The following is an article about the brakes David Laver uses on his racing car that he has very kindly produced for me for use in the Cambridge Austin 7 Club magazine.
We have had a brief exchange of mails about it and thought it might be a good idea to put before you 'listers' both to encourage comment and just to spread information that others might profit by. Any comments that you have will be gladly recieved. If anyone has any direct experience in the use of hydraulic conversion that would be useful as would any experience of the use of the late Girling cross shaft which allows the operation of the rear brake by a single rod running to the rear of the car, perhaps overcoming some of the problems of balancing each rear wheel
Thanks for any input
After my Cadwell race report Hugh asked for some technical details of the car. The most thought has gone into the brakes so I?ll cover these first. If anybody reads to the end and still wants more I?ll talk about another aspect of the car in another issue. The engines have been quite a saga, the gearbox isn?t standard, I made the body myself from scratch, the rad cowl is chopped down, I've had problems with steering, the dampers don't yet do anything, and various bits have dropped off. There is plenty to cover given your appetite. Anything in particular for next time? I hope with brakes there is something of use for everyone, even with a completely original car, so here it goes :
My brakes are very much non-standard but still operated by open cables. Hydraulics are not the easy solution they appear, even with money no object (I keep some premium bonds to justify time planning for such a day). With hydraulics lack of use will soon lead to seizure and many of the fastest racers mange very well without. All a hydraulic system does is transmit push from pedal to shoes - there is no innate advantage like with a servo or a twin leading shoe set-up. Hydraulics also bring the problems of increased unsprung weight, trouble mounting a master cylinder, brake line fracture through chassis twist and vibration, making a handbrake mechanism, and not least cost.
So why do all modern cars have hydraulics instead of cables? Cables have disadvantages of friction, slack and lost motion, the need for a balance mechanism, and manual adjustment. On a low mileage car or a racer these are less of a problem than with something used everyday. Today it is also worth thinking about using very soft linings and adjusting the parts a quick (or paid for) service doesn't reach ? packing the fixed ends of shoes and adjusting lever to the ?bite point' can make a huge difference to lost motion and balance. Any car used a lot and not maintained is beyond hope ? all those two shilling bangers of the 1950s have given Austin 7 brakes a worse name than they deserve.
Beyond the general problems with cable systems Austin 7s have some all their own. I've found that a lot of people know that the front axle will rotate from braking force and let the brakes off in an 'anti-servo' effect. However less are aware of the more important problem that it is impossible to achieve balance both for initial bite and emergency stop. You either need to adjust so they all come on at the same time or adjust so they all pull evenly at all out panic stop ? choose either a veer at first touch or leap about in an emergency with less than full braking. Why is this so? The crux of it is that from brake pedal to offside rear is a straight run while the path to near side rear is via a spring. This spring is the cross shaft working as a torsion bar and the flex in its mountings. Stiffening the near side bracket is a help but the only way to get a consistently even pull between rear brakes is to pull them from a half moon swivel mounted from the middle of the cross shaft; just as for the fronts. Having made that modification you may as well keep going, remove the nearside and centre bearings and let the cross shaft swivel from the off side mount. This permits the middle part of the shaft move fore and aft to balance front and rears. In this context balance is an equal tension in all four cables, the equivalent of equal hydraulic pressure in all brake lines. It's the tension in the cable that is important not the movement of the cable. In a worn system the cable will move a considerable distance to take up slack in cables, loose cable ends on levers, worn cam bearings, worn shoe ends, and pull the lining to the drum before any useful work is done. Only once the 'lost motion' has been taken up does the cable compress the lining a little, stretch a little, and pull the mountings and axle out of shape in its attempt to rub linings on drums and through friction convert kinetic energy into heat.
My car is not ideal in layout but is certainly a good deal better than standard. I have a conventional steering box on a wedge, this lifts the brake pedal and changes its angle relative to the cross shaft. To get everything back into line I welded a fillet above the old tie bar hole and drilled a fresh hole. Into this goes a yacht rigging D-clip, to a yacht rigging bottle screw, to a heavy stainless cable. I replaced the cross tube with a plain piece of thick walled tube with substantial levers welded on. These levers have many holes to be able to trade off pedal motion for leverage. The driver's side of the tube swivels in a standard bracket but is 'loose' such that the other end can move fore and aft. It is constrained in this movement by a large exhaust clamp hidden under a leather gaiter so as not to alarm the MOT man. Levers in the middle of the tube above and below pull bottle screws attached to standard half moons and ?front? brake cables for both front and rear. The levers on the front brakes are longer than the rears to give more braking force for a given cable tension ? the tension being even front and rear. Front levers have brake pull off springs to brackets brazed to the back plates. I have semi-girling brakes from a late Ruby that provide adjustment for the static ends of the shoes. This type of brake has a wide cast drum and mine have been lightly skimmed. Pressed drums can't really be skimmed back to true, will have been distorted to a bell shape, and no doubt have the friction face pitted with rust. The handbrake is mounted outside the car with separate cables, each with a bottle screw, to each rear wheel. I do without balance in the hand brake mechanism to ensure fail safe operation. Also when I heave on the handbrake I am usually after a sudden veer for childish fun on someone's nice grass car park.
It has taken a while to reach this set-up, and have a few plans to improve it further. The first problem must be familiar to all beginners and was that with any steering lock the brakes would bind. I adjusted the front cable to have some slack. Next the standard pull off springs were pulling the brakes on but with the lever pulled the 'wrong' way. I removed these and let the shoe springs pull the lever ?straight? such that any movement would push the shoes apart. The levers then pulled over centre to lock the brakes on. This was fixed by packing the shoe ends where they touch the cams to adjust for the drum skim and lining thickness. Next the bottle screws would vibrate loose. This was most disconcerting but an easy one to fix with lock nuts. I then noticed the rear brakes were prone to lock. This was because by the time the front cable slack had been taken up the cross tube had reached the limit of its constrained fore-aft travel. My fix for this to date is to fit some weak-ish pull off springs at the front such that the front cable ?droop? is taken out and there is some tension against ?droop? in the rear cables. This lets me adjust cables such that the cross shaft is towards the front and when cables pull taught it falls somewhere near the middle.
What next? As mentioned above the cross shaft needs to be able to move more than now. I will allow an inch fore and aft on the near side rather than the quarter inch it has now. Packing brake shoes on the cam is a pain and I will instead add brackets to the back plates with screw adjustment to push the lever just to bite point. Spring loading the cross tube fore and aft will enable independent adjustment of front and rear cables. The handbrake is also important as an emergency brake, I will re-route the cables to be of equal length such that the stretch is even. I will keep with the thin handbrake cables as the stretch acts in a small way to balance ? at the very least it prevents one side taking up all the motion before the other has any useful force applied. To combat fade I fancy some cold air scoops on the back plate and vents to let the hot air out again but fear that in wet weather these will make things worse. This 'summer' every event has been wet. Maybe the scoops need doors or bolt on covers.
Others have used bowden cables to combat the effect of axle twist. I don't think this twist is significant and most of the benefit is the elimination of slack in the cables. Maybe worn shackles make this more of a problem. Another solution to that of slack is to guide the cable down the radius arms with eyelets. If I had seen this before starting I would have tried it. As for front axle rotation I have heavy type radius arms and plan to brace from the kingpin tops back to about half way down the arms.
It takes a lot to reach the level of hydraulics but it is possible. Alas without twin leading shoes and with those tiny drums braking is always going to be weak and very prone to fade but better some than none. Better to improve those brakes now than fix a dented front end for the want of it. At least give softer linings a go and adjust both ends of the shoes.
10th November 1998
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