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Old 12-15-2010, 08:49 PM
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brake bias questions

front calipers 1.77 bore size

rear calipers: 1.687

master cyl. bore: 15/16

front rotor diameter: 13 inches

rear rotor diameter: 11.1 inches

front tire height: 25 inches

Rear tire height: 27 inches

so, now that we have all the specs in play, can you help me figure out if this is a good idea? seems like ill have about neutral bias without the bias adjuster in play. is this correct? let me know what you think. im hoping to get a great braking, predicable car out of this swap.


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Old 12-16-2010, 08:28 AM
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I would imagine that a mechanical engineer might be able to determine some approximate figures for you.

A few more figures are very critical to any calculations though. Overall weight of the vehicle. Weight on front end. Weight on rear end. Tire diameter doesn't make much difference. Tire width does, as it will effect how much surface contact each tire has. The tire compound is important as different compounds will "grip" the surface differently depending on the surface conditions (wet, dry. icy, etc.) Even the shocks you use should be considered. You mention that the master cylinder bore is 15/16" so I assume you are running manual (no power) brakes. For best results with manual brakes, the pedal ratio should be in the 6:1 or preferably 7:1 ratio. This is so that the pressure applied by your foot will be in a realistic range (think one foot rather than both feet) for the master cylinder to be fully utilized.

OK, after saying all the above, here is a simple test you might consider performing to decide whether you need to install an adjustable proportioning valve in your rear brake line. The same test can be used to determine how any adjustments are actually affecting the front to rear brake bias.

Find an empty parking lot or stretch of road that has no traffic at the time you are testing. The road surface should be wet (not ice) or have a loose gravel surface. Accelerate normally up to about 35 MPH and apply the brakes fairly hard. Do NOT go much faster at this time. After adjusting the proportioning valve to get the results you want, then you can repeat the test at slightly higher speeds. A spotter with a video camera can be very helpful to see the results. What you are looking for is which wheels lock up first. By today's standards, it is preferable that the front brakes lock up just before the rear brakes. If the rear brakes lock up first, the vehicle will have a tendency to "swap ends" which is very dangerous and "out of control". This bias is usually easily adjusted with an aftermarket adjustable proportioning valve.

One note concerning the adjustable proportioning valve. When the knob is turned fully clockwise, the valve is wide open. This is the opposite of what you would expect. When the valve is turned fully counter clockwise it restricts the flow by approximately 60%. A good starting point for the adjustment is about halfway position. Do a brake test as above. Adjust accordingly about 1 turn and re-test. Repeat until you get the results you want.

Last edited by Frisco; 12-16-2010 at 08:34 AM.
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