Steering, Suspension, & Brakes

tips, technology, tools and techniques related to non-driveline mechanical components

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Moderator
Curtis Jacobson
Portland Oregon
(4300 posts)

Registered:
10/12/2007 02:16AM

Main British Car:
71 MGBGT, Buick 215

authors avatar
Ever been tempted to crossdrill your own brake rotors?
Posted by: Moderator
Date: March 26, 2008 05:15PM

I was, once upon a time...

I carefully drilled a set of Brembo rotors for my MGB. Here's what they looked like:

Rotor-A.jpg

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:

Rotor-B.jpg

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
Cypress, TX
(2336 posts)

Registered:
10/24/2007 02:46PM

Main British Car:
'76 MGB w/CB front, Sebring rear, early metal dash Ford 302

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Re: Ever been tempted to crossdrill your own brake rotors?
Posted by: rficalora
Date: March 26, 2008 06:59PM

Did you do anything to ensure the bit & rotor didn't get too hot while drilling them?


Moderator
Curtis Jacobson
Portland Oregon
(4300 posts)

Registered:
10/12/2007 02:16AM

Main British Car:
71 MGBGT, Buick 215

authors avatar
Re: Ever been tempted to crossdrill your own brake rotors?
Posted by: Moderator
Date: March 26, 2008 09:11PM

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.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5284 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Ever been tempted to crossdrill your own brake rotors?
Posted by: BlownMGB-V8
Date: March 26, 2008 09:22PM

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
(4300 posts)

Registered:
10/12/2007 02:16AM

Main British Car:
71 MGBGT, Buick 215

authors avatar
Re: Ever been tempted to crossdrill your own brake rotors?
Posted by: Moderator
Date: March 26, 2008 09:29PM

As I recall, they were chamfered. The chamfer wasn't very deep though, and went away with wear.


MGBV8
Carl Floyd
Kingsport, TN
(3181 posts)

Registered:
10/23/2007 11:32PM

Main British Car:
79 MGB, Buick 215

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Re: Ever been tempted to crossdrill your own brake rotors?
Posted by: MGBV8
Date: March 27, 2008 10:16AM

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
(265 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?
Posted by: MG four six eight
Date: April 09, 2008 01:15AM

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.



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

authors avatar
Re: Ever been tempted to crossdrill your own brake rotors?
Posted by: V6 Midget
Date: April 09, 2008 09:59AM

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.
BillYoung-D.jpg
BillYoung-E.jpg


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