[W126 Coupe] Brake upgrades

[Richard Hogarth]

Thank you Daryl for your excellent analysis!!
-axlehead at bellsouth.net

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From: mbcoupes-bounces at mbcoupes.com [mailto:mbcoupes-bounces at mbcoupes.com]
On Behalf Of Daryl Armstrong
Sent: Thursday, August 11, 2005 12:56 PM
To: Mercedes Coupes Mailing Lists
Subject: Re: [W126 Coupe] Brake upgrades


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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