Does Your Rx "jump" Over Holes And Cracks?

[LexRexBlue]

Does anyone else experience their RX400 "jumping" or lurching over the smallest holes or cracks in the road while coasting or braking? It drives me crazy and I have a problem at the bottom of the hill near my street. There is a Stop sign and when I try to stop the car it lurches into the intersection instead. Since I need to use this street several times a day it is a problem. The dealer readjusted the skid control, but it isn't helping.

[kikogirl]

Does anyone else experience their RX400 "jumping" or lurching over the smallest holes or cracks in the road while coasting or braking? It drives me crazy and I have a problem at the bottom of the hill near my street. There is a Stop sign and when I try to stop the car it lurches into the intersection instead. Since I need to use this street several times a day it is a problem. The dealer readjusted the skid control, but it isn't helping.

I have the same problem with you. I was almost hit the car in front of me one time. I'm trying to do more research this problem, It is danger.

[larrytrk]

Mine does it over speed bumps on my street, especially in the rain. I think it's related to the brake regeneration cycle & ICE. Dealer had no comment!

[LexRexBlue]

I'm glad I'm not the only one. The dealer is researching the problem, but Lexus will have to come up with the fix. Since moving isn't an option I need a "fix".

[wwest]

I'm glad I'm not the only one. The dealer is researching the problem, but Lexus will have to come up with the fix. Since moving isn't an option I need a "fix".

My guess would be that this problem lies at the feet of the engineers designing the operational aspects of the regenerative braking system.

The RX400h is a fairly heavy vehicle as for as highly fuel efficient HSD systems go, say like the Prius and Camry. So the engineers undoubtedly needed to squeeze every possible "erg" from the regenerative braking system's ability to recharge the battery. But the "rub" is that there is a "top end" limit to just how fast you can "pour" electrical current into the hybrid battery. And I'm not talking HEAT from the recharge cycle, that certainly is a concern, but one of a longer term. There is a chemical process, a fairly slow chemical process, that occurs within the battery during recharge, this process cannot be "hurried" absent damaging the battery in the short term.

So, let's say the hybrid battery is getting to the point wherein it needs, seriously, to be recharged. And at about this same time you apply braking to a fairly high level in order to stop quickly. The hybrid battery cannot accept the rapid charging rate cognizant with the level of regenerative braking that is suddenly available, so the engineers have decided to "chop off" these peak charging rates in order to spread them over a longer period that will recharge, but not damage, the hybrid battery.

The result...??

The overall braking system does not as quickly transition from mostly regenerative braking to actual friction braking in order to stop as quickly as you desire. At least not as it might otherwise do with a battery charge already at a more "acceptable" level.

And the inconsistency of this "effect" results in the driver not being able to learn when to use harder pressure on the brake pedal.

None of the above is anything close to a "for sure" answer, just a best guess. But for those of you experiencing this problem might want to pay closer attention, as a trial, to the hybrid battery charge level and adjust your brake pedal foot pressure accordingly.

Oh, one other point. If just one wheel should lose traction with the roadbed (pothole or crack??) as you are trying to come to a quick stop, even a moderately quick one, the regenerative braking functionality might be suddenly DISABLED due to the resulting BRIEF (so brief you may not even take note) activation of ABS, INSTANTLY and COMPLETELY disabled. Otherwise, like actual engine compression braking on a FWD vehicle, regenerative braking can, and will, interfere with the functionality of the anti-lock braking system.

This QUICK transition from a reasonably heavy regenerative braking level into ONLY friction braking might not be as smooth as one would desire. It is entirely possible that the brake pedal pressure level resulting in fairly heavy regenerative braking forces might not result in braking forces quite as heavy once the system (suddenly) transitions into friction braking ONLY. Suppose, for instance, the brake rotors happen to be slightly wet, or maybe even a bit of surface rust from not being driven for 8-12 hours.

[RX400h]

Good points, WWest. I'm sure vehicle weight has something to do with it, since few Prius owners complain of this (at least from what I've seen).

[LexRexBlue]

Thanks, the service advisor had a similar theory although the battery is fully charged! He is researching it further. He suggested braking earlier before the bottom of the hill. If I almost stop 6 feet beforehand it doesn't jump, but this isn't real practical when I'm rushing to work. In fact, my battery is almost always charged (green). Believe it or not the cracks and holes span the whole width of the car so that it is both front wheels that are going over the holes and cracks. By the way the holes aren't deep--about 2 inches.

[08rx400h]

I don't have the problem with my '08 400h. Yours is a '06, right? Perhaps Lexus already noticed the problem and have a fix on newer 400h's. Anyway, this is a dangerous situation, you should pursuit Lexus about this matter.

[wwest]

I don't have the problem with my '08 400h. Yours is a '06, right? Perhaps Lexus already noticed the problem and have a fix on newer 400h's. Anyway, this is a dangerous situation, you should pursuit Lexus about this matter.

I still think that if you are braking as you cross a pothole or crack in the roadbed that inadvertently results in activation of ABS the regenerative braking system will be INSTANTLY disabled. That might result in a very noticeable perdubation in braking "feel" as the system must now quickly transition from regenerative braking to "full" frictional braking ONLY. Remember that a fair volume of hydraulic/brake fluid must be "moved" (and "pumped"??) before a significant level of frictional braking will come "on line".

It's even possible that in the above instance the ABS firmware knows not to actuate the brake fluid pressure release solenoids but first "kill" regenerative braking. Now that I think about "that" in more depth it would actually be unwise for the ABS to "dump" brake fluid pressure in this instance as now the important thing is to get that pressure to "rise" to the occassion.

It also occurs to me that the hybrid battery may not necessarily need to be even close to being fully discharged for the regenerative braking system to be operating at a fair high level during actual braking.

Hybrid systems MUST "take" whatever level of regenerative braking is available, when and where it becomes available. Even if the hybrid battery is almost fully charged a "wise" engineer would still use the highest level of regenerative braking available. Like the camel at the watering hole, you never know when the next opportunity of recharging the battery might come along. So the programming rule for regenerative braking might be...

"Drink" as much as you can whenever, where ever the opportunity arises.

[s10]

I don't have the problem with my '08 400h. Yours is a '06, right? Perhaps Lexus already noticed the problem and have a fix on newer 400h's. Anyway, this is a dangerous situation, you should pursuit Lexus about this matter.

I've noticed this already a few times on my 08 RX400H when stopping while driving over a small pothole or similar.

Although the car does stop normally, and I don't think it is a dangerous "defect" it surely feels strange since you think the brakes stop working for a fraction of a sec.

[wwest]

I don't have the problem with my '08 400h. Yours is a '06, right? Perhaps Lexus already noticed the problem and have a fix on newer 400h's. Anyway, this is a dangerous situation, you should pursuit Lexus about this matter.

I've noticed this already a few times on my 08 RX400H when stopping while driving over a small pothole or similar.

Although the car does stop normally, and I don't think it is a dangerous "defect" it surely feels strange since you think the brakes stop working for a fraction of a sec.

And if my theory is correct there CANNOT be a fix, at least not an easy one.

But maybe pre-charge, lightly pressurize, the brake pistons like one of the european marques does if the windshield rain sensor triggers....?