Another Ice Mode Thread - Page 4 - LotusTalk - The Lotus Cars Community
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post #61 of 70 (permalink) Old 09-16-2014, 03:53 AM
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Originally Posted by glagola1 View Post
I'm not exactly sure what you are saying by "With no dampers the weight is on until the car rocks forward in pitch." ??? I'm also not sure how what you have typed is essentially any different that what I typed.

Anyway, it's been my experience that adding rear rebound keeps the car more stable under trail braking... in all the cars I've ever driven in anger. David, I'm confused how it seemed to have no effect or make it worse. Am I correct in remembering that you are running 500/800 on street tires? Yowza. That probably makes it very difficult to tell what your damper settings are doing at all.
What I am trying to convey, is that with no dampers the weight is exactly proportional to the height of the end in question.
So when you first jam on the brakes there is no lightening of the rear or increase in weight on the front until after the car has pitched forwards about the pitch axis.
On the other hand the rebound (or compression) has zero effect in steady-state braking. When one jams on the brakes the dampers are reacting to the velocity in pitch, which is a velocity in compression on the fronts and in rebound for the rears. To little front compression and the front should lock because there is not enough downwards force. To much rear rebound and the rears will get too light (lighter than they would in steady-state braking) and the rears will lock.

In general adding rear rebound will not help to keep the rears from locking, and too much will make the rear go light. (Maybe too little rear rebound might prevent weight transfer to the front?? Or if the pads have a high initial bite, which diminishes then less rear rebound might also help. Lastly a lot of front compression and little rear rebound would make the car raise at the rear and stay the same at the front, so the center of mass would move up. That s clearly ideal for maximising initial braking, but ignores all the other duties of the dampers such as controlling chassis and wheel modes as well as cornering)

So David's observation that it made no difference or made is worse is exactly what I would expect.
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post #62 of 70 (permalink) Old 09-16-2014, 05:18 AM
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Hmmm, not in my experience. Adding rear rebound has always made the rear of the cars I've raced more stable on trail braking.

Holmz, I can see your point of view if and only if you are talking about so much rebound that the spring is basically neutered and that the tire is getting held up after hitting bumps. That's high speed valving. I'm not considering that amount of rebound when I'm speaking about tuning. I'm talking about speeds in the 3"/second range where tuning for chassis control is done.... not bump control.
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post #63 of 70 (permalink) Old 09-16-2014, 01:07 PM
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Originally Posted by glagola1 View Post
Hmmm, not in my experience. Adding rear rebound has always made the rear of the cars I've raced more stable on trail braking.

Holmz, I can see your point of view if and only if you are talking about so much rebound that the spring is basically neutered and that the tire is getting held up after hitting bumps. That's high speed valving. I'm not considering that amount of rebound when I'm speaking about tuning. I'm talking about speeds in the 3"/second range where tuning for chassis control is done.... not bump control.
It is actually low speed valving that I am talking about, but it does sound like high speed damping the way I wrote it.

And I am not talking about experience and perception of how the brakes feel, or a point of view, but rather the basic physics and dynamics of what happens.
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post #64 of 70 (permalink) Old 09-16-2014, 01:09 PM
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Originally Posted by glagola1 View Post
Hmmm, not in my experience. Adding rear rebound has always made the rear of the cars I've raced more stable on trail braking.
....
Keep adding more, and you'll get to worse at some point.
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post #65 of 70 (permalink) Old 05-07-2019, 06:39 AM
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Thread revival:

So a few years of competition have passed since this thread was left for dead and in those years, I have experienced the occasional ice mode even with biased pads and later on with OEM 2 pots on all four corners. It was rare but did happen from time to time.... so the effect of bias is strong when tuning for ice mode elimination but it hasn't been the final answer, IME.

I've recently installed BOE's bias cage and left ABS intact. What this has effectively done is remove vacuum assist from the equation. If you study the schematics of the OE brake booster, you'll see that it's a pretty complex dynamic device with multiple chambers and operation modes. What I've noticed since this install is the complete elimination of ice mode. I can wail on the brakes like a gorilla and all that happens is good, smooth ABS. It's the first time in 5 years where I totally trust the brakes.

Bottom line: I think the brake booster is somehow responsible for what we call "ice mode". It would at least explain the hard pedal.
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post #66 of 70 (permalink) Old 05-07-2019, 07:41 AM
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Quote:
Originally Posted by glagola1 View Post
Thread revival:

So a few years of competition have passed since this thread was left for dead and in those years, I have experienced the occasional ice mode even with biased pads and later on with OEM 2 pots on all four corners. It was rare but did happen from time to time.... so the effect of bias is strong when tuning for ice mode elimination but it hasn't been the final answer, IME.

I've recently installed BOE's bias cage and left ABS intact. What this has effectively done is remove vacuum assist from the equation. If you study the schematics of the OE brake booster, you'll see that it's a pretty complex dynamic device with multiple chambers and operation modes. What I've noticed since this install is the complete elimination of ice mode. I can wail on the brakes like a gorilla and all that happens is good, smooth ABS. It's the first time in 5 years where I totally trust the brakes.

Bottom line: I think the brake booster is somehow responsible for what we call "ice mode". It would at least explain the hard pedal.
Definitely dual master/bias cage will make a difference and allow the retention of ABS. I think you will still in rare situations experience what some call "ice-mode" due primarily to the ancient Kelsey-Hayes ABS system used on the S2 variants.

For your reading pleasure, here is the explanation from a Lotus engineer of what we were experiencing and referring to as "ice-mode" back in the day ......

"The symptoms being described are a result of the Electronic Brake Distribution (EBD) system operating. This system is also referred to as Dynamic Rear Proportioning (DRP) and is, as the name implies an electronic system which, through the ABS control valve block restricts the line pressure to the rear brakes automatically to a pre-programmed algorithm. You can consider it as an electronically controlled proportioning valve which measures parameters like the rate of deceleration and rate of pedal application and uses this data to anticipate a rear wheel lock-up and then reduces the braking effort at the rear wheels as necessary. If the ABS system is left to do this, it can only react to a wheel as it starts to lock and therefore the car can already start to spin before the ABS can start to work. In extreme circumstances, if the driver brakes very suddenly the EBD system can lock off the pressure to the rear wheels completely; what pressure was at the rear brakes as the EBD system engaged remains there and the rear brakes are still working as a result, but further increases in pedal effort will not increase the braking at the rear of the car because the pressure to the rear brakes cannot increase. When this happens the brake pedal goes hard, as it is now pushing against the front callipers and a closed valve only, instead of against the front and rear callipers. The rear callipers are single piston and therefore quite flexible, so they are a major factor in making the brake pedal feel 'soft'. When the valve closes, the brake pedal pressure no longer flexes the rear callipers, hence the increase in pedal hardness. The front brakes are still working just as well as before the valve closed and will give more braking if the pedal effort is increased, while with the rear brakes working as hard as they can the braking is NOT affected. The problem is the driver feels like braking is reduced (even though it is not) because of the change in pedal feel. If the driver continues to push hard on the pedal, the car will continue to slow as fast as it possibly can in the circumstances. If he increases the pedal effort the front braking effort will increase and the rear effort will remain where it was. If he was to back off the pedal for a fraction of a second, the valve will reopen and the rear brakes will operate as normal again, with the pedal feel going back to normal.
In the case of releasing and re-engaging the pedal the car should not be able to slow any faster than it was with the system engaged unless either 1: the driver triggered the system in the first place by stamping on the pedal too fast or 2: the system triggered because a rear wheel was unloaded when the brakes were applied and would have locked up but is now fully loaded once again and able to sustain a greater braking torque. If the rate of deceleration does improve when the pedal is reapplied then it is telling the driver that he is over braking either in terms of the ultimate ability of the brakes (cause 1 above) or the track condition (cause 2 above) and needs to adjust his driving style to suit. If the system were not fitted or disabled and he continued to drive that way he would be in danger of spinning when applying the brakes.
The suggestion that the system is running out of vacuum is just plain wrong. The system carries an internal reservoir of vacuum sufficient for three full brake applications. As with every servo system ever fitted to a car there is a one way valve which prevents the vacuum being lost when the car is on boost. The only way this reserve can be depleted is if the driver is maintaining boost while applying full brakes, i.e.: left foot braking very badly. In this instance I would argue that depleting the vacuum is probably a good thing as it should provide him with a warning that he is doing something awful to the car and it may reduce the speed of impact when he finally hits something as the brakes fade to nothing!! In normal use the throttle is closed when the brakes are applied, there is therefore no boost and the vacuum is automatically replenished as it is used."

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post #67 of 70 (permalink) Old 05-07-2019, 07:49 AM
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Perhaps the manual brake cage eliminates the possibility of "1: the driver triggered the system in the first place by stamping on the pedal too fast"?
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post #68 of 70 (permalink) Old 05-07-2019, 08:04 AM
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Funny story:
I had a conversation with the sales team from a major ABS/ESC supplier for a hypercar project I was involved in. One of the 'upgrades' that was sales pitched to us was 'winter mode'. Needless to say I was not impressed.

I have a MINI that also has an ABS ice mode in which braking is severely reduced under certain conditions. Cost me a garage door repair once. So it isn't just Lotus, it is an industry issue of pandering to the average (ie terrible) driver at the expense of performance for skilled drivers. Don't expect this trend to go away any time soon.

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post #69 of 70 (permalink) Old 05-07-2019, 08:20 AM
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Don't expect this trend to go away any time soon.
A locked front tire cannot steer, so it is better to pick what you hit than be a passenger and hope you stop in time. In a true ice scenario, it is very valuable.

The big problem with Lotus is that we can't leave anything alone and put wheels, tires, pads, rotors, etc on the car that it wasn't calibrated for. When you think of ABS as trying to make a controller that can balance a broom in your hand (both are classed as unstable controls problems) then you understand how such a small change can massively hurt the controller's ability to do its intended purpose.

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post #70 of 70 (permalink) Old 05-07-2019, 08:23 AM
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Perhaps the manual brake cage eliminates the possibility of "1: the driver triggered the system in the first place by stamping on the pedal too fast"?
As in previous post, anything you do the brakes without telling the ABS will interfere with its intended operation. Reducing the master cylinder boost (or removing it entirely) will tame the ABS unless it falls so far behind that it tries to "catch up" too aggressively. I briefly had a stent calibrating race ABS, and I recall one time getting me to update their calibration just because they changed their boost.

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