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Cruise Control Decel Function.

2

Comments

  • wwestwwest Posts: 10,706
    Well no, absolutely NOT..!!

    But I suppose I should have added a caution note to not do this test on a low traction surface.
  • robertsmxrobertsmx Posts: 5,525
    So when does low traction surface become desirable anyway? Your argument is a FWD will lose control but a RWD will stay planted, doesn't make sense at all. Do you realize that more weight on drive wheels = more traction?
  • rorrrorr Posts: 3,630
    "So when does low traction surface become desirable anyway"

    Well, a low traction surface is desireable (to a point) when one starts considering roadbed friction and it's relationship to rolling resistance.

    As you know, rolling resistance coupled with air resistance are the big bugaboos when one is attempting to maximize fuel economy. So, while it might be beneficial to tuck in tight behind large trucks to reduce (and possibly eliminate) most air resistance, it is also beneficial to take all reasonable steps to reduce rolling resistance.

    The tire/roadbed interface is the prime culprit in rolling resistance. We can take some steps to reduce this by inflating the tires to a high pressure. However, this has it's limitations. One benefit to colder weather (approaching 0deg C) is that the cold temperatures affects the rubber compound in the tires, making the tires harder, and thus further reducing rolling resistance. If you couple this with a slick roadbed surface, you have a combination that can't be beat.

    I've found my best hypermiling experiences to occur when tucked 6' off the bumper of semi's with my tires inflated to the maximum sidewall indicated pressures on ice-slickened roadways. Such experiences also give me a great cardiovascular workout without actually exercising (although I usually leave indentations in my steering wheel).

    However, I can say that wwest does bring up some valid concerns regarding possible front wheel lockup. However, I've found it to NOT come necessary from lift-throttle conditions (since, as has been pointed out, such conditions cause a weight transfer to the front wheels, increasing traction).

    Instead, I've found that sudden engine loads imparted by the AC compressor can bring about front wheel lockup on icy roads; particularly when tucked up tight behind a semi has reduced the aero downforce to the front of the vehicle. This situation seems to be particulary a problem with my Toyota since I have SUCH a problem keeping the windshield properly defogged using the standard defroster settings. I've considered upgrading to a stronger compressor, but the concern over front wheel lockup has precluded me front taking this path.

    Perhaps the next innovation to be considered is remapping the AC compressor/clutch engagement software to the OAT sensors to progressively engage as temps approach freezing. Of course, this is just a half measure to address this potentially deadly issue since you still have a system which can unexpectedly impart undesired engine loads to the drive wheels. Perhaps windshield wipers mounted INSIDE the cabin to clear the screen as the OAT approaches freezing is the best solution.
    .
    .
    .
    Or maybe not.
  • wwestwwest Posts: 10,706
    Now I really feel as if my leg is being pulled....

    The efficiency of the A/C for helping to prevent windshield condensation and/or help to remove windshield condensation is strictly a function of local climatic conditions.

    Throughout most of our world the colder the climate the lower will be the RH, relative humidity. Basically that means that the colder it is where you are trying to use the A/C to help prevent or remove windshield condensation the less likely it is that the will be of any help at all.

    The surface temperature of the A/C cooling evaporator will never be driven below ~34F by design. To do so would result in freezing the condensate and eventually blocking all system airflow. So All modern day systems disable the A/C compressor as the OAT declines below ~34F.

    What that means to you is that the dewpoint of the incoming fresh airflow must be above 34F in order for any dehumidification of the airflow to happen at all. And that 34F number is under ideal conditions. For instance an extremely low blower speed which will allow the airflow transit time through the cooling evaporator to be long enough for it to be cooled to its dewpoint.

    In reality if the OAT is below ~47F, most of the time you may as well have the A/C turned off.

    The BEST way to keep your windshield defogged, or remove condensation once it has formed is to HEAT the interior surface of the windshield as much as possible and as quickly as possible.

    Turn up the temperature setpoint to maximum, the blower speed should follow, and then switch to defrost/defog/demist mode.

    Once the windshield is cleared this method will no doubt begin to discomfort you with all of the heated airflow reflected off the windshield to your face.

    Once that happens then turn the blower speed down manually while leaving the heat up and in 3D mode.

    I NEVER allow my A/C compressor to operate at any time during the winter months and Lexus has provided two C-best options to facilitate that.

    And by the by most newer cars have a variable displacement A/C compression so the engine load can be varied in a linear way as more or less cooling is required.

    Additionally most modern day engine/transaxle control systems, ECUs, will "bump" up the air/fuel mixture feed to the engine just prior to engaging the A/C compressor clutch so that no "drag" on the engine is felt at the drive wheels.
  • wwestwwest Posts: 10,706
    Now I really feel as if my leg is being pulled....

    The efficiency of the A/C for helping to prevent windshield condensation and/or help to remove windshield condensation is strictly a function of local climatic conditions.

    Throughout most of our world the colder the climate the lower will be the RH, relative humidity. Basically that means that the colder it is where you are trying to use the A/C to help prevent or remove windshield condensation the less likely it is that the will be of any help at all.

    The surface temperature of the A/C cooling evaporator will never be driven below ~34F by design. To do so would result in freezing the condensate and eventually blocking all system airflow. So All modern day systems disable the A/C compressor as the OAT declines below ~34F.

    What that means to you is that the dewpoint of the incoming fresh airflow must be above 34F in order for any dehumidification of the airflow to happen at all. And that 34F number is under ideal conditions. For instance an extremely low blower speed which will allow the airflow transit time through the cooling evaporator to be long enough for it to be cooled to its dewpoint.

    In reality if the OAT is below ~47F, most of the time you may as well have the A/C turned off.

    The BEST way to keep your windshield defogged, or remove condensation once it has formed is to HEAT the interior surface of the windshield as much as possible and as quickly as possible.

    Turn up the temperature setpoint to maximum, the blower speed should follow, and then switch to defrost/defog/demist mode.

    Once the windshield is cleared this method will no doubt begin to discomfort you with all of the heated airflow reflected off the windshield to your face.

    Once that happens then turn the blower speed down manually while leaving the heat up and in 3D mode.

    I NEVER allow my A/C compressor to operate at any time during the winter months and Lexus has provided two C-best options to facilitate that.

    And by the by most newer cars have a variable displacement A/C compressor so the engine load can be varied in a linear way as more or less cooling is required.

    Additionally most modern day engine/transaxle control systems, ECUs, will "bump" up the air/fuel mixture feed to the engine just prior to engaging the A/C compressor clutch so that no "drag" on the engine is felt at the drive wheels.
  • oldharryoldharry Posts: 413
    Willard West wrote:

    "The surface temperature of the A/C cooling evaporator will never be driven below ~34F by design. To do so would result in freezing the condensate and eventually blocking all system airflow. So All modern day systems disable the A/C compressor as the OAT declines below ~34F."

    This is incorrect, as most modern systems recirculate warm air with in the vehicle. Hence cooling the cabin air for dehumidifying before reheating it is not disabled in modern vehicles at ~34F.
  • andys120andys120 Loudon NHPosts: 16,643
    I NEVER allow my A/C compressor to operate at any time during the winter months and Lexus has provided two C-best options to facilitate that.

    Most of the owners manuals I have seen recommend occasionally engaging the A/C compressor during the winter to keep the seals from drying out.

    2000 BMW 528i, 2001 BMW 330CiC

  • wwestwwest Posts: 10,706
    Most modern vehicles will not even allow you to operate the system in recirculate mode for more than a few minutes unless the A/C is in operation and I stand by my position that the A/C will automatically terminate operation with the outside climate near or below freezing.

    And by the way it is NEVER a good idea to use the recirculate mode during the winter months due to the possibility of the cabin RH rising too high from human metabolism. Additionally when the windshield does eventually fog over due to improper recirculate use the system will automatically switch to fresh inlet airflow when you use the defrost/defog/demist mode. NOW the A/C will be disabled for certain sure!

    If you can find documented evidence to refute my position I will be more than glad to listen and issue an apology.

    In the Toyota/Lexus line the A/C indicator will turn off automatically ay ~33F and you can manually turn it back on but in actuality the compressor itself will not be enabled even though the indicator is now on.
  • oldharryoldharry Posts: 413
    ""...never stops retarding the wheels until motion ceases..."

    And this is different from..??

    Put your FWD automatic transaxle in the 1st gear position, accelerate to, say, 15 MPH. Now take your foot off the gas and tell when you ICE stops retarding the wheels."
    ------------------------------------
    Do that with your RWD stick or automatic, and then comment on the control you have. (Hint do not try it in traffic)

    Harry
  • fezofezo Posts: 9,359
    This topic would be a lot better with English subtitles.....
  • wale_bate1wale_bate1 Posts: 1,986
    Just follow the body english - you'll catch up...
  • rorrrorr Posts: 3,630
    "This topic would be a lot better with English subtitles....."

    Right-o chap. I was jus' thinkin' that same thing me'own self.

    Now I really feel as if my leg is being pulled....

    "Crikey, what a load of bilge....."

    Sorry mate, can't help with the rest. I've no idea whut'tha bloody-hell 'es sayin' either..... ;)
  • wwestwwest Posts: 10,706
    Engine braking a RWD with STICK shift on a slippery surface can be fairly beniegn providing the driver has enough "stick" experience to know to use the clutch to modulate the level of engine braking on the rear wheels.

    With a RWD automatic on an extremely slippery downhill run I would always advise a quick shift into neutral and then maybe judicious use of the e-brake in the same manner as the clutch in the above case.

    It's the automatic transmission that's the "wild card" in these instances.
  • rorrrorr Posts: 3,630
    "With a RWD automatic on an extremely slippery downhill run I would always advise a quick shift into neutral and then maybe judicious use of the e-brake in the same manner as the clutch in the above case."

    Okay - back to seriousness.

    Braking forces (whether they be due to engine compression or mechanical systems) cause weight transfer to the nose of the car. In fact, as you've pointed out before, the front brakes do vastly more work under normal braking than the rear brakes PRECISELY for this reason. Weight transfer.

    So, physics tells us that under braking (or actually, ANY vehicle deceleration) you get weight transfer. Meaing, the force imparted DOWN through the wheels to the road is DECREASED at the rear of the vehicle and INCREASED at the front of the vehicle.

    Now, the friction force available to brake is the product of that normal force ('weight') x the coefficient of friction. I'm going to assume that the coefficient of friction is identical for both the front and rear wheels (since the coefficient of friction is a function of the tire design and roadbed surface). Under vehicle deceleration, you get weight transfer FROM the rear TO the front, therefore the available friction necessary to slow the vehicle INCREASES at the front and DECREASES at the rear.

    Prior to the advent of ABS systems, what usually happened with RWD vehicles (whether using the brakes or through engine compression) is that braking on icy surfaces caused the rear of the vehicle to unload and the REAR wheels to lockup. Result? Vehicle skid ([non-permissible content removed]-end coming around).

    Using the e-brake on ice is, to put it bluntly, stupid. Not only do you BYPASS the ABS system, put you are braking using ONLY the rear brakes, the very end of the vehicle MOST LIKELY to lock up on icy surfaces.

    Now consider FWD and engine compression on ice. The braking forces are acting ONLY on the end of the vehicle which GAINS traction under vehicle deceleration.

    Honestly wwest, I think the disconnect that we are having is with the concept of weight transfer and how it affects available traction.
  • wwestwwest Posts: 10,706
    You forget...

    Braking at the rear, controlled, moderate, modulated, "non-locking" braking at the rear, will always help to hold the vehicle in line. Much like a "bucket" type drag anchor in a boat drifting downstream. As a matter of fact early anti-lock systems only affected the rear brakes for exactly the reason you state.

    Because of the dramatic disparity of braking capability at the front vs the rear, braking at the front is only really controlable (modulate-able) via ABS.

    And yes, there are times when ABS provides more of an interference factor for adequate braking and directional control simultaneously than otherwise. I have always campaigned for ABS to be disabled unless the VSC system detects that the vehicle is deviating from the desired direction of travel.

    What good is ABS, other than oftentimes elongating my stopping distance, if the vehicle is not deviating from the desired "line" of travel and I have no reason to want to change the direction of travel?

    Yes, applying the rear brakes in a controlled manner via the e-brake defeats the Anti-lock system, but there are times when that is exactly the desireable outcome.

    Personally in these type of conditions, slippery roadbed, especially downhill, I would much rather have the option of reserving my front tires' traction coefficient for directional control.

    A good modification of current ABS might be to eliminate braking on the front entirely in these circumstances and allow ABS to control rear braking. But how would the system go about determined which is which? When is the roadbed slippery enough that front braking is undesireable?
  • rorrrorr Posts: 3,630
    "You forget...

    Braking at the rear, controlled, moderate, modulated, "non-locking" braking at the rear, will always help to hold the vehicle in line...."


    No, I did not forget. The operative words you are using are "controlled and modulated".

    What we've been discussing (I think) are the effects of lift throttle (engine compression braking) on slippery surfaces. You seem (correct me if I'm wrong) to be trying to tie the 'evils' of FWD powertrains to loss of control on ice due to engine braking. The ABS doesn't enter the picture since the ABS system has no control over engine compression. And, as you've pointed out it is difficult to control/modulate engine compression in an automatic equipped vehicle (regardless of whether it is FWD or RWD).

    So, getting back on the engine compression topic: if you lift throttle in a RWD (non-hybrid) vehicle, the ABS system doesn't enter the picture. Only engine braking is present and it is 100% to the rear. Yes, rear-drag braking is preferable for directional stability SO LONG AS THE WHEELS ARE UNLOCKED. However, YOU bring in the spectre of extremely slippery surfaces. I'm simply pointing out that in YOUR scenario, you are MORE likely to lock up the rears due to engine compression because the engine braking has caused a weight transfer OFF of the rear and therefore REDUCED the available traction.

    "Because of the dramatic disparity of braking capability at the front vs the rear, braking at the front is only really controlable (modulate-able) via ABS."

    Ummmm, no. Earliest ABS systems, (particularly on trucks) used ABS on the rears only for a reason. The front brakes were much easier to modulate and the rears had a much higher tendency to lock up due to.......weight transfer under braking. Hence, ABS added to the rear.

    "And yes, there are times when ABS provides more of an interference factor for adequate braking and directional control simultaneously than otherwise. I have always campaigned for ABS to be disabled unless the VSC system detects that the vehicle is deviating from the desired direction of travel."

    So, not only are you of the opinion that FWD is inherently dangerous, but ABS is as well? As far as stopping distances go, I was under the impression that ABS only elongated stopping distances on surfaces like snow, gravel/sand where locking the wheels would allow a wedge of material to build up in front of the tire and increase the coefficient of friction. On any other surface (which would be 99% of the time) ABS should be not only more controllable but shorter as well.

    "Personally in these type of conditions, slippery roadbed, especially downhill, I would much rather have the option of reserving my front tires' traction coefficient for directional control."

    You are assuming the traction coefficient is static. It is not. The coefficient of friction IS static (assuming the road condition is the same from front to rear), but the amount of available traction varies depending on the load (force) applied through the tires to the roadbed. Increase the load, increase the traction (like one does by putting sandbags in the trunk of a RWD car when driving on ice).
  • wale_bate1wale_bate1 Posts: 1,986
    "Even slight engine braking on an extremely slippery surface, an icy bridge deck comes to mind, can easily result in loss of control even on a RWD vehicle, but the potential for loss of control of a FWD in these insatnces rises dramatically in comparison.

    Be careful out there..."


    Now, exactly, or even imprecisely, how many incidents of this chilling phenomenon have been documented out there, even via anecdotage?

    Damn dangerous - someone oughta sponsor some legislation.

    WW, could you start the ball rolling on this one? Just as a public service? I'm sure I'm not alone in feeling a certain degree of gratitude.
  • wwestwwest Posts: 10,706
    that needs to be said has now been said...

    Anything additional will only be argumentive.
  • wale_bate1wale_bate1 Posts: 1,986
    "On any other surface (which would be 99% of the time) ABS should be not only more controllable but shorter as well."

    You know, that seems intuitive, but from what I recall reading, on-track testing with ABS on and with it disabled wasn't conclusive on distance. It was very conclusive indeed on directional control though. I don't think OAT was approaching 34, though...
  • lancerfixerlancerfixer Posts: 1,308
    Seems to me that was the original intent of the thread.

    E-brakes going downhill on slippery surfaces. Indeed! :lemon:
  • rorrrorr Posts: 3,630
    "...from what I recall reading, on-track testing with ABS on and with it disabled wasn't conclusive on distance."

    I'm not that surprised. How effective it is in a straight line is probably dependent on how easy the brakes are to modulate, and the quality of the driver. For panic situations, I'd imagine that ABS beats having all 4 locked up for a novice.
  • wale_bate1wale_bate1 Posts: 1,986
    Precisely so, on both points.
  • wale_bate1wale_bate1 Posts: 1,986
    Wait, what about a slow-speed panic in an auto-FWD, OAT<34?
  • Kirstie_HKirstie_H Posts: 10,890
    Yo, hombre. Nice to see you crawl outta the woodwork.

    wale_bate1 - I, too, would be interested in more info on how pervasive a problem this is. While reminders to drive alertly and with caution are always appreciated, I haven't seen a lot of documented consumer complaints about this.

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  • rorrrorr Posts: 3,630
    lancerfixer.....now where do I know that handle from..... ;)

    "While reminders to drive alertly and with caution are always appreciated, I haven't seen a lot of documented consumer complaints about this."

    I think there's more than a few folks wondering the same thing. Given the pervasiveness of FWD vehicles over the last 25 years or so (and the fact that the vast majority are equipped with automatic transmissions and see duty in adverse road conditions), one would think if there was a problem it would be getting a bit more airplay.

    I'm just hoping that some driving novice lurking in here doesn't think that they should be bypassing the ABS system and using the e-brake instead..... :sick:
  • wale_bate1wale_bate1 Posts: 1,986
    "I'm just hoping that some driving novice lurking in here doesn't think that they should be bypassing the ABS system and using the e-brake instead..."

    Is that wrong? :blush:

    Can't be; look at Jason Bourne...
  • wwestwwest Posts: 10,706
    It would take an idiot to think that ABS would need to be bypassed in order to use the e-brake.
  • 210delray210delray Posts: 4,722
    Given the pervasiveness of FWD vehicles over the last 25 years or so (and the fact that the vast majority are equipped with automatic transmissions and see duty in adverse road conditions), one would think if there was a problem it would be getting a bit more airplay.

    You forgot one thing:

    it's a conspiracy!

    We consumers have been kept in the dark.

    Seriously, thanks to you and the others that have done a good job questioning or debunking some of the theorizing that's been going on here.

    Meanwhile, I'm parking my RWD, rear-only ABS Frontier (with manual transmission, no less) in anticipation of adverse road(bed) conditions tomorrow -- snow and ice. And (horrors!) the FWD, 4-wheel ABS, automatic Camry is going to get me to work! I may even use the defroster!
  • wale_bate1wale_bate1 Posts: 1,986
    But what if they were just using the e-brake to bypass the ABS? What then?
  • rorrrorr Posts: 3,630
    "It would take an idiot to think that ABS would need to be bypassed in order to use the e-brake."

    Maybe I should rephrase that then:

    "I'm just hoping that some driving novice lurking in here doesn't think that they should be bypassing the ABS system BY using the e-brake instead....."

    Better? After all, I'm only repeating your sage advice from post #47:

    "Yes, applying the rear brakes in a controlled manner via the e-brake defeats the Anti-lock system, but there are times when that is exactly the desireable outcome."
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