Ideal Brake Force Distribution - Definition and self test
Background. It is accepted awd theory - and physics - that a locked center differential (and torsen center - not applicable to 80) under braking will result in Ideal Brake Force Distribution. This means that the brake loading (up to lockup) will follow *exactly* weight distribution. Specifically, the extra braking force of the front brakes (defined as 'overrun brake torque) is transmitted through the locked driveshafts to the rear tires.
Defining Ideal Brake Force Distribution: Let's say under maximum braking, the weight distribution of the 80 is 78% on the front axle /22% on the rear axle. On a 4700lb 80 that means that the front axle loading is 3666lbs and the rear axle loading is 22%. In 'ideal brake force distribution' theory, you would then want exactly 78% of the braking force available to the front axle and 22% to the rear axle.
In Brake Proportioning systems found on the 80, you have a variable proportioning valve for rear braking. This helps in maintaining a 'better' (vs fixed) brake force distribution thru varying payload weights and distribution. As the rear of the truck settles from weight, so does the increase in rear brake force. These proportioning systems are a compromise of best estimates and hardware dependent. The closest any of these systems get to 'ideal brake force distribution' is only during ABS *activation*. However, how close they get to ideal depends on several factors, and no 'extra' brake force (over run brake torque) can be transfered to the 'other' axle.
Bad pads, weak springs, changing shocks, changing COG, changing payload all effect how close to the factory design brake force distribution gets. How far you stray from the factory design, is directly correlated to how far you will stray from ideal brake force distribution.
To test or feel how close *your* specific truck brakes/hardware is to ideal brake force distribution is easy if you have a center locker (CDL switch) installed. Lock CDL and brake in a straight line, this yields the ideal brake forced distribution given what you have on your truck at the time you lock the CDL. Now, unlock the CDL, and try the same brake test. Notice how much more the chassis wants to dive, and how much more brake effort and distance is required? My conclusion is that the brake force distribution with the stock proportioning valve is massively biased to the front brakes on the 80. This also follows accepted application of brake hardware in production vehicles.
In terms of chassis dynamics, the less you upset the chassis weight distribution, the better the performance you will have under braking and handling. IME with the 80 chassis (TJM, airbags, Blizzacks, otherwise stock chassis), the comparison of ideal (CDL locked) vs stock (CDL unlocked) leads me to conclude that the 80 is massively overbraked in the front compared to the rear. I suspect this is a combination of inherently innacurate rear prop valve, and the algorythums in the ABS computer.
Next up in Part II: ABS in the 80 and Select Low Principle
HTH
Scott Justusson
Background. It is accepted awd theory - and physics - that a locked center differential (and torsen center - not applicable to 80) under braking will result in Ideal Brake Force Distribution. This means that the brake loading (up to lockup) will follow *exactly* weight distribution. Specifically, the extra braking force of the front brakes (defined as 'overrun brake torque) is transmitted through the locked driveshafts to the rear tires.
Defining Ideal Brake Force Distribution: Let's say under maximum braking, the weight distribution of the 80 is 78% on the front axle /22% on the rear axle. On a 4700lb 80 that means that the front axle loading is 3666lbs and the rear axle loading is 22%. In 'ideal brake force distribution' theory, you would then want exactly 78% of the braking force available to the front axle and 22% to the rear axle.
In Brake Proportioning systems found on the 80, you have a variable proportioning valve for rear braking. This helps in maintaining a 'better' (vs fixed) brake force distribution thru varying payload weights and distribution. As the rear of the truck settles from weight, so does the increase in rear brake force. These proportioning systems are a compromise of best estimates and hardware dependent. The closest any of these systems get to 'ideal brake force distribution' is only during ABS *activation*. However, how close they get to ideal depends on several factors, and no 'extra' brake force (over run brake torque) can be transfered to the 'other' axle.
Bad pads, weak springs, changing shocks, changing COG, changing payload all effect how close to the factory design brake force distribution gets. How far you stray from the factory design, is directly correlated to how far you will stray from ideal brake force distribution.
To test or feel how close *your* specific truck brakes/hardware is to ideal brake force distribution is easy if you have a center locker (CDL switch) installed. Lock CDL and brake in a straight line, this yields the ideal brake forced distribution given what you have on your truck at the time you lock the CDL. Now, unlock the CDL, and try the same brake test. Notice how much more the chassis wants to dive, and how much more brake effort and distance is required? My conclusion is that the brake force distribution with the stock proportioning valve is massively biased to the front brakes on the 80. This also follows accepted application of brake hardware in production vehicles.
In terms of chassis dynamics, the less you upset the chassis weight distribution, the better the performance you will have under braking and handling. IME with the 80 chassis (TJM, airbags, Blizzacks, otherwise stock chassis), the comparison of ideal (CDL locked) vs stock (CDL unlocked) leads me to conclude that the 80 is massively overbraked in the front compared to the rear. I suspect this is a combination of inherently innacurate rear prop valve, and the algorythums in the ABS computer.
Next up in Part II: ABS in the 80 and Select Low Principle
HTH
Scott Justusson
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