Moderator Curtis Jacobson Portland Oregon (4578 posts) Registered: 10/12/2007 02:16AM Main British Car: 71 MGBGT, Buick 215 |
Ever been tempted to crossdrill your own brake rotors?
I was, once upon a time...
I carefully drilled a set of Brembo rotors for my MGB. Here's what they looked like: Now obviously this isn't the same pattern other MGB owners have used. I felt it made sense to use larger holes but fewer of them, primarily because I figured it would be advantageous if the holes spanned the whole surface area of the brake pads. Patterns of 1/8" holes generally don't accomplish this. The weight savings wasn't great... the stock rotors weigh about 8.0 pounds, and the drilled rotors weigh about 7.8 pounds. The most noticeable effect of my experiment with cross-drilling was increased noise under braking. Seriously. I can't really testify that I noticed any change in stopping distance or susceptability to "fade". Here's what I started to see at about 12 or 13 thousand miles: Small radial cracks, originating from the drilled holes. A couple further observations: (1) all of the visible cracks were on the outboard (wheel) side of the rotors. (2) the number of visible cracks was 7 on one rotor, 5 on the other. (3) all twelve cracks started at holes in the middle rings of holes... not the innermost or outermost rings. (4) the number of cracks that seemed to be going inward seemed about equal to the number going outward. (5) if it weren't for the cracks, these rotors would have lasted 10's of thousands more miles. (Incidentally, in the photo above I see three cracks: one "inward" from the hole in the center of the photo, and two "outward" from holes higher in the photo.) I caught the problem, and removed-and-replaced the rotors in preparation for a "track day" early this month. I can't speculate on how quickly (or slowly) the cracks might have propagated across the rotors. --- One small additional observation... the second photo (above) shows what proportion of the Brembo rotor surface is swept by standard MGB pads. "MGB V8" pads are larger, but the additional surface area is all INWARD (toward the axis) compared to stock. As you can see, there's only about an 1/8" ring of unswept rotor inboard of the stock swept area. --- Have any of you guys tried crossdrilling your own rotors? Were your results similar? For the time being I'm now running stock, unmodified Brembo rotors. I probably won't mess with drilling my own rotors in the future... but crossdrilled rotors have been proven in countless other applications. What tricks make them work for Wilwood, Porshe, Yamaha, etc., etc., etc.? |
rficalora Rob Ficalora Willis, TX (2764 posts) Registered: 10/24/2007 02:46PM Main British Car: '76 MGB w/CB front, Sebring rear, early metal dash Ford 302 |
Re: Ever been tempted to crossdrill your own brake rotors?
Did you do anything to ensure the bit & rotor didn't get too hot while drilling them?
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Moderator Curtis Jacobson Portland Oregon (4578 posts) Registered: 10/12/2007 02:16AM Main British Car: 71 MGBGT, Buick 215 |
Re: Ever been tempted to crossdrill your own brake rotors?
Yep. Cutting oil and sharp drill bits on an industrial drill press at appropriate speeds... plus, as I recall, I drilled all the holes 1/8" before moving on to bigger bits.
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BlownMGB-V8 Jim Blackwood 9406 Gunpowder Rd., Florence, KY 41042 (6477 posts) Registered: 10/23/2007 12:59PM Main British Car: 1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS |
Re: Ever been tempted to crossdrill your own brake rotors?
Curtis, in all likelihood the biggest problem was failure to chamfer the edges of the holes, as the sharp edge can easily create stress risers. I suspect the noise had something to do with the hole pattern but chamfers would have helped there also. I have drilled rotors before but it doesn't really apply since they were stainless motorcycle rotors. There's not a thing wrong with using larger holes. (I used 3/8" and had good results.) But part of the calculation is to figure how much surface area is increased with a given hole size. Obviously at some point there is no gain. The size of the chamfer will affect this also.
Jim |
Moderator Curtis Jacobson Portland Oregon (4578 posts) Registered: 10/12/2007 02:16AM Main British Car: 71 MGBGT, Buick 215 |
Re: Ever been tempted to crossdrill your own brake rotors?
As I recall, they were chamfered. The chamfer wasn't very deep though, and went away with wear.
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MGBV8 Carl Floyd Kingsport, TN (4541 posts) Registered: 10/23/2007 11:32PM Main British Car: 1979 MGB Buick 215 |
Re: Ever been tempted to crossdrill your own brake rotors?
Curtis,
That last pic does a real good job illustrating why I don't believe in drilled rotors. 1. Cracked rotors 2. Reduced rotor surface 3. Drilling rotors doesn't help anything (slotting does). [flashoffroad.com] [www.superchargersonline.com] |
MG four six eight Bill Jacobson Wa state (325 posts) Registered: 10/23/2007 02:15AM Main British Car: 73 MGB Buick 215, Eaton/GM supercharger |
Re: Ever been tempted to crossdrill your own brake rotors?
I drilled mine as per the special tuning manual. The smaller holes, so far no cracks after approx 11k miles. They are the thicker MGC rotors, which may help reduce the chances of cracking.
Was a time consuming job though! Is it worth it? I think it helps reduce brake fade. For what it's worth the new ZO6 Vette's come with cross drilled rotors. The holes look like they are about 1/4 diameter. |
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V6 Midget Bill Young Kansas City, MO (1337 posts) Registered: 10/23/2007 09:23AM Main British Car: '73 MG Midget V6 , '59 MGA I6 2.8 GM, 4.0 Jeep |
Re: Ever been tempted to crossdrill your own brake rotors?
I cross drilled the rotors on my Midget and so far haven't had an problems. I do think I'm getting reduced fade as Bill says, but I also routed ducts from the front valance to the dust shields so that I have a good supply of cool air being blown through the discs when I'm moving. (as evidenced by the amount of brake dust on my wheels) I have noticed a bit of reduced life on the pads. I used a 1/4" hole with a chamfer done with a 3/8" bit.
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