Brakes for beginners

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Brakes for beginners

Postby F1300Tony » Mon Feb 18, 2019 7:53 pm

Unfortunately I have received one example of positive feedback about these articles, so I will continue!
Having great brakes helps reduce lap time, but they have even greater value when it comes to the race battle.
In cornering the slip angle is the difference between the angle the wheel points and the direction of travel. If the front slip angle is greater than the rear you have understeer. Any one type of tyre has its optimum slip angle. If you are not reaching this optimum you are are not going fast enough, and if you exceed this angle you are going over the top of the curve on the graph and you get less grip. Exceed it by too much and you are in recovery mode, and that might mean going very sideways or it might mean the tow truck. In cornering the slip angle is the sum of the the tread rubber squirming against the road and the sidewall deflection. By squirming I mean the local deflection of the tread rubber. In braking the tyre has a similar optimum for under-rotation. For a race tyre this may be the wheel rotating 2-3% slower than the speed of the car. If you ask the tyre to brake and corner at the same time you are using up more of your available grip. Rob Wils-n suggests you think of it as of the 100% rubber squirm available you can use 70% for cornering and 30% for braking, but while this is a useful concept it is not a mathematical exactitude.
The brakes work by converting kinetic energy into heat, and all that heat has to go somewhere. Initially it goes into the discs, and then it is dissipated from there to the air and conducted into the wheels via the hubs. The discs have be sufficiently thick to do this job without getting too hot or pad fade is inevitable. Do not press the brake pedal on a stationary hot car for any length of time because that part of the disc in contact with the pad will get hotter than the rest of it and lead to distortion or cracking. If the discs get too hot on circuit you can consider ducting air to them. Mallocks did a lot of work on this and found the most effective way was to mount the end of the duct face on and very close to the disc. Discs should have no more run out than 0.003”, but some would say some is useful to knock the pads back. Some race callipers have light springs behind the pistons to hold the pads in contact to avoid g forces causing knock-off but I have always removed those without problems.
I have Googled graphs that show the lateral grip of a race tyre increasing as a straight line with increasing vertical load up to a point where the former drops off. This is at a vertical force way beyond anything a light car like a 750 could achieve. This becomes interesting when thinking about weight distribution.
Under braking the tyres create a rearward force at ground level and the car’s momentum can be viewed as acting on the centre of gravity of the car, likely to be about 500mm above ground on a 750. This force will be the weight of the car times the deceleration g. Looking from the side and taking moments about the point that a tyre touches the ground it is clear there is a considerable transfer of weight to the front wheels and an equal reduction of weight on the rear.
For maximum braking performance it is vital to get the balance of the brakes on the front and rear optimised so rear brakes do as much work as possible which means they are close to locking when the fronts lock, even when the weight transfer is at its greatest. I think a lot of cars run with the balance too ‘safe’ so the rear brakes are not doing as much as they could be. Obviously in the wet the weight transfer is reduced so it is worth adjusting the balance bar to put a little more effort to the rear, say about 1 -1.5 turns but do not forget to put this back the normal for the dry.
So how do we find this magical optimum balance? It depends on the relative size of the master cylinder to the slaves, and the relative size of the front and rear brakes. Having got that in the right ball park by looking at other cars and what you had before, it is down to the balance bar. Your balance bar should have a minimum of 4mm free end float, or it will not work the good system properly if the other fails. The idea is that when one piston goes full travel then the other cylinder with still work without the whole thing binding up and putting full pedal effort onto one cylinder. And it is a fundamental rule of motor racing that when it fails you will be going into the hairpin at Mallory, and it will be because a copper pipe has fatigued.
With a new brake system or a new car, to set the balance roughly initially you will need a helper with a steady foot or some sort of adjustable mechanism to apply pressure to the pedal. The idea is to adjust the balance bar until the rears wheels can be just about turned at the same point the fronts are just locked. For safety’s sake you want to go out onto the circuit being pretty sure the fronts will lock before the rears. I tend to scratch the discs radially with very coarse emery tape to bed in new pads in about 5 reasonably gentle laps. You are probably going to need more than that to get used to a new car, but when you are ready and the tyres are fully up to temperature you need to start pushing the braking to find out which end starts to smoke first. With an enclosed car you may need a helper to observe from the side of the track. You can then adjust the balance bar. To get more braking effort on the rear you need to move the centre piece towards the rear master cylinder but this adjustment is VERY SENSITIVE. I would never move the bar to the rear by more than 1 full turn, unless I was very sure the rears were doing nothing, but it is OK to move to the front by 2 or more turns. The object is to keep moving more onto the rear until the rear lock first and them come back until the fronts definitely lock first. This will probably need a full day’s testing in the present four session arrangement, so it is pretty essential to note the position by counting threads, and to lock the thing up solidly. The process can be speeded by use of temperature daps, an infra-red pyrometer or a wet finger to measure the rear disc temperature. For reasons I do not understand even with this optimum set-up, if you brake too much during the left-hander under the bridge at Snett it will still oversteer and it will be scary.
If you are saddled with rear drums (H750F) on the rear things are much more difficult because they are less consistent. Old Mallocks used Morris Minor twin leading shoe front brakes on the A series axle. Drums are less good at removing heat, and they take forever to bed in even if the linings are machined to the drum diameter. Do not adjust too closely because as they heat up the clearance closes up and you will find yourself losing 1000rpm down the straight. I did a full season of hillclimbing a 500 car and still didn’t bed in the brakes.
Winter is the time to think about whether the braking effort is right. Some people like a light force on the pedal because it make heel and toe easier and it is not so far away from the feel of the road car. Some like a big force because it makes finding that ideal level of under-rotation easier. If you want more mechanical advantage you can reduce the diameter of both master cylinders to the next size down (easy) or increase the pedal leverage which means making a new pedal. This was the route to find better brakes a few years back: we were leg muscle limited!
Do not assume you need race pads for racing. I used half race pads on the front and a road compound on the rear, which makes judging the pedal force for the first corner a whole lot easier.
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Re: Brakes for beginners

Postby GregSwan » Mon Mar 04, 2019 11:49 am

Second positive feedback... I look forward to the next instalment :)
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Joined: Wed Aug 29, 2018 1:47 pm

Re: Brakes for beginners

Postby F1300Tony » Sat Mar 09, 2019 8:32 pm

It seems I need to debunk the idea that allowing the central ball of the pedal bar to drift sideways a little changes the brake bias. I normally avoid posting mechanics or maths, partly because I have forgotten most of what I did know, and partly because it bores most people. So you may want to stop reading now!
The pedal pivot is at right angles to the master cylinder bores.
The push rods are the same distance apart as the centre lines of the master cylinder bores.
Let's say the force from the rear master cylinder clevis, direction in line with the centre line of the car, is Pr, the front Pf.
Let's say the distance from the centre of the central ball joint to the centre of the rear master cylinder clevis is r, and to the front clevis f.
Taking moments about the central ball joint:- Pr * r = Pf * f
Therefore Pr/Pf= f/r. Also the force from the pedal = Pf +Pr
Now we come to what happens when the central ball moves to the side a bit. The push rods are now out of line with the car centre-line, but still parallel. With a rod with a ball joint (or swivel) at each end the force can only act in line with the rod. What happens is the force in each rod is increased a bit, because in addition of the original force straight ahead there is also a small sideways force. (Google 'vector') This puts a small sideways load on the pedal bearing, and this is balanced a sideways load on the master cylinder.
But nothing else changes, so with the central ball joint moving sideways the bias ratio remains the same.
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