[flash]

for every action there is an equal and opposite reaction - the springs absolutely create force - in fact, you can calculate the frequency at which they do that

i could care less where the center of gravity is - while it affects the math, what i am concerned with is how much weight change is on the tire as a result of the springs

springs are applying a constant force - while at a constant rate, they are not static - that net force is determined by how much weight is on the tire

there are 2 sources of load in a straight line - the force downward from the weight change, which is vertical, and the force due to the forward momentum change, which is horizontal

take a picture of your car braking - you will see a lot more than .3" in height change

you can run your calculator all day long - change the spring and see for yourself - if your rear wheel rate is higher than the front, your rear springs are costing you braking

but we are now way off of the original topic

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[Willem968RS]

Sorry guys, but you got far to technical for me.. lost you a few posts ago!

Anyway, inspecting the car it seemed there was air in the front-brakes, so they “bleeded” (?) the brake lines.. Also brake-cooling ports were mounted on the brake-duckts, and I’m going to try <!--sizeo:2--><!--/sizeo-->softening the compression on the front shocks and/or reduce the rebound on the rear before changing the springs.

The race is next saturday, so I'll let you know if things are better!
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[Willem968RS]

finaly able to add some pictures of car, brakes, paddlebox and airjacks at Spa:

   

   

   

   

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[Willem968RS]

<!--sizeo:3--><!--/sizeo--><!--fonto:Calibri--><!--/fonto-->Well.. Did my race yesterday and it didn’t went all that bad, but not thanks to the brakes! I understand that every car is different, but it is still a mystery to me how the difference between the braking of my old S2 (with olmost gone Toyo R888 semi-slicks) and even my 964 streetcar is so enormously better than this car!<!--fontc-->
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<!--sizeo:3--><!--/sizeo--><!--fonto:Calibri--><!--/fonto-->We tried softening the compression up front and reducing the rebound on the rear. We also looked at the temperature of the brakes and that was 1400° C front and 1100° C rear. Played with the brake balance when the lockups were first in the rear and gave the balance one click at a time to the front until lockup was front. It was manageable, but the braking was just far less than I am accustomed!<!--fontc-->
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<!--/fontc--><!--sizec--><!--/sizec-->We also found out that, for some reason 944 type I rear axle trailing arms and wheel hubs are on the car. So if I want to put the ABS back on I would have to upgrade that first..<!--fontc--></span><!--/fontc--><!--sizec--></span><!--/sizec-->

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<!--sizeo:3--><!--/sizeo--><!--fonto:Calibri--><!--/fonto-->Where now going to try different brake-cylinders for a possible better feel and possibility of better set-up of the paddle-box/brake-balance and if that doesn’t work we are going to install big reds.. <!--fontc-->
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[flash]

no way you hit those temps - something wrong there - cast iron melts long before that (1150C to 1200 C)

as for the changes, you went exactly the wrong direction with the shocks - you should have INCREASED the front compression to help prevent nose dive and it's resultant weight transfer, and INCREASED the rear rebound to slow down the rate at which the rear pushes up

rear lockup happens when there is not enough weight on the tire to keep it rolling - to reduce rear lockup you need to put more weight on the rear tires - this means softening up the rear spring rate and/or the tendency of the rear to lift in rebound

front lockup is most likely due to the over-sized brakes for the tires on the car and/or the suspension geometry - too much camber results in lower net contact patch areas, and especially during straight line braking, can result in lockup

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[Johannvb]

Sharp looking car, good luck on getting it sorted.

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[Willem968RS]

<!--sizeo:3--><!--/sizeo--><!--fonto:Calibri--><!--/fonto-->Flash, it seemed quite high temps to me too, but that’s what it said on the (borrowed) display and we looked to make sure the reading was not in Fahrenheit! So guess the meter was wrong although it worked fine on the tires?!<!--fontc-->
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<!--sizeo:3--><!--/sizeo--><!--fonto:Calibri--><!--/fonto-->I tried what Eric K suggested to get more weight to the front, but will try to go the other way around next time. On the other hand, by now I’ve tried a lot of different setups and did experience the difference in handling bumps and corners, but not in braking. So guess the way to go is maybe try some other brake cylinders or (softer?) front springs or just go for the big reds right away!?<!--fontc-->
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Thanks Johann.. You seem to live nearby, but never saw you on a track? Do you race?

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[flash]

yeah - something goofy there for sure - those things should be glowing red at that temp - my rotors are usually about 500 degrees when i have really been cooking on them - i have read a couple of other reports of 1100 on hardened rotors, but that's it, and i've never seen it in 30 years

again, softening the front is the exact opposite direction i think you need to go - i think you need to soften the rear springs, increase the rear rebound, and firm up the front - this will likely lead to some understeer, but that is easy to correct with some bracing, some suspension changes, and changes in tires

stoptech has a good bit of info about braking and how increasing rear braking is the key to preventing lockup - in a perfect world all four corners would hunker down the same during braking - however, physics gets involved here, and there is no escaping front loading - but, limiting it is the key to the best braking

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[Willem968RS]

<!--sizeo:3--><!--/sizeo--><!--fonto:Calibri--><!--/fonto-->You mentioned stoptech before, but I can’t find the section and had no luck with the “search topic” button?!<!--fontc-->
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[Johannvb]

Willem, I do not race. I don't have that kind of budget, but if ever you need a pitcrew member, let me know [img]style_emoticons/<#EMO_DIR#>/wink.gif[/img]

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[flash]

stoptech.com

it's in their tech papers

on this site, always use "more options" when searching - then you can add the filters to help find things

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[Eric_K]

Shocks don't control the amout of weight transfer but the rate of transfer. The idea of the shock change was to get the weight to the front quicker and before the brakes overpowered the front tires. The problem I hear is the front brakes locking too quickly even with the bias moved as far rearward as possible (ie just before rear lockup) A shock change in any direction will probably not cure the problem. Either the tires are rock hard and lost all grip or there is another problem. There hasn't been handling complaints so I guess the tires are okay. Even with large front brakes you should still be able to brake to the maximum of the tire grip. With Michelin slicks it should have lots of grip. My guess is it is a hydraulic issue, like a mis-sized master cylinder or the pedal is mis adjusted. Is it lIke it goes from not enough brake pressure to too much in a very short amount of pedal travel?

I race against an e46 M3 that runs giant 6 pistons front calipers and small (single piston?) stock rear brakes and nothing can out brake it. I think your setup has lots of potential.

968 rear arms have 944 part numbers and are interchangeable with late 944's.

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[flash]

exactly the wrong thing - i am trying to PREVENT getting the weight to the front, and keep it on the rear where it belongs

brake lockup is due entirely to an upset in frictional forces between the tire contact and the rotor/pad - in a perfect scenario the frictional resistance of the tire equals that of the brake, and the wheel slows down evenly - add too much weight, and while you don't get lockup, you do get overheat, and you lose a lot of the braking capability due to the other end not grabbing - too little weight, and you get lockup - balance is everything - it's not as simple as mere hydraulic pressure shift - the frictional surfaces of the brakes, as well as the tires need to be balanced too

increasing the compression will slow down the transfer, as will the other things i listed

far too often i see people trying to increase front braking, which only results in out of control braking, and loss of speed in turns - it also frequently results in longer braking distances - if you get the rear brakes to work properly, they do more of the work, and this balances the car going into a turn

the idea is to set up the braking so that there is less rear to front weight transfer - this allows you to stay on throttle longer, which means faster entry and exit

in a perfect world, all 4 wheels would slow at the same rate, with the same friction, and the suspension would actively resist rear to front weight transfer, while allowing downward transfer equal on all 4 - we can't do that though (yet)

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[Willem968RS]

The stoptech-site indeed has a super quantity of braking information.. Great stuff!!



I also found a section at tiltonracing.com were you can fill in the specs of the car and the brakes, to determine what kind of master cylinder size you need for the specific paddlebox you have in the car. When I’m sure to have that right I can move on to the setup..

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[flash]

yup - that site is a wealth of information

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[Torbjorn968]

I am still trying to understand. How does nose dive give increased weight transferr? There is a lot of opposites in the answears above and here is one more: a nosedive would lower the center of gravity therefore reducing the weight transfer? I think that it would be helpfull to if we could define in high school physical terms what weight transferr really is. For me it is weight distrubution between forward and rear axels and the way to describe this is a force diagramme which includes contact patches, G forces and Center of Gravity. A car with a high center of gravity will have more weight transferr, equall to a kid being thrown of his bike breaking to hard. A Low CG would resul in the reverse.

Interreresing reference to Wilhelm: although my own brakes gives me a littlle over a G I have a freind in a 924 Turbo with standard brakes hitting the same numbers you quote for your previous car.

//TL

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[flash]

this is silly - this is basic physics - when the front dips, weight shifts forward - you can even see it on corner scales in a static condition - lower the front 1/2" and it will increase the weight on the front wheels by about 100lbs - the effect is magnified when you add G forces - stop that from happening and you will have better braking in the rear, which will reduce rear lockup due to the weight on the rear wheels

having driven a momentum car for 25 years, i had to focus on improving brake balance to go faster - it is the exact opposite of what most people think

at this point i give up - try it for yourself and see the results, or don't - i know it works

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[Willem968RS]

No doubt about it Flash, I was already convinced.. Gonna try it next time on track!

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[flash]

lol - no worries - clearly i am unable to explain some things so that it is universally understood - i'm not wigged out about it - i just don't know how else to explain it

have fun - when you get it right, you will blow the minds of the guys you are out-braking - they'll all come poking around your car to see what rule you have broken - it's really sort of a pain in the ass, but it's fun at the same time

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[Seven30]

What a fantastic thread, it's the first I'm reading on this forum and it makes me look forward to the other posts [img]style_emoticons/<#EMO_DIR#>/smile.gif[/img]

Willem, if you are adjusting the brake bias; Have you considered keeping the yellow Pagids up front and going with more grippy Pagids in the rear?

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