Brake Cross Shaft

[David Aylmore]

       
The Devon A7 Club are dismantling an A7 chassis to prepare it for use as a teaching aid for young enthusiasts. 
After a considerable struggle, and by drilling out the central bar,  I have managed to separate the two tubes.
 BUT 
I still can't work out how the cross shaft should work. 
As far as I can see, the bar that goes through both tubes locks them together so how can one ever revolve inside the other?

[Robert Leigh]

When a turning force is applied to a shaft and something is resisting that force at a different point on the shaft it will twist within its length between the two points where force is applied. With the earlier braking system where the pedal rod applies force at the offside end of the shaft the offside cable gets more tension when braking than the nearside does because the transverse rod twists; the harder the braking the more the twist. With the later system the brake pedal force is transferred to the centre of the outer tube, and the pin at that centre transfers the force to the inner rod at is centre. Any twist in the rod under braking is equalised on both sides giving similar braking effect on both back wheels assuming that both rear brakes are in similar adjustment.

[David Cochrane]

With the earlier braking system where the pedal rod applies force at the offside end of the shaft the offside cable gets more tension when braking than the nearside does because the transverse rod twists; the harder the braking the more the twist.

However a far more significant factor is the weak supporting brackets - it is rather instructive to lie under the car and watch the brackets flexing while your assistant presses the brake pedal!

[John Cornforth]

The tubes DO revolve around each other at the outer end, just not very much ! There should be a zinc bush between them at that point.

To give a rough example, when you use the foot pedal "all out" it transmits about 140 lbs-ft of torque to the centre via the outer tube. Half of this is transmitted to the front brakes, and the other half (via the connecting pin) symmetrically and therefore equally to the rear brakes i.e. 35 lbs-ft per side. So that's a differential torque of 175 lbs-ft between the inner and outer tubes at the offside end. Those two tubes are pretty substantial, I don't know how much differential angular movement there is at this torque loading but it will be tiny. This is of no consequence however, as once slack is taken up its the cable tension that matters, not any visible movement. The cable tension for this example will be about 150 lbs per cable.

This all works fine until the tubes rust together, and/or the offside bearing wears. Then you get the all too familiar condition of the foot pedal transmitting tension directly to the offside rear brake but via the whole cross shaft to the nearside rear brake. You can sort of compensate for this imbalance by slackening the nearside rear brake cable, but the rear brakes will then only ever be balanced at one setting. Best to balance at full braking to avoid skids, and accept imbalance at lower braking levels. A flat gravel driveway is a good test !

[Dave Mann]

Some years ago I freed a rusted assembly off, first I drilled and tapped the outer tube only for a grease nipple close to the center bearing then the OS half of the shaft was submerged in deox C after first taping over the the shaft journals. It took about a week before grease exuded between the tubes at the OS end    

[David Aylmore]

Thanks Robert, that makes some sort of sense.