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.

[Jaffar]

Sorry to drag up an old thread.

I am currently renovating a 1936 Ruby brake cross shaft. Do I understand correctly that the inner and outer shafts are locked together with a pin running through them at the midpoint so that there is no movement at that point and the outer shaft twists a small amount as you apply the brakes?

[Chris KC]

Hmm, not exactly - the idea is that instead of having the pedal on one end of the shaft & trying to turn levers at opposite ends of that shaft by the same amount - which inevitably fails due to the flexibility of the shaft - the pedal is attached to an input shaft which feeds the torque in at the centre of the main shaft, thus permitting the levers at each end of the main shaft to receive equal turning force/ angle.

[John Cornforth]

This is one of those areas where intuition sometimes doesn't help.

Whenever mechanical brake systems are discussed, there is always much talk of movement and angles i.e. the things you can see. However, movement and change of angle only occurs because slack is being taken up or items are bending, twisting, stretching or compressing, albeit slightly.

What really matters is the force on the shoes resulting from the tension in the cables and the torque in the shafts, things that are invisible.

So yes, the Ruby cross shaft is designed so that torque from the foot pedal is transmitted down the outer shaft and through the welded pin to the centre point of the inner shaft. I don't know what angular change this results in, but both shafts are pretty substantial so it must be tiny.

The later Semi-Girling cross shaft was a further development, the pin was dispensed with and a swinging arm front/rear compensator used instead. It's not well known that this apportions cable tension at 66 % to the front and 33 % to the rear, unlike the nominal 50/50 split of the previous setup. Presumably as braking improved it was felt more important to avoid the rear wheels locking up first when braking hard on low friction surfaces.

[Robert Leigh]

The 66/33% split given by the late system allows some advantage from weight transference under braking, the tendency of the rear of the car to lift and the front to 'dive', so there is potentially more friction available on the front wheels when going forward.