Drilled Disc Rotors
Steve and I were working late in the Team Kawasaki
RoadRacing shop, Santa Ana, Ca., just before Christmas, 1972. We were preparing
4 bikes for Yvon. Morris Mag Wheels was just next door, through a divider wall.
An engineer that still works at Bertea Corp in Orange County, Dave Hussey, was
doing his moonlighting job, retro-designing Morris Mag Wheel kits.
Dave came next door to see us about 7:00 as he liked to do, take a break, etc. We spoke back and forth for a minute or two, then Dave picked up a brake disk, said "heavy". Steve and I both said "Yup". Dave asked if he could take the disk with him for an hour or so next door, he had an idea. We said "OK" and continued to work on the bikes. Dave's ideas were usually good ones, no reason to doubt him now, no matter what he was going to come up with.
Dave returned about an hour later, with a paper pattern and our disk all Marks-A-Lot marked up with lines and dots. We were skeptical at what he had drawn, 72 holes in the disk. He stated that it wasn't his idea, but had already been used in 1965 Porsche racecars, and was of sound design.
Next day, I went into Kawasaki R&D, literally the other side of our building from Morris, same building complex, and stealthily made a milling machine template to hold a disk in a rotary table, then, went back and hand punched the pattern onto a spare disk. All 72 holes. Steve and I drilled for two nights in secret. Result was a reduction of each disk from 3-3/4ths pounds down to just over half its original weight, just under 2 pounds each.
Steve and I snuck out to Ontario Motor Speedway for a 45 minute test of the disks one day in early January, 1973, managed to not let the cat out of the bag. We kept the "Holy Disks" secret until Daytona, second day of AMA testing.
What Dave Hussey had done was to lay out a pattern as such:
He measured the swept area of the pads on the disk, no0t all the distance from the hjub to the edge, just what the pad swept. He then divided this swept area into four sectors, inside to outside, literally three circles of division, at 1/4th and 3/4ths, and half way. he then set the pattern to start with two holes, 2 rows of holes, 1/4th and 3/4ths centers, at zero degrees. Did same at 15 degrees, then 30 degrees, all the way around the disk.
The center row was done the same way, every 15 degrees, but staggered 7-1/2 degrees from the first set at zero.
Hussey figured the holes should be twice the thickness of the disk, with a very small debur relief. So...we measured the disk thickness, came up with 6.0mm. Roughly .480 inches. Steve and I, being the kind of people we are, rounded the hole size off to a nice 1/2 inch. That's what we drilled all 72 of the holes to. We used a drill bit made of cobalt, obtained from McFadden-Dale Hardware in nearby Anaheim.
Why did we go to al this trouble? Weight. A stocker Kawasaki disk weighed 3-3/4 pounds, and that was heavy for two reasons, harder to control the brake weight under braking, extra unsprung weight was harder to control with the suspension. The Suzuki guys had these really nice ventilated disks, light weight, for the TR750, all we had was street bike disks, so Dave's setup was a real benefit.
As I said, we sprung 'em at Daytona, Steve sitting on the bike in the tech line, me there to. Suzuki Japanese tech came walking by, motioned a "hello", then got a stare at the holy disks, went at full trot to get more Suzuki people, who then went to basic Japanese think position, full leg squat, put fingers oin the holes, jabbered back and forth, etc., looked at the both of us as if we were certifiable crazy, etc. Many others there thought it was some sort of joke from Steve and I, but it turned out to work excellent.
We also did two other things for that Daytona, reversed the fork sliders/calipers to get the weight down on the tube as much as possible, and used Z1 calipers, lighter weight than an H2 caliper.
Now, Steve and I have heard every reason just why we did this back then, from this to that, EVERY reason, but the real one still remains, to lighten the unsprung weight as much as possible, NOTHING else. We've heard every engineering reason, physics, space shuttle mystery theory, every reason there is, but they just ain't the real one. Unsprung weight reduction, period. Steve and I figure we'd be pretty well off if we had, say, a nickel royalty from every hole drilled in a brake disk.
In that next year, 1973, Steve and I went through testing a bunch of different configurations, mostly aluminum disks, coated with processes like hard anodizing, specialty stuff like "Banodizing", flame spraying, metal insert materials from places like Red does over at McLeod Clutches, even straight uncoated aluminum of different levels, all kinds of stuff. We didn't find a viable setup until very late 1975 and what Harry Hunt did with his coated aluminum disks. We always came back to the five sets of drilled disks we made that winter of 1972/1973.
Now, all of you that know more about this than me can flame way, but two things will not change, excess weight IS the ONLY reason we drilled the disks with Dave Hussey's borrowed Porsche pattern, and Dave Hussey, Steve Whitelock and I did it first.
That's the way it actually happened.