[Willem968RS]

After racing two seasons with a 944 S2 I wanted something nicer and faster so I bought a 968 CS racecar build by Havik Racing (see you tube/a racecar is born).
The problem is that the brakes lock up very easily. The data shows I can only get it to brake at a max of 1 G, while the S2 can do 1.3G’s with “big blacks” (and ABS).. The car has big Audi RS brakes in front and standard rear brakes. The pads were changed to Pagid yellow, which worked a bit better than the Carbonne Lorain it had, and I tried to shift the brake balance to the rear, which made the rear brakes lock! Furthermore the car has a Tilton paddle box, 3-way AST shocks, M030 sway bars, 18”x8,5J Wheels with Michelin 24 slicks front and rear.. Any ideas anyone? Are the brakes to big? (I hear that brakes can never be big enough?!), are the rear brakes to small? Do I need a different cylinder in the paddle box?

Thanks!

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[flash]

there are so many variables that can cause rear brake lockup that it would be nearly impossible to diagnose online

we would need to know:

symptom of lockup (straight line, trailing only, entry only, etc)
same size wheel all around?
sway bar setting if it locks up in corners
corner weights
spring rates
shock type and settings (never heard of AST)
tire sizes and pressures
alignment settings
if the lockup is in corners, what chassis bracing has been done?

then there is:
what are the specs of the audi brakes? (never heard of anybody doing that)
what are the pressure specs on the tilton box? (never heard of anybody doing that on this car either)

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[Willem968RS]

Indeed that is a lot of variables Flash!
The lockups started at front and later after getting the balance to the rear, the rear locked up.. It seems that the wheels don't have grip when braking, while in corners the car handles like a dream and makes more than 1.4G's! I only brake in a straight line... think that braking in corners will make it even worse and make the car spin?!
Wheel size 24 front & rear.
The rest I have to check (tomorrow, its getting very late in Holland now!)
As for the AST suspension.. We have a number of very competitive suspension making companies in Holland. You probably know Koni? Have a look at http://www.ast-suspension.com/index.asp..
The Audi RS brakes are the same as on the Lamborghini Gallardo. I tried to include some photo’s, but can’t figure out how!

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[flash]

wheel size 24? what is that?

brake lockup is usually more about weight transfer than it is an issue with brakes themselves, though having too much brake up front can quickly cause rear lockup - i know it sounds backwards, but that is what happens - it does that because the big front brakes cause the front end to dive, transferring all the weight to the front - this means that there is nothing to keep the rears on the ground, and they lock up

do you have any specs on the front brakes?

check out the stop-tech site - they have a great explanation of brake bias

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[Willem968RS]

Sorry for my poor explanation Flash.. As you can see I am a rookie and also fairly new at the game!

Obviously we have different measurements here in Europe. 24 is used by Michelin for 240mm wide tires that I run front and rear on 18 inch (8,5J) wheels..
Your explanation about the weight transferring sounds very logic, but I also have lockups at the front when I move the brakebalance forward.. How do you think I could cure the problem then, bigger brakes at the rear (or will that make the problem even worse?!) or stiffer springs up front to minimize the diving? I had stiffer springs, but they created understeer at the slow corners!

I will get some more info on the car and the brakes when I take the car home tomorrow and look in to the stop-tech site..
Do you know how I can insert a few pictures or is that not possible as a rookie?

Thanks very much for helping me out!

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[flash]

well, you have probably bypassed the ABS with either the tilton or the big brakes, so, you will get lock up at some point - no getting around that unless you figure out how to reengage the ABS - the best you can do is to try to balance it out by getting the spring rates and corner weights established

just as a side note, the small tires you are running in the back aren't helping either, though i understand the benefits of having them all the same

you should be able to add pics - when you hit "add reply" the buttons pop up - if you only hit "fast reply" you have to hit "more options"

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[Jim Child]

You have too much brake and/or not enough tire. Are rules an issue? If not get rid of the 8.5 inch wheels and go with 10's all around with appropriate size tires. People over here are having great success running 285's on 10 inch wheels all around. Even with that change you might find that you still have too much lockup. In my opinion the car doesn't need anything larger than the M030 brakes with good ducting. That's what I have, and my braking is fantastic. Most of my passes occur under very late, very heavy braking. Also, I'd do whatever it takes to restore your ABS. Why waste money on flat spotted tires?

Good luck!

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[firefish]

Since you are running slicks I'm guessing that you're also running with a lot of neg camber. Having too much neg camber would at a point cause a degradation in brake performance whilst still performing great in the turns (the 968 lacking the power/torque for it to be a problem during acceleration).

I haven't tried the Pagids but I've heard both good and bad about them. I'm currently having Trickstuff TS69 which is a great pad with good bite and a predictable behavior. Since you're saying that you're a rookie it might well be that the Pagids are over the top for you and that you're having trouble getting them up to the correct temperature - perhaps try and install a fancier street pad instead to see if that works better for you?

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[flash]

it's unfortunate that we see so often this kind of a problem - somebody goes out and pays good money for a mod they think will work on this car, and then come to find that it actually made things worse - i'm willing to bet, that while you have the "sensation" of better braking with the big front brakes, that your stopping distances actually increased - happens all the time with guys who go to big reds, but fail to do the rear or balance the car

unfortunately, the cure will not likely be inexpensive

but, we're here to help

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[Johannvb]

Hi Willem,

I cannot offier you any advice on your problem, but I wonder if you know these guys:
Race for fun - http://www.raceforfun.nl/
Beekman Racing - http://www.porscheperformance.nl/

They have prepared a couple 968s for racing and would also be able to have a look at your car. It always helps to have someone with experience looking at your car.

Good luck!

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[Eric_K]

Your braking system has been completely changed so I expect it would behave differently than stock. Is Havik available to help? Seems they would be in the best position to help since they put the system together. The master cylinders may be larger making the braking harder to modulate. It appears your bias changes are responding. You definitely want the fronts to lock before the rears. The Michelin slicks should have plenty of grip. They are not old are they? A rock hard old slick will have very poor performance. As mentioned above I would check for excessive front camber. You have big front brakes, put them to work. I would try softening the compression on the front shocks and/or reduce the rebound on the rear to allow the weight to transfer to the front quciker.

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[Willem968RS]

Wow… What a response! Great to have so many guys taking the trouble to help me out.. I really appreciate that, thanks guys!

I bought the car almost completely as it is now. Almost nothing is standard anymore. They only had one short test with the car after it was build so it needed a lot of fine-tuning and we had to solve a lot of small problems first.

Flash; you’re right about the ABS it was already deleted completely, but I didn’t bother because I had raced the S2 before without ABS, and it worked fine for me. I understand that there will be lock up at some point, but as I mentioned it is far sooner than the S2.

The spring rates in mm’s/kg’s are front: 180mm/110kg rear: 140mm/110kg
Inches/Ibs front: 70.92inch/242.51 Lbs rear: 55.16inch/242.51 Lbs

Empty weight of the car: 1140 kg (2568.39 Lbs)
With 101 kg in the driver’s seat and 25 kg of fuel: 1263 kg (2784.44 Lbs)
Corner weights : front-L 320kg/705 Lbs front-R 312kg/687
Rear-L 328kg/723 Lbs rear-R 303kg/668

I would love to get some wider tires, but I understand it’s best to run the same tires front and rear and because of the big brakes/wheelhubs in front I won’t be able to get any wider than 240mm because they will run against the fender. Enough space in the back, so try wider tires there?
On the other hand they don’t seem to suffer in the corners, because the corner speed is fine (>1.4G)

Jim; rules are no issue, because I race in free class, but as mentioned, in front 240mm is the max now or I would need to widen the fenders. (or get other brakes!) Restoring the ABS will be a big issue though. Everything in front is far from original, so the only option would be a costly custom race-ABS.

Firefish: the camber in front is -3°, so it would indeed be a good idea to try a bit less! After changing the Carbonne Loraine pads for the Pagid yellows there was a little more feel in the brakes, but it didn’t cure the problem. I surely don’t have a lot of years (this is my third season) of experience in racing, but when I wrote rooky I meant on the 968 forum. I managed braking without ABS before, so I should be able to manage.

Flash: as mentioned, the mod was already on the car and I often hear that brakes can never be big enough, but if I can’t get them to work, I’ll get rid of them!

Hi Johannvb; I know both firms. In fact Raceforfun (Harry Verkerk) did a lot of work on my first car (the S2) and runs Ben’s white 968 that I compete against and who is also active on this forum! Beek(man) is the firm where my car was in the first place when I bought it. They did some tuning on the engine, but now my car is run by Geeris Porsche service in Heeze. They finished the car and did a fantastic job on a lot of issues, but up till now weren’t able to solve the braking problem. They have some ideas we yet have to try, but as the next race is in two weeks I wanted to try my luck on this forum as well.

EricK: Havik build the car and did a perfect job building a car that was/is very beautiful in my opinion. I fell in love with it the first time I saw it and just had to have it. They are only a bit too far away from where I live. The Michelins are not new, but seem to have enough grip in the corners. I will try the changes on the shocks!

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[flash]

i can't figure out your spring rates, so i can't figure out your weight transfer - normally here we see a lb rate for 1" of compression

you can get wider tires up front - i have 255mm front tires on a 9" rim - plenty of room for more too, if i set the ET farther inboard - some guys are running 265s up there

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[Torbjorn968]

Hello Flash!

Help me understand, in which is weight transferr affected by spring rates? I picture this as a vector from the center of gravity of the car. At 1 G breaking the force vector acting on the car points 45 degrees forward and the weight transferr is represented by the point this vector intersects the ground.

With soft suspension the car will dive more and the rear will raise possibly increasing the CG above ground depending on which effect is the biggest, forward dive or rear raise. A suspension which is stiff at the rear in relation to the front will reduce weight transfer.

The race engineer I use calculated extra breaking force of big reds to 17%, the main rationale to go to big reds seems to be to absorb heat. 17% more force forward seems reasonable given that wider R tyre compound forward will increase breaking force and thus weight transfer.

//TL

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[flash]

again, this is on the presumption that we are looking to INCREASE rear braking and DECREASE front braking, which is what these cars tend to need, and especially if large brakes have been fitted to the front, but not the rear

spring rate affects braking weight transfer in that stiffer front springs will resist the shift forward - softer ones will permit more transfer - stiffer rear springs will also increase the rapidity with which the rear of the car is lifted (bad) - softer ones will act more slowly, reducing the transfer effect (good)

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[Torbjorn968]

Trying to understand this.

Springrates are given in pounds per inch. This goes both ways, for compression and expansion. A stiff spring needs lots of preassure to compress it a certain distance, in the same way it needs a lot of reduction in preassure to expand. If the springrate would be infinitive nothing would happen in either direction, compare this to shock and rebound between a stiffly sprung car and a standard car. The force, when the car is resting, generated by the soft spring and the stiff spring is the same, in both cases the weight of the car needs to be supported. A stiff spring will increase it's force quicker, but also loose it quicker than the soft spring. If I am right then a soft spring will expand more than a stiff spring given a loss of preassure.

Let us simplify wheelbase to 100 " and assume a CG (center of gravity) of 20" from the ground and 50/50 weight distribution. Let us also assume that breaking does not effect the CG relative to the ground. 1 G of breaking force would give you a force vector from breaking equivalent to the weight of the car, and a force vector vertically the same length. The combined force vector would point 45 degrees forward. At 1 G of braking weight transfer equates to 20 " of the wheelbase which means that we now have 70/30 weight distribution. 1 G seems a common number on R tyre compound which in combination with wide racing wheels increases grip by about 50% from the rubber available when the car was designed. At 0,7 G (reasonable when the car was new) the weight transferr is 14" meaning a 64/36 distribution. This means that R tyre compound and max size wheels increases the weight on the fronts by 10% and decreases the weight on the rears by about 20% compared to the orignal rubber.

Let us now introduce a shift of CG in relation to the ground due to nose dive and the tail raising. A 40% loss of weight on the rears is about 0,4 x 1500 lb = 600 lb. With 1000 lbin at the rears on my car this means a combined spring rate (left and right) of 2000 lbin. This would mean that the spring expands 600/2000 or 3/10 of an inch which is not a lot and would have a neligible effect on weight transferr.. The forward dive will also be small at 600/1600 given the springrates on my car. If we softened the springs at the rear to half this amount then we might get close to a typical US setup if I remember numbers I have seen correctly, 800 lb forward and 500 lb at the rear. The front would react with the same amount of dive while rear would raise another 3/10 of an inch.

Added to this is the spring effect given by the rubber for which I do not have a clue but on a stiffly sprung car this effect might be larger than that of the suspension.

Have I missed something here such as the leverarm given by the calipers pinching on the rotors? In this case the effect might be larger but stiffer springs at the rear will still prevent weight transfer by avoiding that the rear is raised during breaking thus increasing the height of the CG above the ground. If the CG would raise by 1", which implies the rear raising 2" while nothing happens at the front, this is a small increase of weight transferr (1" at 1 G) so I think Wilhelms problems might be found elsewhere.

To me a stiff forward spring setup does not fight weight transferr, it reduces the nose dive which is a result of the weight transfer. Softer forward springs in my book would reduce weight transferr due to more nosedive (which you may not want for other reasons) reducing the CG distance to the ground (again, the force vector intersecting the ground).

Given the increase in weight acting on the fronts in relation to reduced weight on the rears it seems reasonable to increase breaking forward by quite a bit on a car setup for racing? 70/30 compared to 64/36 would, everything being equall, equate to 31% more to be applied forward. Given 17% extra on the big reds for a given pedal preassure a situation where a stock car might benefit from more breaking force at the rear might still mean that big reds forward is in the ball park of what is an ideal increase forward for R tyre compounds. As I mention above rubber from the 90s combined with original wheelsizes allowes 93 g on a skid pad. Numbers I have seen with modern sports tyres (michelin pilot sport an example) is something like 1,15 where 1,4 can be seen on wide R tyre compund.

Sorry for being long winded, this is both because if I am thinking load and that it is always good being explcit of assumptions when having a discussion.

Back to Wilhelm: is the car stabile during breaking? if you have a snaking effect of the car alterating preassure between left and right might lead to lockup. I have had this problem. I would not change camber for breaking since you spend more time cornering. Flash: need that under car brace to see if I can reduce optimal cornering camber :-)

//TL

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[flash]

you're overthinking it - weight transfer and nose dive are tied, not independent or one resulting in the other - the front is tied to the rear too - what one end does during braking, the other does the opposite - there will always be weight transfer as a result of change in velocity - as the weight transfer occurs, the relative weight of the rear end of the car decreases, thereby increasing the effective spring rate, or amount it raises the car at that end - the reverse happens to the front - the more weight transfer, the worse the change - but, if you stop the nose dive, and you reduce the weight transfer - you do that by increasing the ratio of front rate to rear rate

just try it - lighten up the rear springs relative to the front - you will suddenly get more rear braking, and less nose dive

the down side is that you may find yourself then fighting understeer, which may cost you as much speed in entry as the nose dive did

this is a design problem with staggered wheels

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[Torbjorn968]

you're overthinking it - weight transfer and nose dive are tied, not independent or one resulting in the other -

This is what I am saying puttting the weight transferr and the resulting nose dive/rear raise in numbers. The breaking will by definition be the chicken and the dive/raise the egg. It will ofcourse increase weight transferr in a spiraling process unit equilibrium is reached. Do you agree with how I have outlined the basic physics with the force vector describing the weight transferr? And the result the change in weight in axles resulting to dive/raise in proportion to spriing rates?

Even with a softer sprung car the effect of raising the CG should be small given that a 1" raise of the CG roughtly eqates to 4% extra weight transferr. To have the CG rasing that much you need soft springs at the rear allowing the rear end to raise. If spring rates where infinitive, no suspension at all at the rear, no raise at all bar the rubber could occur. My agument there fore runs contrary to yours in this sense.

This may be where we depart this time given our earlier camber argument :-). Dont forget about those under car braces!

//TL

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[flash]

lol - no arguments - it's all fun - i know what i know based on having tried each method, on many cars - it's more organic than math allows

the rates aren't static - that's the flaw in trying to use the basic math - as one increases, the other decreases, and you need to then recalculate based on the new end weights - this continues, as you said, in a "spiraling" fashion - it takes some pretty hairy calculating to figure out what is really going on, to determine what to do

fortunately i have done this so long that i can feel it in a car, and get really close very quickly - i can also usually tell from watching the car as it brakes and turns

how much tipping there is depends on the resistance of the springs - a 1G vertical increase in load would remove all of the weight from one end of the car and place it on the other - the issue is that you can't think like that though - there is vertical load and there is linear load - if you are in a turn at the time, there is also lateral load - these all have to be calculated together - it's been way too long since i did that, and i'd have to go back to the books to do it, but it can be done, assuming you have the right measuring equipment

the linear load generates the trigger for the change in vertical load - the vertical load is determined by the forces applied by the springs, both front and rear - in a perfect world, we would have a reverse spring that would pull the rear back down during braking - we don't, so the best we can do is limit how much the rear pushes up during braking, as well as limit the amount the front compresses

if you were here, i would swap the stuff out for you and show you

lol - and it's "braking" not "breaking" (although if the car isn't set up right, you could be doing some breaking too)

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[Torbjorn968]

lol - no arguments - it's all fun - i know what i know based on having tried each method, on many cars - it's more organic than math allows

the rates aren't static - that's the flaw in trying to use the basic math - as one increases, the other decreases, and you need to then recalculate based on the new end weights - this continues, as you said, in a "spiraling" fashion - it takes some pretty hairy calculating to figure out what is really going on, to determine what to do

---------------------------------------------------

>> If you use a force vector analysis shift of horisiontal CG has a limited effect on weight transferr, and it would take a rather large change in spring rate to make a difference to the 3/10 of an in which would be the rear extension of my springs assuming static spring rates. Even a 1" raise of CG (which would be 1 " dive at the front with 2" raise at the rear) would only give a shift of 1 " at 1 G.

how much tipping there is depends on the resistance of the springs - a 1G vertical increase in load would remove all of the weight from one end of the car and place it on the other -

>>Which is what happens when your kid flies over the steering bar on his bike having sqeezed the front brakes past maximum :-)

the issue is that you can't think like that though - there is vertical load and there is linear load -

>>With linear you mean horisontal?


if you are in a turn at the time, there is also lateral load - these all have to be calculated together - it's been way too long since i did that, and i'd have to go back to the books to do it, but it can be done, assuming you have the right measuring equipment

>>Let us stick to a simplified straight line situation. I assumed 1 G for the horisontal G force and put it into a verbally described force diagram (sketch it up!) getting a weight transferr which then determines the change of vertical force at front and rear. Only G forces and CG (and shift of CG) determines weight transfer, that can be seen sketching the diagram. There is an effect of a raising CG resulting in longitudal shifts as well, but this is small since the angle rotating around the front axel of the car is shallow. On a bike lifting its rear wheel it can quickly get large... (because the angle from the CG to the contact point of the front wheel is large)

the linear load generates the trigger for the change in vertical load - the vertical load is determined by the forces applied by the springs, both front and rear -

>>Here we differ:

>>The horisontal load gives the vertical load change due to weight transferr which is then resisted by shocks and springs. Think of it in a steady state with the car at rest. The forces of the springs does not determine vertical loads, this is determined by weight, weight distribution and gravity. The springs resists the forces created by these factors. Think of a person with legs spread appart on rubberblocks (or a wrestling mat) transferring weight from one leg to the other: the rubber does not create the forces, the weight shift does, the rubber deforms and resists it.

>>If the spring would apply a greater upward load than the gravity and weight gives then the car would accelerate upwards until steady state is reached.

in a perfect world, we would have a reverse spring that would pull the rear back down during braking - we don't, so the best we can do is limit how much the rear pushes up during braking, as well as limit the amount the front compresses

>> here we agree, the object is to limit CG raising due to the rear raising . A spring with an extreemly high spring rate, let us 1000 000 lb in, will hardly compress at all in the steady state loaded by the cars weight at rest, therefore it will not expand any appreciable amount when the forces are released, even when the weight transferr (2,5 gs needed in my example above) is enough to have zero vertical force on the rear wheels.

if you were here, i would swap the stuff out for you and show you

>>Would love to, perhaps it is something else than reduction of weight transferr that makes a softer rear end better in your experience while braking?

lol - and it's "braking" not "breaking" (although if the car isn't set up right, you could be doing some breaking too)

>> :-)

[/quote]

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