Brakes - Fundamental considerations
When looking to increase their cars performance, most folks immediately turn to considering higher engine power outputs. Because of this pre-occupation brakes tend to slide down the improvements priority list. However, they are far more important - being able to accelerate at warp speed to hyper-space in the twinkling of an eye is all very well, but disastrous if you can’t stop. You will make a very big hole in the scenery, and possibly end up wearing an wooden over-coat.
Getting the braking system sorted out can help to make the car quicker, as you will be able to more confidently, therefore maximising speed in any given situation. Being one of those ‘sciences’ used in the automotive industry I hope to shed some light on the subject as it's another of those where many questions from confused and mislead folks that clog up my e-mail board.
The advent of the Metro, and the brakes used on it has spawned a lot of activity in this direction. It was soon discovered that they could be applied to a Mini pretty easily, and the later vented ones opened up a new area to exploit. The odd thing I have concluded about this, following all the questions I have been asked about them, is that a greater portion of those trying to apply them to their respective Minis are doing it more for ‘pose-points’ than improvement. I get a large number of enquiries from racers of all types too. Adding these into the above equation, there seems to be a number of points not understood about brakes and sizing.
It would seem that the adage ‘big is beautiful’ can be applied as readily to this topic - most folk being hell-bent on fitting the biggest stoppers they can onto their trusty steed - as many others. Not altogether a bad thing, but not necessarily the way to go either. There is such a phenomena as too much braking power. Exactly how brakes work - friction and torque - are closely coupled to disc and pad types, covered in the 'Brakes - how they work' article. Here I want to look at the more fundamental points.
On the Mini, a certain set of data is fixed because that’s all we can use - without spending absolutely pots of money. The maximum sensible wheel size we can use is thirteen-inch. Even then the internal dimensions of the rim can be limiting. Most modern Minis make use of the twelve-inch wheels fitted as standard now, although a change to alloy twelve-inch wheels with wider tyres is almost ‘de riguer’, then the classic ten-inch wheel type. That leaves us with a selection of relatively cheap and available disc-brake set-ups to consider, ignoring the drum-brake set-up, as they are not a wise option for the performance enthusiast - being barely adequate when well serviced on a standard Min!
I am going to largely ignore the old 7” diameter Cooper brakes, as they are desperately inadequate. It wasn’t so much the diameter that spoilt the design, but a combination of disc thickness, caliper and pad design. So we have the 7.5” diameter S/1275GT type, the 8.4” diameter later GT and post 1985 Mini type, and the Metro 8.4” diameter ones - available as solid or vented. Check out the table for a condensed comparison of the aforementioned.
DISC NO. OF PISTON EFFECTIVE LOAD ON ONE TOTAL TOTAL
TYPE PISTONS DIA. AREA SIDE OF DISC PAD SWEPT
(DIA) (IN.) (SQ.IN.) (Psi) AREA AREA
7.0 2 1.618 2.056 1164 13.8 101
7.5 2 1.715 2.318 1506 17.3 122
8.4 2 2.000 3.142 2041 16.6 134.46
8.4 4 1.417 3.154 2049 16.81 138.82
The above figures for the 4-piston calipers are the same for the current alloy-type calipers available from Mini Spares. The figures were also calculated with the standard fitment master cylinder bore size, as this is another feature that can influence brake performance.
This clearly demonstrates the ‘bigger is better’ syndrome - as the disc diameter increases, so does the important data. It is interesting to note that not much more pressure is developed by the 4-pot caliper over the 2-pot one, and the pad and swept areas are similar. So why bother?
Pad stability. This little statement encompasses everything from more even and slightly increased pressure to even pad wear. The action of the disc on the pad creates two different wear planes - rotational forces and the speed differential from inner to outer edge. These wear the pad in a taper from top to bottom, and outside to inside. Resulting in the whole pad surface area not being used to maximum effect. The normal two piston caliper has very little control over these, using more caliper pistons for a similar sized pad will control the pad attitude far better, giving better contact area, evening out the wear rate, improving brake efficiency and effectiveness.
Using multiple pistons allows the piston centerline to be moved outwards slightly, improving brake effectiveness by making the torque arm longer (see 'Brakes - How they work'). This principle can also be used to optimise braking for any given disc size without the need to go to a bigger brake set-up. The benefit here is keeping the un-sprung weight down, as bigger brake set-ups are invariably heavier - unless vast sums are invested in a full AP Racing set. Correctly designed and applied, a multi-piston caliper on a 7.5” disc will be at least as effective as a two piston one on an 8.4” disc, but generally superior.
Un-sprung weight. This point needs serious consideration when changing disc brakes around, as it has a direct effect on damper performance. The damper is there to control the action of the whole wheel assembly from wheel nuts to suspension arms. I would aver that all dampers produced as after market items (Koni, Spax, etc.) base their damper rates on the old ten inch wheel/7.5” disc assembly weight. This means they are not really properly valved to deal with the increase in weight imposed on them when standard disc assemblies are changed for say the vented four piston Metro kit. The weight increase is immense. The caliper alone is 25% heavier, let alone the vented disc - check out the table below! This scenario is much worse when the standard dampers are retained. When used in competition, the effect is heightened by high wheel speeds when hitting bumps. Give this some serious thought, standard are just not up to the job.
CALIPER WEIGHT COMPARISON
4 Piston MSC alloy caliper 1.15kg (alloy pistons)
S/1275GT 7.5” disc caliper 2.66kg
Post 1985 8.4”disc caliper 3.46kg
Metro 4 piston caliper 3.48kg
Inertia. Again, weight related. The bigger and heavier disc requires more power to accelerate it. Once at speed, it has a flywheel/gyroscopic effect. The flywheel effect means it takes some slowing down, making the car push more when braking. Its gyroscopic tendencies resist change in direction, so has a heavy effect on the steering, making turning slower, promoting under-steer by contributing further to the pushing effect
Temperature. Brakes function through friction. Just how effective they are is dependent on them reaching certain temperature levels. Metro vented discs were made for a car far heavier than a Mini, and are therefore unlikely to reach optimum operating temperatures when used on one unless certain steps are taken. Brakes running too cool are worse than those running a little on the warm side as pads are prone to grabbing when cool, causing the wheel to lock up. Cramming a cut-down vented disc and associated parts into a ten-inch wheel may cause over-heating because the heat cannot escape. This heat can cause problems in other areas too - like wheel bearings, CV joints and gaiters. After all, it has to go somewhere! Yet they may not be getting hot enough. Then there’s the un-sprung weight and inertia effects to be added - is it all worth it, or is there a better way? Such as just fitting a good alloy four-piston caliper instead to the 7.5” disc along with suitable pads.
How do you select what you need? Difficult to give an exact application list, as many want the bigger disc set-up for pose value more than effectiveness. Availability has much to do with it too, like the cheapest set of discs is the one you have. Here are some things to consider other than the above.
Correctly applying a four-piston caliper to a 7.5” disc will improve the torque arm effect and pad control, therefore braking efficiency when used in conjunction with a ten, or maybe twelve-inch wheel set-up. However, fitting thirteen-inch wheels will reverse the effect. The torque action of the wheel diameter combined with the bigger tyre footprint area will be much greater and overcome the brakes to a certain extent. Conversely, using the full 8.4” diameter disc with a ten-inch wheel fitted with 145/70/10 tyres may create too much braking, causing the wheels to lock up prematurely. Retardation is only assured whilst the tyre is gripping the road, as soon as the wheel locks up, grip is lost, and retardation rapidly reduces.
If the car is used in competition, what sort of use do the brakes see? On a hill-climb or sprint car, acceleration and nimble handling are paramount. Minimal brake use sees low temperatures. Is it really necessary therefore to use the big, heavy vented discs over the smaller, lighter 7.5” ones? What wheels are you using? For circuit racing where speeds are very high, and hot/sticky slick tyres are being used, will the small discs cope? The tyre grip will almost certainly overcome them. Biggest is probably best here, but be sure to get a good balance between size and weight.
If you are not sure which direction you ought to be going in, ask questions. Talk to experts for advice, look at what the competition are using - in particular the class leaders. Just to illustrate, a few years ago, cut down vented discs with huge AP calipers became the vogue in a particular formula. A short while later, the leaders were converting back to solid discs, and smaller four piston calipers as these gave a superior all round result. You do not spend around £600 on disc brakes to swop them for £300 worth just for fun! As with all things automotive, honesty is what it’s all about. Be totally honest in your approach - it will pay dividends.
Useful part numbers:
C-AJJ4022 Alloy 4-pot caliper set, 8.4" disc.
C-AJJ4023 Alloy 4-pot caliper set, 7.5" disc.
C-AJJ4021 Alloy 4-pot caliper spacer kit for vented disc.
C-AJJ4028 Complete 7.5" disc-brake conversion, drums to discs.
C-AJJ4029 'Metro' vented disc/4-pot caliper conversion, standard
disc set-up to vented/4-pot.
C-AJJ4024 Steel braided brake hose kit to convert Metro 4-pot
caliper to single line feed.
C-AJJ4027 5-line steel braided hose kit, includes clutch flex hose,
C-AJJ4027B As above but with Verto-type clutch hose.
C-AJJ4026 4-line steel braided hose kit.
C-AJJ4030 Pair front steel braided hoses.
C-AJJ4031 Pair rear steel braided hoses.
C-AJJ4031B Pair rear steel braided hoses with banjo fittings at
Wheel cylinder, replaces flexible pipe and radius arm
mounted metal pipe.
GSJ268 Pre 1984 Metro hub swivel-pin, required to fit Metro
to Mini suspension arms.
C-AJJ3364 1.5-degree fixed negative camber bottom arms.
C-AJJ3364A 2.0-degree fixed negative camber bottom arms.
C-AJJ3364B 2.5-degree fixed negative camber bottom arms.
All general service parts are available separately; please see relevant sections on the site.