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11" 949 Racing Big Brake kit 90-05 (2 wheel)
Caliper: Dynalite 4
Pad: Carbotechs XP8/10 and 1521s on street
Rotor: Directional Vane 949 Rotor (L & R)
Fitted July 2011
Upto 20 track events and about 35,000km daily and interstate driving.

At QR track day yesterday.
Third session, noticed a slight 'grinding' under heavy braking (160>70kph)
Fourth session this became more noticeable.
I thought my xp10 pads must have been down to the bones, and i had brought along a set of XP8s to swap over just in case.
Pulling one wheel off revealed it wasnt the pads.
Boths fronts are the same with about 4 cracks visible on each rotor.
I suggest everyone with wilwood check them and keep an eye on them.
I have ordered a set of SuperMiata Directional Vane friction rings (rotors) for 949 11" BBK
from Emilio as I believe these are an uprated version safer for track use.

That's pretty good life out of a set of rotors used on the track, I'm impressed they lasted that long.

I used to get about 10 race meetings, or two set of pads, out of my road type rotors before they were at minimum thickness and starting to show signs of cracking, being $150 a pair with all machining done to fit the race car it was cheap enough to consider them consumables, especially when the pads are dearer than the rotors.

Might be worth ordering a second set to have on hand Stu in case these ones don't last as long.

You are right Matt
I shouldn't be surprised they are stuffed, when i look at the work they have done.
Poor things have had a hard life with me jumping on them with both feet

Good rule of thumb would be to replace rotors after how ever many pad changes to the current set you last put in. This should mean you are replacing rotors before they are cracked, and if you don't have a new set ready to go you have spare rotors to get you through the rest of the day.

Checking rotors for cracks and measuring thickness, as well as the condition of the pads is on my usual list for track day prep, and also after the track day when changing back to the road wheels. If any dramas are spotted I generally have time to get another set of rotors on the car.

Nothing worse than horrible noises, or a big bang quickly followed by your foot going to the floor to ruin your day.

I must use some really hard high co-efficent pads, cos my rotors never last as long as the pads.

And that most standard rotors are very soft compared to what I would call race type rotors, hence the cheap price tags these days.

dunno if my $400 slotted rotors (purchased 5 years ago so sure they'd be more $$$ now) would be what you call soft standard rotors. maybe i dont heat up the pads enough or make them chew up quicker. more track time maybe is needed.

My DBA slotted rotors lasted 55K with numerous track days and pad changes before cracking.

Until recently, I used to swap from road to track pads on my slotted rotors but found that I wasn’t getting the full advantage of the track pad due to bedding issues.

Then I noticed that all the dedicated track cars don’t run slotted rotors, so after some enquiries to the manufacturer and those at the pointy end of race competitions, I now have a dedicated race pad matched to a stock Mazda rotor.

I swap out my road slotted rotors and pads for track days. It’s a pain in the arse to do but only takes a few minutes’ more than just swapping pads. The improved braking performance is noticeable and has enabled me to keep up with other more highly modified MX5’s.

Now the obvious question which will be asked, like I did, is "what about the slots taking away the gasses between the pad and rotor face during hard application at the track?"

Well besides the “look” appeal, it seems slotted rotors work if you are running a performance street pad and are exceeding their designed temperature range at the track but if you have the correct temperature range pads fitted all the slotting does is reduce the friction area and therefore your braking performance.

Also to be taken into consideration is that certain brands of pads don’t work well with slotted rotors, ie you’ll wear out an expensive set of CarboTech’s very quickly with slotted rotors compared to unslotted.

Interesting, CB, worth thinking about.

Minx, thanks for posting the warning. I reckon those cracks point to either a design or manufacturing fault, even though the rotors pretty old. I'd still consider reporting that back to your supplier or to Willwood.

Thanks CB - good info

project.r.racing wrote:dunno if my $400 slotted rotors (purchased 5 years ago so sure they'd be more $$$ now) would be what you call soft standard rotors. maybe i dont heat up the pads enough or make them chew up quicker. more track time maybe is needed.

Rotors and pads should be matched to the car and the driving. It is quite easy to overbrake a car, that is have too big and hard a brake pad and/or rotor set up for the car.

I was taught many years ago by a well respected race mechanic in Sports car racing (sports 1300's) that you use the smallest and softest brake pads with the smallest rotors and calipers you can get away with as the braking performance will be better as they are working at the optimum, rather than a large and hard pad that doesn't get up to temperature because it isn't being worked hard enough. The other advantage is lower weight than large brakes. Cast iron is heavy.

I have used Hawk DT70 pads using a Wilwood dynalite caliper successfully on RDA TP Magna rotors for a 240Z, weight was 960kg ready to race, plus me. The rotors lasted about a season of racing and two sets of pads. At the end of the season the rotors were under size and shaped like an hour glass. I tried slotted rotors, after one event I had standard rotors, so that didn't last long. Rear brake set up was Volvo 2 spot calipers with a heavy duty road pad and standard rotors. The rotors were never changed and I got about 20 events out of a set of rear pads, usually replaced as they got cooked. I also used a 1" M/C unboosted and had great feel and only once had any dramas when I used a fluid not quite up to the task, flushed through with Penrite Sin and all good again.

On my NA6, I use slotted rotors with Hawk HPS pads front and rear. This setup works well for me and after 4 track days and about 5k of road use, I don't change brakes, I still have about half the pad life left in the front, I'll run then down the bottom of the slot in the pad, the rears are a little better and the rotors are holding up well.

MattR wrote:On my NA6, I use slotted rotors with Hawk HPS pads front and rear. This setup works well for me and after 4 track days and about 5k of road use, I don't change brakes, I still have about half the pad life left in the front, I'll run then down the bottom of the slot in the pad, the rears are a little better and the rotors are holding up well.

I’m currently running the HPS front and rear for the road, mainly because it’s a good road pad that doesn’t dust up like the EBC’s.

I tried the HPS on the track a number of years ago and found “gassing” to be a big problem. The car refused to brake after a few laps, no matter how much pedal pressure was applied. I’d expect that slotted rotors would fix this problem but you’re still giving up precious time on the track when you exceed the operating temperature and the brakes fade.

Charlie Brown wrote::
Then I noticed that all the dedicated track cars don’t run slotted rotors, so after some enquiries to the manufacturer and those at the pointy end of race competitions, I now have a dedicated race pad matched to a stock Mazda rotor.

I swap out my road slotted rotors and pads for track days. It’s a pain in the arse to do but only takes a few minutes’ more than just swapping pads. The improved braking performance is noticeable and has enabled me to keep up with other more highly modified MX5’s.

Now the obvious question which will be asked, like I did, is "what about the slots taking away the gasses between the pad and rotor face during hard application at the track?"

Well besides the “look” appeal, it seems slotted rotors work if you are running a performance street pad and are exceeding their designed temperature range at the track but if you have the correct temperature range pads fitted all the slotting does is reduce the friction area and therefore your braking performance.
:

Aren't slotted rotors supposed to be better in the wet too water dissipation?

Made sense to me when I read it but as always, if someone has empirical proof that it's not the case, I'm prepared to believe the evidence.

You got proof it does?

Surely the 150-300 degree temps the brake componenets run at is good enough for water dissipation, since water seems to dissappear in a cloud of steam at around 100 degrees.

Just thought the slots were for out gassing.

I'd be keen to see some pics of the rotor with the top hat removed. The pics show the crack adjacent to the mounting hole for the top hat and it appears the crack may have begun there? If this is the case it would be interesting to see if the crack aligns with the thread end of the disc rotor itself. Looking at the rest of the disc surface it does not show any signs of heat cracks so it seems it related to the way it is mounted.

Any chance of some more pics?