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Guest Message by DevFuse
 

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Lets talk brakes...


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3 replies to this topic

#1 WJM

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Posted 30 August 2004 - 11:56 AM

Ok....brake bias.

car level...enter turn, hard braking...car pushes.

car lower in rear...enter turn, hard braking...neutral...

car REALLY LOW in rear...enter turn, hard braking...LOOSE! DRIFT! CORRECT! GAS! ok...i jsut drifted the corner.

The strange thing is...the lower it gets, the more TOE IN and more NEGATIVE camber it gets...so..it theory, the lower, the tighter....not loose. Supposidly, lift it, it gets loose. Not the case.

here is what Im tihnking...bias is changed when the car pivots front to rear...less brake rear as the car tilts forward...and vice versa....thats what the stock proportioning valve does...right?

is this true? I have had this happen.

Also, installing stainless lines on the front only...is that going to create more front brake than front?

Maybe its time to redo the brakes, split them F/R and install a valve...

#2 All_talk

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  • Thorp (Ellensburg)

Posted 30 August 2004 - 01:20 PM

Before you go messin with the brakes lets look at the problem and be sure your solution addresses the real cause.

There are three “in turn” conditions to consider, braking, rolling, accelerating, your oversteer on braking might be due to a general (rolling) traction imbalance that’s amplified by the weight shift.

Run a corner without gaining or loosing much speed, run it fast enough to induce a slide (please do this in a safe place), a wet road or parking lot will let you do this at a much lower speed. Does the car under or oversteer?

The solution for rolling oversteer and braking oversteer can be different.

The rear toe-in would normally help, but under hard braking the rear is so unloaded it will have little effect, it will most likely just induce understeer on corner exit (accelerating) where there is more weight on the rear. If I had to make a guess based on what you’ve said so far I would bet the loss of traction is due to excessive negative camber causing less tire contact. Remember, negative camber is used to compensate for body roll, and has to be balanced against spring rates, sway bar rates and center of gravity.

Changing the brake bias will effect straight line braking as well, from the factory most cars are very forward biased because its safer (for most untrained drivers) to understeer upon lockup. Adding more front bias to solve your oversteer problem may result in the front brakes doing vertualy all the work. Suspension tuning should come first.

Gary

#3 WJM

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Posted 30 August 2004 - 01:59 PM

Before you go messin with the brakes lets look at the problem and be sure your solution addresses the real cause.

There are three “in turn” conditions to consider, braking, rolling, accelerating, your oversteer on braking might be due to a general (rolling) traction imbalance that’s amplified by the weight shift.

Run a corner without gaining or loosing much speed, run it fast enough to induce a slide (please do this in a safe place), a wet road or parking lot will let you do this at a much lower speed. Does the car under or oversteer?

The solution for rolling oversteer and braking oversteer can be different.

The rear toe-in would normally help, but under hard braking the rear is so unloaded it will have little effect, it will most likely just induce understeer on corner exit (accelerating) where there is more weight on the rear. If I had to make a guess based on what you’ve said so far I would bet the loss of traction is due to excessive negative camber causing less tire contact. Remember, negative camber is used to compensate for body roll, and has to be balanced against spring rates, sway bar rates and center of gravity.

Changing the brake bias will effect straight line braking as well, from the factory most cars are very forward biased because its safer (for most untrained drivers) to understeer upon lockup. Adding more front bias to solve your oversteer problem may result in the front brakes doing vertualy all the work. Suspension tuning should come first.

Gary


Straight rolling...it seems neutral...if a teeny pushy...under gas it will loosen up suprisingly....in its current state...level.

With the stiffer springs it was looser...but not a drifter loose.

In the wet is pushes bad...this i believe is due to the very shallow tread.

The negative rear...is at most 2.0 deg...at its lowest possible squattage...i dont that would be 'exsessive' considering the nature of these tires...the sidewalls are kinda soft, they like to roll.

Ive not been driving it hard since the brake line install, as i am waiting for something to break while going gently with all the other changes I've had....i need to empty all the loose stuff out and go flog on it sometime today/tomorrow and see if the lines have indeed changed the bias.

What is annoying is that I cannot change the toe on the rear. My plans are to go with 300 lb springs rear before the 24 HR autocross...and the fronts will remain the same...but some new KYBs will be installed.

#4 All_talk

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  • Thorp (Ellensburg)

Posted 30 August 2004 - 02:23 PM

Your right 2deg shouldn’t be excessive, have you tried chalking the tires and adjusting the pressure?

I’ve done some recent reading on brake line upgrades, the consensus seems to be that unless your OEM lines are in REALLY bad shape the upgrades will make no noticeable difference. Most of the people claming big changes in pedal feel did other changes/upgrades to the brake system along with the lines. Just passing that along as I have no personal experience.

You can change the rear toe, its adjusted at the three bolts that mount the di-angle arm to the trailing arm, by moving them for and aft. Note: this is also where you adjust the camber by tilting this connection and one adjustment will effect the other.

Minfo might help get to the bottom of the oversteer issue:
Spring rates

Damping rates

Sway bar sizes

Caster, camber and toe settings

Gary




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