Nervous Breakdown

Sometime ago in an issue long, long ago in a Calver's Clinic, I did a short piece on vented or grooved discs, with a promise to look at brakes a little closer at a later date. I have to confess that this is a little later than I originally intended due to other developments going on at the time. However, with people preparing cars for the forthcoming sports season, and many questions on this matter being asked - now is as good a time as any. [ must point out from the very start that brakes and their application is a very involved science, containing all kinds of formulae and many more ways to achieve required results based on pure experience. Bearingthis in mind I am going to try and explain as much as possible as simply as possible, and eliminate all the mathematics. The idea is to help those confused or misled to grasp the basic yet important principles of brake systems. This is made somewhat more simple as we are purely dealing with the MinLThere are so many factors that influence the end result it is difficult to know where to start. The essential point to remember is that the
brake's main function is to convert kinetic energy into thermal energy - or more simply motion into heat. More commonly called friction.
The next crucial factor is dissipation of that heat. Simple, innit? The hard part, is which selection and combination of components to use to achieve the perfect set up. As always a  number of influencing factors need to be considered, placing them in some sort of order of priority. Before this is possible, some idea of what affects what, in the braking system, needs to be understood.The beginning is always a good place to start - and the source of all braking is the input, Le. the driver. Or more correctly the driver's foot. Although this varies greatly, it is not an easy component to change, so next up is the pedal. The pedal has a direct influence on the system as it has a specific ratio, computed from the difference in distance from the pedal to the pivot pin and the pivot pin to the master cylinder cotter pin. We will leave this alone as altering this is not that easy, and when correct selection of the other components is made, really does not need changing. For the record though, reducing the length of the pedal to pivot distance will mean less pedal travel, but more driver input - increasing it has the exact opposite effect. A reduction in ratio will give a firmer pedal with less 'feel' , an increase the opposite.
Right. next up is the master cylinder. Discussing this component at this particular point is somewhat out of synch as the important factor here - .its bore size - is directly affected by the main component inthe system, the caliper type used. Having said that, for most practical and cost purposes, many will be wanting to use the master cylinder they have on the car. Practically all Mini master cylinders have a .70 inch bore size. The only different one was the .75 inch, fitted to the old 997 Cooper - an extremely rare beast indeed. The clutch master cylinder has been used by some as a brake master cylinder as this has a .75 inch bore, but is not recommended as it does not have any internal return spring and can therefore leave the brakes permanently on. Do not contemplate this adaptation. If the car is being built for competition, or a master cylinder will need to be purchased , then one with the correct bore size to suit the rest of the brake system is a far better bet and is surprisingly cheap - but more on this later.
From the master cylinder there are the necessary hydraulic pipes, both solid and flexible. Not really a great deal to discuss here, except that using steel braided lines is a far better bet than the standard the correct tubing for brakes and should therefore be avoided. As far as solid pipes go, the copper type are great and easy to use as they are very easy to form and do not kink when bent. If you are making up your own pipes, ensure you achieve correct rubber items and are not really much more expensive - but much longer lasting. Beware cheap steel braided lines though - they may not be using flaring at the ends, and correct flare type for the fitting into which the pipe is going. As for what to use where, I firmly believe in as much solid pipe as possible, using flexible only where necessary. Nearly all the cars I have driven that have been plumbed in using all steel braided pipes have had spongy pedals. This is caused by the hydraulics taking up the 'slack' in the system. When the pedal is pressed it is possible to see the braided line flexing. The only way around this is to anchor the pipe every inch along it's length. Altogether a heavier and vastly more expensive installation that metal pipes. In a competition car, keep all pipes inside the car for maximum protection.
Now the most discussed, argued, and again relatively misunderstood part - the front brakes. Basically the calipers and discs. First of all it is necessary to go back to the 'friction' thing. There are two situations to consider here. The first is that where discs are being fitted where they do not already exist, (be it a competition car, or car that has no brakes at all, or one that has drum brakes), the second where the disc/caliper type is being changed for an existing fitment. Some believe that changing from what they have to something else will give
them better brakes - but this is not necessarily so. Changing from drums to any type of disc is always an improvement - excepting the early 7 inch Cooper type discs that is. Again, bearing the friction thing in mind, it is important to establish if the disc set up you have is performing correctly - that is working at the correct temperature, as all brake set ups have a
certain temperature band that optimises their performance. If the set up runs too cooly, the braking performance is every bit as poor as those that are running too hot (creating what is commonly known as 'fade'). So before changing the disc set up you have to a different type, it is highly advisable to establish the temperature performance of your existing set up. This is easy to do by investing in a special paint kit that indicates temperature by discolouration. The paint is applied to the edge of the disc, different paint shows different levels of temperature. The kit costs around £45, but will last ages - so a group purchase would split the cost. Perhaps by a club or group of like minded friends/colleagues. Now, it is important to establish the main usage of the car, as this ultimately will determine the braking capability you will need. This directly affects the total brake package required to include disc type, caliper type, pad type. Other influencing factors are wheel type and size, tyre type (if in competition whether a treaded type tyre or slick), and vehicle weight to name a few - though
most important. Before component selection can be made, appreciation of the components is needed.
Starting with the disc. The bigger the diameter the better, basically. The reason behind this is that more effective torque can be applied by a larger diameter disc. An illustration of this is if you spin a wheel it is far easier to slow it down by applying pressure to the outer edge (tyre) than applying pressure to some point nearer the centre (ie the hub). Points that restrict
diameter are rim size and offset. Don't forget, the caliper sits astrided the disc and therefore forms the outermost measurement. Then there is the subject of vented or not. Vented is  best as it cools more efficiently, but is only necessary where the brakes are getting too hot in there specific application. Another consideration is that they are also heavier. Next is slotted or not, and if so how many slots? lt is important to understand that the slots are there for one reason and one reason only, to clean the pad face. As the brakes are used, the temperature and friction creates debris on the face of the pad. If this debris is left, then the brakes are less efficient. The slots remove the debris and ensure maximum pad efficiency. As for how many slots, I did go into this in issue No.6 - but briefly on a Mini sized disc, four is enough. The slots need to have a sharp edge to be most effective. Slotting discs has probably been subjected to as much discussions as drilled discs did when they were all the rage. Much crap was argued about those too. In fact, drilled discs are more effective at debris clearance than slots as they very rarely become clogged up. However, practically all car type discs suffer from premature cracking if drilled, a combination 'of material type and thermal
loadings created by constantly changing temperatures. One other point I will make about discs is to ensure when buying replacement discs that they are of decent quality. Avoid  seemingly very cheap discs as this usually indicates that an inferior material has been used. Some of the imported aftermarket discs are absolutely diabolical. It is for this reason that
Mini Spares have produced their own in England. Maybe a couple of quid more expensive, but far superior in quality.
Calipers. Hmmm. OK, we'll disregard the early tinney Cooper 997 and 998 7 inch discs as these are pathetic far all sorts of reasons.
The vogue requirement is to use four pot (four piston) calipers as they are seen as the answer to better brakes. This ain't necessarily so. The reasons for multi piston calipers is  misunderstood by many. The main reasons are to apply a more even pressure across the pad and control the pad attitude. It is not to apply MORE pressure as many people believe -
although when the area of piston pressure is computed, the multi piston caliper usually has slightly more and therefore does actually give a slight increase in pressure. It certainly is not a massive increase. When a single piston operates on a pad, the rotation of the disc creates two wear angles. The first is a top to bottom taper as the pad is dragged in at the bottom edge, the second is more wear on the outer edge caused by the fact that the outer edge of the disc is travelling faster than the inner edge. Multi piston calipers create a much more even pressure, therefore reducing this uneven wear rate - and increasing the effective pad to disc friction area. Some calipers have a smaller piston at the leading edge to balance this further - but is only really used in big calipers or where more than two pistons per caliper half are used. Whilst contemplating which set up would be best, it is interesting to compare the pads for different set ups. Funnily enough the biggest pad of the three most common set ups (7.5" S, 8.4" Mini, and The Metro or four pot type) is the one for the smallest disc, givingit a greater surface area on the disc. It is also thicker than the tyre used in the four pot caliper. Hmmm - why, eh? Well, this is all to do with the heat aspect, and uneven wear rate. The heat build up in a small diameter disc requires a pad that can help dissipate heat, as the disc cannot achieve this as easily as a larger diameter one. Also, the wear rate is usually
higher on a smaller diameter disc, and bearing in mind the uneven wear created by a single piston per pad coupled with the requirement of reasonable service intervals, a thicker pad is needed. The smallest and thinnest pad is the one for the four pot caliper, proving that the wear rate is more even and has a cooler running disc.
Once the disc and caliper type have been chosen, it will be necessary to consider the master cylinder, as the four pot caliper will require more fluid movement to operate. The standard Mini .70 inch bore really is not sufficient. It will do, but will give a long pedal. The optimum size is .875 inch (7/8 inch), and is what I have always used on my racers since going to four pot calipers or big two piston calipers. Fortunately there is a Girling master cylinder that bolts directly in place of the standard one. It necessitates the manufacture of a push rod (I  make them from an old Mini one) and a remote reservoir. Sounds a bit complicated, but is dead easy and only costs around £70 all up.The main point to get to grips with here is that the amount of hydraulic pressure produced at the pedal is INVERSELY proportional to the bore of the master cylinder. Basically, if the front brakes are locking up too early, you need to increase the master cylinder bore size. Conversely if you need to grip the steering wheel in both hands and literally stand on the pedal to lock the brakes, the a smaller bore size is required in the master cylinder. A smaller bore gives a higher line pressure for any given pedal pressure, but has more pedal travel. A larger bore size gives a lower line pressure for any given pedal pressure, but less pedal travel. Less driver input is needed with the smaller bore cylinder, but will have more feel. More driver input is needed with a larger bore cylinder, but gives less pedal travel and therefore less feel. Driver preference will govern the end application here.
Pads. There are all sorts of pad materials about, giving varying types of performance. The important factor is to select a pad that is going to run at it's optimum temperature range in your particular application. A full race pad will be of no use whatsoever on the road, and a road/standard pad will very very rarely prove satisfactory in a circuit racer. Again, very serious and sensible consideration of the cars use needs to be undertaken here. When changing pads, always change both sides. If changing pads where differing conditions are to be experienced (road car driven to hill climb, raced, then driven home, etc), make sure you mark the pads so that they go back in exactly the same place they came out of - remembering that they do not wear totally evenly. Whilst changing pads where slotted discs are used, clean the slots out to ensure maximum performance. Oh, and something else about pad fitment. I saw recently an article in a 'specialist' car magazine an article covering pad replacement where it was suggested that if the pads did not slide in, that they should be tapped in with a hammer. NOT, I think!! If the pads do not slide in, there is something wrong, and the pads will not work efficiently. Thoroughly clean the areas where the pad backing plates slide so
that they slide freely in and out.
To the rear.Not much to be said here, other than make sure you select the correct rear wheel cylinder bore size to suit the front brakes. Not so much of a problem if a brake bias valve is fitted in the line between front and rear brakes (not possible on diagonal split brakes). Perhaps in a further issue I'll list what bore sizes are suggested to go with which caliper/brake system types - once I've worked them out!! Other than a change to alloy drums to decrease the weight and help with heat dissipation (very expensive these days) or a conversion to disc brakes (very very expensive) there is little that can or needs to be done. Use of competition linings is advisable as they are less prone to grab when used enthusiastically - vastly reducing the likelihood of lock up, and therefore unwanted rear end steering.
Points to consider ......... Before lashing out your hard earned readies, think logically and sensibly about what the car is going to be used for, and whether any compromises you will have to make can be lived with. A good instance here is a car that is going to be used in hill climbs. Now, this is a short blast from standstill with no chance to run around putting heat into the braking system, and where a lightweight car is desirable. Are four pot, vented discs really necessary? Are you really likely to get them up to full operating temperature? How much unnecessary weight are you carrying? Bearing in mind the fact that a bigger, ventilated disc is heavier, and acts like a gyroscope when spinning at speed and is therefore very reluctant to deviate from it's established course.A car driven on the road is unlikely to need hard pads as this can create some heart-rate boosting situations. Like hammering down a motorway for miles on end, then having to slow down or stop suddenly. No heat in the pads, no heat in the discs, no stoppey the car !! Conversely, if the car is driven, well,  progressively around town or twisty roads, then an uprated pad would be better than a standard type. In a future issue, I'll do a resumee of the available pads and their relevant pro's and con's. OK, I'll sort the rear wheel cylinder thing out as well, and look at the use and application of brake servo units.
Keith Calver

 

 

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