| Author | Message | ||
     Robert Noel Reddington Grand Master Username: bob_uk Post Number: 606 Registered: 5-2015 |
If the brake pedal is pressed with 45 lbs of force the pressure at the front calipers should be 500 psi. At 80 lbs force the pressure should 1000 psi. 2500 psi is available if the pedal is pressed hard enough. To check fit gauge in lieu of caliper bleed nipple. Reference Chapter G workshop manual. | ||
| Randy Roberson Grand Master Username: wascator Post Number: 529 Registered: 5-2009 |
I can just see it on the front page of our local news: car hobbyist drowned by RR363 fluid pressed pedal too hard to test his system test failed! | ||
| Robert Noel Reddington Grand Master Username: bob_uk Post Number: 610 Registered: 5-2015 |
Hopefully when the pedal is pressed with both feet nothing pops. The brake pedal is a heavy duty forging. 1966 Ford Transit LGY 703D with under floor master cylinder. I stamped very hard on the brake pedal and the master cylinder push rod bent up. No brakes. Good hand brake though. The local Ford guys were amazed. Later Transit were different. | ||
| Jan Forrest Grand Master Username: got_one Post Number: 865 Registered: 1-2008 |
I was under the impression that braking effort was more about the deflection of the pedal than the pressure on it - at least for the later Shadow 1s and all Shadow 2s. | ||
| Robert Noel Reddington Grand Master Username: bob_uk Post Number: 641 Registered: 5-2015 |
Jan, To deflect the pedal one must put pressure on it. So it pretty much the same thing. The data comes from chapter G workshop manual. The master cylinder pushes back which applies the power brakes. If the master fails the 2 prongs hit the spacer and the prongs push back. On models without the master the rubber cone pushes back instead. Remember physics at school. When one sits down on a chair the chair legs push back to match the weight on the sitter else the chair would collapse. | ||
| Jan Forrest Grand Master Username: got_one Post Number: 866 Registered: 1-2008 |
However the pedal pressure is still divorced from the caliper pressure. It isn't just a multiplication as on other cars. On the later Shadows the only 'back pressure' comes from the springs installed to give the pedal some 'feel'. Without these we might have ended up with a brake switch as fitted to the Citroen DS. All on or all off with very little control between the extremes. | ||
| Brian Vogel Grand Master Username: guyslp Post Number: 1719 Registered: 6-2009 |
Jan, You are correct. The system in these cars uses the brake pedal as nothing more than a way to control the amount of opening in valves that allow continuously variable opening between the limits of fully closed and fully open. The amount of force that needs to be applied to the brake pedal is controlled by a mechanism that has little effect on overall braking on the master cylinder era cars and no effect at all on braking in the post-master-cylinder cars. One could choose to change the springs that are used to create the pedal feel to virtually any spring that will hold the pedal such that the valves remain closed when one's foot is not depressing the pedal. In theory one could make the brake pedal feel anything from ridiculously feather light to so hard to depress that you can barely get it pushed. The only thing that affects the degree of braking is how the pedal position corresponds to opening the valves, not the amount of pressure required to get it there. I do get Bob's point, though, about the fact that there are standard springs used and because of this one can map the amount of force necessary to move the pedal that has those springs attached to the amount of braking power applied. But since one could change the springs that mapping force map could be radically changed without braking power relative to pedal position changing at all. Brian | ||
| Robert Noel Reddington Grand Master Username: bob_uk Post Number: 644 Registered: 5-2015 |
Brian, The springs are just to hold thr pedal up the the valves shut------ until hydraulic pressure is up and then some of what you feel is actually hydraulic pressure. Try this. Pump the system down and start car with foot on the pedal you well feel tbe pedal pulse and push up against your foot. This is a quick way of telling if the pump is working ---- well one on the pumps or maybe both. The braking can't be mapped by changing springs. Because the geometry of the brake valve internals control that. | ||
| Brian Vogel Grand Master Username: guyslp Post Number: 1720 Registered: 6-2009 |
Bob, I don't want to get into hair splitting and do realize that, as you said, the hydraulic pressure is a part, but a small part, of what you feel when you push a RR/Bentley brake pedal. You are misreading what I mean by mapped. The opening of the valves is what determines the braking force at the calipers. If you used a wimpy spring you might go, say, from fully closed to fully open by barely touching the pedal with a 50% travel position achieved by a feather light touch. If you used an industrial strength "could hold a garage door open" grade spring you'd have to push on the pedal much harder to go from closed to fully open, and the 50% travel position requiring the application of significantly more force. Regardless of the amount of force you exert to get to the 50% travel position due to the spring in use, the braking system is not exerting even the slightest bit more force at the calipers whether you barely had to push the pedal to achieve 50% travel or nearly gave yourself a hernia to achieve that amount of travel. In the end, you can always "map" the braking force relative to force required to achieve it on the pedal if you know what it actually is based on degree of valve opening based on how much force a given spring set requires you to exert to get to that degree of opening. I'm not saying that using a stronger spring changes anything about the braking force at the calipers, just how much exertion the driver must put in to pushing the pedal to achieve a given braking force. Brian | ||
| Robert Noel Reddington Grand Master Username: bob_uk Post Number: 647 Registered: 5-2015 |
The hydraulic bit is greater than you think. The pedal will push back with quite some force. The spring in the brake valve is not that strong and the brake pedal lever ratio is about 8 to 1 which means without hydraulic pressure the driver wouldn't be able to modulate the brakes. Not withstanding that the feel is partially artifical and wholly artificial on later cars. At a guess the spring is not quite as strong as an engine valve spring. If the valve was a simple tap held shut by a spring then the brakes would be on or off with no in between. Air brakes are the same the brake valve is held shut by air pressure. The designers work out the internal dimensions so that its just right (hopefully). If you study air brakes on trucks the little nouces are easier to understand because the bits are biggger. The easiest way to alter brake pedal weight is to shorten or lengthen the the actual brake pedal lever ratio. My brakes feel about right for the car. I think RR got it right. So all this is really academic  | ||
| gordon le feuvre Frequent User Username: triumph Post Number: 90 Registered: 7-2012 |
I was told by someone at Hythe Rd when I was on the Shadow training course in Feb.1967 that the original prototype "Shadow" brake system used a tennis ball under the brake pedal to give the system feel!! The discussion came up as I had started my apprenticeship in a Citroen dealer, very handy when I moved to the R-R dealership. | ||
| Robert Noel Reddington Grand Master Username: bob_uk Post Number: 648 Registered: 5-2015 |
Gordon, A Citroen service engineer called the brake valve "the dozer valve" I never found out why. I have worked on a few Citroëns and found it all perfectly straightforward. The steering was a bit difficult to understand but I got there in the end. The CX was really good to drive. The brake pedal if pressed at a certain pressure would start dancing under foot. The pressure was above wheels locked so I used to frighten the kids at traffic lights with stories of evil cars that fight back. The BX was more conventional than the CX which had very powerful brakes. The later ones thank god had abs. On air brake valves the pressure holding the the valve shut could be adjusted by a small amount on some systems. Approx 10 psi either way on an 120 psi system. Less pressure lighter pedal and visa versus. I used to set the back pressure to the recommended setting and seal the cap with a lead seal. The drivers otherwise would interfere with the setting, yet often failed to drain the water from air tanks. I had one truck with air tanks almost full of water which meant no reserve brake capacity and in 3 pumps no brakes till the compressor catches up. Sort of similar to having no nitrogen charge in a sphere. Except its not 2.5 tons of car with no brakes but 25 tons with no brakes. The water doesn't compress so it leaves little room for compressed air. It doesn't do the relay valves etc a lot of good. The proto types of the Shadow hydraulics used CAV diesel injection in line pump elements. CAV although not involved did supply production knowledge of how to mass produce the close tolerance bits and bobs, as did Citroën. |
