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Perrot shaft front brakes

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  • Perrot shaft front brakes

    It is often stated in the various articles on brakes that you must check that the outer spade of the shaft should be at 90 degrees the king pin axis in order to avoid having the inner wheel brake from being brought on when you put it on full lock. My simple question is why?

    I thought the point of the design of the system with the spherical joint mounted over the king pin axis was to prevent this inner wheel braking on full lock from occurring. Indeed if this 90 degree is critical then it follows that any braking when the steering is applied will be very dangerous as the outer spade cannot be at 90 degrees and this will cause the inner brake to be brought on harder than the outer brake on cornering and braking.

    It is clearly fundamental that the spherical joint must be properly lubricated with no solidified grease, grit or dust otherwise this will prevent the necessary free movement of the various surfaces. If the joint is 'tight' then this will mean alignment of the joint will not be correct giving rise to the inner brake being operated on full lock.

    The CT document on setting the brake linkages makes the point that on the rear brakes once the system has been set up the clevis fork on the operating rod must be screwed up by 2 full turns from the position where the pin fits without moving the lever. For the front brakes the same point is made for the two front rods that they must also be tightened up from the zero position. In both cases this is to ensure that any play in the cam operating system is taken up.

    Had anyone determined how much the Perrot shaft actually rotates before the shoe begins to move?

  • #2
    I have never actually measured the angles, it's not very far, but I always check that they are well greased and do rotate with no sticking. I remember when rebuilding the front axle years ago John Dodd advised that a little play was beneficial, now there is more! Could it be that the 90 degree "rule" is to ensure that both sides are the same and the shafts haven't twisted? Or could it be that inferior non-Talbot versions of the design suffer from geometrical weakness?
    A nasty experience a couple of years ago when the nearside wheel fell into a big hole in a puddle, it pushed the axle far enough over for the off-side spade to come out. Very fortunately the brass d-shims stayed in place and I was able to get it back together at the roadside, whew. Thorough checks later revealed nothing really wrong, spring shackles and axle U-bolts OK. Anyone else had such an occurrence, nothing about that in Whieldon/Davies.

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    • #3
      Wheildon/Davies's problem was that the retaining plate on the drum end was forced off by some magnificent potholes in Algeria. I suppose the axle came up so fast that the Perrot shaft hammered against retaining plate If I remember correctly the studs were forced so hard they jumped their threads in the aluminium brakeplate resulting in their outer ends rubbing on the drum. There is a photo of me with the drum off sorting out the problem. At the same time we probably broke the top leaf of the front spring. We had one spare with us, but within a few hundred miles we had broken all three top leaves. We discovered that a Mitsubishi, I think, pick up rear spring was the same dimension from the eye to the centre bolt on one half, but longer on the other half. It was not difficult to find a man with a welding torch who could shorten the other end and roll it around our silent block bush. These springs were thicker than our originals, and they never broke. But I digress.

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      • #4
        Hi Garth,
        Thanks for taking the trouble to reply.
        I have now measured the shaft rotation before the front shoe begins to move and get a figure of 10 degrees. Winding up the clevis fork 2 full turns does not remove the full amount of play but there is a slight increase in the amount of brake application on full lock. There is virtually none without the 2 turns on the clevis pin. It would be nice to know how much is too much! I wonder if I can put the car on my MOT man's brake tester on full lock and get some data. I also wonder whether there is a better lubricant for the joint that has a lower stiction break out force? I also wonder if there is an optimum balance between clearance of front shoe to drum, amount of play in the joint and the amount of servo force being generated by the system? I am rapidly realising how little I know about the detailed dynamics of the front braking system!! Seeing how 3D modelling is used in the design of camshafts it would be great to get a University to produce a 3D model of the front brake design and use this to investigate the interactions of the different factors present in the operation and performance of the front wheel brakes.

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        • #5
          Hi Garth,
          I was re reading your post about the Perrot shaft pulling out of the back plate u/j and had a look at some spares of the joint I have got. I also looked at the relevant spares parts list for your car and for my Scout. What the parts list drawings show quite clearly are that the 'D' grunion bushes have a hole through them and through the spade. This means that the only way the joint can be assembled is by sliding the pinned grunion bushes and the spade arefronl the side and not along the axis of the shaft. In other words the design should not allow the shaft to be disengaged from the hub under the conditions you described assuming I have understood the design. Which makes me wonder whether the cross pins were fitted on your car? Or have I completely misunderstood what happened and it was the inner end of the shaft that pulled out of the chassis mounting?

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          • #6
            I have measured the Perrot shaft rotation from spade at 90 degrees to kingpin axis to first movement of brake shoe at 9 degrees. This is reduced to 3 degrees by shortening v the rods by 2 full turns of the clevis fork.

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            • #7
              Sorry I wasn't clear about my Perrot shaft "disengagement", it didn't pull out of the backplate, as you rightly note it is pinned, but out of the chassis mounting. I know you are a stickler for the detailed how and how much, but I'm afraid I just make sure its bolted properly together. If you had seen how worn out everything was when I got the car 60 years ago, yet still drove, precision was not my main consideration!

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