I didn't think as much into it as you did, but I did a track day and autox in the same week. At Lime Rock, I didn't get into ice mode as the XP8s have much less bite. On top of the better pad, I ran mostly in the rain and the pavement at the track is only two years old. Three days later I did an autox in a lot that is over 40 years ago, I was hitting ice mode on and off at the start and end of the run (70 second) with the same XP8. As soon as I switched to the stock pads, problem went away. While there might be other factors contributing to ice mode, I personally believe rough pavement is a big one for me.

It's not the pavement itself but rather the friction co-efficient, obviously (or not) older pavement has a much smaller contact patch with the tire due to more of the binding media getting eaten away, therefor less friction. Basically once the pad to rotor friction far exceeds the tire to pavement friction then there is problems.

For what it is worth I had ice-mode with the stock brakes and stock pads and RE-11 tires during auto-x, but only once the brakes got hot and on longer 90 second courses after about the 4th consecutive run. I'm guess that the bite of the pad just kept increasing due to the increasing heat.

Yes it certainly can be the decrease of friction level on the old pavement, but the uneven surface on some of the older lots or corners (turn 1 at VIR) can have a field day with the ice mode

Yes, bumps and pavement transitions certainly seem to be an issue as well.

Question:

On an empty road turn off the engine, pump the brakes gently several times until there's no servo and the pedal gets rock hard. (Yup, if you can't think how to do this professionally, without killing some-one, you'd better not try!)

Do you now have "ice-mode" and have to push madly for any effect?

If so then ABS action is probably "beating" the vac servo (which on a 370 is a bit different anyhow).

Cure? Maybe a big vac tank or a diesel vac pump (diesels have no manifold vac)

Just a thought, Fritz

A very interesting thought too! A temporary vacuum tank setup could be installed to test your very interesting theory. Let's hope some of the racers on the forum give it a try and post the results.

The more I think about it you could really be onto something. It's worth doing just to make sure that it ISN'T a lack of vacuum assist.

I don't believe the ABS system is active with the car turned off (I could be mistaken) so I'm not sure what this would prove.

He's only trying to simulate ice mode in a non-track environment.

The essence of the matter is his speculation that ice mode is not a software problem but a power brake assist problem. What he's saying is that heavy braking on rough surfaces requires such aggressive ABS action that the vacuum assist for the power brakes becomes depleted with a resultant loss of braking power. This would explain the reduced braking power and rock hard brake pedal because these are the same symptoms that occur when the vacuum assist is depleted or fails.

If he's right the fix could be as simple as adding a supplemental vacuum canister to increase the volume of the available vacuum assist to meet these high demand situations. His theory is very insightful and virtually DEMANDS investigation to see if it has merit. All I know is that if my track days were being compromised by Ice Mode I'd install a vacuum gauge to monitor the available vacuum in my power brakes and I'd be looking into adding a remote vacuum canister/tank.

I think it has been pretty clearly documented on this forum and others that the Bosch computer is the source...

well, ABS Ice Mode finally got me... - Rennlist Discussion Forums

... all about liability management for bosch / nissan... easier for them to be conservative and kick-in an aggressive ABS system and explain that in the EVENTUALITY of lawsuits than it is to say they 'tuned the ABS in order to keep race cars out of barriers/walls when they are ripping through bumpy transitions under intense braking'

I think we are going to have to just learn to live with it and practice trying to "reset" the icemode (release, re-apply, and again if needed)... or replace the computer with a $10K race ABS computer

I am a skeptic... still, I'd welcome your experiment, Fritz/Guard Dad, and would be happy to be proven wrong and have you find a simple fix.. to confirm, perhaps there is a variable on the canbus we can pick up that will allow us to monitor vacuum pressure?

travisjb, the link was great. Did you notice that one of the posters cited similar problems in the Lotus community? According to that post the problem was solved by increasing the capacity of the vacuum tank and replacing the the check valve. There was a difference of opinion as to if it really was a vacuum problem but I find it to be an interesting theory nonetheless.

Now I don't track my car but I do enjoy tracking down the answers to this sort of problem. Let's face it, spooky brakes are no fun on the road or the track, so I'm interested in this issue just the same as I'm interested in the boiling brake fluid/brake cooling issues that concern many members of the Z community. For me, a bulletproof set of brakes is a requirement on the street just as it is on the track . It could be the computer, in fact it probably is the computer, but I do believe an investigation of the vacuum issue is warranted and easily done and that's why I am advocating for an investigation of the possibility of this being the cause. Hey, if it's not a vacuum issue, well that's one less thing to worry about.

I've heard of the vacuum problem and experienced it in other cars but this isn't it. The ice mode is very easy to reproduce on the stock braking system and it doesn't require full throttle and zero vacuum to get there.

Easiest method to reproduce it is to heat the brakes up with several 60-10 braking attempts, then let the brakes sit and heat soak for about 5-10 minutes. Attempt some more 60-10 on a slightly less than perfect surface, small bumps, few grains of loose sand, downhill incline, or anything else that can take grip away from the rear (in particular) and it will raise it's ugly head. It doesn't seem to happen as often with the AP Racing brakes for whatever reason (less rear bias perhaps). Stiffer suspension also seems to help ward it off. Smoother application of the brake pedal also helps prevent it by preventing BA (Brake Assist) from intervening. BA responds to any sudden pedal application and amplifies the braking force typically resulting in a full ABS application. Setting the car up so that you don't really need ABS also helps. Definitely an evil computer at work here.

We can always try to run different pads to get different bias.

I have experimented with everything from different pads, different tire setups, different calipers and rotors. All setups eventually resulted in Ice mode.

I haven't run with abs fuse for months. Really forces me to think about brake force I apply which is very fun in a retro kind of way. I can run at turns much faster just knowing I will stop now. When I feared Ice mode I started braking early which was a real drag. I'm fairly experienced driver and I almost drove right off track twice with ice mode on the car. Fortunately I was on wide turns and just made huge arc to scrub off speed. Also experienced it at AX a few times.

I like the vacuum discussion. Has anyone tried playing with this idea yet?

I wonder what Mazda does as I never had ice mode in my MX5 or my MazdaSpeed 6

Guard dad, of 7/10/-2011 evaluated my post correctly, and added to it...thanks....especially re the Lotus bit.

To continue:
There was a time (when BMW tried to call ABS by its own designation "ALS")
At about that time ABS was strictly not for competition as it could not keep up.
The ABS pump in full flight should work madly and could outpace the then servo which on Audi and (maybe BMW) was all done for brakes and steering by a single pump, but on most cars was the usual servo set-up.

Just how much should a manufacturer do for you (us!) who wish to simulate emergency stops, one after the other, along with possible VDC use ? Why should the public (us too) pay for you or us to "go off-road" madly.

I'd guess it'd be OK for the car to do the job for two full blooded emergency stops, with VDC in between.

So folk should pay for more than that just to satisfy you (us!) madmen in an autocross/race ?

To race we all accept what's needed, from oil coolers to brake scoops and special fluids. We balance brakes by selecting the coeff of friction between front and rear pads: that's if we cannot get, or are not allowed, twin master cylinders!

I can hardly believe that ABS is compulsory in any competition event. "Allowed" maybe or "in operational condition" (to avoid folk junking the pieces). The exact terms of the regs need stating (here?)....and I'd be happy to interpret them (!) and hopefully find a solution within the regs.
Plainly one does not want or need ABS and VDC, and especially BA, interfering with driver inputs in any sporting event...and "ice mode" is just broody dangerous.

To quote ChriSSlicks "Setting the car up so that you don't really need ABS also helps" (He hasn't actually dumped it so it is still getting used).

And, for myself, I could do without BA on the road .... how can I dump that?

Fritz

I would love to permanently ditch BA but I don't think it is possible.

It's not like we are asking for race caliber ABS systems here, just asking for an ABS system that isn't out to kill the vehicle's occupants. The ABS computer in the 350Z/G35 never had these serious issues, the braking was 100% reliable.