[Torbjorn968]

I have been using EBC yellows on the front big reds after being recommended by the moderator/sponsor of the British 968 site.



I have had increasingly worse vibration problems under heavy breaking and yesterday it became really bad. The supplier of the spare pads I had brought with me was on the track and gave an interresting explanation: the discs where unlikely to be skewed, he suggested transferr of material from the pad smearing on the disc. He also suggested that the 40% left on the EBCs where not enough for them to cope with the heat generated.



The EBCs had work very unevenly from inner to outer surface which I guess is attributable to brake cooling, I will check but belive I have 500 miles on the track which I believe is quick for a pad that should be a top of the line race pad.



With the Trickstff blues now on I experienced no vibrations, but it was wet the rest of the day and the brakes never got really hot. On the way home they where superior for street use with the EBCs, especially at the start needing lots of pedal pressure to stop well. One good point on the EBCs: no squeal. But the Trickstuffs where actually not bad in this respect, much better than the Mintex 1055 previously on the car. The Trickstuffs is recommended by the Swedish supplier as a good and economical alternative to the most expensive race pads such as Performance Friction (which he also sells) so will be interresting to see how they fare.



//TL

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[flash]

VERY common thing to have happen if you did not start with fresh rotors, or at least have them surfaced first - it's a chemical process, and the rotors must be free of those deposits - additionally, a proper break in in required

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[jfrahm]

Yes my Audi A8 gets material buildup every few months and I need to heat up the brakes good and even out the transfer with some hard stops. Some pads might be prone to uneven transfer more than others. My P-cars generally do not suffer this problem due to the way they are driven.



In any car however I have gotten better at not stopping dead on hot brakes and this seems to help. On the street, if you come to a traffic light just leave yourself lots of room and move a foot or so forward every few seconds. On the track, even with a cool-down lap those brakes are often still cracking hot for a while. It may be impractical to do much about that.



-Joel.

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[Torbjorn968]

[quote name='flash' post='57708' date='Aug 11 2008, 02:31 PM']VERY common thing to have happen if you did not start with fresh rotors, or at least have them surfaced first - it's a chemical process, and the rotors must be free of those deposits - additionally, a proper break in in required[/quote]



Rotors where fresh and I actually had a whole track for myself for the break in when they where mounted so done to instructions. Substandard pads.



//TL

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[flash]

bummer - yeah - probably not up to the braking you do - it sucks because now you get to start all over with another set of rotors - i hate that



something i forgot to ask: i assume you haven't made the mistake of only doing big reds on the front - that would upset braking balance, causing too much front load, unless you did other things to compensate (bias valve, etc) - i find it amazing at how much more rear brake these cars really want

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[Big Dave]

TL.

Now that was a big mistake...

Listening to the "sponsor" of the UK site..

EBC pads are utter rubbish...I had some for a VERY short while... NEVER again...

Get some Pagid blue / orange / yellows [ depends on how much heat + grief you give your brakes],,fitted + your braking will be transformed...

They might well be more money, but WELL worth every penny / cent, or whatever currency you have...



Oh and another mod that might interest you, try + get your rotors grooved,, NOT drilled but grooved. I had this done recently+ found the brakes were great, no fade / judder , just clean hard stopping brakes...

I had the grooves machined to the wear limit / min thickness of the disc, that way i know when the discs are worn out...

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[Torbjorn968]

[quote name='flash' post='57720' date='Aug 11 2008, 06:55 PM']bummer - yeah - probably not up to the braking you do - it sucks because now you get to start all over with another set of rotors - i hate that



something i forgot to ask: i assume you haven't made the mistake of only doing big reds on the front - that would upset braking balance, causing too much front load, unless you did other things to compensate (bias valve, etc) - i find it amazing at how much more rear brake these cars really want[/quote]



According the person I bought the new Pads from the rotors did not need change, the residue from the old pads would wear off and the surface are not more uneven than usuall.



Yes only big reds at the front. Would you not think ABS takes care of things increasing preassure all around (or actually reducing pressure where needed) all around when you get close to locking up? If the rears where too small would they not get very hot and wear out the pads quickly? I have very little pad wear at the rear.



What would you do for the rears?



[quote name='Big Dave' post='57731' date='Aug 12 2008, 01:15 AM']TL.

Now that was a big mistake...

Listening to the "sponsor" of the UK site..[/quote]



How many of you have "migrated" overseas? <img src="/forum/images/smilies/968/rolleyes.gif" class="smilie" alt="" />





[quote name='Big Dave' post='57731' date='Aug 12 2008, 01:15 AM']try + get your rotors grooved,, NOT drilled but grooved. I had this done recently+ found the brakes were great, no fade / judder , just clean hard stopping brakes...

I had the grooves machined to the wear limit / min thickness of the disc, that way i know when the discs are worn out...[/quote]



What would the advantage be compared to drilled. Present front discs are drilled, rears are standard but with race pads.

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[flash]

i was afraid of that - you are probably overheating your front brakes - this happens - people generally think that bigger is better, but don't always follow through, and only go halfway - that actually makes things worse - because the larger brakes grab better, they also grab first, and consequently they get the load - as the suspension then transfers the load to the front, it exacerbates that - it dramatically upsets the balance - it can even be felt in a strong nose dive sensation - it fakes you into thinking you have better braking, but in reality, you have front biased unbalanced braking, which will actually cost you in the turns, and can make you spin



if you ran the pads long enough to get them hot, then no, the old material will not rub off - it must be taken off on a brake lathe - it is a bonding process that must be physically removed



no, abs will not do anything for balancing brakes - it only engages at lockup of a wheel - until then, there is nothing



the diagnosis of your brake wear is exactly the opposite of what you are thinking - if there is no wear, then they are not engaging enough - what you want to see fairly is even wear front to rear



there is a bias valve that many of us have changed to add more braking to the rear - the part numbers are:

5/18bar 951 355 305 01 (stock)

5/33bar 928 355 305 01 (what i am using on stock rotors)

5/45bar 964 355 305 00 (what some are using on the track on stock rotors)

5/55bar 930 355 305 01

5/60bar 965 355 305 01



at a bare minimum i would install the 5/45 valve with your setup - the correct thing to do would be to add big reds in the rear - depending on the rest of the setup of your car, you may still need to upgrade the bias valve though to the 5/33



as for drilled versus slotted, there are some ups and downs to each method - there isn't a performance advantage of grooved or slotted over drilled, but some drilled rotors have been known to crack - circumferential grooving is not a good idea, as it creates cold spots which can quickly lead to cleavage fractures - radial grooving though is effective in both cooling and water removal - it can wear pads more quickly than drilling, and make some noise though



i have used both - on the 968, i am currently using drilled with no issues, but i might switch - on the denali, the rotors are both drilled and radial slotted



hope that helps - good luck

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[Torbjorn968]

Trying to understand.



The more grip a car has, and the heavier you brake, the more you will get forward weight transfer which by definition would remove grip from the rear and make it less possible to break heavily at the rear. If you have a bike for example and brake hard enough then you will be on the front wheel only, the harder you brake the less meaning full strong rear brakes would be? The amount of weight transferr should be a funktion of negative G where the object should be to have good enough brakes to be able to lock up both the rears and the fronts given the amount of tracktion available at each corner. And ABS used to manage lockups.



In my case grip is increased by R tyres and wide fronts (255s around the car again). Compared to a standard car this would be equivalent to my bike example? Grip would be increased more at the front than at the rear due to weight transferr? R tyres at the rear would partly compensate for the weight transfer i guess but should not do so entirely due to increasing forward sizes? Simmilar camber around the car (3,5 forward and 3 aft).



The 911s in contrast has a very different weight distrubution which, even with weight transferr, means that the rears are heavilly loaded.



T

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[flash]

well, you're almost onto the right track (pardon the pun) - what you are getting at is right - the idea is to maintain balance - the key to that is to prevent the big weight shift to begin with - big brakes up front cause the onset of rear to front weight shift - what you want to do is have them all 4 pull down more evenly - that means setting the rear up to grab harder than the front, but not to the point of lockup - deceleration will naturally shift enough weight forward to maintain the proper turn in set - but, that first 1/4 second is the key



a good example of an analogy is in a chicane where you are trying to keep the car flat through the S - if you tip the car just a bit, like in a late too hard brake, it will pitch, you then have to swing it the other way, and you end up losing speed through those corners - but, if you keep it balanced, it will fly right through flat out - optimal braking is the same - get the car to pull down evenly, and you will slow down faster - let that front end dip, and you lose the ability of the rears to brake, lose control, and cook the front brakes



manufacturers set cars up front biased to accommodate the average driver, who could easily have the rear end snap out in a turn if they biased the car to the rear - that is because most drivers brake well into a turn - knowing that, and not wanting to be sued for building a car designed to kill people, manufacturers take the conservative approach



as for the tire sizes and such, the 968 is a balanced car and should have balanced sizes - the reasoning behind the offset sizes was a styling thing, and a carryover from the 911 - the 968 really wants all 4 to be the same size - unfortunately we can't fit as big of a tire up front as we can in the rear - i've got 255s up front now, and 285s in the rear - lol - i just can't convince myself to run a smaller size back there than i can fit



an interesting example of a better brake design is on my denali of all things - they put much bigger rotors on the rear than on the front - the car is very much front heavy, yet they realized that more rear braking meant better control



if it wasn't so messy of a job, i'd probably bump up from the 5/33 to the 5/45 valve - i'm just too lazy to do that these days though - besides, i'm working on an aluminum hat rotor set for the oem caliper

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[rustech]

[quote name='flash' post='57757' date='Aug 12 2008, 10:39 AM']as for the tire sizes and such, the 968 is a balanced car and should have balanced sizes - the reasoning behind the offset sizes was a styling thing, and a carryover from the 911 - the 968 really wants all 4 to be the same size - unfortunately we can't fit as big of a tire up front as we can in the rear - i've got 255s up front now, and 285s in the rear - lol - i just can't convince myself to run a smaller size back there than i can fit[/quote]





I disagree with the idea of having the same tire all around unless it's to meet a budget.

Don't forget power oversteer. A 255 in the back would lose a lot of traction when powering through a turn compared to your 285s+. Balance while maintaining speed will be different then balance under power. On a rear wheel drive car, the more meat the better, use the suspension to balance it out.

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[flash]

i doubt it - i don't think there is enough power to do that - once i put the lsd in there, the power oversteer problem went away - rear traction is not a problem - i have to work to get the rear to kick out - i may actually drop down to a 255 myself to try to induce a bit of oversteer - it's a little too neutral for my tastes



as i said though, i am having a difficult time convincing myself to do that - hard to do what seems like less than i could, even though i know it would probably be better



but, we are digressing from the pad vibration issue

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[Renn68]

yeah - never been impressed with ebc pads (green/red/yellow) on a variety of cars. Pagid with OEM Porsche rotors seems to do the trick if you can live with the brake dust.

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[Torbjorn968]

[quote name='Renn68' post='57843' date='Aug 14 2008, 04:55 AM']yeah - never been impressed with ebc pads (green/red/yellow) on a variety of cars. Pagid with OEM Porsche rotors seems to do the trick if you can live with the brake dust.[/quote]



Recommendations in Sweden seems to indicate that Pagid is no longer #1 but rather PFC with Trickstuff as a good and cheaper alternative.



I have done some interviewing of Swedish racers and they seem to indicate that with wide fronts and R tyres the rears will not need to do a lot of work.



Weight transferr will be proportional to the negative G the fronts can produce. This will be variable with front tire with, tyre compond and forward camber. What you can do with the rear is to fill in what is left in terms of rear grip. The indications I got was that bigger rear breakes could make a marginal difference so the recommendation was to upgrade my rear pads to match what I have forward. People riding on narrower fronts where at times actually reducing rear breaking (on 944s without ABS) to make the car more stabile.



Weight transfer can be reduced by a good chassis: with less rise of the rear of the car the center of gravity will not raise as much during breaking. Spring rates and rebound settings on the dampers come in here as well as the ride height of the car.



//TL

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[flash]

i think we are approaching this from 2 entirely different directions, with philosophies completely at odds with each other - i don't think i can help anymore, so i'm going to beg out of this one - good luck

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[Big Dave]

Not sure if my input will help,, BUT.

Ive fitted the front STANDARD callipers of my 968 onto the rear of the car..Meaning it now has larger pistons on the rears...

The car is currently running M030 front brakes with Dunlop D01J track biased tyres in 235-45-17 fronts + 255-40-17 rears. With Pagid Blue pads all round. It has braided hoses + Castrol SRF fluid The front discs are plain 928S4 ones that ive had CNC grooved [6 off]. The rears are M030 drilled discs. ALL discs are Genuine Porsche items. Standard brake bias valve.

So far i HAVNT experienced any judder , fade , pad transfer, etc etc , just clean braking....

PS, this ISNT on my cabrio....As i have another 968 JUST for track work....

Hope its of help....???

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[Torbjorn968]

Hello Dave!



Does make sense for a cost efficient rear upgrade doesn't it.



Flash: below I am trying to reason the oppsite way, starting at the rear. But the proof of the pudding is in the eating isn't it? So the question is what has been proven on the race track. What does the fastest US cars use?



Let us assume that we only use the rear to stop the car. The Center of Gravity will then be above the center of breaking force (the contact with the tarmac. This will result in a momentum in a rotationm around an axxis where the breaking force is applied which will result in a weight transferr and increase the downforce on the fronts. This will reduce the amount of breaking force which can be applied at the rear where the rears will lock up at some point. The rest of the breaking will have to be done at the front which will result in even more weight transferr reducing the amount of breaking force which can be applied to the rears.



If the car is entirely without suspension and the tires have no compliance then the weight transferr due to breaking at the rear will be identical to that due to breaking at the front simply because the center of gravity is the same in relation to the contact with the tarmac.



Now we intruduce suspension travell I think, but need to ponder this, that the rear breakes mainly would lower the front, and that the front breakes would tend to lift the rear to a greater extent with a resulting greater weight transferr breaking at the fronts. This difference will be smaller if the suspension is stiff.



//TL

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[flash]

ok - i'll try this once more, as i want to help you solve the problem, but i think we are going in opposite directions



you were on the right track, but then you took a left - yes, the transfer happens as you describe - the results are almost as you describe - however, this is the UNDESIREABLE effect - the solution is the opposite of what you have done



the key is to prevent much of the transfer that happens as a result of rear lift, which is what causes the rear brakes to lose ability - the reason you lose rear braking is the suspension is pushing up - a softer rear suspension would help prevent some of this - these cars like a softer rear suspension than you would think - they figured this out in the cup cars - keep the rear down and the car will stop faster, and push through the corners faster - some transfer is needed, but not nearly as much as what you are experiencing



the increasing of braking capability in the rear also helps limit rear to front shift, as it maintains a balance better thereby allowing the fronts to grab along with them at a more equal rate, and reducing the rear lift



it's similar to the camber issue - the wrong thing to do is increase negative camber to square the outside wheel, when the real problem is that your inside wheel is lifting off - net contact patch area is the key to maximum grip - likewise net braking force and weight balance is the key to maximum braking - two wheels with a lot and two with less will not brake as well as four with a lot



you can easily see this with a laser thermometer - just like with tires, where you are trying to keep temperatures and wear even across the tire left to right and and front to rear, the goal with brakes is equal temperatures and wear front to rear - you may never reach that goal, as other factors are involved, but it does not change the philosophy, and the closer you get, the better the car will stop



then there is the whole braking understeer/oversteer issue that happens when you experience so much weight transfer - you may well be trying to solve one problem one way, when in reality the cause is something entirely different

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[Eric_K]

TL-What you need is a Tilton adjustable proportioning valve (or similar) and an ABS switch. Turn off the ABS. Keep dialing the bias rearward until the first sign of the rear locking first (Obviously use caution while doing this). Then go one click forward on the bias. Turn the ABS back on. You should then have the maximum braking of whatever your setup is at the moment.



FWIW, I'm running the front calipers on the rear on slotted rotors. Currently, I have mo30/s4/993 calipers on the front with slotted rotors, but I'm switching back to the 993TT Big Reds that I have refreshed. I run whatever setup I have parts for at the moment. I run the above mentioned Tilton prop valve and Pagid Orange Pads. I have had good luck with PFCs as well.

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[Torbjorn968]

Thanks Eric!



Being able to turn ABS off, that is a thought, also for training purposes. It would help me get used to breaking in the E 30 M 3 which is currently my greatest difficulty.



OT: lap times in the M 3 is coming down and we nailed a 2nd place in the latest race <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" />



//TL

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