The Bahrain round, right?

Not dumb...er, well, if it was, then we were equally dumb, let's put it that way.

New pads or new disks alone would not solve the problem. It might band-aid it for a while, but in short order it would come back.

We did have 'bad' disks turned and took a cut of about 0.015". Separately, we also had some ground taking maybe 0.008" In both cases, it was marginally better than 'bad', but not as good as new. They also returned to 'bad' quickly. I'm sure the reduced disk mass didn't help.

We cryo'd disks. This gave a different feel to the brakes, which was not altogether good, and they lasted a similar (if shorter) mileage to baseline disks. Cryo'ing disks and pads made for a very 'grabby' combination that was difficult to be smooth with. It was different than when the disks were 'bad', but not good. We had a heat-treat process done that was just terrible from the word go. That one was almost comical it was so bad.

On the car we ran them scalding hot and as cool as we could possibly make them. Cold didn't help the brakes (only hurt aero) and hot was enough to destroy pads and boil fluid.

The manufacturer would take the 'bad' disks and test them. This might have meant getting them back to the factory and throwing them in the bin. They never reported anything of consequence. They were always deemed 'good condition used disks'.

The upshot of the whole deal was that we would just have to throw a lot of new brakes on the car. I'll be damned if I know what the deal was. I always felt that there was some chemical change happening with the disk and/or pad at high temps that was sending things pear-shaped. I never did figure it out.

I think I felt a few more gray hairs pop out just writing this post and remembering what a nightmare that was.

Carroll Smith- Every FSAE-er and solar car racer should read and understand Race to Win, just for context. He has his hobbyhorses, but at least he presents a sensible baseline. Sometimes it is sensible to break his rules, if you know enough to understand why he made them in the first place. On the other hand sometimes people who should know better break his rules and come a cropper. Ahem, nuff said.

So much for my suggestion... if grinding won't help, fat chance scuffing will do anything. Is this an experienced brake manufacturer that supplied the rotors? Beats me.

We've been given aggressive pads to clear this problem. If carbide is the problem then aggressive pads won't help. They'll make it worse.

I'm assuming FB went through the whole range of brake pads chasing it. I don't think the problem could be cementite inclusions without producing a vibration. This case sounds almost like the rotors were made from white iron.

Reminds me, slightly OT, of my first Mini, a tuned 850, for what it was, and the time, quite a quick little car. The brakes were not too good, and as I couldn't afford to switch to Cooper discs I was seduced by an advertisement for Minifin replacement drums. Swopped the drums, took the car for a gentle test, gave the pedal a tentative prod - and my world instantly changed. My immediate thought was that somehow I'd been diverted onto a railway track. The vibration was astonishing. Took it back to my friends garage where he put the drums on a lathe and with a dial guage assertained that one drum was eccentric by 300 thou. I'll write that again - 300 thou.

Hi all,

Long time no post here.

I've been using C-C brakes in prototypes and single seater.
On a F3000 class car even did some back to back testing.
I also used big cast iron brakes in sprint and endurance racing.

First point to me is of course difference in weight for unsprung and rotating mass
As for stopping power they are the same really.
If your driver brakes hard enough he will lock up. If needed you can change the ratio in the pedal and use a bigger mastercylinder.
Friction coefficient nowadays is high enough at the sweet spot for any material.

The main difference is variation of that coefficient through temperature.
For racing brakes, they need to be warm enough or friction is to low that is true for any material.
For what I saw through data. Good bite came at around similar temperature for C-C and cast iron.
Around 250 degrees C will be good just before the attack.

Main difference then is that when over heated, cast iron brakes just lose their friction coefficient completly while carbon keep braking well.
Big issue for carbons used in this way is very quick wear.

If you think about it, F1 brakes have problems to finish a GP such as Monza. There the aero and mostly drag is the most important. So reducing ventilation of the brakes just come as a natural way to achieve drag reduction.
So clearly F1 is under ventilating the brakes to gain in aero. As they run high budgets they don't really care as long as they last the 1 hour 30 minutes.

For the tests I did on F3000 car, swapping cast iron and C-C, brake distance was the same. Feeling was a bit different but my driver was used to both materials so no issue here.
Pressure graph were not significantly different. Advantage came from unsprung mass reduced and car was better going up and down kerbs.
Damping had to be adjusted.
Also, the C-C could do with less ventilation. Blanking the brake ducts gained me a little speed in straight. Could have been better with redesigned ducts. As for wear, I did not try to completely over heat them as I did not have a F1 budget but the brake duct could be blanked quite a lot compared to cast iron.

On the other side, Le Mans, as RDV pointed, is very tough on brakes. The Hunaudieres straights and chicanes are terrible.
Long time to cool the brakes then really hard stops. But brakes do have to last 24 hours and they do.
They also provide quite constant braking power through the race.

When I did a 24 hours race with a high aero and big grip car (performance similar to prototypes) fitted with steel brakes, we tried many levels of brake cooling but there was simply no way to make them last 24 hours.
Solution was to fit quick connectors on brake lines and change them a bit after mid race.

C-C do have an advantage in wear, C-C do weight less, C-C can be over heated badly and still brake.

On braking performance alone C-C or cast iron doesn't matter, as if it is not your tire grip the limiting factor your brake system is too small.

Also C-C brakes are less prone to absorb dust or whatever trash material on their hot surface. As metals are always "hungry" to charge whatever is laid on them when they get warm, C-C are a very dense matrix and just to get the whole stuff bonded together it takes months in an autoclave.
Also, carbon is the result of combustion, it is completely inert. It does not interact with anything. So chemical reactions are just limited to the resin bonding the whole stuff together. In carbon brakes resin is a ridiculous fraction of the material whereas cast iron is the whole material.
That makes for a constant braking performance through the life of the set.

Basicaly, if you run few km per year, C-C are not what you'd go for. The cost would be prohibitive.
But if you do run quite a lot of kms, say endurance, C-C will definitely be cheaper in the end.
After how much distance the C-C beome cheapper to use? that will depend on each car and racing program

Ben

Thanks very much Ben.

When I was a university student, Carroll Smith's view were first presented as a serialized article in Sports Car Graphic, if my memory serves me well.

Because one of the key elements of Smith's articles was fasteners, particularly aerospace cap screws, you could have a lot of fun walking around the checking out who had read the latest articles and who had yet to catch up.

http://www.carrollsmith.com/main.html

Great stuff, Ben.

The equipment was from _the_ US supplier of brakes. No need to throw them under the bus too badly, but the company has always seemed to know what they're doing. We only had a couple legal brake pad options, which we tried. We considered having illegal pads made for testing just to see if they helped things. The problem with that is that if they worked, then we'd have cheated and not wanting to cross that particular bridge we made a conscious decision to not do it.

We consulted another brake manufacturer that we'd worked with previously. This was 'spec. series' stuff we were using. He thought that being it was not in competition against other products that they very well may have shipped manufacturing to China and been using somewhat lower quality materials. He then admitted that they did the same thing when they had to.

Ultimately, I think we might have just been asking too much from what we were using. You don't expect a set of tires to last an entire weekend, so why would you ask that from a set of brake disks? That was my argument that finally gained some traction. I guess that coupled with trash cans full of brake disk boxes throughout the paddock every night was enough to convince our management we weren't the only ones struggling.

So even testing non-approved parts in a private testing session or even on a brake dyno is against the series rules? I'd think that such testing could potentially be useful diagnostically if only to avoid chasing down dead ends trying to address the problem even if the non-approved parts tested were never used on race weekends.

Spot on Ben...nothing to add.

(PS-sorry about blowing you away at Okayama.... )

Ok thanks for the remark on the post,

As for Okayama, fair enough you won that one, but 22 seconds in three hours in not blown away.
We'll see next year with more up to date machinery

Well, it was an odd-ball pad shape, so we couldn't just phone someone up and get a set. If that were the case, I'm sure we would have tried others. Our alternate pad company was willing to work with us by using our old backing plates and making a mold for cost (I'm sure to sell our competitors 'test-only' pads!).

In the end, it was kind of a thing the engineering dept. didn't want to get into for a couple reasons. First, we would use our R&D budget for something that we couldn't race. Second, if we did find an advantage, then we'd probably end up racing it illegally (against the engineer's wishes). Sometimes it's better to not give yourself those types of options.

I really would have liked to know what was going on. It would have been a good off-season project. Those types of things in the middle of the season get triage treatment. Off-season you can do it the right way. Of course, by the next season, the series was gone and no one cared.

The standard street-friendly cast-iron brake rotors carried by forged aluminium carriers weigh 18 pounds less than the optional, track-ready carbon-ceramic rotors.

Quoted from

http://www.insideline.com/features/2011-mc...first-look.html

Perhaps someone can explain?

There's not a word on the brake system in there

Lots of things covered here and very informative. Hopefully I can add some insight

I have done some production car studies on "Disc Thickness Variation" or DTV. We used capacitive proximity probes to measure runout on the discs and also to induce run out on the discs. These were then run for durability and NVH purposes. Certain suspension set ups are more sensitive to DTV than others. Our conclusion of this came about because you can have wildly different DTV on two different cars and one is crappy to drive and the other is fine. Another study looked at wheel bearing tolerances and how they affect DTV sensitivity. You can induce DTV by over torquing one of your lug nuts. It definitely gives you that "warped" rotor feel. The aside to this is all the pads and rotors were bedded in in a prescribed manner. It was a certain amount of stops from a certaind speed at a certain temperature. Obviously the temp aould go up and down but no stop could commence until the brakes were under a certain temp. Which led to an easy job in winter and a long drawn out one in summer. it would be very difficult for a "normal" person to bed in brakes like we did.
Although I have helped do these studies I may not totally understand them. My feeling on DTV was that the rotor isn't of similar strength all the way around. Some areas may be "softer" than others which causes the pad to wear out a part of the rotor faster than others. Someone who knows more about casting and steels could set me straight though. It always seemed there was more DTV from the middle to the outer diameter of the rotor. The other cause of DTV, which we induced, was the mismatched torque on lug nuts.
As an aside to this we did a very large study for an American racing team about a brake vibration problem. Mostly there problems came from not bedding in the brakes properly. In a racing environment, I assume bedding in the brakes is the last thing on your mind. The racing team had to buy a brake dyno and match rotor and pads in order to eliminate this problem. The team had been through multiple suppliers of rotors, pads, and calipers before they believed us. It did save them from losing a driver though.
It has been my understanding that pad build up and DTV are two different problems. One being caused by the person and one an inherent manufacturing problem or suspension compromise.

Not to quibble but overtorquing will not produce TV. The thickness of the rotor remains constant, at least in the short term. The rotor is simply distorted, which is runout -- TV and runout being two different things. Disc brakes are far more sensitive to TV than to runout. That said, this was indeed a common field service problem in the '80s. A typical lug nut torque spec might be 75 ft lbs but many impact wrenches are capable of 200+, pulling the rotor out of true. Rusty scale or other material trapped between the hub and rotor or rotor and wheel, same deal. The problem was solved with improved service procedures and by the introduction of the torque stick.




Buildup and TV are two different things but closely related in this way: Non-uniform buildup of pad friction material causes insufficient cooling/localized overheating of the rotor face. Under these local regions of buildup cementites aka iron carbides form in the iron. These regions are much harder than the surrounding iron, which produces variations in friction coefficient over the 360 degrees of the rotor's rotation, and also produces uneven wear -- the iron regions wear much faster than the carbide regions, producing TV. Re-posting a photo from earler (below) see how the hard spots are shaped like they were smeared onto the rotor face? Well there you go.

By the way, when a rotor is this bad, often you can run your hand over the rotor face and feel the hard spots standing above the rest of the surface.

If you want to see something interesting sometime, watch an old-fashioned brake lathe with negative-rake carbide cutters trying to cut serious hard spots out of a rotor. It's essentially carbide-to-carbide and the lathe will start dancing across the floor like an old washing machine. And when you examine the rotor, the hard spots have barely been cut, only the rest of the rotor, so the hard spots stand even higher above the rotor face than they were before. And looks like the rotor has grown warts.

In resurfacing, there is an interesting problem known as "phonographing." If the rotor is cut with too fast a feed, there will be a single groove, perhaps imperceptible, running from the back to the front. Doesn't cause too big a problem with rotors (maybe a clunk on application) but with drum brakes, it causes the shoes to screw themselves in and out on application, makes them grabby as hell until it wears off. One thing not explored here yet is surface finish on iron rotors. Generally you want to be shooting for around 50 Ra. Earlier I mentioned that TV is more critical than runout... to give you an idea, .001" TV can cause noticable judder but it will take at least .003" runout, more with floating calipers. One problem with runout is pad knockback, producing long pedal travel.

So why do it? Looks like a new rotor might be the best solution.

I think you only try to cut rotors with cementite inclusions when you don't realize what's going on.

Some brake lathes use an intermittent "step" feed to avoid this effect.

Yep, new rotors are often the way to go, especially with prices so low these days. Rotors can be as cheap as pads anymore on the more common models. But some rotors might not be so cheap or available, and thus in need of saving if possible, and newer lathes with much stiffer arbors and a positive rake angle on the cutters can usually handle the less severe cases. If the inclusions don't run deeper than the minimum refinishing thickness, the rotor can be saved. Two-piece racing-type rotors can be disassembled and refinished on a surface or Blanchard grinder.

While all this is relatively recent with braking systems, with clutches it has been a problem since the beginning of time. If you ever had a clutch replaced and was told the flywheel or pressure plate "could not be saved," or got the car back with a grabby or chattering clutch, that's generally the problem -- cementite inclusions in the friction faces, aka hard spots.

I am still puzzling over FB's case. If I had to guess, it seems the rotors were cast in a material that was simply unsuitable for braking use. If there is still a rotor on hand it would be easy enough to find out. Just break the rotor with a hammer and look at the edges. If they are gray and pebbled (broken around the individual crystals) it's gray iron. If they are white and smooth (straight through the crystals) it's not.

That stuff is gone with the wind, man. All I've got left is a bunch of shirts I don't want, a Lista cabinet and a couple cases of brake clean left to remember it by.

Whatever was going on, the manufacturer was mum about it and just acted like we were the only ones that would even mention it.

After a particularly good ass-reaming by management after one race I happened to find myself in an elevator with an older engineer that I was competing against. As we stood there I decided to just ask. "If you guys run your brakes over 200 miles, do you find that everything goes to shit, the bias moves all around and the car won't stop?" Maybe it was poor form. We weren't really friends, he was just a guy I'd say 'Hi' to in the bar or paddock, but I was at wit"s end.

He just laughed a little and said, "I don't have the slightest clue what they do at 200 miles. We're lucky to get them past 125."

The manufacturer knows how to make a good product, but they know how to make money, also. Sometimes those two things don't run hand-in-hand.

Glad I'm not the only one wondering what to do with their obsolete uniforms.

Is proper brake bedding still required? How about with the C-C brakes?

The rotors warp because either the rotors are bad, or the pads are bad, or sometimes it's the driver who is "fecking stupid". The way to "unwarp" the brakes is to temporarily change your braking technique, and by doing so re-do the bedding process. Of course in pro racing the only proper technique is to stomp and hold. But if it's your own car, or you just do not want to throw away rotors, the driver can substitute one long brake application with several short ones. He should never allow the rotors to feel "warped", he should change his braking technique immediately and re-bed them. If that does not help in the long run, then either the rotors are defective, or the pads are not made for the actual operating temperature range.

Another way to "unwarp" the rotors is to install very aggressive and abrasive pads and drive the car with cold brakes. That does the same thing as turning then, it makes them thinner with a uniform thickness. Not sure about the cementite/carbide though, I don't think I ever had that.

Also, I read in a some paper an excessive disk runout can indeed cause a DTV after a while.

Yeah unfortunatly the case is relace the rotors before the pads!!The pad material for road cars is that agressive it just eats up the rotors.I have relaced rotors on 40000km cars because the still serviceable OEM pads have eaten the rotors.
Yet I have to early 70s cars here with 5 times that km using old style Bendix asbestos pads that have almost unmarked never machined rotors after 35 plus years. they dont bite quite so hard but still do not do a bad job.
I often wonder though about the iron they use these days as it seems to be softer and seems get 'hot spots ' easier too.

Cheap Chinese knockoffs vs usa made?

Nah-economics at work. If I make rotors that last 35 years on a car likely to have a service life of 15, I'm wasting money. On the other hand, if I design everything to be reliable for 3 years and serviceable for perhaps 7, I can sell more parts and more cars and spend less (per unit) money doing it. Having said that, there are OE engineers on this site that could likely provide more accurate lifespan constraints.

Actually, there is. George has quoted directly from the article (last sentence under "Carbon-Fiber Chassis").


The article has described the relationship backwards. Carbon-ceramic rotors are roughly 1/2 the weight of the same size in iron (that is rotor itself; bell/mounting system will be essentially the same for both). Carbon-carbon rotors are roughly 1/2 the weight of carbon/ceramic. 

Another aspect of c-c is that, as the heat builds during a single application, the CoF continues to rise (until the start of oxidisation, that is, which I believe is at about 850 C). The driver has got to be sensitive to this phenomenon, as it makes locking more likely: at the same time that your speed is falling and you're beginning to reduce brake pressure, the braking effect is increasing.