:21 PM
#31
Rennlist Member
Thread Starter
The other thing most forget about is how critical and how much the engine braking is after a downshift.
Guess what the force at the wheels is, at a blip downshift into 3rd and then into 2nd.
anyone??????
Guess what the force at the wheels is, at a blip downshift into 3rd and then into 2nd.
anyone??????
08-13-2014, 03:39 PM
#32
Rennlist Member
Thread Starter
Mark,
If, by some miracle, the rear of the car was as light as you suggest under hard braking, it would quickly be the front of the car if there was any steering applied. I might recommend that you try the different rear pad compounds with the rear bias valve removed and the ABS disabled. Find some deserted space to try a few high-g stops with small steering input similar to what you'd be doing 'trail braking', then try with hard braking and no steering lock at all. Your goal is to fid that combination of pads that gives you almost-locked on the rear at max braking force available on the fronts. Then fit bias valves progressively higher to lower to find the one that compensates for your extra pedal effort needed when the fronts fade. Then the ABS again for track condition surprises only.
There's no single textbook case that predicts the perfect solution for your car, your driving style, and your track conditions. You can theorize all you want about what's going on. An hour or two of testing, without your pre-conceived prejudices, will tell you a whole lot about what will work best for you in the end.
If, by some miracle, the rear of the car was as light as you suggest under hard braking, it would quickly be the front of the car if there was any steering applied. I might recommend that you try the different rear pad compounds with the rear bias valve removed and the ABS disabled. Find some deserted space to try a few high-g stops with small steering input similar to what you'd be doing 'trail braking', then try with hard braking and no steering lock at all. Your goal is to fid that combination of pads that gives you almost-locked on the rear at max braking force available on the fronts. Then fit bias valves progressively higher to lower to find the one that compensates for your extra pedal effort needed when the fronts fade. Then the ABS again for track condition surprises only.
There's no single textbook case that predicts the perfect solution for your car, your driving style, and your track conditions. You can theorize all you want about what's going on. An hour or two of testing, without your pre-conceived prejudices, will tell you a whole lot about what will work best for you in the end.
point is, there is only 300lbs or so on each of the rear wheels. there is some brake bias today, but ill admit, based on the graphs, its not much and it stays "not much" regardless of how hard i press the pedal because of the bias valve. but , how much force can you put on a rear tire before it breaks free longitudinally?
do you know how much force is applied during the downshift using engine RPM?
one measure of break away force, is the near 1g acceleration of a 450hp 928. you can barely get a chirp by flooring the car in 1st gear. thats with half the weight of the car sitting on the rear tires. 400ft-lbs, 8:1 gear ratio, thats 3200ft-lbs, and 3200lbs of acceleration force at the tires.
but, if you were to take the 2000lbs off the rear tires and change it to 500, that number jumps down to less than half that. we are talking grip potential of the rear tires.
under decel, and threshold braking, there is some substantial braking force in the rear + the decel engine force. care to guess what the decel force is to the rear tires in a 4th to 3rd downshift, and then 3rd to 2nd downshift, with high RPM blips?
in the end, i dont think there is much to be gained, and certainly you run a risk of engaging abs in the trail braking sections of a turn entry, for a marginal gain in the straight line braking sections if you optimize the rear brakes.
i agree, going to the largrer diameter rotor in front, with no bias change will lengthen stopping distances, because less pedal pressure will use less of the rear brakes. But, the main reason i went to a larger diameter rotor, was to get more bite when the brakes start to fade, which requires more pedal pressure , which should equal some things out. (based on the assumption that small or large rotors , can both threshold brake the front tire grip, until fade., (just using more or less pedal pressure to do it)
just for the reason alone of less pedal pressure needed for any initial brake sequence, is reason to up the bias valve. the question is how much did it help and how much did i loose as far as braking force in the rear.
the graph is pretty interesting (pedal pressure front vs rear with bias valve). looks like the rear bias valve gives a constant rear brake force almost regardless of the pedal pressure when braking with moderate pedal pressure to start.
08-13-2014, 04:02 PM
#33
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Last edited by dr bob; 08-25-2014 at 07:10 PM. Reason: [Done]
08-13-2014, 04:12 PM
#34
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Last edited by dr bob; 08-25-2014 at 07:10 PM. Reason: [Done]
08-13-2014, 04:46 PM
#35
Former Sponsor
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.
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.Try it.
Don't forget that those Cup Cars also have a limited slip you can only dream about. The higher percentage trailing throttle limited slip keeps the rear tires going straight.
I'm going to be making these, soon.
08-13-2014, 04:54 PM
#36
Rennlist Member
Thread Starter
To your response about how little you depend on the rear brakes, how light you think the rear is, and using the massive amount to engine braking to slow the rear down. Get the rear brakes closer to the fronts so you don't have to depend on full engine braking (and you still don't have enough total rear braking force). Then you can use the throttle to control the car's rotation better. I'll guess from ypour descriptions that you do a hard downshift and use the 'impact' of a hard clutch release to unsettle the rear and get the car to start to rotate.
Everything you've said so far points to the need for better brake balance. You manage to work around the shortcomings of the car pretty well. But this is relatively low-hanging fruit. Still driving on the worn-out shocks and tired original suspension bushings? More low-hanging fruit. You regularly point out that you have great results and reliability considering all the low-hanging fruit you refuse to pick.
Everything you've said so far points to the need for better brake balance. You manage to work around the shortcomings of the car pretty well. But this is relatively low-hanging fruit. Still driving on the worn-out shocks and tired original suspension bushings? More low-hanging fruit. You regularly point out that you have great results and reliability considering all the low-hanging fruit you refuse to pick.
the engine braking adds to whatever i have for rear brakes now. point is, the rear end is very lightly loaded under max braking force at the tires.
I guess the fronts are splitting near 2400lbs, while the rears are dealing with 600lbs. if you go thourgh some basic calculations, there is not much to be gained by maximizing the rear, and it could bite you in the trail braking turns. i will get the other bias valve and see how it works.
the engine braking is near 200hp !!!! thats 200ftlbs , and this goes down to about half before the next downshift. at 3rd gear, thats 900ft-lbs or 900lbs of force on the rear wheels...... then, in 2nd, thats near 1200ft-lbs, or 1200lbs of decel force on the rear tires. It doesnt seem like there could be a lot more available on the rears. just go through the calculations, or extrapolate from what we know.
you can use the decel rates i have found over the years to get your start.
3000lbs, 130mph at the start of the braking point, 50mph starts brake release for trail braking and the turn in. (around the turn speed is 45mph at turn 2 laguna) the entire braking time is 4 seconds, with the last second where have major fade.
by the way, as far as worn out shocks, and other things, laguna is a little unfair to compare due to its smoothness. i can drive around these problems pretty easily. if it was sebring.. the car would be undrivable. sears is a real handful as the car is today. have to be real careful on transitions due to lack of compression control on one corner, and weak rebound control initially on the other . its ugly i know, but still working.
as a note, when the ABS was "thunked " up, that was air in the lines and the pump was not working. we fixed that and its worked ever since. the holbert S4 had all the wires clippped so after splicing it up , it worked the same too. as designed. and ive done all the tests on gregorys pristine braking system 88 S4. same exact response. skilled foot beats ABS 100% of the time. except on ice!
08-13-2014, 04:58 PM
#37
Rennlist Member
Thread Starter
The ABS on the 928's is not much different than on a 964.....If you start to feel the pedal bounce.....push harder. If your system is working, it will stop, without locking the tires.
Try it.
Don't forget that those Cup Cars also have a limited slip you can only dream about. The higher percentage trailing throttle limited slip keeps the rear tires going straight.
I'm going to be making these, soon.
Try it.
Don't forget that those Cup Cars also have a limited slip you can only dream about. The higher percentage trailing throttle limited slip keeps the rear tires going straight.
I'm going to be making these, soon.
in otherwords, i stand on the 928 brakes, any 928, ive done this with many!! it just skates forward. I can destroy the slow down rate with a sensitive foot. no comparison.
08-13-2014, 05:00 PM
#38
Former Sponsor
its is as light as i have suggested. its a pure function of decel rate. with huge slicks up front, and the cars ive race (and seen their data aqu), its near 2gs. this means that less than 25% of the cars weight is on the rear tires.. and i have BIG tires on the rear.. racing as long as i have, you wont see me swapping ends, even if there was too much bias or the rear was light. as long as you hands are quicker than the rear end movement, and you match the movement with your hands, its pretty easy to control. you can see this in rain races too, where there is no grip at all.
point is, there is only 300lbs or so on each of the rear wheels. there is some brake bias today, but ill admit, based on the graphs, its not much and it stays "not much" regardless of how hard i press the pedal because of the bias valve. but , how much force can you put on a rear tire before it breaks free longitudinally?
do you know how much force is applied during the downshift using engine RPM?
one measure of break away force, is the near 1g acceleration of a 450hp 928. you can barely get a chirp by flooring the car in 1st gear. thats with half the weight of the car sitting on the rear tires. 400ft-lbs, 8:1 gear ratio, thats 3200ft-lbs, and 3200lbs of acceleration force at the tires.
but, if you were to take the 2000lbs off the rear tires and change it to 500, that number jumps down to less than half that. we are talking grip potential of the rear tires.
under decel, and threshold braking, there is some substantial braking force in the rear + the decel engine force. care to guess what the decel force is to the rear tires in a 4th to 3rd downshift, and then 3rd to 2nd downshift, with high RPM blips?
in the end, i dont think there is much to be gained, and certainly you run a risk of engaging abs in the trail braking sections of a turn entry, for a marginal gain in the straight line braking sections if you optimize the rear brakes.
i agree, going to the largrer diameter rotor in front, with no bias change will lengthen stopping distances, because less pedal pressure will use less of the rear brakes. But, the main reason i went to a larger diameter rotor, was to get more bite when the brakes start to fade, which requires more pedal pressure , which should equal some things out. (based on the assumption that small or large rotors , can both threshold brake the front tire grip, until fade., (just using more or less pedal pressure to do it)
just for the reason alone of less pedal pressure needed for any initial brake sequence, is reason to up the bias valve. the question is how much did it help and how much did i loose as far as braking force in the rear.
the graph is pretty interesting (pedal pressure front vs rear with bias valve). looks like the rear bias valve gives a constant rear brake force almost regardless of the pedal pressure when braking with moderate pedal pressure to start.
point is, there is only 300lbs or so on each of the rear wheels. there is some brake bias today, but ill admit, based on the graphs, its not much and it stays "not much" regardless of how hard i press the pedal because of the bias valve. but , how much force can you put on a rear tire before it breaks free longitudinally?
do you know how much force is applied during the downshift using engine RPM?
one measure of break away force, is the near 1g acceleration of a 450hp 928. you can barely get a chirp by flooring the car in 1st gear. thats with half the weight of the car sitting on the rear tires. 400ft-lbs, 8:1 gear ratio, thats 3200ft-lbs, and 3200lbs of acceleration force at the tires.
but, if you were to take the 2000lbs off the rear tires and change it to 500, that number jumps down to less than half that. we are talking grip potential of the rear tires.
under decel, and threshold braking, there is some substantial braking force in the rear + the decel engine force. care to guess what the decel force is to the rear tires in a 4th to 3rd downshift, and then 3rd to 2nd downshift, with high RPM blips?
in the end, i dont think there is much to be gained, and certainly you run a risk of engaging abs in the trail braking sections of a turn entry, for a marginal gain in the straight line braking sections if you optimize the rear brakes.
i agree, going to the largrer diameter rotor in front, with no bias change will lengthen stopping distances, because less pedal pressure will use less of the rear brakes. But, the main reason i went to a larger diameter rotor, was to get more bite when the brakes start to fade, which requires more pedal pressure , which should equal some things out. (based on the assumption that small or large rotors , can both threshold brake the front tire grip, until fade., (just using more or less pedal pressure to do it)
just for the reason alone of less pedal pressure needed for any initial brake sequence, is reason to up the bias valve. the question is how much did it help and how much did i loose as far as braking force in the rear.
the graph is pretty interesting (pedal pressure front vs rear with bias valve). looks like the rear bias valve gives a constant rear brake force almost regardless of the pedal pressure when braking with moderate pedal pressure to start.
If you are not wearing rear brake pads at the same rate as the front pads (assuming you haven't screwed up and still have the original size rear calipers), you've got some serious work to do....probably in more than one spot.
For example, if you really only have 300 pounds on the rear tires under braking.....you need to stiffen up the front of the car, considerably, to keep the car from diving so much under braking.
08-13-2014, 05:06 PM
#39
Former Sponsor
greg, Ive experimented many times. instructed with cars of more recent age. the 928 ABS is way too slow of a frequency controller. if you engage it, the car will skate off into the distance. should i video tape a test. full engagement vs threshold braking?? ive done it a 1000 times.. when abs engages, ive lost the corner. if i threshold brake, i can stop 2x as fast or slow 2x better rate. it brakes an releases so slowley, that the car will skate down the road. modern ABS , like the new bmws, porsches and ferarris ive driven, you can stand on the brakes and its as fast as me threshold braking. big diff.
in otherwords, i stand on the 928 brakes, any 928, ive done this with many!! it just skates forward. I can destroy the slow down rate with a sensitive foot. no comparison.
in otherwords, i stand on the 928 brakes, any 928, ive done this with many!! it just skates forward. I can destroy the slow down rate with a sensitive foot. no comparison.
Twenty year old thinking always results in twenty year old results.
Like I said, if you are locking the tires up, using the ABS....you've got some serious work to do, in several different areas.
08-13-2014, 05:14 PM
#40
Rennlist Member
At the First OCIC in Wichita KS they did a DE track day at the track in Hallett OK. I was planning to take the 90GT I had just purchased from Jarrod Wise when the local 944 tuner shop offered to pay me and my expenses for the day if I took my white 88 944 Turbo S. They bought some dyno time for some intercooler testing using my 944 Turbo on the mobile Dynojet that was available for the event.
My personal goal for the track that day in the 944 Turbo was practicing my late braking/trail braking on corner entry. However, I had put on new pads the day before and not taken the time to bed them in before the first session. Towards the end of the first session I experienced the worst boiled the fluid brake fade one could possibly have.
Hallett's second turn is the tightest turn on the track, a left hander entering from the end of the fastest straight. The turn is named the stop sign and if you continued straight you don't just go off the track in about 50 ft, down hill, you go into a grove of trees. Late in the first session I was headed into the stop sign, glanced down and was accelerating past 140mph. Moved to the outside and started watching the markers leading into the corner. My brake point came up and I hit it. To my surprise the pedal just went thump against the floor, No Brakes!!
Without taking time to think I just reacted and immediately pumped the pedal a couple of times...nothing. Without further hesitation I did a scandinavian flick sending the car sliding sideways pointed towards the inside of the turn. As I slid sideways it dawned on me that I seemed to be slowing pretty well, and maybe if I downshifted and got into the throttle I could make the corner and did just that. As i got back in the throttle i counter steered to keep the rear from coming around too much and amazingly enough the car continued it's sideways slide, but arced towards and into the apex. Keeping on the gas and counter steering I straightened it out and was exiting the turn heading down the next straight.
Did a quick glance back at the corner worker, he was standing up and giving me a thumbs up with both hands. I kind of coasted blankly through the next two turns not even going fast enough to touch the brake pedal. On the next turn I gingerly tested the brakes and sure enough, soft spongy pedal and almost not brakes. It wasn't until then I realized I had boiled my brakes. So I used the last three laps left in the session to just cruise around the track, letting everyone pass me, and NOT use my brakes so they could cool as much as possible.
Kim Crumb was riding as an instructor in the track prepared 928 directly behind through that corner. He came over to me after the session and told me he saw what I did, and wanted me to know, "You only have so many of those in you!"
So, with all this talk about brake bias, ABS, and trail braking and car setup, my fastest time through that corner was with no brakes at all!
My personal goal for the track that day in the 944 Turbo was practicing my late braking/trail braking on corner entry. However, I had put on new pads the day before and not taken the time to bed them in before the first session. Towards the end of the first session I experienced the worst boiled the fluid brake fade one could possibly have.
Hallett's second turn is the tightest turn on the track, a left hander entering from the end of the fastest straight. The turn is named the stop sign and if you continued straight you don't just go off the track in about 50 ft, down hill, you go into a grove of trees. Late in the first session I was headed into the stop sign, glanced down and was accelerating past 140mph. Moved to the outside and started watching the markers leading into the corner. My brake point came up and I hit it. To my surprise the pedal just went thump against the floor, No Brakes!!
Without taking time to think I just reacted and immediately pumped the pedal a couple of times...nothing. Without further hesitation I did a scandinavian flick sending the car sliding sideways pointed towards the inside of the turn. As I slid sideways it dawned on me that I seemed to be slowing pretty well, and maybe if I downshifted and got into the throttle I could make the corner and did just that. As i got back in the throttle i counter steered to keep the rear from coming around too much and amazingly enough the car continued it's sideways slide, but arced towards and into the apex. Keeping on the gas and counter steering I straightened it out and was exiting the turn heading down the next straight.
Did a quick glance back at the corner worker, he was standing up and giving me a thumbs up with both hands. I kind of coasted blankly through the next two turns not even going fast enough to touch the brake pedal. On the next turn I gingerly tested the brakes and sure enough, soft spongy pedal and almost not brakes. It wasn't until then I realized I had boiled my brakes. So I used the last three laps left in the session to just cruise around the track, letting everyone pass me, and NOT use my brakes so they could cool as much as possible.
Kim Crumb was riding as an instructor in the track prepared 928 directly behind through that corner. He came over to me after the session and told me he saw what I did, and wanted me to know, "You only have so many of those in you!"
So, with all this talk about brake bias, ABS, and trail braking and car setup, my fastest time through that corner was with no brakes at all!
08-13-2014, 05:20 PM
#41
Rennlist Member
Thread Starter
EDIT... i didnt mispeak.. i re-read it.... it skates off into the turn... not lock up, but the braking force is so much less, its like you hit ice or are locked up, but we are not locked up. in otherwords, ABS is good for rain and snow, not on the track when talking about 928 ABS. I can prove this at any time, very easily.
20 years of racing here Greg, more experience in a 928 racing than anyone here now. I think i know the car pretty well by now.
what are you talking about. i engage ABS and its way sllower than theshold braking. what can i say. do you think ABS should work better? again, i drive many other cars and do this test the 20 year old ABS doesnt work near as good as threshold braking.. in fact, if i race you and you use it on a 928, ill fly by so fast it will make your head spin.
08-13-2014, 05:27 PM
#42
Rennlist Member
Thread Starter
I have all what you say on video. in fact, even with boiled brake fluid you stilll get some pressure. i was at laguna seca, hitting the main straight and blew the brake lines. no brakes... nothing. i was able to do what you talk about, by downshfting and trailbraking like turn in. saved the corner and the car. same thing happened to a less experience driver at thunderhill and he rolled and flipped the car.
great you were able to use the 600 to 1200lbs of force to slow the car with the engine, along with turn in friction at the tires.
BUT, and read this real carefully, doing what you did, in now way would be the fastest way around that turn. once you break traction, you are slower than if you were able to keep it. if you didnt break traction, then you were just using the brakes in an area where you didnt need to before.
08-13-2014, 05:37 PM
#43
Rennlist Member
I have taught a driving clinic several times for the local PCA autocross group. The first actual driving I let them do is straight line braking from 80mph to a complete stop.
It's set up so you can measure by how many cones you pass how quickly you can bring your car to a stop. With plenty of run off room. I have to really stress in the clinic to get on the brakes hard then as you feel the cars weight go forward press harder. Most don't hit the brakes hard enough initially and then trigger the ABS too soon. The first time they do it right it's like a BIG light coming on and their stopping distance is almost cut in half.
I use to try to tell them 80% on initial braking, but that usually translates to 60% on the pedal. Have to just tell them if the ABS kicks on immediately when they hit the brakes, they need to ease up a tiny bit, then stand on em. Hitting brakes hard enough at first to immediately go into ABS is rare though.
Young drivers seem to learn the quickest as they don't have the years of ingrained limo stops built up yet. One older 964 driver could never get it. It could do it in he Z06, but would always trip the ABS too soon on his 964. He just couldn't trust his brakes in the 964 he had done the ABS slide so many times.
It's set up so you can measure by how many cones you pass how quickly you can bring your car to a stop. With plenty of run off room. I have to really stress in the clinic to get on the brakes hard then as you feel the cars weight go forward press harder. Most don't hit the brakes hard enough initially and then trigger the ABS too soon. The first time they do it right it's like a BIG light coming on and their stopping distance is almost cut in half.
I use to try to tell them 80% on initial braking, but that usually translates to 60% on the pedal. Have to just tell them if the ABS kicks on immediately when they hit the brakes, they need to ease up a tiny bit, then stand on em. Hitting brakes hard enough at first to immediately go into ABS is rare though.
Young drivers seem to learn the quickest as they don't have the years of ingrained limo stops built up yet. One older 964 driver could never get it. It could do it in he Z06, but would always trip the ABS too soon on his 964. He just couldn't trust his brakes in the 964 he had done the ABS slide so many times.
08-13-2014, 06:03 PM
#44
Rennlist Member
When the pedal goes thump against the firewall and continues to when you pump it, there are NO BRAKES. Not fade, no brakes.
And yes i did know the limit of my car to begin with. At that time me in my 944 Turbo was the car to beat. In fact, at the local autocross the ONLY car to get within 4 seconds of my 60 second lap times was ME in my 90 GT.
I've learned that despite the rule of thumb of sliding is slower i.e., drift more than 10-15 degree slip angle, that there are some circumstances and situations where putting the car out of control and catching it, while dangerous and very risky, is faster.
And yes i did know the limit of my car to begin with. At that time me in my 944 Turbo was the car to beat. In fact, at the local autocross the ONLY car to get within 4 seconds of my 60 second lap times was ME in my 90 GT.
I've learned that despite the rule of thumb of sliding is slower i.e., drift more than 10-15 degree slip angle, that there are some circumstances and situations where putting the car out of control and catching it, while dangerous and very risky, is faster.
