Too much rotor drilling?

plattey

plattey

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So I drilled my front brake rotor on my '82 hoping to save some weight and in order to get the holes to overlap when the pad swept by, the hole diameter was increased to 3/8". Let's just say that the disc is much lighter now but the braking power compared to before is no-where at the same level as before with the stock disc. The pattern I used was pretty aggressive and I'm afraid that I removed too much surface area. The setup lacks the initial "bite" as it did before. I'm using the stock replacement pads from Mikes and ss braided line with a rebuilt stock master cylinder.
 
I've read the hole dia shouldn't exceed the thickness of the disk. I don't know why or if it's valid. If you put on new pads at the same time, it can take pads awhile to become as effective as they will get. Don't settle for less braking than you had before...
 
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Come on now you can't post that and not show us. Swap in an undrilled disk and compare braking action.
 
I have often heard that when drilling your own, choose a pattern that will not place 2 holes in the same radial sweep under the pad at the same time, and maintain at least 1.5 times the thickness of the rotor between any 2 holes. I have also heard the hole size to disc thickness 1:1 ratio mentioned above.
 
Looks like too many, compared to current high performance rotors. Pattern looks too tight, as in, holes too close together.

$0.02.
 
2 more cents; the chamfers increase the effective size of the holes beyond your already generous hole size. I think I would do a comparo swap and maybe use smaller holes next time. On the "I heard" front I believe chamfers are considered to be a no-no also. It creates a ramp for FOD to get sucked between the rotor and pad.
 
Bit O quick math, a 1/32 chamfer would increase the area of the holes by 36%!

3/8 hole area .11 sq "
7/16 hole area .15 sq "

I have extra rotors, I even have a pair of slotted XS1100 rotors that are bolt on.
 
Yes, your holes are both too big and beveled - 2 no-nos when drilling discs. Leaving the holes sharp edged cleans the pads better. Also, if you've read the "Minton Mods" article, he states that a properly drilled disc will actually start to polish itself out in use. The score lines on it will start to go away. After several thousand miles, mine is starting to do this. All the score lines have disappeared in some spots.

Here's the theory behind hole sizes. When you drill a hole, you create more surface area (the sides of the hole) but at the same time, you lose some (top and bottom surface of the hole). Make the hole too large and you lose more than you gain. The rule of thumb is not to make the holes any larger than the disc is thick. Go larger than the thickness and you start losing surface area. The stock 650 disc is 7mm thick so staying at or slightly smaller than 1/4" for the holes is good. If you have 5mm discs, go with 3/16" holes.

I've done a couple using variations of the same pattern. First was my original 7mm thick disc with 1/4" holes .....

OldDisc.jpg


I currently run a 5mm thick SR500 disc with 3/16" holes (but more of them) .....

ATKForkBrace2.jpg
 
I'll keep that in mind for when I drill the next rotor lol. I think the only thing that this current rotor is good for is a large paperweight....otherwise I was thinking that a modern sportbike caliper would work ok with this rotor since the modern pad would have more surface area than the current stock '82 pad. A r6 caliper with the blue pot covers would look pretty sweet chillin up at the front haha
 
You may be on to something there. Folks put the early R1 caliper on the front of Road Stars they do single disc and 21 inch front wheels on. It is a great unit, and a pair of them stop an 800 pound bike quite well. May be worth a shot if you can pick one up cheap.
 
I'll keep looking around, i have some quarter inch thick aluminum plating that i could make an adapter plate out of to retrofit a modern caliper to the xs fork.
 
Many seem to think they need to upgrade stuff but the original components are very good and work very well if set up properly. I wouldn't be here now writing this if they didn't. I've had to panic stop with them and they responded superbly. I had that front tire howling, right on the edge of adhesion. Braking doesn't get any better than that. Upgrade to more powerful stuff that will lock and skid the front wheel and basically, you're fucked. When skidding starts, stopping stops.
 
^I feel similarly about rear disks. I think they're pretty much for sales appeal. Once you begin stopping hard and the weight goes forward, it doesn't take much power at all to skid the rear wheel. And you don't want the rear to start skidding. I have the pedal set real low on mine to try and make it hard to lock it up under that circumstance. Still plenty of grip for dragging it for low speed riding though.
 
xjwmx said:
^I feel similarly about rear disks. I think they're pretty much for sales appeal. Once you begin stopping hard and the weight goes forward, it doesn't take much power at all to skid the rear wheel. And you don't want the rear to start skidding. I have the pedal set real low on mine to try and make it hard to lock it up under that circumstance. Still plenty of grip for dragging it for low speed riding though.
Click to expand...

I concur, I am always told drum my rear brake is usless and i should change it. When i am in a real heavy braking situation i can stand on the pedal and it won't lock up. Much safer
 
Usually i just end up downshifting and engine braking combo that with using the front brake for slowing myself down on the paved road unless there's a gravel road ahead of me. Then I slow down as much as I can before the gravel then rely on the rear brake to slow me down to decrease the risk of the front end from sliding out.
 

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