richrf

I believe that only Lexus engineers can clearly define what is happening. They have to look at the computer program itself and see what it is doing with the braking and ABS systems under a specific situation. I believe that there is a nexus of conditions which is causing the systems to operating in an unorthodox (i.e. a sense of acceleration) mode while the car should be clearly slowing down.

Thanks for all of your comments and thoughts on this issue. Much appreciated.

RX_330

It's not a sense of acceleration. What you're feeling is a decreased rate of deceleration in that split second it switches from regen braking to mechanical braking. Regen braking is much more sensitive and responsive by nature than mechanical braking and since mech brakes are rarely used on hybrids they may have condensation, surface rust, or anything else that might decrease the initial bite. Either that or the rotors/pads might simply be too cold on initial bite since they're very rarely used in normal driving.

This is not an issue or a problem per se, as it is just the nature and difference between regenerative and mechanical braking. Sure it could be designed a little better or more seamlessly. Perhaps have the mech brakes hug the rotors under certain deceleration threshold so if it needs to switch the pads will be right up to the rotors already, and this should dry off, clear off, warm up the brakes before it needs to do any work.

The reason for the switch to mech brakes from regen on bumpy roads is because ABS doesn't really work with the regen brakes. More obviously so on non-AWD hybrids. It switches to mech brakes so you can get ABS action on all four wheels.

RX_330

This is a completely irrelevant and not an issue. CNN is a joke and they're just digging up irrelevant information to try to cause a storm again. What those documents reveal are results from pre-production testing. Problems are forced to happen, normal, and expected. You have to be able to create a problem in order to be able to prevent it successfully.

Anyway, it doesn't matter because that system isn't even sold in the US and all problems discovered in pre-production testing were resolved before the vehicle went on sale and into production.

richrf

Thanks for your comments While I understand the scenario you are trying to describe, it does not describe the sense of sudden lurching or lunging forward that I am feeling as does the passenger. It is not simply a slower, slowing down. It is a sense that the brakes are being released and the car is moving forward suddenly. It happens only under certain conditions such as a slippery road, maybe going over a bump or something similar. There are lots of moving parts in this moment and I believe they are cooperating in an unsafe manner.

This may be the case. I have no idea. We are all guessing. Only the engineers can determine for sure based upon the underlying design of the system. There is certainly no rust. There may be condensation on the surface. Cold may be playing a part. But none of these scenarios gives me any sense of safety.

Fundamentally, I should not feel that the car is lurching forward while breaking. It is an extremely unique sense that is unnerving. A sense that no driver should be experience while braking. It could lead to all other types of unusual reactions from both the driver and the passengers.

Then the whole design may need to be reworked so that the car does not behave in this manner.

RX_330

What you are describing and what I am describing are exactly the same thing. The sudden lunge forward is due to that split second where it switches over and there is no braking action.

So you have the regen braking slowing you down, now for whatever reason the car senses it should switch over to mechanical brakes (full battery charge, more braking power, etc.), so regen stops, and mech takes over. Now, in that moment between the two systems disengaging and engaging you have 3500 lbs moving forward still - that's a lot of momentum. And since you have nothing slowing it down, it's going to push forward. It doesn't accelerate, that's the wrong term to use in this instance, it just pushes forward under its own momentum. You cannot overcome basic physics and that's what this is.

It happens in such a small amount of time though. It's no different than ABS which automatically releases brake pressure for fractions of a second. In many instances (low traction conditions such as snow, gravel, dirt) ABS actually increases stopping distance, however the added stability and control it provides is considered a better tradeoff.

What I've been describing is absolutely 100% normal operation for hybrids. This is how it was designed to work. However, I've not driven nor ridden in your HS. If you truly think what you're feeling is completely different than what I've described all along, then by all means have your dealer pursue the issue.

noku

Seems that richrf's car switch over from regenerative to regular braking may be taking longer than others, thereby creating a much more noticeable effect while driving. Might want to ask the technician if its possible to have this checked out. I usually creep to a stop so something like this may not be as pronounced in my case nor do I have snow/ice here and not many metal grating type roads to drive over. Might be wise to use the "B" position while braking in such conditions as much as possible. It should help with a sudden loss of braking during a really short span.

TheAirman

This is a common complaint with hybrids and has been since the beginning using regenerative braking systems.

The perception of no brakes that the original poster feels is real, and a serious issue to him/her. But most of the time it's only a perception and if affects different people differently. Unless, of course, the OP's car is really flawed and has a true problem with the brakes. Only the Lexus Tech and the OP know for sure, and I can't wait to hear the outcome. Please keep us posted.

Issues like this were common when anti-lock brakes were first introduced on cars years ago. People had the perception that their brakes were not stopping their car on slippery surfaces because the wheels didn't skid. But regen braking combined with anti-lock opens up a whole new can of worms.

Here's some good reading on this touchy subject:

http://www.hybridcars.com/safety/sor...eel-26604.html

richrf

Thanks for the detailed explanation. This may or may not be what is happening, but at least it is an explanation. Much more than what I am getting from Lexus or my Lexus dealer at this time, both of whom are pretending nothing is going on simply because a computer read out is saying everything is OK. To use a computer read out as the final arbiter makes not sense. Of course the computer will be saying everything is OK as long as it is doing what it is programmed to be doing.

If this is standard behavior for all hybrids, then something is very wrong. To have a car feel like it is loosing control and lurching (accelerating) forward while braking is counter-intuitive and dangerous since any driver will have a very unsettled feeling and possibly react in a unpredictable manner. Just consider the feeling that you might have if you are simply beginning a braking process and all of a sudden it feels like you are accelerating. Why did any engineer ever allow such behavior?

Whatever is happening, I think that Lexus should investigate it and come forward with a clear answer and if indeed this is normal behavior then it should be declared for everyone to know and make appropriate decisions based upon the facts of the matter. I hope to get an official answer from Lexus. At this point I am not at all satisfied with their level of service or commitment to the quality of their engineering and cars.

Again, thank you very much for your detailed description of the matter. It is far more than I have received from Lexus to date.

richrf

Thanks for your comments. They are very helpful.

The whole process of braking, whether it be under slippery conditions or uneven conditions, has become quite a stressful situation nowadays since I the behavior of the car is unpredictable. What I do is just drive much slower than normal and press very slowly and just observe. Driving no longer is "second nature', if you understand what I mean. The whole experience is much less relaxing than any car I've ever had in the past, especially after having the lurching experience twice in the last month. There is just no way to relax. I don't know if I will be able to become relaxed again during the dry months of the Spring and Summer. I hope so. Right now everything is tentative.

richrf

Thanks for the link. I registered my own complaint at the NHTSA website so that there is an additional record.

https://www-odi.nhtsa.dot.gov/ivoq/

I agree with you that this is a whole new can of worms and if this is standard behavior, then Lexus does have a major engineering problem. The biggest problem is that the behavior evokes a very unpredictable response from drivers and passengers because the feeling is so unusual. Much different from any ABS system I have ever used. Never did I ever have the feeling for lurching forward (accelerating) while braking. It is so against the grain of what any driver has experienced. The only time one gets this feeling is during a brake failure and that is how it feels.

I'll keep everyone informed of any results. So far, Lexus is playing see no evil, hear no evil, speak no evil. It is really quite insulting.

richrf

Hi,

I met with a Field Technician today for what he thought was going to be another road test but I just pulled out the article which TheAirman provided a link to (thanks much for the link!) and the discussion went into a whole another direction.

First, there was a direct acknowledgement of the issue. (I call it a problem but no reason to quibble over semantics). He then proceeded to give me a detailed description of how the regen brakes are activated and how under specific conditions that have been mentioned in this thread there is an abrupt changeover from regen brakes to mechanical caliper brakes. It is at this moment that the car feels like it is lurching. It appears that caliper brakes are only activated under 5 mph (if the car is coasting to a halt) or if there is a sudden force applied to the brakes.

So, when the ABS system detects one of the wheels spinning and the car is coasting but still over 5 mph, it does that regen to mechanical switch which feels so uncomfortable. The recall on the 2010 models was suppose to tighten up the interval using a software upgrade. It did. But there is no way to do away with the issue unless there is actual overlap.

What we decided to do was try to decrease the number of times this event occurs by softening up the tire pressure from 36 to 33. Tire pressure is suppose to be 32 but the dealer keeps it at 36 so as not to activate the low tire signal. Simultaneously, the dealer lowered the signal activation threshold from 32 (factory setting) to 26. Hopefully, with a slightly softer tire, the ABS will not be activated as often, especially here in Chicago where pot holes and uneven streets is the rule not the exception.

The problem is also more noticeable on lighter cars, again because of the greater tendency to engage the ABS system on these type of cars. This may be why it is not showing up as much on the heavier hybrids.

This is a core dump of what happened. If anyone has any questions, I will try to answer them. Whether or not I will ever get use to it is hard to say, since the sudden motion is unnerving. However, at least I understand what is happening and therefore can feel better after the event. I don't know why Lexus is not training its dealership staff to be more aware of this issue so that there is an immediate explanation (I know for sure that no salesperson will reveal it at time of sale. That would be too much to ask). My service representative was aware of the problem but this was the first time she had heard a detailed explanation. She was listening very intently.

beavlexus

Hi All. I live in the great state of Oregon, haven't noticed any brakes/lunging problems yet. We purchased a used 250H early Feb. no black ice problems either.

Thx, BL

Magellan55

richrf, glad you got an explanation from Lexus, and help here on the forum!

If anyone else experiences this bad enough to make you feel uncomfortable, by all means file a complaint with your dealer. It's the only way Lexus will know the extent of the problem, so they apply some engineering resources (SW patch or whatever) to address it. This is how recalls get started.

TheAirman

I'm glad you finally got some answers.

But softening the tires? Wow, it never occurred to me! That's a new one for the record books!

I know on my "other car", and keep in mind you may think the streets of Chicago are bad, but they don't hold a candle to the streets of Pittsburgh, but anyways, when I am going down a certain hill on my way to work and the roads are wet, when I hit this frost heave bump slowing down to a stop, the tire that bounce into the air, the ABS activates on it. That cause a feeling of lost braking power for the second or two that the ABS in activating on the bounced wheel. So I know the feeling you get must be unnerving.

In my 2011HS, which was made in October of 2011, I haven't felt anything like that yet. In fact, twice now someone has cut across traffic in front of me and I have had to panic stop. To the point of activating the brake assist and pre-collision system. I can tell you the bakes on the HS are really good! So could it be your car specifically?

And it's no wonder your dealer listened so intently. I myself think it's REALLY scary that the dealers who sell them, know so little about hybrids.

Thanks for the info. Keep us updated, and now with this info I'll be looking for that same thing to happen on my car.

rajeev6

Richrf,
Thanks for posting the explanation - although I don't understand all of it but I'm not familiar with all the technologies involved with cars either. With others (more technologically savvy in this area) accepting this explanation I feel more relaxed now.

As Airman said, on 2 different occasions, I had to slam hard on my brake too - to the extent that the bottom of nose of the car almost touched the pavement but my car never hesitated in braking. It's a 2010 bought in May of that year. I also live in cold climate and yes we do have a lot of potholes too (thanks to the snow plows) and even thru the winter, I kept tire pressure to around 37-38.

I'm beginning to think that the design of switch from regen to mechanical was done correctly, but due to mechanical parts involved, and since not all mechanical parts are 100% compliant to required size and shape, this issue is popping up in some cars and is fixable by reconfiguring the lag thru software for that particular car. Just my thought