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d3c0y

Brake Bias

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I was wondering two things:

 

  • Does anyone know what the factory brake bias for a 240Z is?
  • What sort of brake bias are people running on their race cars?

I'm aiming for around 5% more rearward bias than factory, but i dont know what that is, or how to put drum brakes into my bias calculator to get a baseline.

 

My current proposed setup will have 70% front bias which I think is way too much. If anything i want it too far rearward so I can use a bias adjuster to turn them down.

 

 

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Edited by d3c0y

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I might have this wrong but this is how I understand the 240Z system works.

 

The little brass unit which sit above the diff is a reservoir set-up which initially allows more fluid to flow to the rear cylinders when the brakes are applied. This is required because the return springs in the drum brakes which drag the the shoes off the drum displace more fluid than that displaced by the retracting front discs.

 

The other little doovalacky in the engine bay is merely a safety device which ensures at least two brakes will work if a brake on any corner fails;

 

I don't know that there is an actual brake bias feature in the system other than the piston sizes of the slave cylinders.

 

I'm not 100% on this!

Edited by PeterAllen

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Hi Peter,

 

Thanks, but I know how the stock system works. What I want is a starting point for the new brake setup i'm building for the car in terms front/rear bias. I want to have the bias 5% more rearward than stock at a minimum. Ideally i want to shift the bias too far rearward so i can then turn them down with a proportioning valve.

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Remove the fixed PSI Line Pressure Reducing valve, fitted to rear line, in engine bay, then fit a screw top type ( not five click lever type) Willwood Line Pressure Reduction valve.

This goes from a higher % at full open, compared to the stock Nissan fixed unit.

 

The brass blck at rear, bolted up to floorpan on RHS, is a residual pressure valve, its has a small poppet valve inside, and is desgned to hold a little bit of residual pressure to rear drums, so that pedal retains height, due to rear drum return springs not being able to pull rear slave cylinders back in.

 

As far as % front to rear go, you need to have accurate car weight, accurate spring rates, and calculate fore/aft pitch ( weight transfer rates), then you look at the swept area of pads, piston area, master cylinder(s ?) piston area, rear slave sizes/area, rear shoe swept area.

The drum brake racer boys in Nascar get all serioys about this, especially on road courses.

I have a few books on this subject.

Applicable to four wheel disc cars too.

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The way to shift the bias forward is to cancel the factory proportioning valve, prior to regulating the rears with the aftermarket adjustable proportioning valve. I think you already know this.

 

The 240z up to 73 had the factory proportioning valve at the rear RHS (and some believe it has residual pressure function too).

 

After 73 we get the factory proportioning valve on the firewall.

 

It's not really possible to give a percentage of front to rear bias on a standard 240z as it is not linear. Some people throw 70:30 around which is probably a good average, but the system is designed so that the harder you press the brakes, the more the rears get reduced.

post-101333-0-69526600-1452216123_thumb.jpeg

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Thanks for the answers guys. Maybe i need to go deeper into what I am doing so we are on the same page.

 

Basically I am building an entirely new brake setup consisting of:

  • Porsche Boxster 986 4 piston monoblock calipers all round (They have different size calipers front caliper pistons - 40/36mm - rear caliper pistons - 30mm/28mm)
  • 15/16th master cylinder (which is what the Porsche has)
  • Custom braided lines (required to fit the calipers anyway)
  • Obviously custom brackets to mount calipers
  • 323mm 2 piece DBA front rotors 26mm thick (from an Rx-8 sport package - 47mm rotor offset - 8% increase in size)
  • 260z hubs
  • Handbrake caliper setup
  • Willwood Line Pressure Reduction valve

At this stage, I was thinking of going S13 Silvia front rotors for the rear (easy to get in 2 piece 280mmx22mm) or Z31 front rotors. (274mm x 20mm - correct width for the caliper). 

 

The Boxster comes factory with:

  • F Disc 298mm x 26mm
  • R Disc 292mm x 20mm

From what I have read, a rear engine car will have a more rearward brake bias from the factory than an front engined car. So what I am trying to do is have a more rearward bias than a factory zed to allow for fine tuning but at the same time run the smallest rotor I have to at the back for unsprung weight advantage.

 

Roberto's answer has been helpful for giving me a ball park to base my targets around at 70:30 and probably aim for 65:35 as a minimum. I think the Boxster is pretty close to 50:50 and obviously I need to figure this out too.

As Jason has suggested previously I can do this using % increases and decreases from the stock setup, but this becomes hard with drums at the back. 

 

The other question i need to answer is will a caliper designed to work with a 292mm rotor work on something as small as 280mm as that might be another limiting factor?

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Put an adjustable brake bias valve inline with your rear brakes and go for a spin.

Hit the brakes as hard as you can so as you lock the rear and then back the valve off to suit. Repeat this process until the system is set up for your car the rear no longer lock and then do not adjust. This is how I have dialed in my system. I installed the valve under the driver seat.

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Jake,

Just fit it all up, try it out, the Willwood screw in valve should solve most ssues.

If still an issue, ie way to much rear bias, you may need to go a tandem master setup, and run different size masters, along with the fine adjustment bias that will be available, in a twin madter setup.

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More worried about not having enough rearward bias with a 280mm rotor that is my issue here. I don't want to go and make up all this gear and order rotors to go, oh it's too front biased. I'll keep calculating and post up what i am getting, for group thoughts.

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Put an adjustable brake bias valve inline with your rear brakes and go for a spin.

Hit the brakes as hard as you can so as you lock the rear and then back the valve off to suit. Repeat this process until the system is set up for your car the rear no longer lock and then do not adjust. This is how I have dialed in my system. I installed the valve under the driver seat.

 

I like the under the seat idea, do you have a pic of your setup and how you ran the lines through the floor?

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Just following the line to your back brakes and cut into add a joiner and drill a hole in the floor connect to adjustable proportioning valve in my case a willwood unit then come out of that back through the floor and re conect to brake line with correct joiner.( It is Just a tap)

See photos for info just zoom in as I took these during the build about a year ago. Put your valve wherever you are comfortable with. The engineer was happy with this idea. A few rubber grommets in tne floor for the lines to enter/exit and a bit of silicon to seal against the weather.

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post-101458-0-72502300-1452692070_thumb.jpg

post-101458-0-05386800-1452692205_thumb.jpg

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Yeah nice, thanks for that. I think in-cabin adjustment is the way to go. I have the same valve too.

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