1 Topic by TJenkins121 (edited by TJenkins121 2011-01-18 05:35 AM)

  • Registered: 2009-12-30
  • Posts: 38

Topic: Brake Bias

I did a brake pressure sweep on our car.  Necessary brake input 'felt' to relate to 250psi front and 215psi rear @ the calipers.  I have 4 piston calipers with 1.75" non-staggered pistons on the front and 1.375" non-staggered pistons in the rear.  Calculations resulted in 65/35 bias (65% front).  From one website, I read that vehicles from factory come with 60/40 bias, where other websites said the stock combo valves on older cars resulted in a very front biased system (but they did not quantify).

Drivers complained that the car was very unstable under braking.  Our springs do amplify longitudinal load transfer under braking, but I would still think that 65/35 would be a more-than-adequate setpoint for this application.

I'm thinking that there may be a residual valve in the rear portion of the master cylinder?  It was made for rear drum and is now handling disc.  However, I'm not sure that little pressure buildup would cause excessive instability?

Thoughts?


forgot to add.....66/34 @ 150psi front; 65/35 @ 250psi front; 63/37 @ 400psi front.  Appears to be very linear which would say that rear lockup is possible with increased longitudinal load transfers during extreme braking...but the complaint is unstable during all braking.

  • Dave
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  • Registered: 2009-10-09
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Re: Brake Bias

Our cars are unstable sitting still let alone moving, crap I'll go one more our whole team is unstable.;)

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  • JThw8
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Re: Brake Bias

TJenkins121 wrote:

Thoughts?

I think you've already put more thought into it than most folks smile

Your bias sounds about right, and you've been more scientific about it than we've ever been.  We threw a $30 bias adjuster in our car and played with it till it "felt right"

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  • FJ40Jim
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  • From: Lancaster OH
  • Registered: 2009-05-02
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Re: Brake Bias

If the car feels unstable during most braking, then it's not a bias adjustment.

Find and correct the deflection/compression in the suspension that is allowing toe-out under braking.

Jim C.
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  • From: Gilbertsville, PA
  • Registered: 2009-12-22
  • Posts: 3,251

Re: Brake Bias

TJenkins121 wrote:

I did a brake pressure sweep on our car.  Necessary brake input 'felt' to relate to 250psi front and 215psi rear @ the calipers.  I have 4 piston calipers with 1.75" non-staggered pistons on the front and 1.375" non-staggered pistons in the rear.  Calculations resulted in 65/35 bias (65% front).  From one website, I read that vehicles from factory come with 60/40 bias, where other websites said the stock combo valves on older cars resulted in a very front biased system (but they did not quantify).

Drivers complained that the car was very unstable under braking.  Our springs do amplify longitudinal load transfer under braking, but I would still think that 65/35 would be a more-than-adequate setpoint for this application.

I'm thinking that there may be a residual valve in the rear portion of the master cylinder?  It was made for rear drum and is now handling disc.  However, I'm not sure that little pressure buildup would cause excessive instability?

Thoughts?


forgot to add.....66/34 @ 150psi front; 65/35 @ 250psi front; 63/37 @ 400psi front.  Appears to be very linear which would say that rear lockup is possible with increased longitudinal load transfers during extreme braking...but the complaint is unstable during all braking.

huh?

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  • From: Columbus, OH
  • Registered: 2009-05-26
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Re: Brake Bias

I had an "unstable under braking" condition in my '66 Pontiac .. and it turned out that one of the bolts holding the tie rod (sort of - this was a primitive car) to the knuckle was completely loose.  You could move the wheel a couple inches with the car off the ground.  Amazingly the car drove great other than under braking.

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  • From: Life's great in 21158
  • Registered: 2009-05-01
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Re: Brake Bias

JThw8 wrote:
TJenkins121 wrote:

Thoughts?

I think you've already put more thought into it than most folks smile

Your bias sounds about right, and you've been more scientific about it than we've ever been.  We threw a $30 bias adjuster in our car and played with it till it "felt right"

That's exactly what we did, too.  We aren't good enough drivers to notice the difference between the rear drum brakes being locked up or missing.

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