So are we concluding, with respect to materials staying the same, dimpled rotors are the best followed by normal rotors?

 

I'm not. I'm saying blanks is the best, but if you want to go down this road there's X*inifinity threads out there arguing about it.

 

If I had to order them best to worst, for the "weekend warrior" without a sponsor budget for rotors and pads, I would rank them:
solid>dimpled>slotted>dimpled cross drilled>cross drilled> cross drilled and slotted (because of cracking issues AND wear issues- worst of both worlds )

But I'm not a brake expert, I just make knives out of carbon steels. And all I know is that when I heat a blade for heat treat and quench it too fast (or sometimes even leave it to air cool) it'll crack. Folks have tried to tell me that rotors are made of mild steels (very low carbon content) but I can't beleive that. If I took a bar of "wrought iron" or today's A36 mild steel heated it to the point it was almost liquid and quenched it, it wouldn't show any signs of cracking or warping like rotors tend to.

Just my $0.02 for what it's worth.

 

I know that this is WAY, way OT...but I find that very cool! Knives are a hobby of mine, though I'm more into the collecting/using side of things than the making side. I have a few nice customs and a bunch of production knives. I'd like to try my hand at knifemaking one day, but I think I'll probably start with putting together some of those Darryl Ralph kits before forging my own!

Emre

 

Me neither. I thought most rotors, at least for road use, were cast iron, which has a very high, >2%, carbon content. Mild steels run 0.1-1% or so, e.g. SAE 1020 is 0.15-0.25%.

Dave

 

A thousand pardons, but that may not be completely true, particularly if one is using an aggressive brake pad. I installed a pair of new slotted front rotors AND new Ferodo DS2500 pads back in May. With my wife and I double-dutying the car, we have done: 1 day at Road America, one day at Blackhawk Farms (Blackhawk is typically a bit harder on brakes, but the day we tracked we had to suffer through intermittent rain), and one day at Grattan; our pads are still thicker than the backing plates. I would imagine the pads are eating the rotors.

I also had AP Racing calipers with 2-piece slotted rotors on my '93 Z and, once again, double-dutied, we never went through pads that quickly.

 

I'd like to know what the problem was then. A single set of DS3000's lasted me an entire season on a blank rotor. A brand new set of DS3000's lasted one weekend with slotted rotors. It seemed obvious to me there was a problem. It was an expensive problem too and I'm not about to try it again. I'm not discrediting that it's not possible for it to be something else, but so far it seems that all signs point to the slotted rotors.

djh,
Carbon content is around 3.5% depending on the casting process and temps. I have a nifty SAE doc about it if you're interested. Shoot me an email if you are. c.v.fowler@gmail.com

 

Porsche cast there rotors with the holes in them which like someone said before eliminates the cracking problem. I also think if the rotors are made properly there arent any problems with drilled rotors most people with the cracked ones from what i have found have had crappy rotors that companys just put them on a C&C machine and drill their own holes.

 

I was just doing some searching, as I recently put some slotted rotors on my Evo, and I completely agree with Bogie about the topic. I had some rather agressive Project MU street sport pads on my Evo, and those pads really wore out the factory (solid) rotors pretty quickly. I since have put some Hawk pads on with slotted rotors, and I've taken the car out for some hard driving on the street and a track day, and the pads aren't too warn and the rotors are looking and feeling fine. The Project MU pads were extremely grippy and while I do miss the performance some, I'd rather not replace the pads/rotors quite that often. Not having the constant squeaking and excessive brake dust is nice too

-Trevor

 

Good info, but we were talking about wearing out the pads too quickly, not the rotors. All the same, all of us want to replace rotors or pads as little as possible.

 

Rotors are cast iron - high carbon - flake type (gray iron)

It has fantastic thermal properties and is cheap (heat capacity and heat transfer figures are high due to the high carbon content)

It is poor at heat cycling and rapid heat absorbing due to it's microstructure. Drilling holes in it is - ahem - silly as it exacerbates the materials weaknesses. This is simply amplified w/ race pads.

Friction and wear characteristics can be improved by alloying and proper microstructure as well as heat treating.

Drilling is bling - but thats OK if thats what you want - but it's not so good for the race track.

-Better iron works a little better/lasts longer

-Slotting increases torque (slightly) and bite (noticeable with lower friction street or combo type pads - less so w/ full race types).
-It also reduces glazing of pads but increases pad wear (more so under track braking type conditions) and correspondingly dust .

-Slotting may be more beneficial when car serves double duty - and/or driver preference for brake feel. It may also be good when laying on cold brakes at 120MPH trap speeds when bite is a good thing or autox runs where pads have no warmup time.

The Ferrodo's may be something of an exception. They are known to be a great - but fast wearing pad in general. It seems as if they don't like slotting though as I had another customer experience the exact same thing as you.

Meanwhile - another customer runs Carbotech XP-12's on slotted rotors and does not have that issue.

Perhaps the exact machining of the slots has some effect i.e. how sharp is the slot edge - thats is something to try and figure out.

 

While on the subject of rotor cracks, I was wondering at what point do rotor surface cracks become a concern? Right now, I can kinda feel some of the surface cracks w/ my finger nail, but they're not that big.

As far as rotor thickness goes, my rotors are still far from the service limits.

 

I know those rotors far too well.

I tend to like the fingernail test - if you can't feel them they are not there. (SAE?) We might differentiate between heat checking and cracking - but surely one eventually leads to the other.

Crack propogation in brittle materials (fast fracture - omg flashback to school) is a difficult science - but it might be safe to say the bigger the gap the more leverage the expansion forces have. I don't like them when they are near the edge as things can happen rapidly.

 

The sizes of the cracks seem to have stabilized. Sometimes they seem smaller than other times. It might just be pad material filling up the gaps.

I understand what you're saying about the leverage. I'll inspect my rotors for cracks nearing the rotor edges. That is a good point.

 

I'll correct myself

Fast fracture is when it breaks below yield - so we hope that never occurs as it's unpredictable.

Thermal fatigue is a bit more stable to predict. I know the heat checking can stay that way for a long time before anything else happens.