The following was copied and forwarded to us by a Ferodo / BrakeTech dealer who thought similarly that confusion indeed exists relative to the questions raised in the letter from Greg regarding sintered metal pads versus organic brake pads on cast iron rotors:
> …actually this was from the "instructions" that came with my rear rotor. This was several months ago and I
> can't say I recall exactly what the issue was, but believe they said sintered pads would cause excessive
> wear on the brake tech rotors. It was immediately followed by a recommendation to use Platinum series
> Ferodo pads (of course).
> Anyway, I forgot about it until I bought a set of brake techs for the front. Called the phone # listed
> on their web site to ask which pads they would recommend and was told that only Ferodo sintered pads
> should be used. Funny how that works, but then what do I know? I've asked people at a shop I trust and
> they've not heard of any reason why sintered pads and CI cause problems. Perhaps it's just marketing.
As such, we'd like to take a stab at clarifying some basic points on this rather convoluted issue:
There's really two very elemental dynamics in play when it comes to material life expectancy [rotor and pad] at the friction couple:
Friction related abrasion [on the rotor pad track]
Thermal stress distortion [i.e.: warping of the rotor and/or backplate]
Although this is the simplistic view, I'll focus here since this forms the basis of our recommendations:
1/ MATERIAL ABRASION.
Most sintered metal pads are designed specifically for use on stainless steel rotors, which is typically a harder material in general than cast iron. As such, some of these friction materials do indeed gall the rubbing surfaces to varying degrees. If this occurs on stainless, the softer cast iron will normally suffer to a greater extent. Since there are so many different sintered materials out in the market (with the vast majority specific to stainless steel) we must as a general rule of thumb recommend to our customers that they not use them on their iron rotors as the potential for premature problems is real and potentially expensive.
Furthermore, using sintered metal pads (any) on Gray iron or individual mold casting rotors can be a recipe for disaster.
2/ THERMAL DYNAMICS.
Regarding sintered pads on Ductile iron, Gray iron, carbon steel or stainless for that matter; using the base assumption that the friction material isn't overly abrasive [causing galling, scoring, etc.], the real issue is thermal capacity.
All conventional metals used in rotors have their own thermal dynamics to deal with; total mass, shape, lightening/venting hole type (placement and quantity) combined with inherent thermal conductivity of the resident material all play an integral role in material stability at peak operating temperatures. What we're essentially talking about here is resistance to distortion [warping, coning, etc.].
This is a complicated and convoluted subject that has few simple answers as there are so many variables to factor in. But one basic truth is that the sintered pads, including our new SinterGrip series, run hotter at the interface than do "most" organic pads...by as much as 150 degrees [F] in our testing. If you're running near the edge of thermal capacity with a particular rotor type and design and high performance / racing organic pads (like the carbon based CP series: 911Star, CP211 and the new CP1), switching to a racing sintered pad will surely exacerbate the problem.
In majority of instances where a problem occurs, thermal stress induced distortion is the culprit.
Another area of potential confusion is the many and significant differences between the classes of iron; the two main categories being primarily the Ductile irons and Grays irons. There simply isn't sufficient room here to expand on those differences here without taking up excessive room...have to leave that for another time.
We manufacture the BrakeTech full-floating rotors from computer controlled Continuous Cast [billet] nodular ductile iron supplied by America's oldest and most experienced foundry specializing in advanced composition cast iron. Cutting from billet is clearly a more expensive process than many others but we do this to have greater control over material matrix, eliminate porosity and have the most consistent micro-structure possible.
Does all this extra care and material selection guarantee they won't warp? Nope. There's very few guarantee's in this world (outside of death and taxes) and in the rarefied atmosphere of racing, even less...we just build our brake rotors to be the best possible given the material technology of today.
In final answer to Greg's question regarding our recommendation of the Ferodo SinterGrip ST pads on the AXIS rotors; we've dyno tested them extensively and have discovered no significant abrasion related issues with this combination. After over 200 dyno runs, they didn't distort either. But bottom line when it comes to thermal stress distortion; as noted above, there's no guarantee this won't happen under the right circumstances.
Hope this provides some insight.
- All dyno testing was performed with Ferodo sintered metal pads
- All BrakeTech full-floaters are Cryogenically treated, substantially improving wear resistance and maximizing their overall performance
- Do not extrapolate or presume that any of the above infers that one can run any sintered metal pad [brand] on the AXIS/Iron rotors without the potential of serious consequences and damaging them as result