mark kibort

Again
U are totally missing the point
And I don't let the clutch out during a turn
Waisted text right there
I'm am easier and smoother than any one u will see
Btw
The tires i used were over 6 years
They are not 3 seconds faster
I still use the sets of 06 toyos ra1s

GregBBRD

"Again
U are totally missing the point"

You can't possibly stop a vehicle using only the two front brakes and the two front tries, modulating the pedal to prevent lock up, as quickly as an ABS system can stop a vehicle bringing all four tires right to the very edge of lock-up.

You could be the best driver the world has ever seen....and this would be true.

There's a limited amount of ways to reduce lap times.....Better driving, more power, higher cornering forces, lighter weight, better braking.

To reach the plateau you are currently on, you probably improved your driving ability, you have better tires, and you have increased horsepower.

You are stuck on that current plateau until something else changes.....you've been doing the same damn thing for 15 years, with the brakes....no wonder you are not any faster!

On top of that, you are already having brake fade issues!

What's the next, most economical way to improve your lap times? Buy a set of brand new tires every weekend? Take your engine out, send it to me, along with 50K and get more power?

How about keeping the front end from diving under braking, get all 4 tires involve in the braking, bring all 4 tires to the very point of lock-up by using ABS?

I've been saying this, from my very first post.....

RKD in OKC

When I got my GTS it had the suspension setup by DEVEK.
Hypercoils, Devek front anti-swaybar (the blue one), and externally adjustable Koni Reds (rebound).

The first thing I did was set the konis to full soft and check the steady state turn balance of the anti-sway bars. And added Ott-Steroid links to the rear to firm it up and adjusted the front bar to get the a good balance in the apex. Where adding power at the limit both front and rear would slide out in a bigger radius, then letting off power the front would grip more and rotate into the corner, throttle steer in 4 wheel drift.

Next came adjusting the front rebound so when adding throttle for turn exit did not lift the front too much and induce understeer, but also wasn't so firm that would hold the front down enough to get immediate rear wheel spin and power oversteer.

THEN came BRAKING.
I was getting brake fade, pedal got spongy towards end of lap, and at the end of the day the front rotors were blueish, rear rotors looked unused.
When ABS kicked in I could feel the pulsing pulling the steering wheel back and forth.
Most of the braking was being done by the front brakes or in the least they were triggering the ABS.

Immediately went with higher temp brake pads, EBC Yellows. And because of my experience with setup on a 944 Turbo and 928 90GT I went straight to a 5/55 brake bias valve over the 5/18 stock one. Then worked on adjusting the rear rebound to just soft enough the rear would start to rotate if I put in just a little steering input under heavy straight line braking.

Setup was pretty good. Both front and rear rotors were bluish at the end of the day and the ABS pulses were mainly felt in the brake pedal not the steering wheel. Was getting the fastest laps on street tires matching times and sometimes even setting FTD against cars running Hoosiers. Not too bad for a heavy old 928, pretty happy with setup.

Then at one event the accelerometer under the driver's seat came unplugged. Didn't know at the time that was what had happened. Spent an entire event with ABS light on and PSD Controller OFF in the display. Besides the obvious one rear wheel spin limiting acceleration, noticed the rear tires were consistently the tires locking up under braking. Got me to thinking. When ABS is working are rear brakes triggering ABS before fronts get maximum braking/weight transfer and is that limiting overall braking potential? Can't add weight to rear with more rebound firmness or will lose turn in trail braking rotation. So, planning to try a lower bias valve. Got a 5/44 to try, but haven't been able to drive in anger since my foot operation. The 5/55 that worked well on the 944 Turbo and 90GT might be too much for the 928 GTS because the GTS has more tire stagger.

Also as a note: Thought the whole idea of double de clutch down shifts under braking was to rev match so gear changes on would have the least effect possible on braking and car balance. Basically to involve the engine in rear braking the least amount possible but still have the transmission engaged and ready when needed. My goal shifting is to be so smooth I could shift mid-apex without upsetting a 4 wheel drift.

As far as what cup cars have or don't have my old skool mechanic once told me. "Hang a bag of poop on your front bumper and start setting fast laps, then see how quickly everyone else starts hanging a bag of poop on their front bumper."

I'm not trying to tell anyone what to do or what not to do or what will or won't work for them, Just sharing my experiences setting up my car.

mark kibort

A lot of what we are saying to one another are responses to misunderstood points or thoughts
Sorry . Regarding brake % use
I just got your point
Ur talking pedal pressure
I re read you're post and caught the comment " big guy like you and the abs hits at 60 %" and a light went on
I'm talking system presure to get max grip of the tires
Big diff

Next
I don't believe our abs is good enough to stop better than me
I'll easily be able to prove this
If you are right I could be a litttle faster

Next I have fade because of brief over temp of pads at one section of track
I don't thing rear bias can reduce 25% stopping distance
But easily proved

You still didn't explaine how rear abs can engage with Car in gear
Up until turn in point
Also ,
Confused over your comment of your drivers downshifting and not letting out clutch
Of course all this is braking before turn in point as I have said many times before
This discussion is 130 mph and all the downshifts before release after turn in before apex

RKD in OKC

Oh Yeah, almost forgot.

On the 944 Turbo and 90GT went to higher temp pads and frozen slotted front rotors along with the 5/55 rear bias valve to keep the fade monster away. Used the temp paint and was getting 900° on the front rotors. They didn't fade every event, but front rotors would warp, then start fading regularly. Slotted frozen rotors stopped the warping problem. With the big blacks on the GTS all I needed was the higher temp pads and 5/55 bias valve and am still getting away with using stock rotors.

My ol' skool mechanic told me about cryotempering 911 push rods back in the day using dry ice and alcohol to keep them from bending.

GregBBRD

My statement about not letting out the clutch was not as carefully worded as possible.

My point was that we never want a driver to use the engine as a brake, on a downshift at high rpm....unsettling the car and beating up the drivetrain. Of course we would want the car to be in the appropriate gear and the clutch released, when the time comes to accelerate. This can generally be done very smoothly, at a later point in the corner, without a "downshift gear brake".

I've never given much thought to what happens when rear ABS engages when the engine is coupled to the rear tires at high levels of rpms and off throttle. I just let whatever is going to happen occur.

The sequential transmission Cup Cars (without ABS) can be made to avoid rear tire lock up by high rpm downshifting (without using the clutch), quite effectively.

Therefore, I'd naturally make the assumption that the coupling of the engine to the rear tires would "assist" keeping the rear tires in staying "away" from lock-up and the ABS would have less "work" to do.

I certainly do not see it being a problem....

mark kibort

Greg,
(this is long, but try and be patient and read carefully!)

You did say, you were looking at some data and trying to make sure that the drivers went through the downshifts and HAD NOT let the clutch out to avoid engine braking. you also seem to equate, engine braking with letting the clutch out quickly, which are not the same. letting the clutch out quickly, increases rear wheel decel forces for a moment, but after the clutch is let out, these forces are linear, decreasing with engine speed.
you also said for me to engage ABS and and then push harder to engage rear ABS.... insinuating that you must know you can engage Rear ABS, which you usually cannot.
As you said, you haven't given it much thought, but i do and have used and experimented with those forces for many years now. just because i dont have a million dollar budget, doesnt mean i cant get relevant testing done. Some use laser alignment machines. I use trig and a level. I can still get very close to the aliment machines for the simple settings. Just because i use video and a stopwatch and redline shift markers at the track, doesn't mean i cant tell when i have a power or driving style issue.

So, you have to think about this one clearly. We have 3 channel ABS. this means that the fronts, with respect with each other, have speed sensors that trigger a ABS intervention, if the speed differential is too great.... it cant be any, it has to be a percentage, as going around tight turns under braking will obviously have one wheel spinning faster than the other. The rear wheel ABS sytem is a single channel i believe it works off the differential speed of the fronts. And, i dont know what percentage the ABS will kick in.

That said, getting back to you not really thinking about it that much, you dont want, or really even have, ABS kicking into the rear wheels, in gear. you cant. its coupled to the engine. if by chance you were coasting, or were threshold braking and heal toeing and didnt let up on the brake between the shift, ABS could kick in. but, if you are in 4th gear and on a threshold brake sequence from 130mph to 80mph..... ABS in the rear will not engage. it cant. but if it did, you would see the engine RPM drops all over the place, it would be rattling the driveline and the forces on the driveline and gear box would TREMENDOUS! (because of the gear reductions) the speedo and TACH would show the ABS intervention, and this never happens. ( I believe , because we can see on the video the speedo and RPM gauge)
go into a turn and that rear wheel inside wheel liifts off the ground and locks up against the forces of the LSD , what happens? If it was 4 channel, it could fight that. but 3 channel..... I dont think anything happens....... but, even if it did, there is so much engine braking, that it might not matter ..... leading into engine braking discussion.....
Greg, you absolutely dont want harsh clutch release on downshift, unless you do it on purpose to have the car step out in a tight turn. so, on a straight line braking sequence.... the driver lets out the clutch on a 4th to 3rd shift, with a nice blip and maybe even doesn't push the pedal to the floor as he comes out of 4th, blips, (spins up the driveline by the small amount of drag ) and has a clean, reduced wear downshift into 3rd.
as that clutch is released slowly, you are tranfering the braking force of the engine in decell RPM mode.. (AKA engine braking). its forces are extremely easy to measure on the dyno, but not many do. I do. i also measure coast down HP . this gives you a baseline to see if anything has changed rollling frictionwise, how it was strapped to the dyno, and if you have something else that is changing in the engine or driveline. (lack of compression, bearing wear, or something major)

That engine braking power is THERE and if you blip and shift and are at the high RPM as most all racers are, you have near max to 2/3rds of the engine braking force.
the reason i dont give much credence to the excitement Jeff has for rear bias settings, is that the engine is doing about as much brakingforce as the rear tires can produce on the road during a 1.5g decel.

andersons car with 11:1 compression (as well as mine), or any others , (sans small differences in cams and larger differences in displacement), will produce up to 1400ft-lbs at the rear tires!! thats 700lbs of decel force on each tire. if the car in a 1.5g slowing condition, has only 300lbs on each tire, you can see that there is not any more room for additional braking force. ( look a friction coefficients of tires to see how much over gravity that a rear tire can grip. its not much more than 1.4 of the weight on any tire) any force acted on to the rear brakes, just goes up in heat as the car now trades the decel force of the engine for the rear braking force of the brakes, and will keep the forces constant, if the clutch is pushed in for a downshift, if you heel toe.
I dont think a car's engine in decel, and producing 1/2 to 1/3 of the engines torque, wears or stresses anything, more than using 1/2-1/3rd engine power on accel. this engine braking is used in so many areas where you are not touching the brakes. feathering the throttle, lifting for kinks, etc. the slower those turns, the more the forces are applied to the gear box and driveline,but still less than WOT forces on it.

Now, you want to damage gearbox and sheer driveline shaft.. the hotly contested concept of letting the clutch out too fast, and getting a decel chirp, can snap drivelines . We both know dirivers that have not matched RPM on downshifts, and just let out the clutch to have CV joints break or main shafts shear. could be a reason we see so many rod bearing failures too, as a mis matched downshift from 4th to 3rd, can double the forces to the driveline (and thus engine bearings) vs WOT pretty easily and equal standing start forces on the CV shafts.

mark kibort

I just did the temp paint test. on the inner diameter of the rotor, i got over 1000 degrees during a test to fade. i would imagine the pad at the contact point was quite a bit higher. i think the entire problem is too much temp for a 12.6" rotor. again, 130mph to 45mph in 4 seconds , cant be done with the weight of the car. remember, a 911 with a different weight balance, has an instant 20% adavantage because it has double the weight on the rear and 20% less weight up front vs a 928 on a 1.5g slow down.
you shouldnt have warping... that usually is due to the lugs not being tightened evenly . you have seen my rotors over the last 15 years. they never have warped but once, when my mechanic at the time and pit helper, tightened the wheels with a rattle gun at the shop and i didnt check them. (some 80ftlbs some 120ftlbs)

RKD in OKC

So..,All this experience racing the 928 and you are just now having brake fade problems? What changed?

mark kibort

Great question......

what changed was 50hp and stickier tires. The rate of KE change went through the roof, and buried the little 12.6" rotors. 130mph to 45 vs 122 to 45mph, over the same distance. It produced enough heat in the pads to make them fade.. in my video, i actally called it out to remember to watch where it happened, because it was so distinct.. I went deep on purpose, to see if could get the car to slow and during the last 1 second of braking , out of 4... it was like i hit a patch of water.

trying to see if anyone else has run this fast to 45mph with a 3000lbs with 12.6" rotors. all the responses are "oh yeah, ive raced there with my 911". or "my vet has 12.6" rotors", or "i get to 130 and ive slowed to that point."
These guys dont understand that 911s, get a 20% advantage on the front brakes for the same KE rate change. vets have huge pads, helping with heat dissipation and force per sq area. and without looking at how fast you slow down, 130mph to 45mph can easily be 20% less rate of KE change, if you brake a little earlier an softer.

again, its only laguna, not even the most demanding braking areas of Sears or thunderhill. and its only one turn approach for the last second. and there is no spongie pedal. no boiling of the fluid. I dont think air flow can cure this... too much heat to remove, too quicly. I dont think bias can work. i think just being in gear with engine braking is near capacity of the rear tires opposite thrust force. I certainly do not think the 928 ABS system can compete with threshold braking at all , based on years of experience there.

mark kibort

all i do is drink.... very thirsty?

Now, didnt you say the ABS was 3channel?? I originally thought it was 4 because we have sensors at all 4 wheels. i thougth 3 channel ABS has only one sensor in the rear diff like a truck 4x4.

RKD in OKC

Water injection like the racing truck tractors use?

dr bob

[ ]

mark kibort

so, hence the single line out to the rear brakes out of the ABS control unit? (out of the bias valve?)
so, if one wheel is going slower than the other side, the rear abs kicks in? does it reference the front wheels as well?

But again, as i elaborated to greg... why all the effort to get individual abs to the rear, because when in gear, the lifting wheel has all the decel force and you wont be able to get that one to lock up, but if you do, then the outer wheel gets the power with our whimpy 25ftlb LSD.
it also makes a case for rear brakes during trail braking, because engine braking during the turn, will not be that effective, as decel power goes to lifting wheel just as it goes to the lifting wheel under acceleration .

mark kibort

GREG????

you made the comments... care to elaborate?

since you probably didnt read the response below. Ill paraphrase based on how i think you misread and misinterpreted........
yes, Greg, when the clutch is released, fast or slow, you have engine braking. more unsettling, and for an instant, if released fast.... but, even if released slow and smooth, as i advocate and practice, to NEVER unsettle the car, THERE IS ENGINE BRAKING...... all the time you are off the gas!! that force is substantial, and using a quick calculation, can be almost at the limit of what the rear brakes can apply. do you have any idea what it is? have you ever measured it? probably not. I have and it is a force to understand and consider if you want to interact in the conversation from an informed perspective.

YOU SAID:My statement about not letting out the clutch was not as carefully worded as possible.

My point was that we never want a driver to use the engine as a brake, on a downshift at high rpm....unsettling the car and beating up the drivetrain. Of course we would want the car to be in the appropriate gear and the clutch released, when the time comes to accelerate. This can generally be done very smoothly, at a later point in the corner, without a "downshift gear brake".

I've never given much thought to what happens when rear ABS engages when the engine is coupled to the rear tires at high levels of rpms and off throttle. I just let whatever is going to happen occur.