Cruise Control Decel Function.

wwestwwest Member Posts: 10,706
Apparently the latest versions, designs, for cruise control have been seriously revised.

Since late in the last century manufacturers have been doing almost everything possible, conceivable, to improve the safety factor of vehicles with automatic transmissions when operating on low traction surfaces, most especially FWD or front torque biased AWD vehicles wherein engine compression braking will be the most detrimental.

It has now become common knowledge throughout the industry that engine braking on the front wheels will oftentimes interfere with the operation of ABS, potentially to the detriment, obviously, of the owners/passengers.

For those with long term stick shifting experience think about how often you wish for a clutch as you drive along in wintertime with your automatic shift transmission, especially a FWD one.

Most new owners manuals state quite explicitly that engine braking cannot be attained absent a manual downshift and in some cases not even then unless you disable, completely turn off, cruise control.

This whole widspread episode of throttle delay, 1-2 second downshifting delay/hesitation has arisen as the result of widespread industry adoption of a new automatic transmission shift pattern/sequence adopted late in the last century.

The technique involves quickly upshifting these electronically controlled transmissions/transaxles upon any FULL lift-thottle event wherein should the current gear ratio be retained would result in a significant level of engine braking. The idea is to improve the safety factor by virtually eliminating engine braking that cannot be overcome by the operator absent a quick shift into neutral, an action currently recommended by the AAA, but of itself fraught with peril.

So, rather than retarding the timing, as was previously done, to reduce the road speed of cruise control, I wouldn't be at all surprised to see the use of rear braking only via the traction control system to initially slow the vehicle.

But it is now pretty clear that applying the brakes, in total, to do this will be potentially safer, overall, than the use of engine braking which cannot be alleviated by the anti-locking braking system should it subsequently be needed.

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....
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Comments

  • andys120andys120 Loudon NHMember Posts: 22,669
    What'd he say? :confuse:

    You don't have to be a genius to figure out that disengaging your CC can result in some engine braking and every owner's manual I've ever read cautions against the use of CC in slippery conditions.

    "Tempest in a teapot?"

    2001 BMW 330ci/E46, 2008 BMW 335i conv/E93

  • wilcoxwilcox Member Posts: 582
    "Be careful out there...." --- ;) We can all agree to that!
  • tariktarik Member Posts: 344
    Maybe I don't fully understand, but are you saying that new programming of the gearbox controllers (or the CC) will shift up if you lift-off quickly enough? Or do you suggest to manually upshift in that moment?

    Well, sure sounds like some bogus last-millenium-advice to me :sick:
  • wale_bate1wale_bate1 Member Posts: 1,982
    Why would anyone be using CC in inclement conditions in the first place?

    This is some bizarre pseudo-corollary to the old climate control compressor theory, isn't it?
  • rorrrorr Member Posts: 3,630
    "....but of itself fraught with peril."

    Have we been attending writer's workshop conferences again?

    In all honesty, I'm at a loss as to just WHAT is the villian in all this:

    Is it the cruise control? Is it the programming in current automatic transmissions? Or is it that old bugaboo, FWD powertrains?

    I mean, the TITLE of this thread has to do with "Cruise Control Decel", but you spend your entire post discussing engine braking, ABS, automatic trannies, FWD, and the slippery conditions "fraught with peril".

    Uh, why not just NOT use cruise control in icy conditions? Isn't this the standard instruction in every owner's manual?
  • wwestwwest Member Posts: 10,706
    Yes, Rule # 1, NO ONE should be using cruise control on adverse roadbed conditions. But then NO ONE should EVER drive with a Blood/alcohol level above 0.08....

    Absent being able to enforce rule # 1 what would you have the automotive industry do?

    Personally I would disable cruise control when raining or if the OAT were below ~35F. And I most certainly would disable it if TC, Traction Control, activated. It would not be re-enabled absent a complete and full stop and an engine restart.

    Big Brother strikes again...?

    The post was intended as simply an advisory of the fact that the cruise control systems seem to be evolving away from the use of engine braking and into the use of actual braking.

    And my guess is that this "evolution" is the result of the fairly "recent" discovery by the industry that old style automatic transaxles and FWD were/are not a good match.
  • rorrrorr Member Posts: 3,630
    "Big Brother strikes again...?"

    Yes. And no thanks.

    Perhaps if the wiper speed has been set to 'high' I could understand having the CC disabled. But OAT below 35F? Ah, no. Living in rural central Texas I see temps below 35F virtually every morning in the winter - yet we see 'adverse roadbed conditions' which would actually WARRANT no CC maybe 1-2 times a year.

    Personally, I think the best 'solution' (Houston, do we even HAVE a problem?) is driver training.

    "...what would you have the automotive industry do?"

    On this issue? Nothing. How much nanny-engineering do we need? Radios that turn themselves off when vehicle speed exceeds 45mph? Mirrors/seats/steering wheels that CAN'T be adjusted unless the vehicle is stationary? Adaptive throttle controls that restrict vehicle speed based on tire/brake wear indicators?

    In all honesty, I'm not entirely sure just WHERE the discussion should be going on this issue. Are we supposed to be debating engine braking vs. wheel braking as a means by which the CC decel functions? Or is this whole thing just a left handed way to attack the safety aspects of FWD powertrains again?
  • wwestwwest Member Posts: 10,706
    And what about those bridge decks with an OAT ~35F...??
  • rorrrorr Member Posts: 3,630
    "And what about those bridge decks with an OAT ~35F...??"

    What about 'em?
  • wale_bate1wale_bate1 Member Posts: 1,982
    Well, some of 'em need painting something fierce, to begin with...
  • rorrrorr Member Posts: 3,630
    Yep, they sure do.

    Of course, I'd let it warm up a bit first.....
  • robertsmxrobertsmx Member Posts: 5,525
    Okay, none of that made sense to me. I do know a lot of vehicles (Infiniti M35/45 is a good example) that have nanny features controlling what can and what cannot. For example, with wiper running, cruise control can no longer be engaged.

    Now this one...
    It has now become common knowledge throughout the industry that engine braking on the front wheels will oftentimes interfere with the operation of ABS

    How exactly does that happen?
  • rorrrorr Member Posts: 3,630
    "How exactly does that happen?"

    Good question.

    All this time, I thought it was a closely guarded secret and now I find that it is common knowledge...... ;)
  • wwestwwest Member Posts: 10,706
    At fairly low speeds on a highly slippery surface the engine can provide just enough "braking" such that your anti-lock braking feature, the ability to release the brakes and thereby keep the front wheels turning just enough to maintain directional control, will be compromised.

    Ford was just awarded a patent concerning the use of regenerative braking for hybrid vehicles wherein the primary regenerative source is at the front wheels (where it sorta has to be anyway).

    The patented technique involves substantially reducing the level of regenerative braking if the OAT is near or below freezing, and additionally, otherwise, to disable regenerative braking entirely the instant ABS activates during actual brake pedal pressure.
  • andys120andys120 Loudon NHMember Posts: 22,669
    ...the "recent" discovery by the industry that old style automatic transaxles and FWD were/are not a good match.

    I am unaware of any such discovery, please enlighten us as to what "the industry" has discovered.

    I wonder if you could also explain what on earth the relationship might be between OAT and regenerative braking. :confuse:

    2001 BMW 330ci/E46, 2008 BMW 335i conv/E93

  • oldharryoldharry Member Posts: 413
    I have owned several FWD and several RWD vehicles over the years, and question Mr. West's assertion that engine braking is not desirable on FWD vehicles.

    That the front wheels are braking without the rear gives a handling situation similar to the RWABS on my 94 C-1500. If this is bad, all the pickups with RWABS are unsafe.

    When I had a stick shift FWD, I frequently used engine braking to avoid spinout from rear wheel lockup during slipperly conditions.

    Driver training could convince you, Mr. West.

    Harry
  • oldharryoldharry Member Posts: 413
    wwest wrote:

    "At fairly low speeds on a highly slippery surface the engine can provide just enough "braking" such that your anti-lock braking feature, the ability to release the brakes and thereby keep the front wheels turning just enough to maintain directional control, will be compromised."

    Actually, at LOW speed the engine does not brake the vehcle, but keeps the wheels turning, enhancing the ability of the ABS to keep the front end steerable.

    Your explanations all sound theoretical, Have you ever driven a FWD in winter enough to be familiar with the actual handling?

    Harry
  • wwestwwest Member Posts: 10,706
    speaks for itself...
  • oldharryoldharry Member Posts: 413
    Can you give a link to the patent. I would like to read it myself, and consult with my son who is a mechanical engineer.

    Harry
  • rorrrorr Member Posts: 3,630
    Here you go Harry:

    http://www.patentstorm.us/patents/6588860.html

    However, I think wwest may be drawing some faulty conclusions (particularly in regard to his interpretation that this is further 'evidence' of the evils of FWD).

    As I understand the writeup (and I'm admittedly not an automotive engineer), in a hybrid drivetrain utilizing regenerative braking, there are several programming maps which the overall braking system consults in a braking event. In 'normal' situations, when the driver first takes their foot off the gas, the system defaults to lift-throttle regenerative braking (referred to as "Compression" regenerative braking because the braking is due to engine compression). Then, as the brake pedal is applied, a secondary component called "Service" regenerative braking is added. The total amount of regenerative braking is the sum of these two (compression + service) components.

    However, the ABS system can only modulate the "Service" portion of the system. And, if the road is sufficiently slippery, only modulating the "service" portion of the regenerative system may not be enough to unlock a locked wheel (since the "compression" portion may offer enough braking force to keep a locked wheel locked). Therefore, in low temps (close to freezing OAT), the braking maps reduce (or eliminate) the "compression" portion of the regenerative braking.

    Now, how all this translates into some sort of global condemnation of FWD powertrains in general, I'll have to let wwest answer. Because I can't find any evidence in the writeup that Compression braking, BY ITSELF, can lock up a wheel.
  • wwestwwest Member Posts: 10,706
    A slight misread of the Patent...

    Actual engine compression is NEVER present unless the Ford hybrid implementation, like the Prius, has a special "mode" to enable it.

    On normal lift throttle events "light" regenerative braking is used to "simulate" actual engine compression braking. Now as the brakes are applied, more and more regenerative braking will be used as brake pedal pressure is increased.

    And yes, if in freezing conditions, or even in rain, the potential for loss of control increases to the point where it is now being acknowledged that both RWD and FWD vehicles can transition into "unsafe" territory without any obvious actions of the driver.

    ABS will only activate with the most severe braking, itself based on roadbed conditions. Severe, HARD braking, is also the point at which regenerative braking will be at the highest level. So yes, it would be best to disable regenerative braking on activation of ABS regardless of road conditions.

    And also yes, this is an especially serious matter for FWD and/or front torque biased AWD wherein loss of traction at the front means TOTAL loss of control.

    And you don't have to be an automotive engineer to know that the front brakes expend/supply ~80% of the braking HP, and 100% of the stearing capability. So having ABS operate correctly on the front wheels is an especially important consideration.
  • rorrrorr Member Posts: 3,630
    "And also yes, this is an especially serious matter for FWD and/or front torque biased AWD wherein loss of traction at the front means TOTAL loss of control."

    Whoops! See, that's where you lose me.

    If you have loss of traction at the front in a RWD vehicle due to ice, don't you ALSO have total loss of control? Or are you saying that while the road conditions led to loss of traction in the front of a RWD vehicle that I somehow still have adequate traction to apply power to the rear?

    The odd thing is that I HAVE experienced momentary loss of control due to ice and lift throttle conditions. The odd thing is that this was in a RWD vehicle. Lifting the throttle caused weight transfer onto the nose; the rear-tires lost traction, and the rear-end started coming around. Fortunately I was only doing about 30 so it was a slo-mo tailslapper. I've been in exactly the same circumstances in a FWD vehicle and lift-throttle DIDN'T cause loss of control to the front wheels.

    In fact, usually the opposite occurs: I've taken turns a smidge too fast in cruddy conditions with FWD and the front end starts to plow. The instinctive thing to do is to get off the gas. Hmmm, lifted the throttle, compression braking cause weight transfer ON TO the nose and traction at the front was regained.

    Seems straight-forward to me.
  • wwestwwest Member Posts: 10,706
    Simple, really...

    Engine braking occurs on the REAR of a RWD vehicle but on the FRONT of a RWD vehicle.

    Which would you rather, assuming both with automatics, be driving downhill on a slippery roadbed?
  • british_roverbritish_rover Member Posts: 8,502
    Engine braking occurs on the REAR of a RWD vehicle but on the FRONT of a RWD vehicle.

    Err care to restate that?
  • robertsmxrobertsmx Member Posts: 5,525
    Which way does weight shifts during deceleration? How does it help or make things worse? Make a case for FWD and RWD. Then we will get to the point of ABS.
  • rorrrorr Member Posts: 3,630
    "Which would you rather, assuming both with automatics, be driving downhill on a slippery roadbed?"

    Honestly not sure; 95% of my driving history since 1978 has been with a manual.

    However, I've had both a 5.0 Mustang and a Toyota Celica driving downhill on glaze ice (here in central Texas we tend to get freezing rain/sleet rather than nice fluffy snow) and I'll tell you that the Celica (and my previous Hondas) was easier to control.

    When headed downhill, braking (whether by mechanical means or through engine compression) causes weight transfer to unload the rear of the vehicle and load the front. Weight = traction. Therefore, weight transfer caused a LOSS of traction at the rear and an increase of traction at the front.

    The problem with the Mustang was that IF the rears locked, the rear of the car wants to lead. Swap ends. And locking the rears due to compression braking was pretty easy since you are unloading the wheels being braked.

    The Celica on the other hand was much more difficult to induce wheel lockup due to compression braking. The weight transfer would LOAD the front of the vehicle (remember: weight = traction) helping to keep the fronts unlocked.
  • oldharryoldharry Member Posts: 413
    My current vehicles are both automatic, and my experiences mirror yours. Slowing/braking downhill on glare ice, the FWD has better control than the RWS. The ABS system on the Impala (05) has no difficulty preventing wheel lockup, while the rear ABS on the truck does not stop the front from locking, and steering is lost with ANY application of brakes under extreme slippery conditions.

    I therefore leave a good interval when driving the truck on ice, and slow well in advance, releasing the brakes to reaquire direction when the front slides out of desired path, then braking again as necessary.

    Under cruise control conditions, wet pavement gives no problem, and I am not foolish enough to use cruise on ice in either vehicle.

    Harry
  • oldharryoldharry Member Posts: 413
    Thanks for the link rorr.

    "Abstract

    The amount of regenerative braking that is applied to the wheels (105) of a vehicle (100) is based on ambient temperature and a lift-throttle event. An ambient temperature sensor (108) monitors the temperature around the vehicle. Based on the temperature, a map is selected (204, 212, 214). If a lift-throttle event occurs, then the map is applied (206, 208). Compression regenerative braking is reduced to zero if an anti-lock braking system event occurs or if the throttle is re-applied, or both (216, 218, 220, 222). "

    This is obviously different in application from a engine driven FWD vehicle, in that, unlike a piston engine, and eletric motor has no 'idle speed' to resist stopping the wheels. Regenerative braking is greater when rotational speed is higher, but unless turned off (per patent) never stops retarding the wheels until motion ceases.

    Harry
  • wwestwwest Member Posts: 10,706
    "...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.
  • robertsmxrobertsmx Member Posts: 5,525
    Do you want him to lose control and crash? :shades:
  • wwestwwest Member 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 Member 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 Member 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 Member 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 Member 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 Member 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 NHMember Posts: 22,669
    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.

    2001 BMW 330ci/E46, 2008 BMW 335i conv/E93

  • wwestwwest Member 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 Member 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 Manahawkin, NJMember Posts: 10,379
    This topic would be a lot better with English subtitles.....
    2015 Mazda 6 Grand Touring, 2014 Mazda 3 Sport Hatchback, 1999 Mazda Miata 2004 Toyota Camry LE, 1999.
  • wale_bate1wale_bate1 Member Posts: 1,982
    Just follow the body english - you'll catch up...
  • rorrrorr Member 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 Member 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 Member 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 Member 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 Member 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 Member Posts: 1,982
    "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 Member Posts: 10,706
    that needs to be said has now been said...

    Anything additional will only be argumentive.
  • wale_bate1wale_bate1 Member Posts: 1,982
    "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 Member Posts: 1,284
    Seems to me that was the original intent of the thread.

    E-brakes going downhill on slippery surfaces. Indeed! :lemon:
This discussion has been closed.