Brake Rotor Friction Characteristics?

[meep]

Are there *really* different formulations of brake rotors which have greater friction characteristics?

I've been told, "yes," by many, but am not sure. The only documentation I've seen so far in deeper reading is that:

1. most domestic vehicles use about 4% titanium in their rotors, which greatly reduces rotor wear, but slightly reduces friction characteristics. The european makes don't use the Ti, which demonstrates why bimmers, volvos, etc., wear through stock rotors so quickly, often being too thin to shave after the first set of brake pads are done.

2. newer rotors, the ones we pay extra for, can have a carbon content added-- the carbon aids in the even dissipation of heat, which assists with even braking characteristics and healthier cooling, while reducing the strength of the rotor overall.

That's it. That's all I've been able to find on technical, actual "white paper" level research on better friction characteristics of rotors. Seems like most of the "these have better stopping power" claims is bunk.

I ask because the newest used car in our stable came with recently turned cheap rotors. They "fit" but they are not oem design, and even with new pads the stopping power is ho-hum at best, so I'm tempted to replace them. Perhaps they are a "lifetime" rotor, maybe they have even more Ti in them, to make them last but at the sacrifice of stopping power. On the other hand, if there's really no difference in what we buy here, it'd be a waste of money.

I like the AZ premium coated rotors. I've always used Raybestos Advanced Tech, but have had to send several back and am done with that. AZ Premiums I installed recently on the back had no flaws I found. I'd be looking at them again in this case.

Any experience here?

-m

 

[Kestas]

I've never heard of 4% titanium added to cast iron for any engineering alloy. Is this for aluminum rotors? You're not specific in your post.

 

[BigD1]

Only thing I can tell you is that I went with Centric 125 high carbon series rotors, and the Centric Posi Quiet semi-metallic pads, and my Volvo 240 has better braking now than it ever has.

"Centric Parts also offers 125 Series High Carbon Brake Rotors for certain vehicle applications. High Carbon Rotors feature advanced metallurgy that greatly reduces the possibility of pad squeal especially with higher friction, European style brake pad compounds. Proprietary Molybdenum and Chromium alloys resist cracking in high heat situations and increase friction couple for improved braking power and stopping performance."

Source: http://www.centricparts.com/products/centric-premium-brake-rotors

 

[meep]

Kestas - for Iron. I'm trying to find what I'd read--- it may have been Si instead of Ti. I can't access some of the sites from work though. I do remember a 4% additive to the iron which improved strength but sacrificed friction characteristics.

This is one of the articles I went through, but it's more on pads/shoes than rotors/drums. Good for pads. https://info.ornl.gov/sites/publications/Files/Pub57043.pdf

I'll keep looking, may have to be tonight.

-m

 

[meep]

not the same article I found earlier, but here is a reference to Ti, and improved wear characteristics.

http://www.pmt.usp.br/ACADEMIC/antschip/freio.pdf

 

[dparm]

Originally Posted By: meep


1. most domestic vehicles use about 4% titanium in their rotors, which greatly reduces rotor wear, but slightly reduces friction characteristics. The european makes don't use the Ti, which demonstrates why bimmers, volvos, etc., wear through stock rotors so quickly, often being too thin to shave after the first set of brake pads are done.



That is absolutely not true. BMW specs very thick rotors for its cars that will last a long time. The heavier rotors will also help with the heat for those harder stops at autobahn speeds. A possible third reason is that due to the no-cost maintenance, BMW wants the rotors to last longer than the free maintenance period so they don't have to shell out for new ones on every car they sell.

BMW does not recommend turning rotors and no dealer will do it.

Go on any of the BMW websites and you will hear everyone talking about how long their OEM rotors last. My M3's lasted over 60k, and would've gone longer had it not been for track use. Tons of guys on M3post are getting two or three sets of pads per set of rotors.

I don't know what the metallurgy is for the rotors, however.

 

[Ducked]

Two or three simple minded questions:

Can't you generally lock the wheels (or exceed the ABS activation threshold if you have it) with pretty much any standard braking system?

If that is the case (I THINK it is, but I could be wrong) why the search for greater friction in your brake disk surfaces?

I'd assumed that pretty much any functional braking system would perform the same in normal operation and one off emergencies, because lock-up sets an upper limit to the performance.

I'd also assumed that "high performance" systems were distinguished by continuing to perform under sustained hard use, when they were pretty hot. In normal use, they'd be no different.

Not so?

 

[meep]

dparm - my info on BMWs is probably dated. good correction. I see you have an '11. Since you track the car and know people, is this a recent change (last 10 years) in their brakes?

The tribology I came across surrounding european vehicle brakes is that they favor performance and handling over longevity. The popular consensus was typically that they (and some of this dates back to the 90's) the rotors would wear faster on the germans than the domestics/japanese - soft rotors, soft pads--- but the combination provided the absolute best in brake feel and performance. The BMW purists and SCCA guys I knew learned to stay oem - if you went aftermarket (unless you did a full brake kit), normal store bought rotors resulted in reduced brake performance). Back then, at least, dealers would not turn them, and if you asked why, they'd tell you it wasn't designed for it/not enough material remaining. It sounds like they have improved their product by adding more material. The vehicles are heavier now, than they were back then.

I suspect it to still be true with volvos - I've had three used and none had original front rotors. All had original rears, but significantly worn.

Those statements, even if out-dated, still align with the metallurgy items I read.

-M

 

[901Memphis]

Whatever minor difference you see in the rotors will be nothing compared to the differences in the friction of the pads.

Even amongst the popular FF friction rating there is a huge swing between the low and high end, the ratings range from 0.35-0.45 coefficient friction within FF, but GG is 0.45-0.55, so getting a good GG pad will make a bigger difference.

 

[Jetronic]

Originally Posted By: Ducked
Two or three simple minded questions:

Can't you generally lock the wheels (or exceed the ABS activation threshold if you have it) with pretty much any standard braking system?

If that is the case (I THINK it is, but I could be wrong) why the search for greater friction in your brake disk surfaces?

I'd assumed that pretty much any functional braking system would perform the same in normal operation and one off emergencies, because lock-up sets an upper limit to the performance.

I'd also assumed that "high performance" systems were distinguished by continuing to perform under sustained hard use, when they were pretty hot. In normal use, they'd be no different.

Not so?


Abslutely correct, but having a bit higher coefficient of friction gives more response from the brakes to the same pedal pressure. That like "appearance of quality", in that it doesn't mean anything in practice, but it can help sell a car.

 

[Linctex]

Good question... I wish the info was available to match the "correct" pas to rotor metallurgy.

I have ceramic brake pads on my 2008 F150,
and I'm not impressed with their stopping power.
I'm also not impressed with the shiny mirror finish the have left on my brake rotors.
There's just so little friction happening.

I'll take fast wear over safety (tires and brakes) any day.

 

[meep]

agreed. the brakes are, for certain, workable. This car is a hobby for me--- so as I slowly work through its bugs and repair the "brokes," I'm taking my time, enjoying it, and enjoying the game of getting it "just right."

As for lockup - one would assume all systems can lockup - but in my truck for instance, with akebono pads and thermoquiet shoes, I could not lock it up when straight and dry, at least not with one leg. I had to two-foot it on a couple of occassions when someone pulled out and I was towing a load. That has since been sorted out with different pads/shoes.

But it is a good question, for us - this is an experienced enthusiast board. We talk about performance a lot, but a lot of things get said about rotors both here and everywhere else, that we can't substantiate. I'm genuinely curious, and found very little out there that was conclusive.

m

 

[Jetronic]

One of the links you posted talked about train iron brake linings: those had different friction coefficients on steel wheels. So yes, small differences in metallurgy can have an impact, but so does bedding in, frequent heat cycling, the weather, etc...

I don't think you can find the optimal combination from researching the internet, but curiosity is always good and needs to be satisied.

 

[CBR.worm]

I've found more of a difference in stopping power from pads, however; I've found better rotors last longer and deal with heat better. I'm fine with a rotor wearing at the same rate as the pads if the combo lasts a long time.

I can also say that I am getting ready to change the OE pads and rotors on my wife's 5 series at 110k miles. Very long life. Thick, heavy rotors and beefy pads.

 

[meep]

@CBR - 110K is impressive for a set of pads, and very healthy for a set of rotors, too.

@Linctex - I think ceramic pads really do best with larger brakes where they aren't overwhelmed but still provide good feel. (personal opinion)

So - agreed, pads have a lot of variation. Rotors are expected to provide less of an impact, but the bottom line, so far, is that most claims for better friction characteristics for rotors may be marketing. I came across some other sites that also suggested Molybdenum and chromium components to the alloy, in each case for longevity, but with no discussion on how that impacted friction/stopping power.

If I swap these out with others, I'll keep the same pads and pipe back in with any results. I'm on the fence - it's money I don't have to spend.

-Meep

 

[Jetronic]

I recently fitted Brembo HP2000 pads. Not cheap, but I liked them from the first second, even before they bedded in on my part worn discs. They immediately had the same bite as the original pads and that improved after bedding in. No codes on them to point towards the friction coefficient, but the brake rollers suggest close to double from the original pads.

That's probably not accurate, but that's how it feels on the brake rollers. These pads aren't as compressible and have a metal shim on the back while the others had a non-metallic lining which had indents in it from the pistons.

For rotors, my main concern would be with heat dissipation and tolerance. I'd rather get some with extra heat treatment to prevent warping than just a random set with a seemingly good price.

 

[dareo]

All brake rotors are made in China out who knows what. I mean have you actually seen one that isn't Chinese anymore?

 

[dparm]

Originally Posted By: Ducked
Two or three simple minded questions:

Can't you generally lock the wheels (or exceed the ABS activation threshold if you have it) with pretty much any standard braking system?

If that is the case (I THINK it is, but I could be wrong) why the search for greater friction in your brake disk surfaces?

I'd assumed that pretty much any functional braking system would perform the same in normal operation and one off emergencies, because lock-up sets an upper limit to the performance.

I'd also assumed that "high performance" systems were distinguished by continuing to perform under sustained hard use, when they were pretty hot. In normal use, they'd be no different.

Not so?



You are correct. If you can engage the ABS, you are at the TIRES' limits, not the brakes'. This is why I chuckle when people declare they are going to buy a big brake kit for their daily driver thinking it will stop the car faster. It won't -- buy stickier tires, instead.

"High performance" brakes are for sustained hard stopping, like when you are on a race track or towing. Even then, I don't know if the metallurgy of the discs is any different. They rely on more cooling vanes, floating/two-piece rotors, duct work, different pad shapes/sizes/compounds, etc.

 

[Garak]

Originally Posted By: dparm
You are correct. If you can engage the ABS, you are at the TIRES' limits, not the brakes'. This is why I chuckle when people declare they are going to buy a big brake kit for their daily driver thinking it will stop the car faster. It won't -- buy stickier tires, instead.

I've been telling that to people for ages, to no avail, even before ABS. If you can lock up your wheels, how is more braking power going to help?

 

[RooflessVW]

Big brake kits are not just about stopping "power." They're more about stopping "capacity." Bigger rotor=longer arm, more stopping force with less generated heat.

Sure you can lock up OEM brakes and engage ABS... But can you do it over and over and over again reliably?