WORK­SHOP

In the sev­enth part of our han­dling series, Neil talks about brakes and how they aren’t the en­emy of per­for­mance.

Mini Magazine - - Contents -

In the sev­enth part of our han­dling series, Neil dis­cusses brakes and the evo­lu­tion of Mini brak­ing sys­tems.

Brakes are the en­emy of speed, right? Wrong – with­out brakes you can’t get the cor­rect en­try speed into cor­ners. They are vi­tal to a fast lap and of course, stop­ping at a cross­ing for pedes­tri­ans on the road.

Cars’ brak­ing sys­tems orig­i­nally de­rived from horse-drawn carts with wooden blocks which pushed on wheels or leather straps pulled around axles. When you com­pare these to the lat­est For­mula 1 com­po­nents they still work on the same prin­ci­ple of us­ing fric­tion to con­vert the ki­netic (mo­tion) en­ergy into heat. Back to those school lessons where we were taught that en­ergy can­not be cre­ated or de­stroyed, just con­verted.

As cars got faster, a new so­lu­tion was re­quired which could han­dle more heat. This re­sulted in the drum brake, which per­formed bril­liantly on a va­ri­ety of land speed record and rac­ing cars. The fa­mous Le Mans Bent­leys re­lied on drum brakes and they be­came a stan­dard com­po­nent on road cars.

When the Mini came along in 1959, drum brakes were the log­i­cal choice. They pro­vided good, con­trol­lable brak­ing in a com­pact pack­age. The drums on the front had twin shoes ar­ranged to give max­i­mum ef­fi­ciency against the for­ward di­rec­tion of the car with twin hy­draulic slave cylin­ders to op­er­ate them.

REAR BRAKE DE­SIGN

At the back, drum brakes were fit­ted, again with two shoes but this time one was more ef­fi­cient when the ve­hi­cle was trav­el­ling back­wards and there was one dou­ble-sided slave cylin­der. Op­po­site that hy­draulic cylin­der was a ca­ble­op­er­ated lever to be used for the hand­brake. This rear brake de­sign ran for the en­tire life of the Mini.

Ev­ery­thing was fine with the drum brakes, in­deed they were fac­tory fit­ted on all four wheels un­til the mid 1980s, un­til peo­ple started com­pet­ing the lit­tle car. The reg­u­lar, heavy brak­ing re­quired when rac­ing and ral­ly­ing gen­er­ated too much heat for the drum brakes to dis­si­pate. So when the Mini Cooper

“As cars got faster, a new so­lu­tion was re­quired which could han­dle more heat”

came along, a new fac­tory so­lu­tion was re­quired.

The disc brake con­cept had been around for al­most as long as the mo­tor car but it wasn’t un­til newer ma­te­ri­als and de­signs that it started to be­come more pop­u­lar. What we’d recog­nise as the mod­ern disc brake is more ef­fi­cient at con­vert­ing that ki­netic en­ergy into heat, bet­ter at dis­si­pat­ing that heat, and on the Mini also re­moves the need for ad­just­ment. The drum brake does need less force to op­er­ate due to the way the pres­sure is ap­plied.

The 997 and 998 Mini Coop­ers came with tiny seven-inch discs at the front un­der the 10-inch wheels. I say tiny, but when you com­pare them to the ~11.5-inch discs on the mod­ern MINI you can see my point. The seven-inch discs and small pads on the 997cc Mini Cooper were not re­garded to be much more ef­fec­tive than the drum brakes they re­placed. In­deed, they are

con­sid­ered to fade (over­heat) faster than the drums. When the 998cc Mini Cooper came along, they were fit­ted with larger brake pads, which made them marginally more ef­fec­tive.

DISC BRAK­ING

The Cooper S re­ally brought disc brakes to the fore of Mini stop­ping power. The 7.5-inch discs fit­ted to this model were cou­pled with a brake servo which am­pli­fied the pedal pres­sure with the as­sis­tance of vac­uum pro­vided by the engine. So, not only did the brakes have a larger sur­face area be­tween the pads and the disc, but also a greater disc area to dis­si­pate heat and the servo to boost the amount of pres­sure the driver could ap­ply to the pads and so squeez­ing the disc. To give you an idea of just how hot these brakes got on the twisty moun­tain passes of the Monte Carlo rally, there are tales of the discs glow­ing so much they be­came vir­tu­ally trans­par­ent af­ter par­tic­u­larly twisty alpine de­scents!

Minilite wheels im­proved the cool­ing of the discs when com­pared with the steel al­ter­na­tives. Third-party al­loy Minifin and Su­perfin drums are avail­able these days which do help drum brake cool­ing and give much im­proved looks.

Fi­nally, 12-inch-wheeled Minis came along on the 1275GT and 8.4-inch discs could be squeezed un­der these wheels, again giv­ing a greater area to dis­si­pate heat from and more pad sur­face area which could cre­ate more fric­tion and a more con­trol­lable brak­ing ex­pe­ri­ence. Ser­vos then started to ap­pear on the 8.4inch disc mod­els from the late 1980s.

As I men­tioned, the rear drum brakes sur­vived all of these changes. The brak­ing force at the rear of a car is much less due to the way which the weight of the ve­hi­cle acts on the front wheels more when you are slow­ing down. If you have too much brak­ing at the back, then it can get rather twitchy and ul­ti­mately lock the wheels up. The rear drums also pro­vide a neat so­lu­tion for the hand­brake. If you have hot disc brakes and clamp them at a given pres­sure, when the disc cools, the metal con­tracts and the pres­sure re­leases. Ma­jor man­u­fac­tur­ers have used this method, own­ers soon learned to leave their cars in gear af­ter a spate of cars rolling off when parked with hot brakes.

Neil Burgess

Words and Pho­tog­ra­phy

Rear drum brake assem­bly. Rear drum ad­juster.

Rear drum slave cylin­der. Su­perfin drum.

7.5-inch Cooper S disc. The 8.4-inch disc.

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