Brakes - Pads, function and parameters
The past few years have witnessed a great deal of activity in the brake-swapping department by Mini enthusiasts world wide. The reasons are quite diverse, ranging from fashion to necessity. Some fit vented Metro 4 pot brakes because it’s the ‘in’ thing to do, giving max pose value. Some believe it’s a performance improvement, usually misguided by a misinformed vendor hell bent on making a sale. Some do it because they’ve been given a Metro, and their Mini’s endowed with drum-brakes. Some fit 7.5” S-type brakes because they want to fit 10” wheels on their late-type Mini. Some fit them because the racing discipline they compete in won’t allow anything else. Some even fit the disastrous 7” Cooper-type because it’s all they have.
Whatever the reason or type of disc-brake set-up used, the effectiveness of the installation can be honed to perfection by the type of brake pads fitted. Indeed, using the correct brake pad on a current set-up could out perform any intended swap, such is their influence.
Just how well your Mini stops is dependent on a number of components getting it together - it’s all-up weight, tyre size/type, suspension condition, damper performance, actual brake set-up installation, even the brake fluid. But without the pad doing its thing, none of these would have the slightest effect. After all it’s this small block of material that generates that which causes retardation of motion - friction. To be able to make a reasonably educated choice of which to use, some knowledge of what they’re about is necessary. There’s a greater choice available than some vendors would have you believe, boosted for us Mini fanatics by current technological advances.
Appliance of science
How brakes actually work isn’t too difficult at all (see 'Brakes - How they work' for further information). The braking system simply turns kinetic energy (motion) into thermal energy (heat) - more commonly known as friction. The greater the friction generated, the greater the stopping power. Nice and easy. So what we want is a brake pad that creates copious quantities of friction, right? Well, no. The trick is to generate just the right amount of friction to suit certain applications. Consequently just banging in a set of ‘race’ pads for your racer, or ‘fast road pads’ for your road car may not give the best result if the brake set up fitted is already over the top. A controlled stop is desired, not screeching to a halt amid a plume of tyre smoke. Getting to grips with brake terminology is a step in the right direction, so walk this way....
Friction - we now know what it is and how it’s produced. The industry give pads a numeric tag that describes their friction coefficient (referred to as ‘Mu’ hereafter) where 0 is worst and 1.0 is best. The highest a pad would be expected to achieve is approx. 0.50. Higher than this and any control would go out the window as a pad designed to just give maximum friction isn’t the answer. Too much friction for any given application will over-come the tyres ability to grip the road (traction). The result is the aforementioned tyre-smoke shrouded, howling stop. Eventually. Wheels locked-up under braking are grossly inefficient, and negate any steering input. Basically you’re out of control. A situation that gets very much worse in the wet as traction is more easily broken. So a pad has to be designed to give maximum friction in a controlled manor for any given application. Just picking a pad that has a high ‘Mu’ will get you in trouble, as this doesn’t describe the whole story.
Bite - the initial friction experienced at the start of a stop. That feeling you get when you first press the pedal. We already know that brakes need heat to work. Generally higher performance pads take longer to get up to working temperature, so initial bite is low - that ‘o my god, the brakes aren’t working’ feeling, making you press harder, then all of a sudden the temperature rises sufficiently to work, and you lock the wheels up. Formula one pilots left foot brake most of the way around the warm-up lap to put heat in their car’s brakes to avoid disappointment at the first bend - going either sailing into the kitty litter (if they’re lucky) or someone else (if they’re not). This practice is all well and good for racing, but on the road you can’t be doing this all the time, so a pad that gives good initial bite must be high on the priority list. Having a pad for racing that can do this and give maximum ‘hot’ performance verges on ecstasy!
Cold friction - the pad’s performance for a heavy application from ambient temperature - such as experienced from braking to a standstill from motorway cruising speed. Many standard road spec pads seem barely adequate at this. A race-type pad may be better at the end, but heart stopping nothingness when you first hit the pedal can cause panic, and an impromptu interface with another vehicle/armco! Most undesirable. A pad with a very stable compound will give the best performance in this instance.
Fade - the drop off in ‘Mu’ from stop to stop. One of those much-abused terms. Although heat is necessary for good braking, excessive heat can cause a pad to become inefficient, even stop working, because the compound has been ‘cooked’. This happens when frequent and successive hard braking stops occur. A rare problem on the road, but one experienced often on the racetrack. A pad that tolerates a broad heat spectrum dilutes these problems.
Friction consistency - how consistent the pads perform during a stop from cold to hot, and from stop to stop - the pads ‘characteristics’. Important points to remember here are that a locked-up wheel is nowhere near as efficient at stopping the car as one that’s rotating under braking, and the slower the car is going, the lesser the braking effect needed. Consistent characteristics are essential as they greatly influence your confidence. Pads that perform erratically will have you worried, predictable pads have you subscribing to the ‘last of the late breakers’ club!
There are three basic transient situations for pad performance in the stop, and in repeated applications. One that starts with a low initial ‘bite’, ‘Mu’ increasing with heat build up is hard to control - necessitating skillful driver input (‘feel’) as the pedal pressure will need to be gently eased off through the stop to avoid wheel lock-up. Not user friendly, but preferred by some racers as the type of compound used generally has high ‘Mu’ levels and fade resistance. One that maintains its ‘Mu’ throughout the stop is tricky to use. The compounds used generally give better than average initial bite, reasonable ‘Mu’, but can fade in arduous situations. Diligence is required from the driver to get the best use of these. One that has a strong initial bite with ‘Mu’ gradually decreasing through the stop is the most desirable - most user friendly.
And these features need to duplicate themselves whenever the pedal's pressed!
Pad wear - how quickly a pad wears in use. The compound used needs to be sufficiently sustaining to endure an acceptable service life. One for road use that only lasts a few hundred miles is as unacceptable as a race pad that barely manages a race. However, some mid ground needs to be accepted when superlative braking is sought.
Disc life - how abrasive the pads are in use, giving rise to increased disc wear. Of great importance on a road car, and a consideration in racing where budgets aren’t in the formula one league! A pad that provides high ‘Mu’ levels with minimal abrasion is obviously preferred.
Effective temperature range - strangely enough, the temperature range where the pad gives its best performance. Operational temperatures vary from zero to 800-degrees Celsius. These are greatly affected by the overall brake set-up and how they’re cooled. Generally the greater the disc mass and the greater the airflow around the disc/caliper, the cooler they run. Road cars generally operate in the region of 0-250-deg C, fast road/competition 0-450-deg C, and all out race 300-800-deg C. Few Minis would reach the upper end of the race scale as they’re neither heavy enough or quick enough to generate such temperatures. Pads that transgress these boundaries give a greater performance envelope than more dedicated ones.
Area of use - the most suitable arena of use for a given pad type - usually designated by road, rally, or race. But as pointed out earlier, this is not a parameter that should be considered carved in stone. The actual application is entirely dependent on what temperature the brakes work at.
Noise/comfort - the amount of squeal/judder generated, and pedal effort needed. Obviously both are completely undesirable on a road car. Squeal on a race-only car is immaterial if the stopping power is what’s wanted. Judder on a race-only car can be tolerated but generally needs to be avoided as it can be confidence destroying for the driver, and may upset your Mini’s handling under severe braking on bumpy surfaces. Excessive effort may tire you, and eventually give you a sore foot!
See 'Brakes - Pad types & choices' for brake pad options/applications.