[W126 Coupe] Brake upgrades

[Daryl Armstrong]

Hi,

I am involved in the business of design and manufacture of big brake upgrades for race and street applications. As of a year ago, we no longer supply direct replacement (stock size) brake components. I do have opinions which I am happy to share.

I'd like to start out by saying that I did NOT join this list with a view to pedaling my wares and this should not be construed as an attempt to promote the company.

Some people like the appearance of cross drilled rotors, they look pretty cool. Most of the time they work pretty well. Many people condemn them for their tendency to crack and in many cases it's justified. There are factors which influence the rate of crack development, these are primarily hole configuration, heat and to a lesser degree, placement of holes. 

Most rotor manufacturers chamfer the surface of the rotor around the holes. If the holes were not chamfered the edges of the holes at the rotor surface would be quite sharp (90 degrees). The sharp edges heat up and cool down rapidly during the braking cycle which causes rapid and repeated localized expansion and contraction and so, fatigue. Repeated severe brake applications result in small fractures growing into big cracks and I suppose if no one notices the problem developing (?) catastrophic failure could result. 

The chamfer angle is typically 45 degrees which means, in round numbers, that the 'sharpness' of the friction surface transition into the cross drilled hole is half that of an un-chamfered hole. This significantly reduces the rate of crack development. A gently "radiussed" transition from the rotor friction surface into the cross drilled hole seems to just about triple the crack development time over chamfered holes (all other variables remaining the same).

Our experience with normal street driven vehicles indicates that cross drilled (or cross drilled and slotted) rotors usually wear out before cracking makes it necessary to replace them. If your daily drive includes 14 miles of 6% down grade through a winding mountain pass, in a 4000 lb car and you like to double the recommended corner approach speeds, drilling probably is not for you. I have such a drive, but of course, I don't know anyone who drives that way ;).

Slotted rotors exhibit many of the positive characteristics of cross drilled rotors without such a tendency to crack. One feature which makes slotting attractive is the tendency of the slot to 'wipe' the pad clean. Slots keep pad glazing to a minimum under severe conditions and pretty much guarantee sustained friction coefficient for the life of the pads. Frequently, on plain rotors, we find the friction coefficient falling off as the pad wears. This seems not to be the case with slotted rotors. The same can be said for pads used with cross drilled & slotted rotors

My recommendation: If you think you will use your brakes to capacity (you experience brake fade) on a regular basis, use plain or slotted rotors. If you like the idea of cross drilled rotors, use them and keep an eye on the surface condition. Tiny hairline cracks are usually no big deal. If they become really obvious, you might trade the rotors out. If you like drilled rotors, you might consider drilled and slotted rotors for the benefit the slots provide. If possible, try to get "radiussed" holes.

In terms of brake effectiveness, our tests have shown little or no difference in stopping distances using plain or drilled / slotted rotors. I would modify that statement to include "within our ability to accurately test....", considering we use real life conditions and 'G' meters as scientifically as real life conditions permit. Also, rotors that look like Swiss cheese are just as effective as plain rotors in terms of stopping distance. This dispels the myth that surface area has a lot to do with how well brakes work. The same applies to pad area.... Big pads last longer than small ones, but they do the same job.

There are other advantages to cross drilling that don't really apply to standard sized replacement parts for street use, but they typically come with the need for some compromise. Actually, compromise seems common to all walks....

Although we no longer deal in direct replacement parts, we have access to a variety of direct replacement parts. I would be prepared to make rotors and pads available to list members on a, cost plus a small bit, basis if it is of interest. Again, not wishing to be a 'vendor' here.

Regards,

Daryl.
  ----- Original Message ----- 
  From: Rockymiranti at aol.com 
  To: mbcoupes at mbcoupes.com 
  Sent: Wednesday, August 10, 2005 2:18 PM
  Subject: [W126 Coupe] Brake upgrades


  Looking into slotted and drilled roters for the 560, anyone have advice and expertise in this area? 


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