Quote:
Originally Posted by GreatOldOne
Code:
Pedal centre to pivot centre 32cm - excuse mixed units - it does not matter
Pedal connection to servo to pivot - 8cm
Therefore my pedal ratio is only 4:1
My master cylinder bore = 22mm = 7/8" = 0.6 sq inch
Now assume:
23” of vacuum
100lbs of applied foot pressure for comparative purposes.
Based on my brake's details:
Booster size “Manual Pressure” “Free Pressure” Total Brake Pressure
7” Metro 666 psi *1 587 psi *2 1253 psi
7” Dual 666 psi 815 psi *3 1481 psi *4
*1 = (100lbs x 4:1) / 0.6sq.inch = 666psi
*2 = Metro booster constant 352lbs / 0.6 = 587psi
*3 = Dual booster constant 490lbs / 0.6 = 815psi
*4 - The dual offers an increase of 18% of the total over the Metro.
Mike - Where did you get the 'constant' figures in the calculation?
I've tried to do the sums my self, but as a bear of very little brain, what little there is is now smoking. 
Taking all the figures as stated, I got as far as:
666 psi Manual Pressure.
Metro Boost Ratio is 1:1.9, so total brake pressure is 666 * 1.9 = 1265 psi
Free Presure is then 1265 - 666 = 599 psi.
Then I get confused. I don't know the boost ratio of the dual 7"... |
Hi Jason
The servo generates a pressure by creating a vacuum on one side and allowing normal atmospheric pressure on the other. Therefore assuming your engine generates the 23" of mercury pressure (sorry about Imperial units, but just using the MBM assumptions), then the servo will always give the same amount of boost. It is not a ratio relative to pedal pressure, but a constant relative to diaphragm surface area - for a constant vacuum).
I calculated the constant from MBM's figures.
Surface area of a circle = pye x r2
1 1/8" bore - Sorry I need to convrt to decimal = 1.125sq.in
therefore (1.125 /2) x (1.125/2) x 3.14159 = 0.994sq.in.
Stated Free Pressure = 355psi.
Therefore the force from the servo was 352lbs
(352lbs / 0.994sq.in. = 355psi)
If you do the same calculation for the 1" bore and the 15/16" bore you will get a constant figure for the servo boost.
1" bore = 0.785sq.in
352lbs / 0.785sq.in = 449psi
15/16" = 0.69sq. in.
352lbs/ 0.785 sq.in. = 511psi
Therefore the servo gives a constant boost of 352lbs force. Note this is not psi, but lbs only. It is a force, not applied per surface area, until directed at the master cylinder plunger, and then it is affected by the bore size.
Hope this helps.
PS : The adaptor arrived in the post this morning - many thanks.
Mike