[ROVERNET - UK] P6 2000 TC servo boost

Netspace vmitps at netspace.net.au
Mon Dec 3 08:40:53 GMT 2007

The air chamber size affects the maximum force it can exert.
To wit:
Assume diameter of 7"
area = (7 * .0254metres)sqaured*pi/4  =  .0248 square metres
Pressure difference maximum is 100kPa, so the force is
F = area * pressure = .0248 *100 * 1000 N = 2480N
The piston is, what, .875"? don't have one to measure right now
this gives an area of .000388 sq metres, so the pressure in the fluid is 
The slave piston area is (guessing size here since I haven't looked at a P6B 
front caliper):
front piston area + rear piston area
(60mm squared * pi / 4 * 4) + (12mm squared *pi /4 * 4)
= (.011sq m) + (.00045sq m)
= .0115 square metres
This times the fluid pressure gives the force on to the discs of 73KN 
(average, I should work out the split front to rear...)
Usual coefficient of friction between pad and disc is 0.4
Now the braking force on the rotor actually involves integrating the force 
elements across the pad, for different rotor radii.  I can't be bothered 
doing that right now, so I shall use rull of thumb that 2/3 of the pad 
distance (going radially) is the radius of action.  This gives about 250mm 
Now the tyre longitudinal force is the inverse of the disc effective 
diameter and the tyre diameter (to contact patch, a bit less than the free 
diameter) times the disc torque braking force.
Tyre diam is roughly .63m, so ratio is .25/.63 = 0.4
This gives at tyre force of :
disc side force * coeff * torque ratio
=  73KN * 0.4 * 0.4
=  11.7KN at tyre perimeter
Weight force on tyre = 300kg * g  (good old F = MA, and assuming that all 
wheels carry equal down force which is rubbish when braking) (totally making 
assumptions here, worse than an evolutionist)
=  2940N (or 2.94KN)
Rubber to road coefficient of friction is around 1.0 on a road tyre (higher 
on track tyres),
so there is Plenty of force to lock the wheel.
If the front wheels took the full weight of the car, they'd handle around 
5KN braking force.  Their ability proportionally drops as the force 

Paul Smith (eh!)

----- Original Message ----- 
From: "Vern Klukas" <vern at inkspotco.com>
To: <rovernet at lyris.ccdata.com>
Sent: Monday, December 03, 2007 3:57 AM
Subject: RE: [ROVERNET - UK] P6 2000 TC servo boost

>I did a little research on this, it appears thw size of the vacuum piston 
>is irrelevant, it is the ratio between the area of the control piston (the 
>little one that operates the airvalve) and the slave cylinder (the main 
>hydraulic bore of the servo) that counts.
> Based on that, the ratio should be 2.78:1, which is the same number as the 
> Girling MkII servo used on the P4 110  ( 
> www.head2head.free-online.co.uk/Rover/servo.htm )
> Perhaps more later, I'm going to poke around a bit more.
> Yours
> Vern
> -- 
> Vern Klukas                             I'm a little . . .
> Inkspot Type & Design
> vern at inkspotco.com
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