Groovy Rotors

[C M Smith <cmsmith@domain.elided>]

Brian Shorey and I had a brief correspondence about grooved and slotted 
brake rotors. Since so many posts seem to be confusing what Brian actually 
said, and we clarified between us, I thought I'd pass along this tidbit.

In the early 70s up to about 1988, SAAB delivered their cars with grooved 
brake rotors. If you bought brand new SAAB rotors they showed up grooved 
right out of the box, as if some bimbo (not brembo, bimbo) had screwed up 
the brake rotor honing. Mechanics and brakemen unfamiliar with SAAB's 
quirks often honed these brand new rotors flat reducing their effectiveness 
and their service life. Used rotors were also treated this way if the owner 
was not on hand to forbid it. Installed with the factory grooves untouched, 
in a very few miles the pads would bed into the same (though "negative") 
pattern of grooves effectively keying the pads into the rotors. This 
improved braking performance and also lead to no squeal and very smooth and 
quiet brakes free of rumble or vibration.

The grooves were concentric and just big enough to be seen easily and felt 
with a fingernail dragged across the rotor surface.

As Brian has correctly pointed out, several times now, these grooves 
increase the effective contact area of the pad on the rotor. Think of the 
coastline of an Island which is indented with fiords or bays. The coastline 
is far longer than the perimeter of a polygon enclosing the island. For a 
brake rotor, the ripples in cross section due to the grooves increase the 
contact area of the pad on the disc. Now whether this was responsible for 
the powerful brakes on every SAAB is debatable. What is not debatable is 
that the SAAB 99 and later 900 had more powerful brakes than any 
equivalent  car available on the market at the time.

SAAB switched to ATE brakes in 1988 (previously they used Girling) and now 
their rotors show up nicely honed from the factory and flat. The newest 
SAABs have slotted rotors. Hmmmm

Cheers

Michael


Michael Smith
Calgary, Alberta,Canada
91 Alfa 164L

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