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+--- Thread: Brake cross shaft- help needed please (/showthread.php?tid=2292)
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Brake cross shaft- help needed please - jonblob33 - 26-01-2019
Hi, can someone on here help re brake cross shaft. How much movement should there be radially between the two tubes, & how relevant is it to balancing my brakes? If I disconnect the front cable & apply two clicks of the hand brake the rear brakes are hard on. When I connect the front cable four clicks of the hand brake locks the front but both back wheels are free to rotate. Any advice or suggestions please.
Kevin. Ps car is 1933 PD.
RE: Brake cross shaft- help needed please - Bob Culver - 27-01-2019
Few realise that there are two concentric tubes on rhs but all tubes operate almost as one; usually seized solid and not likely to be a factor anyway.
It is a logical mystery. A lot of tension is being absorbed in the front sytem in a way which reduces tension to rear. I am not sure of effect but is cross shaft very loose in "bearings"? Is it same brake sytem and lever lengths front and back?
The front axle must be rigidly located by the ball joint and shackles. If the front only are working axle should not move when car rocked forwards and back. Some front lead is desirable but when all is rigid not excessively so. The thread holding the aluminium swivel balance piece is the adjustment for front brakes.
RE: Brake cross shaft- help needed please - jonblob33 - 27-01-2019
Thanks for the reply Bob. King pins& bushes, axle link bushes all new & good as is radius arm ball etc. Cross shaft bushes seem ok. All this came about because I decided to fit a better steering box, got the car on 4 axle stands and decided to try improving the brakes. 50 yrs as an engineer in several fields and a set of “simple “ cable brakes has got me stumped. Luckily not reached the box of matches point yet. ( burn the b*****d & go to the pub)
Kevin.
RE: Brake cross shaft- help needed please - John Cornforth - 27-01-2019
Hello jonblob
As Bob mentions, Austin devised a devious concentric tube which was supposed to convey effort from the footbrake on the driver's side through to the centre of the cross shaft, to give balanced effort Left / Right. The angular movement between tubes is tiny, but it's the forces which matter. When the tubes seize together, there is a bit of a tendency for the rear offside brake to be over-enthusiastic as it gets a direct pull.
Until the Semi-Girling brakes came in in 1936, there was no form of balance bar to equalise cable tension Front / Rear and give equal(ish) braking. Cable length adjustment is therefore critical, and I have found that small adjustments of the front cable (behind its swivel) can be enough to shift braking effort from front to rear and vice-versa.
As far as I know, Austin always used the same brakes and same lever lengths for front and rear, so braking effort would have been equal front and rear. Given that the cars are slightly front heavy, and that weight is transferred under braking, I guess a case could be made for biasing the effort a bit frontwards. This would also avoid the rears locking up under hard braking on a slippery surface with "interesting" results.
Ideally, the brakes should be adjusted for best balance using the footbrake (with assistant), and with the weight of the car on its suspension (axle stands are OK if they are actually under the axles) so that all the cables are in their normal position. They should also be balanced at somewhere near full braking. To give an idea, if you take the weight of the car as 1200 lb, to give 50% braking you are looking for 150 lb braking effort at the tread of each tyre. This is rather more than most of us can muster if turning the jacked up tyre by hand, so you may have to settle for balanced brakes at lighter applications and accept that the balance may not be as good at full whack.
Final adjustment can be made after looking at the skidmarks from braking on a loose gravel surface, often the Offside rear will need slacking a bit to curb its enthusiasm. My local "old car friendly" garage has a rolling road brake tester which is narrow enough for a Seven, and this is even more accurate of course.
RE: Brake cross shaft- help needed please - AustinWood - 27-01-2019
An extra bias is needed towards the front. When the front brakes come on while the car is moving the axle moves back so slackening the brake cables. The more play there is in the system the more pronounced this effect is. Even if all is correct there will still be some movement.
The effect is reversed when going backwards. The axle moves forwards tightening the cables and can lock the front wheels. Also when the front wheels are turned one front brake tends to tighten because the cable ends are not directly below the king pins.
It takes a bit of fiddling to get everything set up correctly but when all is correct the system should work pretty well.
RE: Brake cross shaft- help needed please - Bob Culver - 27-01-2019
Hi Kevin
Not always sure of the experience level of enquirers. I generally pitch any reply at an elementary level anyway as hopefully then useful to more.
Will be interested to learn the outcome. I take it the clicks are counted from full off. With all the general springiness the brakes are daunting for those only accustommed to moderns! As any drag is too much with a Seven, arranging the brakes to come on with the pedal quite low is advantageous but further unnerves many! (Also reduces problems with front brakes when turned sharp. The direction of cotters influences just where the lever balls lie.)
RE: Brake cross shaft- help needed please - Hedd_Jones - 27-01-2019
Kevin
I'm not sure anyone has properly described the construction of the part, though we seem to be clear on its operation.
The outer slides over the offside of the inner shaft, it is fixed by a single pin which is welded solid under where the centre bearing goes. Ive had one in bits, and other than where it fits over the centre of the inner shaft (where it is pinned), and the outermost inch (under the lever fitted to the outer shaft), the outer is a rattling good fit on the inner. Clearly the middle is fixed solid, but you really want the outer end not to be stuck on the inner shaft.
If you fit the outer lever to the inner shaft on the offside, with its cotter tight, you should be able to detect slight difference in rotation if you lever between the two levers. If you cant its stuck. Now dont expect it to move a lot, we are talking a gnats dick, but you should and will be able to detect it. This is the inner shaft twisting in relation to the outer.
I find with some heat, oil, clouting the outer with a hammer and general swearing at it, fairly easily you will detect motion. When you have the job is sorted.
RE: Brake cross shaft- help needed please - "Slack Alice" Simon - 27-01-2019
Hi, Hedd
Are you saying that the outer is most likely to be stuck to the inner at the offside ?
I assume it could be stuck along the whole length, but only is the worst case scenario?
Cheers
Simon.
RE: Brake cross shaft- help needed please - Hedd_Jones - 27-01-2019
its deffinatley stuck in the middle!. The rest has about 1/16 clearance. Except under the outer lever where there is a bearing
RE: Brake cross shaft- help needed please - Bob Culver - 28-01-2019
A couple of idle comments...
The system is not all that "simple". Given no prior hints very very few mechanics or anyone else under the car would likely tumble to the arrangement, intended working, and reason for the concentric shafts? I dont think a description appears in any of the standard texts.
Full bedding of the shoes is much less simple than on moderns where the actuator is free to positon around the drum and the shoes ditto across the drum.
The movement of the front axle rearward has an effect so rolling roads can be deceptive, esp on handbrake performance if assessed only from the rear axle! The four pad type more informative but Seven unlikely to fit on!
(The tendency for axle to spring rearward is worst on specials with light lowered radius rods, hence much of the enthusiam for Bowdenex brakes. Incidentally the action tends to cause the pre Girling radius rods to slowly twist out of vertical, reducing castor.
(I took my Seven to a fancy govt testing station when these were new years ago, but it would not span the inspection pit or fit the brake machine so the brakes were tested the same way as every one man garage did)