Effect of brake servo on brake fluid pressure?
Discussion
I am trying to calculate the brake pressures and forces on my car, but can't seem to figure out the booster/servo effect correctly.
I have a dual 7"/8" servo which, as I calculate it has a surface area of 88.75"^2 and should produce 1001 lbs of force, when the engine is pulling 23"Hg of vacuum (11.29psi).
If pedal input is 100 lbs and the pedal ratio is 5, we have 500lbs of input from the driver, what is the overall force on the master cylinder? I have it as 1501 lbs, which produces overall braking forces of around 1G which you would expect.
What doesn't seem right is if you input a driver input of 10lbs, the MC input is 1011 lbs and the car is still stopping at .75G!
I don't think I am understanding how the servo applies pressure to the MC, can someone enlighten me? I was under the impression that as soon as you touch the pedal, the servo diaphragms see the inlet manifold vacuum and apply that force to the MC, but it would seem not...
I have a dual 7"/8" servo which, as I calculate it has a surface area of 88.75"^2 and should produce 1001 lbs of force, when the engine is pulling 23"Hg of vacuum (11.29psi).
If pedal input is 100 lbs and the pedal ratio is 5, we have 500lbs of input from the driver, what is the overall force on the master cylinder? I have it as 1501 lbs, which produces overall braking forces of around 1G which you would expect.
What doesn't seem right is if you input a driver input of 10lbs, the MC input is 1011 lbs and the car is still stopping at .75G!
I don't think I am understanding how the servo applies pressure to the MC, can someone enlighten me? I was under the impression that as soon as you touch the pedal, the servo diaphragms see the inlet manifold vacuum and apply that force to the MC, but it would seem not...
Vacuum brake servo's are quite cunning things!
They actually apply vacuum to BOTH sides of the diaphram when the actuator rod displacement is zero, A "needle valve" attached to the brake acuator rod allows air to "leak" back into the pedal side of the servo, creating a delta P across that diaphram and a additional force output. The clever part is that the needle valve system is "displacement" controlled, and the system uses the spring rate of the brake hydraulic system as it's "load source" vs an internalrefference spring system. In this way the brake load multiplication is reasonably linearily controlled up to the maximum load condition. Efectively if the driver stops and holds pedal position, the "leak" valve will shut, and the pedal side of the system is held at the current depression (and servo multiplcation), if the drivers pushes hared more air is released until the system is balanced, or the reverse if the driver lifts off the pedal etc.
For calculations involving servo's as a first approximation ignoring friction and lag effects, it is sufficient to just use a "Boost ratio" linearly applied on top of the driver pedal effort. typical servo's range from 1.8:1 to 4.5:1.
useful info here:
http://server11.web-mania.com/users/eaglekit/How%2...
and here:
http://www.mira.co.uk/site/ResearchReports/0235036...
They actually apply vacuum to BOTH sides of the diaphram when the actuator rod displacement is zero, A "needle valve" attached to the brake acuator rod allows air to "leak" back into the pedal side of the servo, creating a delta P across that diaphram and a additional force output. The clever part is that the needle valve system is "displacement" controlled, and the system uses the spring rate of the brake hydraulic system as it's "load source" vs an internalrefference spring system. In this way the brake load multiplication is reasonably linearily controlled up to the maximum load condition. Efectively if the driver stops and holds pedal position, the "leak" valve will shut, and the pedal side of the system is held at the current depression (and servo multiplcation), if the drivers pushes hared more air is released until the system is balanced, or the reverse if the driver lifts off the pedal etc.
For calculations involving servo's as a first approximation ignoring friction and lag effects, it is sufficient to just use a "Boost ratio" linearly applied on top of the driver pedal effort. typical servo's range from 1.8:1 to 4.5:1.
useful info here:
http://server11.web-mania.com/users/eaglekit/How%2...
and here:
http://www.mira.co.uk/site/ResearchReports/0235036...
That would make more sense. Is there anyway to 'guestimate' the boost ratio from the size of the diaphragms and the maximum boost they are capable of generating at max vacuum?
If for example, I have 500lbs input from the pedal and a maximum available boost of 1000lbs from an 88sqin diaphragm at 23"hg for a total input of 1500lbs, which on a 1" MC gives me 1500psi. In this case my boost ratio is 3:1 yes?
If for example, I have 500lbs input from the pedal and a maximum available boost of 1000lbs from an 88sqin diaphragm at 23"hg for a total input of 1500lbs, which on a 1" MC gives me 1500psi. In this case my boost ratio is 3:1 yes?
Edited by Kozy on Monday 8th November 20:48
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