25-11-2020, 10:48 AM
My 1936 "Early" Pearl was originally fitted with the last iteration of Austin brakes. Some time ago I upgraded to a semi girling setup for the front, complete with the later more rigid axle mounting. This improved braking from about 50% to 55%, but I had hoped for more.
The problem seems to be that under heavy braking the rear brake cables get to their limit of movement, which restricts the pull on the front to a little less than it might otherwise be. This may be because the smaller cams on the semi girling brakes result in a "softer" action with more cable movement than the rears. This effect is compounded by the fact that the cross shaft inner and outer tubes are probably seized together, so the offside rear brake gets more than its fair share and can lock up even on a dry road during a (rare) panic stop.
I have at last got enough parts together to make up a fully refurbished semi girling compensated cross shaft, which will hopefully solve the above limitations. Getting all those bushes, washers and felts (home made) correct is like a Christmas jigsaw puzzle !
Whilst looking at this mechanism in detail, I discovered a piece of Austin ingenuity which I wasn't previously aware of. The little swinging balance beam is 2 inches long and pivoted at the centre. The lower end pulls on the front cables. The upper end has a peg which actuates a forked lever to the inner cross shaft, which then pulls on the rear cables via the end levers. Because the forked lever is 1 3/4 inches long, but the end levers are 2 3/4 inches long, the tension on the rear cables is less than that on the fronts by a factor of 0.64. The upshot of this is that the cable tension (=braking effort) is distributed 61% to the front and 39% to the rear, which makes sense when you think about weight transfer.
The Austin Magazine Dec 1936 says ".... and the dual cross-tubes whereby the front and rear braking is automatically compensated and proportioned." (my italics). So that's what they meant.
The new shaft mountings fit the same chassis rivet holes as the old one, so it is a direct replacement. Ground clearance is reduced by 1 inch. There is no handbrake adjustment wingnut, so some tweaking of rear cable lengths is necessary to get the handbrake right. The front cable is then adjusted so that even with a mighty shove on the brake pedal there is still clearance between the balance beam and the "fail safe" fork fingers This should mean that compensation is working and front cable tension is not being restricted. Once this lot is fully set up and road tested I will report back...
The problem seems to be that under heavy braking the rear brake cables get to their limit of movement, which restricts the pull on the front to a little less than it might otherwise be. This may be because the smaller cams on the semi girling brakes result in a "softer" action with more cable movement than the rears. This effect is compounded by the fact that the cross shaft inner and outer tubes are probably seized together, so the offside rear brake gets more than its fair share and can lock up even on a dry road during a (rare) panic stop.
I have at last got enough parts together to make up a fully refurbished semi girling compensated cross shaft, which will hopefully solve the above limitations. Getting all those bushes, washers and felts (home made) correct is like a Christmas jigsaw puzzle !
Whilst looking at this mechanism in detail, I discovered a piece of Austin ingenuity which I wasn't previously aware of. The little swinging balance beam is 2 inches long and pivoted at the centre. The lower end pulls on the front cables. The upper end has a peg which actuates a forked lever to the inner cross shaft, which then pulls on the rear cables via the end levers. Because the forked lever is 1 3/4 inches long, but the end levers are 2 3/4 inches long, the tension on the rear cables is less than that on the fronts by a factor of 0.64. The upshot of this is that the cable tension (=braking effort) is distributed 61% to the front and 39% to the rear, which makes sense when you think about weight transfer.
The Austin Magazine Dec 1936 says ".... and the dual cross-tubes whereby the front and rear braking is automatically compensated and proportioned." (my italics). So that's what they meant.
The new shaft mountings fit the same chassis rivet holes as the old one, so it is a direct replacement. Ground clearance is reduced by 1 inch. There is no handbrake adjustment wingnut, so some tweaking of rear cable lengths is necessary to get the handbrake right. The front cable is then adjusted so that even with a mighty shove on the brake pedal there is still clearance between the balance beam and the "fail safe" fork fingers This should mean that compensation is working and front cable tension is not being restricted. Once this lot is fully set up and road tested I will report back...