Lacking stopping power on RS 314mm brakes
 

I have a 2000 FD3S Type RS with the larger 314mm rotors and the following tyre/brake parts:
- Project Mu HC800 pads
- Project Mu CRD rotors
- G-Four 335 Racing Fluid
- 265/35R18 Federal 595RS-RR (street tyres)
- No brake ducting (just the OEM backing plate/ducks)

Based on everything I have read, my brake set up should be more than enough for these tyres and I should have no issues getting to the limits of tyres (ie, ABS kicking in). Unfortunately, this is not what I am experiencing.

Basically, my brakes are not providing enough stopping power to overcome the limits of traction. I cannot get ABS to kick in (ie, tyres locking if ABS was disabled).

This is most apparent on higher speed braking zones, such as Turn 1 on Winton (180km/h to 100km/h) or Turn 4 at Phillip Island (200km/h to 70km/h).

This problems starts straight after warm up lap so it is most likely not JUST pad fade, but definitely DOES experience pad fade after 2-3 consecutive hot laps.

The following G-plot was taken from a few hot laps. As you can see, I am pulling 1.3G on the corners, but generally only around 0.9-1G under braking. There's a few spikes that go a bit higher, but it struggles to be maintained there.

Attachment 751053

Things I have considered already:
- It is not brake fluid boiling. The brake goes extremely hard - it's not spongy. There are no bubbles in the fluid.
- The check valve for the brake booster seems to be working (via the blowing/sucking test).
- It is not (just) pad fade, since it happens on the first hot lap

This is what the rotors looked like after my latest track day (overall, 4 track days of use on these rotors/pads) and there are clear signs of over-heating on one side.

Driver side:
Attachment 751054

Passenger side:
Attachment 751055

Attachment 751056

Any ideas on what else could be effecting brake force? Do I need better pads?

Or am I just not pressing the pedal hard enough? (In my previous 4 cars, with considerably smaller brakes, I never had issues locking up and had to be careful about modulation so...)

It looks like you may have had some contamination on the pads/rotors. You may need to clean/turn/replace the rotors and pads. Has the setup ever been fine? If so, then something happened. If they've never worked fine, it could be the setup. I am not familiar with those pads. Are they spec'd for your driving conditions? I run Hawk DTCs and really like them. I can go fast and late into corners (on a stock FD). Not sure you can source them down under tho, or if they make pads for the RS calipers

Just briefly looking at the operating range of those pads I would be surprised if overheating is your problem. However, this is highly dependent on how fast your car is. What kind of power are you making? The easiest way to determine what is going on would be to use either temp sensing paint or strips on your calipers or even better on the back of your pads. If they are getting close to or above 800 degrees C, you know you have an overheating problem. But if you are going to be doing a lot of track duty you should definitely spend the small amount of money it costs to get some NACA brake ducts with tubing and either aim them at the middle of the back of the rotors or get some rotor guards that have a flange molded in for attaching a 3" cooling duct.

Is the engine highly ported? My full bridge n/a engine doesn't like to make enough vacuum for the brakes to work well around town but this goes away on the track since at higher RPM when I let off the throttle it still produces enough vacuum to lock up my Nitto NT01s with the stock turbo brakes (FC). Just wondering if you aren't having a booster or master cylinder issue. Is it ever hard to stop other than at the track after a hot lap or two?

What did you do to bed in the brake pads? Did you follow the procedure outlined by Project Mu? This can have a huge effect on pad to rotor performance and durability especially on higher end ceramic pads. My cousin who raced for Honda a number of times in the Continental Sports Tire Challenge series said they tested many different brake pad manufacturers and Project Mu was at the top of the list for wear, durability, and braking performance on their car. So I doubt the quality of the pad is your problem either. Assuming you properly bed in the pads and rotors we really need temperature data to make any kind of informed decision

Pedal goes hard = problem with brake booster circuit.

I had to replace the check valve on my TII. Failing one worked fine around town but got hard pedal when racing. *ass pucker*

Replace the check valve first and then check the booster diaphragm if the problem persists.

My stock FD brakes easily overpowered 265 RS-Rs and the braking force required should be like any other production (consumer) car.

Dragging your brakes (which is what you are doing if you are doing un-assisted braking) heats your rotors/pads more than short effective braking since your brake components have less time to cool down.

+1 on properly bedding

I assumed you meant the pedal still felt solid by that statement but Blue TII is definitely right if you are in fact saying the pedal gets more hard feeling. Is the pedal feel changing when this happens?

Also, your driver side rotor looks good and is getting a nice transfer layer of pad material. The passenger side one is either overheating, not properly bedded, or there is some contamination. You want that nice even layer of "smudging" of pad material be transferred over to the rotors.

Might see if the diagnostic box can tell you if there's something wrong with the ABS....using an LED bridge.

There's more track oriented pads than the 800s in project mu...unfortunately not available in the rx7 shape.

Rotors don't look too bad from a heat perspective, the paint on the hat will turn white and you'll get lots of stress fractures in the surface if you overheat those PMu.

Thanks for the responses all!

Think I've always had problems with this setup. My first two track days were at Phillip Island, where the only heavy braking turn is Turn 4, but I do recall over shooting that corner quite a bit due to lack of brake power, so I think it has always been an issue.

230rwkW at 1bar.

Completely agree and that is on the cards, but that would only manage the pad fade, not the initial issues with a lack of stopping power after a warm up lap.

Stock ports.

On the street it feels fine to me and does seem to at least have some level of brake booster assistance.

10 or so hard stops from ~100-30km/h. Project Mu does suggest a bit more speed, but I live in a residential area and drive to the track so...

Hrmm, I hadn't thought of that. I had only thought about it in terms of completely failed or operational, but I guess there's a possibility of an in between state.

I think this sounds like the best avenue for me to pursue at this stage.

Is there any way to check whether the brake booster itself has partially failed? And the check valve (which passes the "blow/suck" (giggle) test)?

Not sure I agree on that one. Correct me if I'm wrong but from my understanding, brakes are all about energy state transfer. Turning kinetic energy into heat energy. So that'd suggest that regardless, the same amount of heat has to be produced doesn't it?

You'd think that shorter stops are more likely to overheat it precisely due to that same amount of heat energy being generated without a chance for it to cool down, leading to a shorter period of more heat.

Will give it a look, but don't think it's an ABS issue. That'd only change the way it reacted once you lost traction wouldn't it?

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I have another track day on Sunday, so I will pay attention to the brake feel and try to see if I notice anything about the brake pedal feel.

After that, I will order a new check valve and see if that helps, else look at potentially replacing the brake booster itself.

You guys are also probably right that the HC800s probably aren't the best pad for what I'm doing. I originally went for them due to the high coefficient of friction, but that was before I really understood the concept of coefficient of friction vs temperature, and the HC800s probably fall off at higher temperatures.

I got cheap PMU pads through my sponsor, so at this stage would rather stick with the PMU brand. Might try RC09 Club Racers or maybe even one of the dedicated motorsport pads PMU sell next like the Racing777...

I would urge you to put some temperature monitoring paint or strips on the back of the pads as well. As long as you are not operating outside the pads working range (up to 800 C) there really is not a lot of reason to upgrade to more expensive pads. Just based on the temp range, those pads should compare to the g-loc, carbotech, etc pads that are proven to work well on our cars

You are exactly correct in the first part of your response which proves what I said above.

It takes the same amount of energy to brake the car down from a set speed to a set speed whether you apply the brakes hard for a short amount of time or more softly for a longer amount of time.

The difference is if you brake hard for a shorter amount of time there is more time when you are not putting additional heat load into the brakes for them to cool between braking events.

In addition, the greater the temperature difference between an object being cooled (brakes) and the cooling medium (air) the more heat transfer. So, getting all the braking heat into the brakes in the shortest length of time is also beneficial to cooling.

This second part would be true if you were on a long straight just accelerating to a set speed and braking down to a set speed.

Harder braking in this scenario would result in more braking events in a given length of time.

However, on the track the braking zones are all a set distance apart. You brake the same number of times per lap whether hard or soft.

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Now in real life, if you are braking harder for a shorter length of time on track you have more time to be accelerating prior to braking so you will be putting more heat into your brakes because you will be going faster.

But then you will have faster lap times which is usually the point of racing.

If you unload a corner in the braking zone, the pedal will pulse if you driving anywhere near the limits when it's functioning. Usual prob with the system, are sensors that haven't been unhooked when suspension changes have taken place - and metallic debris.

No need to replace bits willy-nilly, very simple to diagnose the one way valve and booster, without disassembly...basically running the engine, pumping the pedal to see if it rises once switched off, then starting the engine to see if it drops with your foot on it. I'd expect a much clearer explanation in the workshop manual!

If looking for initial response, H21 and 16-03...if you want to start cutting down and machining pads. The 777 is less than the Hawk DTC70 in bite really, still more of a tracking day pad, but might be ok with those tyres.

Ceramic pads are great because you can use them on the street and track and they can handle lots of heat but they don't have much torque or they may FADE!

Try some cheap hawk blues or DTC 70s. They aren't expensive and might be worth a try to see if a higher torque pad is needed.

Didn't have time to source any rotor temp paint before Sunday's track day unfortunately.

I think brake pad operating ranges are a bit more complex then simply not exceeding the specified range.

For example, a friend showed me this. Winmax charts for some of their pad compounds.

Attachment 751028

Attachment 751029

As you can see, all pads vary their coeffecient of friction over the heat range. The more "track orientated" pads maintain their mu over a wide range of higher temperatures, where the "street orientated" ones peak at a lower temperature and fall off.

When I bought these pads, I scratched my head over why the Club Racers had a lower Mu than the HC800s, despite having the same max operating temperature. The same is the case of some of even more hardcore pads in the range. So why pay the extra money for something that seems to offer less coeffecient of friction?

Those charts above seem to illustrate why. Even though the HC800 claims to be operable up to 800 deg C, it probably peaks at quite a low temperature and falls away as you get to higher temperature, where the track pads probably have significantly more coeffecient of friction at higher temperatures, even if their peak may not be as high.

So even though I may not be exceeding 800 deg C, I may have had a lot of the friction drop off.

Good point!

I can definitely get it to pulse. Eg, if I slam the brakes before weight shifts forward, it certainly pulses, as does it on trail braking etc.

At the track day yesterday it started pouring rain, so had a good opportunity to confirm that ABS is working and prevents lock ups.

Unless you reckon that the ABS may actually be preventing the brakes from operating properly somehow? Not getting an ABS light on the dash or anything if that helps.

Tested this.

Pressing the brakes when starting the car - the brake pedal does drop once car starts running. Brake booster works.

Holding the brakes then turning off the car, pedal stays down. Check valve seems to keep vacuum inside.

Will look into those!

Will look into them. A friend ran Hawk Blues on the street with no issues at all!

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Some more observations from yesterdays track day:

1. Definitely getting some major pad fade. After ~2-3 consecutive hot laps, would often find myself overshooting corners by quite a bit due to loss of stopping power. Cool down lap resolves the issue and I get braking ability back.

2. Definitely pushing the pedal as hard as possible. I am getting to the point where pushing harder does not actually yield any increase in stopping power and the pedal does not go down any further.

Given that it seems to hit a limit, would this almost suggest that it is unlikely to be a brake booster problem? You would think brake booster would require model pedal force to stop the car, but would not add a limit to the peak braking force.

3. While the brakes are cool, remain unable to get the brakes to the limit of traction. There are times when the tyres will chirp and the ABS will pulse a bit - eg, when first applying the brakes, towards the end of the braking zone (slower) or while trail braking, but certainly not really at the threshold of traction for most of the braking zones.

4. Somehow I managed to boil my fluid on the first session (not related to the above symptoms). Fluid is PMU G-Four 335 with about 6 months and 4 track days prior. However after each track day, I have been bleeding out about 50mL from each corner to get less heated fluid to the calipers.

Fluid on the back corners was clear (PMU fluid is fluro green but goes clear when it overheats). Fluid on the front corners was black and one side had particles in it. Think the brake caliper seals may be on their way out.

I have had this happen on the first track day I went to, but had no issues on the other 3, where fluid has always come out fluro green.

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Actions from here I am going to try:

- I ordered a booster check valve hose. They're not expensive so might as well give it a go, even if I'm wondering whether it will make a difference.

- Going to get the brake shop to rebuild the front calipers as a result of the issue with black fluid and particles.

- These pads are on their way out, so need to swap them anyway. Will buy pads that are more track orientated and see whether this helps with both the pad fade, but also the overall bite.

- This weekend I will read codes out of the ABS ECU to see if anything is registered.

- Will probably do a complete brake fluid change. It's possible moisture has gotten in somehow.

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Would welcome any other suggestions of things to try or actions to take.

Will keep you posted on the results! Next track day is booked in for end of Sep.

I do have a bit of a timeline in which I have to sort this out. I'm competing in World Time Attack Challenge in early October, and only have one track day planned between now and then to really do testing. I'm willing to throw multiple solutions at the problem in hope something sticks, but don't have a lot of chances to try different solutions and testing them (except for what I can do on the street).

Definitely sounds like pad material is not suited for how fast you are going at this point.

As to brake fluid, Castrol SRF is used by pretty much everyone I know at the endurance events I race as it has a pretty high wet boiling point. SRF is 270C whereas your Project Mu fluid is 221C.

Do you have titanium shims on the back side of your brake pads? These are an effective and relatively cheap way of insulating your calipers/fluid from the heat of your pads. Between switching to SRF, better pads, and titanium shims your only other option is the brake cooling ducts that you really need to have anyway...

Sounds like the ABS and booster are working correctly.

Fluid will go black pretty much every session, if you're doing much more than time attack 3 laps. We normally were rebuilding fronts every 2 or 3 events with endurance pads, with high friction pads, you'd burn the dust boots in 1 and wear out the pads in 2, so 6 months of fluid use wouldn't be typical.

If you can find the pad shape for these in the "good" Pmu compounds mentioned, there's not a huge amount of work to get them to fit the rx7 caliper....just a trim on length, a little bit of welding on the backing plates and slight modification for the retaining pins.

BRAND NEW PERFORMANCE FRICTION 0460.01.15.44 RACING PAD - 01 COMPOUND | eBay

Sounds like the ABS and booster are working correctly.

Fluid will go black pretty much every session, if you're doing much more than time attack 3 laps. We normally were rebuilding fronts every 2 or 3 events with endurance pads, with high friction pads, you'd burn the dust boots in 1 and wear out the pads in 2, so 6 months of fluid use wouldn't be typical.

If you can find the pad shape for these in the "good" Pmu compounds mentioned, there's not a huge amount of work to get them to fit the rx7 caliper....just a trim on length, a little bit of welding on the backing plates and slight modification for the retaining pins.

BRAND NEW PERFORMANCE FRICTION 0460.01.15.44 RACING PAD - 01 COMPOUND | eBay

A pad that I went to early this year for experimentation was a set of Raybestos ST-43s which I dual purposed for daily driving and mountain driving.

It was too much pad for the application, but was SO linear throughout the heating cycle that I couldn't think of a better race track pad for something like your tire size and grip levels.