Semi Girling Ingenuity

[John Cornforth]

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...

[Renaud]

Hi John,
Photos of that mod would be great if possible?
Thanks,

[Bob Culver]

Semi Girling front only was discussed at length a year or three ago and several conceded that with some 4x cable movement at front not very effective, although short levers at front and long rear should assist. A spring used with the radius rod bolt may not assist.
It is not easy to figure the workings of the balance from the handbook pics but, as stated, the action is relatively complicated. In normal panic use I dunno if the beam reaches its stops which alters all again to incrase rear leverage. The setup favours rear brakes for the handbrake; not sure how much is applied to the front.
Based on experience semi Girling with the uncompensated cross shaft works reasonably, although with high pedal pressures and no dip the driver is not very aware of the braking G.
The old crosshafts can be prized with a lever to see if any relative movement available.

[Chris KC]

I try to stay out of discussions like these as all they really do is make the case for hydraulics

I'd certainly agree that something like a 60:40 split is to be aimed for (more or less depending on where your C of G is).

The inherent difficulty as I see it is to separate the question force applied from that of travel and parasitic friction.

Do let us know how you get on!

Personally I have settled for most parts free of excessive free play or friction, an eager sense of anticipation and an impressive horn note. Can't claim I've really cracked it yet.

[dickie65]

I find the horn impossible to use when one hand is steering and the other is hauling on the handbrake whilst trying to rip the seat out with my right leg on the might stop one day pedal.
I used to adjust the brake to pass an MOT then adjust them again for dynamic use.
I managed to lock the front brakes up on the rolling road a couple of times, ( The dog was not on the bonnet at the time )
Wet grass is your friend for setting up brakes as you can see which wheel locks first and adjust accordingly.

[Bob Culver]

The more effective mechanical brakes were very good. I recall Ford Pops at work which were entirely satisfactory. With single circuit hydraulics never 100% confident on long descents and braking to corner at the limit. On other makes over the decades endless stuck pistons, pitted cyls, weeping seals, other oil leaks onto hoses. And worst of all every disturbance of hubs etc triggers the tedious bleeding ritual for which you need a very patient wife.
My RP with semi girling would cause the passenger to brace their hands on the dash although the decel was not apparemt to the driver. The early track rods proved too light and caused wheels to shudder on the road.

[dickie65]

The MOT tester described my brakes as three line.
If you have strong legs the brakes are good.
If old and frail you can not push the pedal hard enough ( I am getting old enough to start losing braking efficency )

[John Cornforth]

Update time !  I have now adjusted and measured my brake setup with late Austin rear and Semi-Girling front, together with the compensated cross shaft.  I hope these notes might help anyone doing the same task. I can measure angles and free play fairly accurately, but efforts and tensions (i.e. the bit that matters) are hard to measure and therefore only approximate.  I used a lever bolted to a scrap wheel which I could stand on.  If the wheel doesn’t turn with my full weight applied, I know the braking effort is at least 180 lb. The photo below shows the cross shaft action viewed from the offside. To get things right, I needed an adjustable front cable which was set for a total length of 72.5 inches.
At rest, the balance lever on the outer shaft faces forwards at 23 degrees to the vertical, whereas the forked lever on the inner shaft sits at 20.5 degrees. The gaps to the forked lever are zero at the front and 0.25 inches to the rear. The handbrake roller has no clearance to its mating lever on the inner cross shaft.
If the footbrake is “enthusiastically” applied, the outer shaft moves round to 12 degrees to vertical and the swinging balance lever turns a little clockwise as it does its job. Pedal travel is just under 3 inches until the foot effort is about 150 lb.  Pedal reserve travel is at least 1 inch. The gaps to the forked lever are now equal, at 0.12 inches each side.  The front cables are pulled back by 0.8 inches, until their tension is about 190 lb giving 300 lb braking per wheel.  Simultaneously, the rear cables are pulled by 0.4 inches until their tension is 120 lb giving 170 lb braking per wheel. As you might expect, cable tension is rather low whilst the slack is being taken up, then rockets up with very little further movement. The outer and inner shafts rotate by 11 and 8.5 degrees respectively. The levers at the wheels move by about 20 degrees at the front and 10 degrees at the rear.
Handbrake operation is rather different, as the compensator doesn’t move.  Near full application of the handbrake (which bears on the inner shaft) moves both shafts by 8 degrees. The front cables are pulled back by 0.5 inch, until their tension is about 30 lb giving 25 lb braking per wheel.  Simultaneously, the rear cables are pulled by 0.38 inches until their tension is 100 lb giving 140 lb braking per wheel. Handbrake lever movement is about 3.5 inches or 5 clicks, and the hand effort is about 30 lb. Clearly, the lion’s share of handbrake action is to the rear wheels, with the fronts giving a bit of a “top up”
I am pleased that the new shaft has given a change towards greater braking at the front, which should solve the previous rear lockup effect. The compensator always has clearance to the fork, so front cable travel is no longer being restricted by the rears “going solid”.  Once some better weather returns I plan a road test with a Tapley meter.

[Bob Culver]

My brain bleeds!. Interesting though. At rear the effect per applied tension is comparable .If you retain Austin brakes at rear, and esp with short levers, this is surprising (although front at an angle which dimishes effective pull there.
The complete semi Girling seems much superior to previous and to semi Girling appled to all wheels of earlier cars, but the latter often seem quite effective. I guess it depends how the uncompensated balance is set up.
I presume the tensions are calcul;ated by lever prportions. It is intereting to compare the mech advantage with hydraulic systems.

[Terry McGrath]

Pistol thermometers cost £12 on Ebay/Amazon. Aim it at each brake drum after a run and you soon find out which is doing the least work. Adjust accordingly.